Spatiotemporal Variability and Tectonic Implications of Very Low-Frequency Earthquakes in the Southwestern Ryukyu Trench

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VLFEs constitute a type of slow earthquakes characterized by surface waves with periods of 20–50 seconds. As they are detectable over long distances, VLFEs are suitable for effective monitoring of aseismic processes in offshore subduction zones with sparse seismic networks. Using waveform data from Japan’s F-net network and Taiwan’s Broadband Array in Taiwan for Seismology (BATS) network, we constructed two VLFE catalogs, namely the FB-catalog (2000–2024), which was based on both networks, and the B-catalog (1998–2024), which was based only on BATS. VLFEs were identified using waveform cross-correlation with template events. Results show that VLFE activity is concentrated along the trench axis at depths of 10–20 km, occurring periodically in swarm-like episodes approximately every 2–3 months. In some cases, these swarms exhibit migration. VLFE epicenters are spatially complementary to thrust-type earthquake zones, suggesting distinct regions of seismic and aseismic slip. VLFE clusters are distributed just south of areas characterized by recurring slow slip events (SSEs) and GNSS-inferred slip deficits. A clear contrast is observed across the subducted Gagua Ridge: Seismicity is more prevalent to the west, whereas VLFEs dominate to the east. VLFE activity has increased significantly since late 2001, temporally coinciding with moderate earthquakes and afterslip, suggesting the activation of slow slip processes or weakened interplate coupling near the trench axis. This finding provides new insight into the spatial distribution and physical mechanisms of aseismic slip along the plate interface—especially near the trench axis where direct geodetic observations are limited. VLFE monitoring can serve as a valuable tool for characterizing interplate coupling in offshore subduction zones. Very low-frequency earthquake slow slip events aseismic slip Ryukyu Trench subduction zone interplate coupling Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 1 Introduction Subduction zones accommodate stress through both seismic and aseismic processes (e.g., Obara and Kato 2016). Aseismic slip can generally be categorized into two types: slow slip events (SSEs) and afterslip. SSEs typically occur spontaneously during interseismic periods, whereas afterslip follows large or moderate earthquakes and decays gradually over time. Understanding whether interplate stress is released seismically or aseismically is essential for evaluating the potential for large earthquakes in a given region. However, detecting aseismic slip remains challenging in areas with insufficient geodetic observation networks. For this reason, this study focuses on a type of slow earthquakes, namely very low-frequency earthquakes (VLFEs). VLFEs are characterized by surface waves with periods of 20–50 seconds and can be detected at distances of several hundred kilometers. This long-range detectability makes them particularly valuable for monitoring tectonic activity in areas with sparse seismic instrumentation, such as the Ryukyu Arc (Ando et al. 2012; Nakamura and Sunagawa 2015). VLFEs have been documented in several subduction zones around the world (Asano et al. 2008, 2015; Ando et al. 2012; Nakamura and Sunagawa 2015; Takemura et al. 2019; Ghosh et al. 2015). Observations show that shallow VLFE activity increases near the rupture zones of large earthquakes due to associated afterslip (Asano et al. 2008; Matsuzawa et al. 2015; Nakano et al. 2018). In addition, VLFEs often co-occur with SSEs in various subduction settings (Walter et al. 2013; Asano et al. 2015; Takemura et al. 2023). Comparative studies of VLFE activity and plate coupling in the Japan Trench and Nankai Trough have shown an inverse correlation between shallow VLFE occurrence and interplate coupling strength (Baba et al. 2020). In the southern Ryukyu Trench, various types of slow earthquakes—including SSEs, VLFEs, and low-frequency earthquakes (LFEs)—can be detected (Ando et al. 2012; Heki and Kataoka 2008; Nishimura 2014; Nakamura and Sunagawa 2015; Nakamura, 2017). Mw 6.5-class SSEs have repeatedly occurred on the subducting plate interface beneath Iriomote Island (Heki and Kataoka 2008; Nishimura 2014; Tu and Heki 2018; Kano et al. 2018). In this region, VLFE activity is concentrated near the trench axis adjacent to the Yaeyama Islands, with swarm-like events lasting several days and recurring every approximately 2–3 months (Ando et al. 2012; Nakamura and Sunagawa 2015). In addition, LFEs associated with VLFEs have been detected 30–50 km landward from the trench axis (Nakamura 2017). The subducting plate in this region lies at depths of 10–20 km, and the spatial distribution of LFEs complements those of SSEs and typical thrust earthquakes. While a plate-coupling region has been identified at the southwestern end of the Ryukyu Trench off eastern Taiwan (Hsu et al. 2012), seafloor geodetic observations indicate that the coupling strength there is weak (Chen et al. 2022). Nevertheless, the VLFE distribution spans the region from 22° to 24° N and from 121° to 125° E, overlapping the presumed coupling zone (Nakamura and Sunagawa 2015). Due to the limited accuracy in epicenter determination in that earlier study, whether this overlap is real remains unclear. VLFE activity near the trench axis in the southwestern Ryukyu Trench likely reflects the slip behavior along the plate boundary, which is otherwise difficult to observe with land-based geodetic networks. Therefore, this study elucidates the long-term characteristics of VLFE activity and clarifies the state of aseismic slip at the subduction interface near the trench axis in the southwestern Ryukyu Trench. 2 Data and methods Waveform data were sourced from continuous broadband seismic observations conducted by the F-net network of the National Research Institute for Earth Science and Disaster Resilience (NIED) in Japan and Taiwan’s BATS network (Fig. 1 ). Since the F-net stations in the Ryukyu Islands were established between 2000 and 2002, enhanced detection capability during this period may have introduced biases in VLFE observations. These potential biases must be considered when interpreting temporal trends in VLFE activity. Two detection methods were used in this study. The first method utilized data from both the F-net and BATS networks to compile a catalog (hereafter referred to as the FB-catalog) covering the period from May 18, 2000, to August 31, 2024. This approach provided high detection capability and improved hypocenter accuracy for VLFEs during this period. The second method relied solely on data from the BATS network, which has been operational since before 1998. This separate catalog (the B-catalog) that was compiled covered the period from January 1998 to August 2024. While this method allows for the evaluation of long-term VLFE activity independent of changes in F-net coverage, it has lower detection sensitivity and poorer hypocenter resolution compared to those of the FB-catalog. For both catalogs, VLFEs were detected and located using a waveform cross-correlation method with template events, following Asano et al. (2015). Both regular thrust earthquakes and known VLFEs were employed as templates (Fig. 1 ). The centroid moment tensor (CMT) solutions of regular earthquakes were selected from the F-net catalog and used consistently for both the FB-catalog and the B-catalog. CMT inversion for VLFEs was conducted using the time–domain inversion code of Dreger (2003). A bandpass filter (0.02–0.05 Hz) was applied to waveform data from both networks, and 300-second waveform segments were used for the inversions. Grid search parameters included horizontal spacing of 0.02° and vertical spacing of 5 km. The centroid time and location were determined as the values that maximized the variance reduction (Figures S1 and S2). The VLFE detection procedure was as follows: bandpass-filtered (0.02–0.05 Hz), three-component waveforms were resampled at 0.25 Hz. Cross-correlation (CC) functions were computed between continuous waveforms at each station and 120-second template event windows. Assuming a surface wave propagation velocity of 3.7 km/s (Nakamura and Sunagawa 2015), the back-propagated CC values were mapped across space and time. Events with an average CC ≥ 0.5 across stations were considered detections. When multiple candidates were found within a 120-second window using different templates, the location with the highest CC value was selected as the VLFE epicenter. Magnitude estimates were based on thrust-type reference earthquakes southwest of Yonaguni Island. After applying the same bandpass filter (0.02–0.05 Hz) to vertical components, the maximum surface wave amplitude was recorded. A relationship between maximum amplitude, epicentral distance, and moment magnitude (Mw) was established (Figure S3 ), with Mw values referenced from F-net CMT solutions. Based on the frequency–magnitude distribution of the detected events, the magnitude of completeness was determined to be 3.7 for the FB-catalog and 3.9 for the B-catalog (Figure S4 ). To assess potential external influences on VLFE activity, changes in Coulomb failure stress (ΔCFS) were calculated using the Coulomb 3.3 software (Toda et al. 2011). These calculations accounted for earthquakes and dike intrusions in the Ryukyu Trench and Okinawa Trough. The relevant source parameters are summarized in Table S3 . Two significant dike intrusions occurred on October 24, 2002, and April 16, 2013. Most events included in the stress calculations were interplate or intraslab earthquakes; nevertheless, the December 18, 2001, crustal earthquake and the March 2002 afterslip near Yonaguni Island were also incorporated. 3 Results Figure 2 shows the spatial distribution of VLFEs based on the FB-catalog. VLFEs follow primarily an east–west alignment along the trench axis near 122.5 °E. These events are concentrated at slab depths between 10 and 20 km, with notable clusters being evident around 123.2° and 123.5° E at approximately 15 km depth. In contrast, between 124.0° and 124.7° E, activity is somewhat reduced and occurs across a wider depth range within the same interval. The horizontal uncertainty of VLFE epicenter locations is approximately 20 km in both the east–west and north–south directions (Figure S5 a). In the B-catalog, north–south location errors are also approximately 20 km; however, due to the absence of F-net stations, east–west uncertainties increase significantly—up to approximately 100 km. Notably, 69% of the VLFEs (Mw ≥ 3.5) listed in Nakamura and Sunagawa (2015) were also detected in the FB-catalog. The spatial distribution of VLFEs and thrust earthquakes appears to be mutually exclusive (Fig. 2 ). No thrust earthquakes occurred within the main VLFE region (23.5°–23.7° N, 123.0°–123.8° E). Instead, thrust earthquakes are concentrated westward between 122.0° and 122.7° E, corresponding to slab depths of 15–20 km and located approximately 30 km west of the primary VLFE zone. Regions where recurrent SSEs occur (Heki and Kataoka 2008; Tu and Heki 2018) and areas of slip deficit (Kano 2021) are located north of the VLFE cluster. In most cases, VLFE swarms remain confined to individual clusters. However, in some instances, activity spreads to adjacent areas (Figs. 4 and S6). For example, in August 2009, activity initiated near 123.0° E and migrated eastward at an approximate rate of 6 km/day (Fig. 4 ). Similar eastward migration was observed in other swarm episodes (see Figure S6). Figure 3 c displays the cumulative number of VLFEs in each subregion (FB-1 to FB-3), along with events of Mw ≥ 3.7, as recorded in the FB-catalog. In FB-1 (23.7° N, 123.2° E) and FB-2 (23.6° N, 123.5° E), VLFEs tended to occur in swarms approximately every 2–3 months. In contrast, FB-3 (23.7° N, 124.5° E) exhibited more sporadic activity, involving one or two isolated events per episode. VLFE activity in FB-1 started in 2002, decreased between 2004 and 2006, and increased again after 2007, with the increase being more pronounced in 2016. Activity declined after 2022. In FB-2, activity started after 2002 and 2004 and increased in 2006 and again after 2016. FB-3 also showed increased activity around 2016. Figure 3 d presents the cumulative number of VLFEs with Mw ≥ 3.9 from the B-catalog. Although event locations in the B-catalog are more dispersed than those in the FB-catalog, an overall east–west alignment remains evident. Epicenter uncertainties are 20–30 km in the north–south direction and up to 100 km in the east–west direction. In both B-1 (122.8°–123.5° E) and B-2 (123.5°–124.3° E), activity was low from 1998 until mid-2001; however, it increased initially in late 2001 and again in mid-2006. In B-1 in particular, activity further increased around 2016. Following the Mw 7.7 Chi-Chi earthquake on September 21, 1999, detection of VLFEs may have been hampered by strong aftershock activity for about one month. However, there were no other clear factors that would have significantly reduced VLFE detectability between 1998 and 2000. 4 Discussion 4.1 Relationship between VLFEs and other slow earthquake distributions The distribution of VLFEs identified in this study is much more localized than that identified in previous work based on manual arrival-time picks of the largest amplitude phases (Nakamura and Sunagawa 2015). The events are concentrated within a narrow (i.e., approximately 15 km wide) zone located south of both the region of repeating SSEs and the slip-deficit area (Heki and Kataoka 2008; Tu and Heki 2017; Kano et al. 2021). This pattern is consistent with the distribution of LFEs reported by Nakamura (2017). Three LFE clusters were identified along the southern Ryukyu Trench (Nakamura 2017), arranged in an east–west orientation between the trench axis and the Ryukyu Islands. The most active cluster, labeled YA-RB (Fig. 12 in Nakamura 2017), is centered at 23.5° N, 123.7° E. LFE activity is also observed within a 30 km radius of this location as well as in the vicinity of 23.5° N, 124.3° E (cluster YA-RC). While LFE epicenter locations have poor resolution in the north–south direction, the correspondence between VLFE and LFE clusters suggests spatial overlap in the along-strike direction. In general, regions of active VLFE occurrence are spatially complementary to areas of strong interplate coupling (Baba et al. 2020). In the southern Ryukyu Trench, VLFE activity is also complementary to the inferred slip-deficit zones; however, these slip-deficit regions are derived from land-based GNSS observations, which makes it difficult to confirm whether a similar deficit exists south of the VLFE clusters, near the trench axis. Notably, VLFEs are sparse and less active in the source region of the 1771 Yaeyama tsunami, i.e., east of 124° E. This observation, also noted by Nakamura and Sunagawa (2015), is even clearer with the improved resolution of this study. The tsunami is believed to have been generated by a large slip (17–30 m) over a 30-km wide area near the trench axis (Nakamura 2009a; Nakata et al. 2024). A similar complementary relationship between VLFEs and tsunami earthquake source areas has been observed in the Nicoya Peninsula. For instance, while the 1992 Nicaragua tsunami earthquake occurred near the trench axis, VLFEs were observed nearby, albeit outside the main rupture zone. This spatial separation has been interpreted as reflecting fault heterogeneity near the trench axis (Baba 2021). In Nicaragua, stress drop estimates for the tsunami earthquake source zone are approximately 1.2 MPa (Bilek et al. 2016), whereas in the adjacent VLFE region to the southeast, the stress drop is estimated to be between 0.1 and 10 kPa (Baba 2021). Although stress drop values for the southern Ryukyu Trench are currently unknown, the spatial complementarity between the VLFE cluster and the tsunami source region suggests heterogeneous frictional properties along the subduction interface. Seismic reflection data reveal the presence of strong reflectors at the subducting plate interface in the southern Ryukyu Trench (Arai et al., 2016). In the Nankai Trough, shallow VLFE activity has been linked to elevated pore fluid pressures (Takemura et al., 2023). These observations imply that VLFE activity in the southwestern Ryukyu Trench occurs in frictionally weak zones characterized by high pore fluid pressure and low interplate coupling. Thus, VLFEs may serve as effective indicators of heterogeneous frictional conditions along the subduction interface. 4.2 Relationship between VLFE clusters and ISZ In the southwestern Ryukyu Trench, VLFE activity is concentrated east of 122.9° E, whereas thrust-type earthquakes are predominantly observed between 122.0° and 122.5° E. This boundary coincides with the northern extension of the subducted Gagua Ridge—a prominent bathymetric feature approximately 25 km wide and 4 km high—subducting obliquely in a northwest direction (Fig. 2 ). The Gagua Ridge enters the Ryukyu Trench near 23.3° N, 122.9° E, and continues northwestward. It is also possible that the ridge extends beneath the Nanao Basin (24.0° N, 122.5° E) (Deng et al. 2023). Previous studies have suggested that the Philippine Sea Plate to the west of the Gagua Ridge has undergone compressional deformation (Wang et al. 2004). In either case, the subduction of thicker crustal material west of the ridge likely alters the stress conditions and influences the seismic behavior. When thick crustal features, such as seamounts, subduct, compression and fluid expulsion are enhanced at the leading edge, promoting seismic activity. In contrast, the downdip region may enter a stress shadow zone, where increased porosity and elevated pore fluid pressure facilitate slow slip phenomena (Sun et al. 2020). A similar mechanism may explain the observed pattern in the southwestern Ryukyu Trench, where enhanced seismicity occurs to the west of the Gagua Ridge and VLFE activity dominates to its east. This spatial contrast strongly supports the hypothesis that large-scale bathymetric features, such as ridges, modulate both seismic and aseismic slip behaviors. These features likely affect fault strength and stress regimes by altering the fluid migration pathways and crustal properties at the subduction interface. 4.3 VLFE activation around 2000 – 2002 The increase in VLFE activity beginning in late 2001 coincided with a series of M6-class earthquakes in the region. Prior to this period, only swarm activity of small VLFEs (Mw < 3.9) had been observed. However, starting in late 2001, swarm activity involving VLFEs with magnitudes exceeding 3.9 began to occur more frequently (Fig. 3 d; see yellow arrow in Fig. 5 b). Three moderate-to-large earthquakes occurred near the trench axis during this time (Fig. 5 a). On November 9, 2000, a thrust-fault earthquake (EQ1; Mw 5.9) occurred at 23.2° N, 124.2° E. On December 18, 2001, a normal-fault earthquake with a strike-slip component (EQ2; Mw 6.5) struck at 23.9° N, 122.8° E, northwest of the VLFE cluster. On March 26, 2002, another thrust-fault earthquake (EQ3; Mw 6.5) occurred near the trench axis at 23.2° N, 124.4° E. Following EQ2 and EQ3, increased VLFE activity was observed (labeled as sequences 1 and 2 in Fig. 5 b, respectively). Assuming that VLFEs are associated with aseismic slip (e.g., SSEs) (Hirose 2010; Davis 2006; Asano 2008), the increase in VLFE activity around late 2001 may reflect the initiation of a shallow slow slip episode near the trench axis. A similar sequence has been documented in the northern Peru subduction zone, where aseismic slip lasting approximately seven months was accompanied by frequent moderate-sized earthquakes, including those with magnitudes up to Mw 6.0 (Villegas-Lanza 2015). In Peru, these included both interplate and normal-fault events—similar to EQ2 in the Ryukyu Trench. In addition, an afterslip related to the March 31, 2002, offshore Taiwan earthquake (Mw ~ 6.8) was detected south of Yonaguni Island (Nakamura 2009b). ΔCFS calculations show that this afterslip increased the stress in the VLFE region by approximately 3–25 kPa. As ΔCFS values of this magnitude can promote fault slip, the afterslip may have sustained VLFE and associated SSE activity over the subsequent years. Similar patterns have been observed in the Nankai Trough. VLFE swarms off the Kii Peninsula in 2009 and 2018 were associated with surrounding SSEs (Takemura et al. 2019). GNSS-A data also revealed long-term SSEs (Mw ≥ 6.6) in strongly coupled shallow regions, coinciding with VLFE activity (Yokota and Ishikawa 2020). By analogy, the activation of VLFE swarms in the southwestern Ryukyu Trench during 2001–2002 likely reflects the onset of long-term slow slip events, triggered by nearby moderate earthquakes and enhanced by stress perturbations such as afterslip. In this region, the lack of offshore geodetic coverage makes direct observation of shallow SSEs difficult. However, the temporal correlation between VLFEs and increased seismicity suggests that VLFEs can serve as proxies for detecting aseismic slip. This highlights the potential value of VLFE monitoring in regions where traditional geodetic methods are not feasible. 4.4 Relationship between the activation of VLFEs in 2007 and 2016 and the surrounding crustal deformation In 2007 and 2016, VLFE activity increased despite the absence of large nearby earthquakes that could have caused significant ΔCFS alterations in the VLFE region (Figure S7). This suggests that the activation of VLFEs during these periods was not directly triggered by dynamic stress changes associated with major seismic events. Examining the broader crustal deformation context, seismic activity and SSEs around Iriomote Island have increased since approximately 2013 (Nakamura and Kinjo 2018; Tu and Heki 2017). This escalation in activity appears to have been initiated by a dike intrusion event in the Okinawa Trough in 2013. However, there was a notable three-year lag before the observed increase in VLFE activity in 2016. Moreover, the ΔCFS imparted by the 2013 dike intrusion was relatively small, i.e., approximately 5 kPa (event 10 in Figure S7), which is likely insufficient to directly trigger a significant swarm of VLFEs. The absence of major earthquakes or substantial ΔCFS alterations during 2007 and 2016 implies that other internal factors, such as slow stress accumulation or fluid migration at shallow depth, may have played a more significant role in VLFE activation. These findings highlight the potential for VLFE swarms to occur independently of external stress perturbations and suggest that internal processes may govern their occurrence during seismically quiet periods. Overall, long-term monitoring of VLFE activity is essential for detecting transient aseismic phenomena—particularly in regions lacking offshore geodetic coverage. Even during periods of low seismic activity, changes in VLFE occurrence may signal deeper tectonic processes at work. 5 Conclusions A comprehensive analysis of VLFE activity in the southwestern Ryukyu Trench provides new insights into the relationship between VLFEs and regular seismicity. VLFEs were concentrated near the trench axis and occurred in swarm-like sequences approximately every two to three months. Their spatial distribution was largely complementary to that of thrust-type earthquakes, suggesting that VLFEs reflect aseismic slip along the subduction interface—especially in offshore areas where traditional geodetic observations are limited or infeasible. Tectonic structures, such as the subducted Gagua Ridge, appear to significantly influence the spatial distribution of both seismic and aseismic activity. In particular, the transition from seismicity to VLFE-dominated regions coincides with the northern extension of the Gagua Ridge, suggesting that crustal heterogeneities play a key role in modulating slip behavior along the plate interface. The sharp increase in VLFE activity beginning in late 2001 coincided with a period of frequent M6-class earthquakes and afterslip, suggesting that transient stress changes may have triggered or enhanced long-term slow slip events near the trench axis. Additional episodes of increased VLFE activity in 2007 and 2016 occurred without any major external stress perturbations, indicating that internal factors, such as stress accumulation or fluid migration, may also contribute to VLFE generation. Overall, this study enhances our understanding of subduction zone dynamics in the southern Ryukyu Trench. VLFE monitoring offers a valuable means of inferring aseismic slip processes—especially in offshore regions where geodetic networks are sparse. These findings have important implications for assessing interplate coupling and evaluating seismic hazard in tectonically active subduction zones. Abbreviations CC cross-correlation BATS Broadband Array in Taiwan for Seismology B-catalog BATS-only catalog CMT centroid moment tensor DCFS difference in Coulomb failure stress FB-catalog Fnet and BATS catalog gCMT Global Centroid Moment Tensor GNSS Global Navigation Satellite System GNSS-A GNSS-Acoustic ISZ Interplate Seismogenic Zone JMA Japan Meteorological Agency JST Japan standard time LFE low-frequency earthquake Mw Moment magnitude NEIC National Earthquake Information Center NIED National Research Institute for Earth Science and Disaster Resilience SSE slow slip event VLFE very low-frequency earthquakes Declarations Ethics approval and consent to participate Not applicable Consent for publication Not applicable Competing interests The authors declare that they have no conflict of interest. Availability of data and materials CMT data were downloaded from the NIED website. Earthquake data were downloaded from the JMA website. Waveform data were provided by NIED (F-net; https://www.fnet.bosai.go.jp/) and BATS (https://bats.earth.sinica.edu.tw/). Coulomb 3.3 is available at https://earthquake.usgs.gov/research/software/coulomb/. The slab geometry shown in Figure 1 was downloaded from the U.S. Geological Survey website (Hayes 2018). Funding This research received no external funding. Authors' contributions MN designed the study, conducted the data analysis, and wrote the manuscript. RY performed centroid moment tensor (CMT) inversions as part of his master’s research, which provided the foundation for part of the VLFE catalog used in this study. Both authors discussed the results and contributed to the final version of the manuscript. All authors approved the final manuscript. Acknowledgements We are grateful to the JMA for providing the earthquake catalog. 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Geophys Res Lett 36:L19307. doi:10.1029/2009GL039730 Nakamura M (2009b) Aseismic crustal movement in southern Ryukyu Trench, southwest Japan, Geophys Res Lett 36:L20312. doi:10.1029/2009GL040357 Nakamura M, Sunagawa N (2015) Activation of very low frequency earthquakes by slow slip events in the Ryukyu Trench. Geophys Res Lett 42:1076–1082. doi:10.1002/2014GL062929 Nakamura M (2017) Distribution of low-frequency earthquakes accompanying the very low frequency earthquakes along the Ryukyu Trench. Earth Planets Space 69:49. doi:10.1186/s40623-017-0632-4 Nakamura M, Kinjo A (2018) Activated seismicity by strain rate change in the Yaeyama region, south Ryukyu. Earth Planets Space 70:154. doi:10.1186/s40623-018-0929-y Nakano M, Hori T, Araki E Kodaira S, Ide S (2018) Shallow very-low-frequency earthquakes accompany slow slip events in the Nankai subduction zone. Nat Comm 9:984. doi:10.1038/s41467-018-03431-5 Nakata K, Goto K, Yanagisawa, H (2024) New source model for the 1771 Meiwa tsunami along the southern Ryukyu Trench inferred from high-resolution tsunami calculation. Prog Earth Planet Sci 11:28. doi:10.1186/s40645-024-00631-0 Nishimura T (2014) Short-term slow slip events along the Ryukyu Trench, southwestern Japan, observed by continuous GNSS. Prog Earth Planet Sci 1:22. doi:10.1186/s40645-014-0022-5 Obara K, Kato A (2016) Connecting slow earthquakes to huge earthquakes. Science 353:253–257. doi:10.1126/science.aaf1512 Sun T, Saffer D, Ellis S (2020) Mechanical and hydrological effects of seamount subduction on megathrust stress and slip. Nat Geosci 13:249–255. doi:10.1038/s41561-020-0542-0 Takemura S, Noda A, Kubota T, Asano Y, Matsuzawa T, Shiomi K (2019). Migrations and clusters of shallow, very low-frequency earthquakes in the regions surrounding shear stress accumulation peaks along the Nankai Trough. Geophys Res Lett 46: 11830–11840. doi:10.1029/2019GL084666 Takemura S, Hamada Y, Okuda H, Okada Y, Okubo K, Akuhara T, Noda A, Tonegawa T (2023) A review of shallow slow earthquakes along the Nankai Trough. Earth Planets Space 75:164. doi:10.1186/s40623-023-01920-6 Toda S, Stein, RS, Sevilgen V, Lin J (2011) Coulomb 3.3 Graphic-rich deformation and stress-change software for earthquake, tectonic, and volcano research and teaching—user guide: U.S. Geological Survey Open-File Report 2011–1060, 63 p., available at https://pubs.usgs.gov/of/2011/1060/. Tu Y, Heki K (2017) Decadal modulation of repeating slow slip event activity in the southwestern Ryukyu Arc possibly driven by rifting episodes at the Okinawa Trough. Geophys Res Lett 44:9308–9313. doi:10.1002/2017GL074455 Villegas-Lanza, J, Nocquet JM, Rolandone F, Vallée M, Tavera H, Bondoux F, Tran T, Martin X, Chlieh M (2015) A mixed seismic–aseismic stress release episode in the Andean subduction zone. Nature Geosci 9:150–154. doi:10.1038/ngeo2620 Walter JI, Schwartz SY, Protti M, Gonzalez V (2013) The synchronous occurrence of shallow tremor and very low frequency earthquakes offshore of the Nicoya Peninsula, Costa Rica. Geophys Res Lett 40:1517–1522. doi:10.1002/grl.50213 Wang TK, Lin SF, Liu CS, Wang CS (2004) Crustal structure of the southernmost Ryukyu subduction zone: OBS, MCS and gravity modelling, Geophys J Inter 157:147–163. doi:10.1111/j.1365-246X.2004.02147.x Wessel P, Luis JF, Uieda L, Scharroo R, Wobbe F, Smith WHF, Tian D (2019) The Generic Mapping Tools version 6. Geochem Geophys Geosys 20:5556–5564. doi:10.1029/2019GC008515 Yokota Y, Ishikawa T (2020) Shallow slow slip events along the Nankai Trough detected by GNSS-A. Sci Adv 6:eaay5786. doi:10.1126/sciadv.aay5786 Supplementary Files GraphicalAbst03.jpg TableS1.xlsx Additional File 1: Table S1. File format: XLSX Description of data: List of VLFEs in the B-catalog. TableS22.xlsx Additional File 2: Table S2 File format: XLSX Description of data: List of VLFEs in the FB-catalog. TableS3.xlsx Additional File 3: Table S3 File format: XLSX Description of data: Fault parameters used in Coulomb stress change calculations. AdditionalFile4.docx Additional File 4: File format: DOCX Cite Share Download PDF Status: Published Journal Publication published 27 Nov, 2025 Read the published version in Earth, Planets and Space → Version 1 posted Reviewers agreed at journal 03 Aug, 2025 Reviewers invited by journal 28 Jul, 2025 Editor assigned by journal 08 Jul, 2025 First submitted to journal 05 Jul, 2025 Editorial decision: Minor Revision 03 Jul, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-7029393","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":491803315,"identity":"e6773f41-c2c0-4e2f-b380-52c33558f99c","order_by":0,"name":"Mamoru Nakamura","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA8klEQVRIiWNgGAWjYHACZgaGCjgnAUrzENJyhmQtjG0YWvAA3dnNjw1+zrOzZ5BIYPvM25aWuOEA88MPDDJ3cGoxu3PMOLF3W3Jig0QC82zethygFjZjCQaeZ7i13EgwPsC77UACg0T+Z2betgqgFgYzoF8O49GS/vng3zkHQA5jhmph/0ZAS45xMm/DAcYGiBaQw3gI2ZJTbCxzLDmxjecBM+Occ2nGMw/zFEsk4PVL+mbJNzV29vzsCcwMb8qSZfuOt2/88LEHd4jBARsQMwEj0LEBGLUMiT0HCGsBAcYfDAz2EOYPIrWMglEwCkbBSAAA0mtRuJBcVaIAAAAASUVORK5CYII=","orcid":"https://orcid.org/0000-0001-7627-0056","institution":"University of the Ryukyus","correspondingAuthor":true,"prefix":"","firstName":"Mamoru","middleName":"","lastName":"Nakamura","suffix":""},{"id":491803316,"identity":"bf64c0fd-be08-4530-b642-ebdbdd7c567d","order_by":1,"name":"Ren Yakabu","email":"","orcid":"","institution":"University of the Ryukyus Faculty of Science Graduate School of Engineering and Science: Ryukyu Daigaku Rigakubu Daigakuin Rikogaku Kenkyuka","correspondingAuthor":false,"prefix":"","firstName":"Ren","middleName":"","lastName":"Yakabu","suffix":""}],"badges":[],"createdAt":"2025-07-02 12:32:03","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-7029393/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-7029393/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1186/s40623-025-02314-6","type":"published","date":"2025-11-27T15:57:03+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":88001069,"identity":"5e473710-2e0a-45a4-b96f-8d6f6b18bd67","added_by":"auto","created_at":"2025-07-31 10:22:28","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":531246,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eDistribution of seismic stations and template earthquakes used in this study.\u003c/strong\u003e Dates next to seismic stations indicate the start of observations. Red triangles represent broadband seismic stations from the F-net network of the National Research Institute for Earth Science and Disaster Resilience (NIED) in Japan, and blue triangles represent those from Taiwan’s Broadband Array in Taiwan for Seismology (BATS) network. Red centroid moment censor (CMT) solutions correspond to regular earthquakes, whereas blue CMTs indicate very low-frequency earthquakes (VLFEs). Circles show VLFE epicenters (from the FB-catalog) for the period 2000–2023. Plate boundaries follow the model of Bird (2003).\u003c/p\u003e","description":"","filename":"Figure1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7029393/v1/0203e8b2eb02ba853b72f4c0.jpg"},{"id":88001070,"identity":"976896b4-6092-4bf3-8303-07b5bd27168d","added_by":"auto","created_at":"2025-07-31 10:22:28","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":991317,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eSpatial distribution of VLFEs in the southwestern Ryukyu Trench and\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003ejuxtaposition with regular earthquakes, slow earthquakes, and tsunami source regions.\u003c/strong\u003e Circles represent VLFE epicenters (FB-catalog; 2000–2023), with colors indicating the number of events at each location. CMT solutions show thrust-type earthquakes from the F-net catalog (2003–2023; depth \u0026lt; 60 km). The source region of the 1771 Yaeyama tsunami is based on Nakamura (2009a); the 2002 afterslip model is from Nakamura (2009b). The slip-deficit area follows Kano (2021), and repeating slow slip events (SSEs) are based on Kano et al. (2018). Contours denote slab depth from Hayes (2018). IR: Iriomote Island; YN: Yonaguni Island.\u003c/p\u003e","description":"","filename":"Figure2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7029393/v1/7be860b42be5c24d2a2f8283.jpg"},{"id":88003120,"identity":"cef68937-5734-4a5f-902c-e278dc196911","added_by":"auto","created_at":"2025-07-31 10:30:28","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":752402,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eCumulative number of VLFE events from the FB-catalog and B-catalog. \u003c/strong\u003e(a) Distribution of VLFEs with Mw ≥ 3.7 (FB-catalog). (b) Distribution of VLFEs with Mw ≥ 3.9 (B-catalog). (c) Cumulative number of VLFEs (Mw ≥ 3.7) in regions FB-1 to FB-3 (FB-catalog). (d) Cumulative number of VLFEs (Mw ≥ 3.9) in regions B-1 and B-2 (B-catalog).\u003c/p\u003e","description":"","filename":"Figure3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7029393/v1/e7cf4adba961f9b20564b09f.jpg"},{"id":88003121,"identity":"e03ad8a7-c18f-4f93-8c7a-5fcf02595cce","added_by":"auto","created_at":"2025-07-31 10:30:28","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":125482,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eSwarm activity of VLFEs showing eastward migration.\u003c/strong\u003eLeft: Spatiotemporal distribution of a VLFE swarm observed in August 2009 (FB-catalog). Right: Spatial distribution of VLFE epicenters during the same swarm episode.\u003c/p\u003e","description":"","filename":"Figure4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7029393/v1/b1b6b762e1ea6896a9911662.jpg"},{"id":88001074,"identity":"9834e3dc-2fbc-4219-ba3d-ac3627c08b6a","added_by":"auto","created_at":"2025-07-31 10:22:28","extension":"jpg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":627024,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eSeismic and VLFE activity near the VLFE cluster from 2000 to 2002.\u003c/strong\u003e (a) Spatial distribution of earthquakes and VLFEs (B-catalog). CMTs are from the ISC-EHB Bulletin (ISC 2022), JMA-CMT (for the 2000 Mw 5.6 event), and gCMT (for the 2001 Mw 6.8 and 2002 Mw 6.4 events). (b) Spatiotemporal distribution of seismicity and VLFEs. Black stars indicate earthquakes with Mw ≥ 6. Green squares denote regular earthquakes with M ≥ 2.0 at depths of 0–50 km. Small orange circles show VLFEs with Mw \u0026gt; 3.3, and large red circles show VLFEs with Mw \u0026gt; 3.9. The yellow arrow marks the onset of enhanced VLFE activity.\u003c/p\u003e","description":"","filename":"Figure5.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7029393/v1/32d42637c7200fb44941711a.jpg"},{"id":97178249,"identity":"7cf9bf79-fa2d-435a-b91f-f30d3cb38a25","added_by":"auto","created_at":"2025-12-01 16:05:36","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":3781440,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7029393/v1/22608aa6-d3ac-4b80-ae41-9192f1d4b492.pdf"},{"id":88001068,"identity":"0417bdd5-7b0d-43e7-b65f-34ef7615364e","added_by":"auto","created_at":"2025-07-31 10:22:28","extension":"jpg","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":604618,"visible":true,"origin":"","legend":"","description":"","filename":"GraphicalAbst03.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7029393/v1/63407bd449ee7185441651dc.jpg"},{"id":88003122,"identity":"c7fd3e51-5f6b-49da-ae5d-7a508917300b","added_by":"auto","created_at":"2025-07-31 10:30:28","extension":"xlsx","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":154566,"visible":true,"origin":"","legend":"\u003cp\u003eAdditional File 1: Table S1.\u003c/p\u003e\n\u003cp\u003eFile format: XLSX\u003c/p\u003e\n\u003cp\u003eDescription of data: List of VLFEs in the B-catalog.\u003c/p\u003e","description":"","filename":"TableS1.xlsx","url":"https://assets-eu.researchsquare.com/files/rs-7029393/v1/e3aa1845325670e2deecbe4b.xlsx"},{"id":88004723,"identity":"253e3fdb-9112-436f-8116-4d82304d1904","added_by":"auto","created_at":"2025-07-31 10:38:28","extension":"xlsx","order_by":3,"title":"","display":"","copyAsset":false,"role":"supplement","size":159269,"visible":true,"origin":"","legend":"\u003cp\u003eAdditional File 2: Table S2\u003c/p\u003e\n\u003cp\u003eFile format: XLSX\u003c/p\u003e\n\u003cp\u003eDescription of data: List of VLFEs in the FB-catalog.\u003c/p\u003e","description":"","filename":"TableS22.xlsx","url":"https://assets-eu.researchsquare.com/files/rs-7029393/v1/e0e0efe21134a2522da0b010.xlsx"},{"id":88001073,"identity":"3c956e59-e9cd-4d02-8fbf-6b4708d78569","added_by":"auto","created_at":"2025-07-31 10:22:28","extension":"xlsx","order_by":4,"title":"","display":"","copyAsset":false,"role":"supplement","size":11681,"visible":true,"origin":"","legend":"\u003cp\u003eAdditional File 3: Table S3\u003c/p\u003e\n\u003cp\u003eFile format: XLSX\u003c/p\u003e\n\u003cp\u003eDescription of data: Fault parameters used in Coulomb stress change calculations.\u003c/p\u003e","description":"","filename":"TableS3.xlsx","url":"https://assets-eu.researchsquare.com/files/rs-7029393/v1/d89518b2f961408a2a1aa6af.xlsx"},{"id":88004722,"identity":"20f73aeb-4cce-4bca-b7b6-230d2da28bad","added_by":"auto","created_at":"2025-07-31 10:38:28","extension":"docx","order_by":5,"title":"","display":"","copyAsset":false,"role":"supplement","size":2193806,"visible":true,"origin":"","legend":"\u003cp\u003eAdditional File 4:\u003c/p\u003e\n\u003cp\u003eFile format: DOCX\u003c/p\u003e","description":"","filename":"AdditionalFile4.docx","url":"https://assets-eu.researchsquare.com/files/rs-7029393/v1/38fa171c63f524fc3d6bd77b.docx"}],"financialInterests":"","formattedTitle":"Spatiotemporal Variability and Tectonic Implications of Very Low-Frequency Earthquakes in the Southwestern Ryukyu Trench","fulltext":[{"header":"1 Introduction","content":"\u003cp\u003eSubduction zones accommodate stress through both seismic and aseismic processes (e.g., Obara and Kato 2016). Aseismic slip can generally be categorized into two types: slow slip events (SSEs) and afterslip. SSEs typically occur spontaneously during interseismic periods, whereas afterslip follows large or moderate earthquakes and decays gradually over time. Understanding whether interplate stress is released seismically or aseismically is essential for evaluating the potential for large earthquakes in a given region. However, detecting aseismic slip remains challenging in areas with insufficient geodetic observation networks. For this reason, this study focuses on a type of slow earthquakes, namely very low-frequency earthquakes (VLFEs).\u003c/p\u003e\u003cp\u003eVLFEs are characterized by surface waves with periods of 20\u0026ndash;50 seconds and can be detected at distances of several hundred kilometers. This long-range detectability makes them particularly valuable for monitoring tectonic activity in areas with sparse seismic instrumentation, such as the Ryukyu Arc (Ando et al. 2012; Nakamura and Sunagawa 2015). VLFEs have been documented in several subduction zones around the world (Asano et al. 2008, 2015; Ando et al. 2012; Nakamura and Sunagawa 2015; Takemura et al. 2019; Ghosh et al. 2015). Observations show that shallow VLFE activity increases near the rupture zones of large earthquakes due to associated afterslip (Asano et al. 2008; Matsuzawa et al. 2015; Nakano et al. 2018). In addition, VLFEs often co-occur with SSEs in various subduction settings (Walter et al. 2013; Asano et al. 2015; Takemura et al. 2023). Comparative studies of VLFE activity and plate coupling in the Japan Trench and Nankai Trough have shown an inverse correlation between shallow VLFE occurrence and interplate coupling strength (Baba et al. 2020).\u003c/p\u003e\u003cp\u003eIn the southern Ryukyu Trench, various types of slow earthquakes\u0026mdash;including SSEs, VLFEs, and low-frequency earthquakes (LFEs)\u0026mdash;can be detected (Ando et al. 2012; Heki and Kataoka 2008; Nishimura 2014; Nakamura and Sunagawa 2015; Nakamura, 2017). Mw 6.5-class SSEs have repeatedly occurred on the subducting plate interface beneath Iriomote Island (Heki and Kataoka 2008; Nishimura 2014; Tu and Heki 2018; Kano et al. 2018). In this region, VLFE activity is concentrated near the trench axis adjacent to the Yaeyama Islands, with swarm-like events lasting several days and recurring every approximately 2\u0026ndash;3 months (Ando et al. 2012; Nakamura and Sunagawa 2015). In addition, LFEs associated with VLFEs have been detected 30\u0026ndash;50 km landward from the trench axis (Nakamura 2017). The subducting plate in this region lies at depths of 10\u0026ndash;20 km, and the spatial distribution of LFEs complements those of SSEs and typical thrust earthquakes. While a plate-coupling region has been identified at the southwestern end of the Ryukyu Trench off eastern Taiwan (Hsu et al. 2012), seafloor geodetic observations indicate that the coupling strength there is weak (Chen et al. 2022). Nevertheless, the VLFE distribution spans the region from 22\u0026deg; to 24\u0026deg; N and from 121\u0026deg; to 125\u0026deg; E, overlapping the presumed coupling zone (Nakamura and Sunagawa 2015). Due to the limited accuracy in epicenter determination in that earlier study, whether this overlap is real remains unclear.\u003c/p\u003e\u003cp\u003eVLFE activity near the trench axis in the southwestern Ryukyu Trench likely reflects the slip behavior along the plate boundary, which is otherwise difficult to observe with land-based geodetic networks. Therefore, this study elucidates the long-term characteristics of VLFE activity and clarifies the state of aseismic slip at the subduction interface near the trench axis in the southwestern Ryukyu Trench.\u003c/p\u003e"},{"header":"2 Data and methods","content":"\u003cp\u003eWaveform data were sourced from continuous broadband seismic observations conducted by the F-net network of the National Research Institute for Earth Science and Disaster Resilience (NIED) in Japan and Taiwan\u0026rsquo;s BATS network (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Since the F-net stations in the Ryukyu Islands were established between 2000 and 2002, enhanced detection capability during this period may have introduced biases in VLFE observations. These potential biases must be considered when interpreting temporal trends in VLFE activity.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eTwo detection methods were used in this study. The first method utilized data from both the F-net and BATS networks to compile a catalog (hereafter referred to as the FB-catalog) covering the period from May 18, 2000, to August 31, 2024. This approach provided high detection capability and improved hypocenter accuracy for VLFEs during this period. The second method relied solely on data from the BATS network, which has been operational since before 1998. This separate catalog (the B-catalog) that was compiled covered the period from January 1998 to August 2024. While this method allows for the evaluation of long-term VLFE activity independent of changes in F-net coverage, it has lower detection sensitivity and poorer hypocenter resolution compared to those of the FB-catalog.\u003c/p\u003e\u003cp\u003eFor both catalogs, VLFEs were detected and located using a waveform cross-correlation method with template events, following Asano et al. (2015). Both regular thrust earthquakes and known VLFEs were employed as templates (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). The centroid moment tensor (CMT) solutions of regular earthquakes were selected from the F-net catalog and used consistently for both the FB-catalog and the B-catalog. CMT inversion for VLFEs was conducted using the time\u0026ndash;domain inversion code of Dreger (2003). A bandpass filter (0.02\u0026ndash;0.05 Hz) was applied to waveform data from both networks, and 300-second waveform segments were used for the inversions. Grid search parameters included horizontal spacing of 0.02\u0026deg; and vertical spacing of 5 km. The centroid time and location were determined as the values that maximized the variance reduction (Figures \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e and S2).\u003c/p\u003e\u003cp\u003eThe VLFE detection procedure was as follows: bandpass-filtered (0.02\u0026ndash;0.05 Hz), three-component waveforms were resampled at 0.25 Hz. Cross-correlation (CC) functions were computed between continuous waveforms at each station and 120-second template event windows. Assuming a surface wave propagation velocity of 3.7 km/s (Nakamura and Sunagawa 2015), the back-propagated CC values were mapped across space and time. Events with an average CC\u0026thinsp;\u0026ge;\u0026thinsp;0.5 across stations were considered detections. When multiple candidates were found within a 120-second window using different templates, the location with the highest CC value was selected as the VLFE epicenter.\u003c/p\u003e\u003cp\u003eMagnitude estimates were based on thrust-type reference earthquakes southwest of Yonaguni Island. After applying the same bandpass filter (0.02\u0026ndash;0.05 Hz) to vertical components, the maximum surface wave amplitude was recorded. A relationship between maximum amplitude, epicentral distance, and moment magnitude (Mw) was established (Figure \u003cspan refid=\"MOESM3\" class=\"InternalRef\"\u003eS3\u003c/span\u003e), with Mw values referenced from F-net CMT solutions. Based on the frequency\u0026ndash;magnitude distribution of the detected events, the magnitude of completeness was determined to be 3.7 for the FB-catalog and 3.9 for the B-catalog (Figure \u003cspan refid=\"MOESM4\" class=\"InternalRef\"\u003eS4\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eTo assess potential external influences on VLFE activity, changes in Coulomb failure stress (ΔCFS) were calculated using the Coulomb 3.3 software (Toda et al. 2011). These calculations accounted for earthquakes and dike intrusions in the Ryukyu Trench and Okinawa Trough. The relevant source parameters are summarized in Table \u003cspan refid=\"MOESM3\" class=\"InternalRef\"\u003eS3\u003c/span\u003e. Two significant dike intrusions occurred on October 24, 2002, and April 16, 2013. Most events included in the stress calculations were interplate or intraslab earthquakes; nevertheless, the December 18, 2001, crustal earthquake and the March 2002 afterslip near Yonaguni Island were also incorporated.\u003c/p\u003e"},{"header":"3 Results","content":"\u003cp\u003eFigure \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e shows the spatial distribution of VLFEs based on the FB-catalog. VLFEs follow primarily an east\u0026ndash;west alignment along the trench axis near 122.5 \u0026deg;E. These events are concentrated at slab depths between 10 and 20 km, with notable clusters being evident around 123.2\u0026deg; and 123.5\u0026deg; E at approximately 15 km depth. In contrast, between 124.0\u0026deg; and 124.7\u0026deg; E, activity is somewhat reduced and occurs across a wider depth range within the same interval. The horizontal uncertainty of VLFE epicenter locations is approximately 20 km in both the east\u0026ndash;west and north\u0026ndash;south directions (Figure \u003cspan refid=\"MOESM5\" class=\"InternalRef\"\u003eS5\u003c/span\u003ea). In the B-catalog, north\u0026ndash;south location errors are also approximately 20 km; however, due to the absence of F-net stations, east\u0026ndash;west uncertainties increase significantly\u0026mdash;up to approximately 100 km. Notably, 69% of the VLFEs (Mw\u0026thinsp;\u0026ge;\u0026thinsp;3.5) listed in Nakamura and Sunagawa (2015) were also detected in the FB-catalog.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe spatial distribution of VLFEs and thrust earthquakes appears to be mutually exclusive (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). No thrust earthquakes occurred within the main VLFE region (23.5\u0026deg;\u0026ndash;23.7\u0026deg; N, 123.0\u0026deg;\u0026ndash;123.8\u0026deg; E). Instead, thrust earthquakes are concentrated westward between 122.0\u0026deg; and 122.7\u0026deg; E, corresponding to slab depths of 15\u0026ndash;20 km and located approximately 30 km west of the primary VLFE zone. Regions where recurrent SSEs occur (Heki and Kataoka 2008; Tu and Heki 2018) and areas of slip deficit (Kano 2021) are located north of the VLFE cluster.\u003c/p\u003e\u003cp\u003eIn most cases, VLFE swarms remain confined to individual clusters. However, in some instances, activity spreads to adjacent areas (Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003e and S6). For example, in August 2009, activity initiated near 123.0\u0026deg; E and migrated eastward at an approximate rate of 6 km/day (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003e). Similar eastward migration was observed in other swarm episodes (see Figure S6).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eFigure \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e3\u003c/span\u003ec displays the cumulative number of VLFEs in each subregion (FB-1 to FB-3), along with events of Mw\u0026thinsp;\u0026ge;\u0026thinsp;3.7, as recorded in the FB-catalog. In FB-1 (23.7\u0026deg; N, 123.2\u0026deg; E) and FB-2 (23.6\u0026deg; N, 123.5\u0026deg; E), VLFEs tended to occur in swarms approximately every 2\u0026ndash;3 months. In contrast, FB-3 (23.7\u0026deg; N, 124.5\u0026deg; E) exhibited more sporadic activity, involving one or two isolated events per episode. VLFE activity in FB-1 started in 2002, decreased between 2004 and 2006, and increased again after 2007, with the increase being more pronounced in 2016. Activity declined after 2022. In FB-2, activity started after 2002 and 2004 and increased in 2006 and again after 2016. FB-3 also showed increased activity around 2016.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eFigure \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e3\u003c/span\u003ed presents the cumulative number of VLFEs with Mw\u0026thinsp;\u0026ge;\u0026thinsp;3.9 from the B-catalog. Although event locations in the B-catalog are more dispersed than those in the FB-catalog, an overall east\u0026ndash;west alignment remains evident. Epicenter uncertainties are 20\u0026ndash;30 km in the north\u0026ndash;south direction and up to 100 km in the east\u0026ndash;west direction. In both B-1 (122.8\u0026deg;\u0026ndash;123.5\u0026deg; E) and B-2 (123.5\u0026deg;\u0026ndash;124.3\u0026deg; E), activity was low from 1998 until mid-2001; however, it increased initially in late 2001 and again in mid-2006. In B-1 in particular, activity further increased around 2016. Following the Mw 7.7 Chi-Chi earthquake on September 21, 1999, detection of VLFEs may have been hampered by strong aftershock activity for about one month. However, there were no other clear factors that would have significantly reduced VLFE detectability between 1998 and 2000.\u003c/p\u003e"},{"header":"4 Discussion","content":"\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\u003ch2\u003e4.1 Relationship between VLFEs and other slow earthquake distributions\u003c/h2\u003e\u003cp\u003eThe distribution of VLFEs identified in this study is much more localized than that identified in previous work based on manual arrival-time picks of the largest amplitude phases (Nakamura and Sunagawa 2015). The events are concentrated within a narrow (i.e., approximately 15 km wide) zone located south of both the region of repeating SSEs and the slip-deficit area (Heki and Kataoka 2008; Tu and Heki 2017; Kano et al. 2021). This pattern is consistent with the distribution of LFEs reported by Nakamura (2017).\u003c/p\u003e\u003cp\u003eThree LFE clusters were identified along the southern Ryukyu Trench (Nakamura 2017), arranged in an east\u0026ndash;west orientation between the trench axis and the Ryukyu Islands. The most active cluster, labeled YA-RB (Fig.\u0026nbsp;12 in Nakamura 2017), is centered at 23.5\u0026deg; N, 123.7\u0026deg; E. LFE activity is also observed within a 30 km radius of this location as well as in the vicinity of 23.5\u0026deg; N, 124.3\u0026deg; E (cluster YA-RC). While LFE epicenter locations have poor resolution in the north\u0026ndash;south direction, the correspondence between VLFE and LFE clusters suggests spatial overlap in the along-strike direction.\u003c/p\u003e\u003cp\u003eIn general, regions of active VLFE occurrence are spatially complementary to areas of strong interplate coupling (Baba et al. 2020). In the southern Ryukyu Trench, VLFE activity is also complementary to the inferred slip-deficit zones; however, these slip-deficit regions are derived from land-based GNSS observations, which makes it difficult to confirm whether a similar deficit exists south of the VLFE clusters, near the trench axis.\u003c/p\u003e\u003cp\u003eNotably, VLFEs are sparse and less active in the source region of the 1771 Yaeyama tsunami, i.e., east of 124\u0026deg; E. This observation, also noted by Nakamura and Sunagawa (2015), is even clearer with the improved resolution of this study. The tsunami is believed to have been generated by a large slip (17\u0026ndash;30 m) over a 30-km wide area near the trench axis (Nakamura 2009a; Nakata et al. 2024).\u003c/p\u003e\u003cp\u003eA similar complementary relationship between VLFEs and tsunami earthquake source areas has been observed in the Nicoya Peninsula. For instance, while the 1992 Nicaragua tsunami earthquake occurred near the trench axis, VLFEs were observed nearby, albeit outside the main rupture zone. This spatial separation has been interpreted as reflecting fault heterogeneity near the trench axis (Baba 2021). In Nicaragua, stress drop estimates for the tsunami earthquake source zone are approximately 1.2 MPa (Bilek et al. 2016), whereas in the adjacent VLFE region to the southeast, the stress drop is estimated to be between 0.1 and 10 kPa (Baba 2021). Although stress drop values for the southern Ryukyu Trench are currently unknown, the spatial complementarity between the VLFE cluster and the tsunami source region suggests heterogeneous frictional properties along the subduction interface.\u003c/p\u003e\u003cp\u003eSeismic reflection data reveal the presence of strong reflectors at the subducting plate interface in the southern Ryukyu Trench (Arai et al., 2016). In the Nankai Trough, shallow VLFE activity has been linked to elevated pore fluid pressures (Takemura et al., 2023). These observations imply that VLFE activity in the southwestern Ryukyu Trench occurs in frictionally weak zones characterized by high pore fluid pressure and low interplate coupling. Thus, VLFEs may serve as effective indicators of heterogeneous frictional conditions along the subduction interface.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\u003ch2\u003e4.2 Relationship between VLFE clusters and ISZ\u003c/h2\u003e\u003cp\u003eIn the southwestern Ryukyu Trench, VLFE activity is concentrated east of 122.9\u0026deg; E, whereas thrust-type earthquakes are predominantly observed between 122.0\u0026deg; and 122.5\u0026deg; E. This boundary coincides with the northern extension of the subducted Gagua Ridge\u0026mdash;a prominent bathymetric feature approximately 25 km wide and 4 km high\u0026mdash;subducting obliquely in a northwest direction (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eThe Gagua Ridge enters the Ryukyu Trench near 23.3\u0026deg; N, 122.9\u0026deg; E, and continues northwestward. It is also possible that the ridge extends beneath the Nanao Basin (24.0\u0026deg; N, 122.5\u0026deg; E) (Deng et al. 2023). Previous studies have suggested that the Philippine Sea Plate to the west of the Gagua Ridge has undergone compressional deformation (Wang et al. 2004). In either case, the subduction of thicker crustal material west of the ridge likely alters the stress conditions and influences the seismic behavior.\u003c/p\u003e\u003cp\u003eWhen thick crustal features, such as seamounts, subduct, compression and fluid expulsion are enhanced at the leading edge, promoting seismic activity. In contrast, the downdip region may enter a stress shadow zone, where increased porosity and elevated pore fluid pressure facilitate slow slip phenomena (Sun et al. 2020). A similar mechanism may explain the observed pattern in the southwestern Ryukyu Trench, where enhanced seismicity occurs to the west of the Gagua Ridge and VLFE activity dominates to its east.\u003c/p\u003e\u003cp\u003eThis spatial contrast strongly supports the hypothesis that large-scale bathymetric features, such as ridges, modulate both seismic and aseismic slip behaviors. These features likely affect fault strength and stress regimes by altering the fluid migration pathways and crustal properties at the subduction interface.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\u003ch2\u003e\u003cb\u003e4.3 VLFE activation around 2000\u003c/b\u003e\u0026ndash;\u003cb\u003e2002\u003c/b\u003e\u003c/h2\u003e\u003cp\u003eThe increase in VLFE activity beginning in late 2001 coincided with a series of M6-class earthquakes in the region. Prior to this period, only swarm activity of small VLFEs (Mw\u0026thinsp;\u0026lt;\u0026thinsp;3.9) had been observed. However, starting in late 2001, swarm activity involving VLFEs with magnitudes exceeding 3.9 began to occur more frequently (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e3\u003c/span\u003ed; see yellow arrow in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThree moderate-to-large earthquakes occurred near the trench axis during this time (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea). On November 9, 2000, a thrust-fault earthquake (EQ1; Mw 5.9) occurred at 23.2\u0026deg; N, 124.2\u0026deg; E. On December 18, 2001, a normal-fault earthquake with a strike-slip component (EQ2; Mw 6.5) struck at 23.9\u0026deg; N, 122.8\u0026deg; E, northwest of the VLFE cluster. On March 26, 2002, another thrust-fault earthquake (EQ3; Mw 6.5) occurred near the trench axis at 23.2\u0026deg; N, 124.4\u0026deg; E. Following EQ2 and EQ3, increased VLFE activity was observed (labeled as sequences 1 and 2 in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb, respectively).\u003c/p\u003e\u003cp\u003eAssuming that VLFEs are associated with aseismic slip (e.g., SSEs) (Hirose 2010; Davis 2006; Asano 2008), the increase in VLFE activity around late 2001 may reflect the initiation of a shallow slow slip episode near the trench axis. A similar sequence has been documented in the northern Peru subduction zone, where aseismic slip lasting approximately seven months was accompanied by frequent moderate-sized earthquakes, including those with magnitudes up to Mw 6.0 (Villegas-Lanza 2015). In Peru, these included both interplate and normal-fault events\u0026mdash;similar to EQ2 in the Ryukyu Trench.\u003c/p\u003e\u003cp\u003eIn addition, an afterslip related to the March 31, 2002, offshore Taiwan earthquake (Mw\u0026thinsp;~\u0026thinsp;6.8) was detected south of Yonaguni Island (Nakamura 2009b). ΔCFS calculations show that this afterslip increased the stress in the VLFE region by approximately 3\u0026ndash;25 kPa. As ΔCFS values of this magnitude can promote fault slip, the afterslip may have sustained VLFE and associated SSE activity over the subsequent years.\u003c/p\u003e\u003cp\u003eSimilar patterns have been observed in the Nankai Trough. VLFE swarms off the Kii Peninsula in 2009 and 2018 were associated with surrounding SSEs (Takemura et al. 2019). GNSS-A data also revealed long-term SSEs (Mw\u0026thinsp;\u0026ge;\u0026thinsp;6.6) in strongly coupled shallow regions, coinciding with VLFE activity (Yokota and Ishikawa 2020). By analogy, the activation of VLFE swarms in the southwestern Ryukyu Trench during 2001\u0026ndash;2002 likely reflects the onset of long-term slow slip events, triggered by nearby moderate earthquakes and enhanced by stress perturbations such as afterslip.\u003c/p\u003e\u003cp\u003eIn this region, the lack of offshore geodetic coverage makes direct observation of shallow SSEs difficult. However, the temporal correlation between VLFEs and increased seismicity suggests that VLFEs can serve as proxies for detecting aseismic slip. This highlights the potential value of VLFE monitoring in regions where traditional geodetic methods are not feasible.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\u003ch2\u003e4.4 Relationship between the activation of VLFEs in 2007 and 2016 and the surrounding crustal deformation\u003c/h2\u003e\u003cp\u003eIn 2007 and 2016, VLFE activity increased despite the absence of large nearby earthquakes that could have caused significant ΔCFS alterations in the VLFE region (Figure S7). This suggests that the activation of VLFEs during these periods was not directly triggered by dynamic stress changes associated with major seismic events.\u003c/p\u003e\u003cp\u003eExamining the broader crustal deformation context, seismic activity and SSEs around Iriomote Island have increased since approximately 2013 (Nakamura and Kinjo 2018; Tu and Heki 2017). This escalation in activity appears to have been initiated by a dike intrusion event in the Okinawa Trough in 2013. However, there was a notable three-year lag before the observed increase in VLFE activity in 2016. Moreover, the ΔCFS imparted by the 2013 dike intrusion was relatively small, i.e., approximately 5 kPa (event 10 in Figure S7), which is likely insufficient to directly trigger a significant swarm of VLFEs.\u003c/p\u003e\u003cp\u003eThe absence of major earthquakes or substantial ΔCFS alterations during 2007 and 2016 implies that other internal factors, such as slow stress accumulation or fluid migration at shallow depth, may have played a more significant role in VLFE activation. These findings highlight the potential for VLFE swarms to occur independently of external stress perturbations and suggest that internal processes may govern their occurrence during seismically quiet periods.\u003c/p\u003e\u003cp\u003eOverall, long-term monitoring of VLFE activity is essential for detecting transient aseismic phenomena\u0026mdash;particularly in regions lacking offshore geodetic coverage. Even during periods of low seismic activity, changes in VLFE occurrence may signal deeper tectonic processes at work.\u003c/p\u003e\u003c/div\u003e"},{"header":"5 Conclusions","content":"\u003cp\u003eA comprehensive analysis of VLFE activity in the southwestern Ryukyu Trench provides new insights into the relationship between VLFEs and regular seismicity. VLFEs were concentrated near the trench axis and occurred in swarm-like sequences approximately every two to three months. Their spatial distribution was largely complementary to that of thrust-type earthquakes, suggesting that VLFEs reflect aseismic slip along the subduction interface\u0026mdash;especially in offshore areas where traditional geodetic observations are limited or infeasible.\u003c/p\u003e\u003cp\u003eTectonic structures, such as the subducted Gagua Ridge, appear to significantly influence the spatial distribution of both seismic and aseismic activity. In particular, the transition from seismicity to VLFE-dominated regions coincides with the northern extension of the Gagua Ridge, suggesting that crustal heterogeneities play a key role in modulating slip behavior along the plate interface.\u003c/p\u003e\u003cp\u003eThe sharp increase in VLFE activity beginning in late 2001 coincided with a period of frequent M6-class earthquakes and afterslip, suggesting that transient stress changes may have triggered or enhanced long-term slow slip events near the trench axis. Additional episodes of increased VLFE activity in 2007 and 2016 occurred without any major external stress perturbations, indicating that internal factors, such as stress accumulation or fluid migration, may also contribute to VLFE generation.\u003c/p\u003e\u003cp\u003eOverall, this study enhances our understanding of subduction zone dynamics in the southern Ryukyu Trench. VLFE monitoring offers a valuable means of inferring aseismic slip processes\u0026mdash;especially in offshore regions where geodetic networks are sparse. These findings have important implications for assessing interplate coupling and evaluating seismic hazard in tectonically active subduction zones.\u003c/p\u003e"},{"header":"Abbreviations","content":"\u003cdiv class=\"DefinitionList\"\u003e\u003cdiv class=\"DefinitionListEntry\"\u003e\u003cdiv class=\"Term\"\u003e\u003cb\u003eCC\u003c/b\u003e\u003c/div\u003e\u003cdiv class=\"Description\"\u003e\u003cp\u003ecross-correlation\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv class=\"DefinitionListEntry\"\u003e\u003cdiv class=\"Term\"\u003e\u003cb\u003eBATS\u003c/b\u003e\u003c/div\u003e\u003cdiv class=\"Description\"\u003e\u003cp\u003eBroadband Array in Taiwan for Seismology\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv class=\"DefinitionListEntry\"\u003e\u003cdiv class=\"Term\"\u003e\u003cb\u003eB-catalog\u003c/b\u003e\u003c/div\u003e\u003cdiv class=\"Description\"\u003e\u003cp\u003eBATS-only catalog\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv class=\"DefinitionListEntry\"\u003e\u003cdiv class=\"Term\"\u003e\u003cb\u003eCMT\u003c/b\u003e\u003c/div\u003e\u003cdiv class=\"Description\"\u003e\u003cp\u003ecentroid moment tensor\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv class=\"DefinitionListEntry\"\u003e\u003cdiv class=\"Term\"\u003e\u003cb\u003eDCFS\u003c/b\u003e\u003c/div\u003e\u003cdiv class=\"Description\"\u003e\u003cp\u003edifference in Coulomb failure stress\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv class=\"DefinitionListEntry\"\u003e\u003cdiv class=\"Term\"\u003e\u003cb\u003eFB-catalog\u003c/b\u003e\u003c/div\u003e\u003cdiv class=\"Description\"\u003e\u003cp\u003eFnet and BATS catalog\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv class=\"DefinitionListEntry\"\u003e\u003cdiv class=\"Term\"\u003e\u003cb\u003egCMT\u003c/b\u003e\u003c/div\u003e\u003cdiv class=\"Description\"\u003e\u003cp\u003eGlobal Centroid Moment Tensor\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv class=\"DefinitionListEntry\"\u003e\u003cdiv class=\"Term\"\u003e\u003cb\u003eGNSS\u003c/b\u003e\u003c/div\u003e\u003cdiv class=\"Description\"\u003e\u003cp\u003eGlobal Navigation Satellite System\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv class=\"DefinitionListEntry\"\u003e\u003cdiv class=\"Term\"\u003e\u003cb\u003eGNSS-A\u003c/b\u003e\u003c/div\u003e\u003cdiv class=\"Description\"\u003e\u003cp\u003eGNSS-Acoustic\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv class=\"DefinitionListEntry\"\u003e\u003cdiv class=\"Term\"\u003e\u003cb\u003eISZ\u003c/b\u003e\u003c/div\u003e\u003cdiv class=\"Description\"\u003e\u003cp\u003eInterplate Seismogenic Zone\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv class=\"DefinitionListEntry\"\u003e\u003cdiv class=\"Term\"\u003e\u003cb\u003eJMA\u003c/b\u003e\u003c/div\u003e\u003cdiv class=\"Description\"\u003e\u003cp\u003eJapan Meteorological Agency\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv class=\"DefinitionListEntry\"\u003e\u003cdiv class=\"Term\"\u003e\u003cb\u003eJST\u003c/b\u003e\u003c/div\u003e\u003cdiv class=\"Description\"\u003e\u003cp\u003eJapan standard time\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv class=\"DefinitionListEntry\"\u003e\u003cdiv class=\"Term\"\u003e\u003cb\u003eLFE\u003c/b\u003e\u003c/div\u003e\u003cdiv class=\"Description\"\u003e\u003cp\u003elow-frequency earthquake\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv class=\"DefinitionListEntry\"\u003e\u003cdiv class=\"Term\"\u003e\u003cb\u003eMw\u003c/b\u003e\u003c/div\u003e\u003cdiv class=\"Description\"\u003e\u003cp\u003eMoment magnitude\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv class=\"DefinitionListEntry\"\u003e\u003cdiv class=\"Term\"\u003e\u003cb\u003eNEIC\u003c/b\u003e\u003c/div\u003e\u003cdiv class=\"Description\"\u003e\u003cp\u003eNational Earthquake Information Center\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv class=\"DefinitionListEntry\"\u003e\u003cdiv class=\"Term\"\u003e\u003cb\u003eNIED\u003c/b\u003e\u003c/div\u003e\u003cdiv class=\"Description\"\u003e\u003cp\u003eNational Research Institute for Earth Science and Disaster Resilience\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv class=\"DefinitionListEntry\"\u003e\u003cdiv class=\"Term\"\u003e\u003cb\u003eSSE\u003c/b\u003e\u003c/div\u003e\u003cdiv class=\"Description\"\u003e\u003cp\u003eslow slip event\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv class=\"DefinitionListEntry\"\u003e\u003cdiv class=\"Term\"\u003e\u003cb\u003eVLFE\u003c/b\u003e\u003c/div\u003e\u003cdiv class=\"Description\"\u003e\u003cp\u003every low-frequency earthquakes\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003c/div\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eEthics approval and consent to participate\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent for publication\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no conflict of interest.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAvailability of data and materials\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eCMT data were downloaded from the NIED website. Earthquake data were downloaded from the JMA website. Waveform data were provided by NIED (F-net; https://www.fnet.bosai.go.jp/) and BATS (https://bats.earth.sinica.edu.tw/). Coulomb 3.3 is available at https://earthquake.usgs.gov/research/software/coulomb/. The slab geometry shown in Figure 1 was downloaded from the U.S. Geological Survey website (Hayes 2018).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis research received no external funding.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthors\u0026apos; contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eMN designed the study, conducted the data analysis, and wrote the manuscript. RY performed centroid moment tensor (CMT) inversions as part of his master\u0026rsquo;s research, which provided the foundation for part of the VLFE catalog used in this study. Both authors discussed the results and contributed to the final version of the manuscript. All authors approved the final manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWe are grateful to the JMA for providing the earthquake catalog. We are also grateful to the NIED for the F-net waveform records and CMT solutions and to the Institute of Earth Sciences, Academia Sinica for BATS for the waveform records. Generic Mapping Tools (Wessel et al. 2019) were used to create the figures.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eAndo M, Tu Y, Kumagai H, Yamanaka Y, Lin CH (2012) Very low frequency earthquakes along the Ryukyu subduction zone, Geophys Res Lett 39:L04303. doi:10.1029/2011GL050559\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eArai R, Takahashi T, Kodaira S, Kaiho Y, Nakanishi A, Fujie G, Nakamura Y, Yamamoto Y, Ishihara Y, Miura S, Kaneda Y (2016) Structure of the tsunamigenic plate boundary and low-frequency earthquakes in the southern Ryukyu Trench. Nat Commun 7:12255. doi:10.1038/ncomms12255\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eAsano Y, Obara K, Ito Y (2008) Spatiotemporal distribution of very-low frequency earthquakes in Tokachi-oki near the junction of the Kuril and Japan trenches revealed by using array signal processing. 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Geochem Geophys Geosys 20:5556\u0026ndash;5564. doi:10.1029/2019GC008515\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eYokota Y, Ishikawa T (2020) Shallow slow slip events along the Nankai Trough detected by GNSS-A. Sci Adv 6:eaay5786. doi:10.1126/sciadv.aay5786\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":true,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"earth-planets-and-space","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"epsp","sideBox":"Learn more about [Earth, Planets and Space](http://earth-planets-space.springeropen.com)","snPcode":"","submissionUrl":"https://www.editorialmanager.com/epsp/default.aspx","title":"Earth, Planets and Space","twitterHandle":"@SpringerOpen","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"BMC/SO AJ","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"Very low-frequency earthquake, slow slip events, aseismic slip, Ryukyu Trench, subduction zone, interplate coupling","lastPublishedDoi":"10.21203/rs.3.rs-7029393/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7029393/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThis study focuses on very low-frequency earthquakes (VLFEs) occurring in the southwestern Ryukyu Trench and investigates their long-term activity as well as their relationship with regional seismicity and tectonic features. VLFEs constitute a type of slow earthquakes characterized by surface waves with periods of 20\u0026ndash;50 seconds. As they are detectable over long distances, VLFEs are suitable for effective monitoring of aseismic processes in offshore subduction zones with sparse seismic networks. Using waveform data from Japan\u0026rsquo;s F-net network and Taiwan\u0026rsquo;s Broadband Array in Taiwan for Seismology (BATS) network, we constructed two VLFE catalogs, namely the FB-catalog (2000\u0026ndash;2024), which was based on both networks, and the B-catalog (1998\u0026ndash;2024), which was based only on BATS. VLFEs were identified using waveform cross-correlation with template events.\u003c/p\u003e\u003cp\u003eResults show that VLFE activity is concentrated along the trench axis at depths of 10\u0026ndash;20 km, occurring periodically in swarm-like episodes approximately every 2\u0026ndash;3 months. In some cases, these swarms exhibit migration. VLFE epicenters are spatially complementary to thrust-type earthquake zones, suggesting distinct regions of seismic and aseismic slip. VLFE clusters are distributed just south of areas characterized by recurring slow slip events (SSEs) and GNSS-inferred slip deficits. A clear contrast is observed across the subducted Gagua Ridge: Seismicity is more prevalent to the west, whereas VLFEs dominate to the east. VLFE activity has increased significantly since late 2001, temporally coinciding with moderate earthquakes and afterslip, suggesting the activation of slow slip processes or weakened interplate coupling near the trench axis. This finding provides new insight into the spatial distribution and physical mechanisms of aseismic slip along the plate interface\u0026mdash;especially near the trench axis where direct geodetic observations are limited. VLFE monitoring can serve as a valuable tool for characterizing interplate coupling in offshore subduction zones.\u003c/p\u003e","manuscriptTitle":"Spatiotemporal Variability and Tectonic Implications of Very Low-Frequency Earthquakes in the Southwestern Ryukyu Trench","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-07-31 10:22:23","doi":"10.21203/rs.3.rs-7029393/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"reviewerAgreed","content":"","date":"2025-08-03T14:30:54+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-07-28T08:20:50+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-07-08T13:26:06+00:00","index":"","fulltext":""},{"type":"submitted","content":"Earth, Planets and Space","date":"2025-07-05T06:11:03+00:00","index":"","fulltext":""},{"type":"decision","content":"Minor Revision","date":"2025-07-03T22:47:40+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"earth-planets-and-space","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"epsp","sideBox":"Learn more about [Earth, Planets and Space](http://earth-planets-space.springeropen.com)","snPcode":"","submissionUrl":"https://www.editorialmanager.com/epsp/default.aspx","title":"Earth, Planets and Space","twitterHandle":"@SpringerOpen","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"BMC/SO AJ","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"91afb862-975b-4b6b-8fee-101a3a188223","owner":[],"postedDate":"July 31st, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2025-12-01T15:59:42+00:00","versionOfRecord":{"articleIdentity":"rs-7029393","link":"https://doi.org/10.1186/s40623-025-02314-6","journal":{"identity":"earth-planets-and-space","isVorOnly":false,"title":"Earth, Planets and Space"},"publishedOn":"2025-11-27 15:57:03","publishedOnDateReadable":"November 27th, 2025"},"versionCreatedAt":"2025-07-31 10:22:23","video":"","vorDoi":"10.1186/s40623-025-02314-6","vorDoiUrl":"https://doi.org/10.1186/s40623-025-02314-6","workflowStages":[]},"version":"v1","identity":"rs-7029393","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7029393","identity":"rs-7029393","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}