Seismic Tomography for Subsurface Structures Imaging beneath Hachijojima Volcanic Island, Izu-Bonin Arc, Japan

Seismic Tomography for Subsurface Structures Imaging beneath Hachijojima Volcanic Island, Izu-Bonin Arc, Japan | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Seismic Tomography for Subsurface Structures Imaging beneath Hachijojima Volcanic Island, Izu-Bonin Arc, Japan Adrianto Widi Kusumo, Hiroyuki Azuma, Toshiki Watanabe, Yoshiya Oda This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6195617/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 01 Jul, 2025 Read the published version in Journal of Seismology → Version 1 posted 9 You are reading this latest preprint version Abstract We present a seismic tomography study of the subsurface structure beneath Hachijojima Island, one of the volcanic fronts in the Izu-Bonin Arc, Japan. Seismic observations were conducted over two 7-month periods in 2019 and 2021, utilizing 55 densely installed stations on the island. During these periods, a total of 179 local earthquakes were recorded — 119 in 2019 and 60 in 2021 — resulting in 4671 P-wave arrival times and 3927 S-wave arrival times. The 3-D tomography, derived using the double-difference technique, revealed a shallow low-velocity region between the island’s two main volcanoes, Nishiyama and Higashiyama, suggesting the presence of volcanic sediments near the surface. Additionally, a high-velocity anomaly was identified at a depth of 4–5 km, extending vertically from deeper regions beneath Nishiyama. This feature is interpreted as a magma pathway from past volcanic activity, with high P-wave velocities and elevated Vp/Vs ratios indicating possible fluid presence. At greater depths, low P-wave velocity perturbations and elevated Vp/Vs ratios suggest a magmatic plumbing system comprising a mid-crustal magma chamber at approximately 8–12 km depth and lateral magmatic pathways at 10–20 km depth. Furthermore, a distinct zone characterized by reduced P-wave velocity and increased Vp/Vs is interpreted as a shallow magma chamber with H₂O-saturated magma accumulation. These findings provide valuable insights into the subsurface magmatic processes beneath Hachijojima Island, which are crucial for improving volcanic hazard assessment. Hachijojima seismic tomography double-difference magmatic plumbing system volcano seismology Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Figure 13 Article Highlights Seismic observations using a dense array within volcanic front of the Izu-Bonin Arc, Japan. Subsurface structures imaging beneath Hachijojima Island, Japan, using seismic tomography. 3D tomography revealed the magmatic plumbing system beneath the volcanic island. 1. Introduction Seismic tomography is a widely used geophysical method for investigating the subsurface of the Earth, particularly in volcanic areas. It allows us to obtain detailed information about the geometry, location, and dimensions of magmatic systems (Ardianto et al. 2021 ; Lees 2007 ). Additionally, another parameter appears to be the Vp/Vs ratio can provide insights for fluid and melt content evaluation (Koulakov and Shapiro 2021 ). Seismic tomography has been successfully applied to study volcanoes around the world (Bushenkova et al. 2023 ; Komzeleva et al. 2021 ; Koulakov et al. 2023 ; Nugraha et al. 2019 ; Widiyantoro et al. 2018 ) and particularly in Japan, such as Hakone volcano (Yukutake et al. 2021 ), Naruko volcano (Okada et al. 2014 ), Sakurajima volcano (Alanis et al. 2012 ), Onikobe volcanic area (Nakajima and Hasegawa 2003 ), and Aso volcano (Sudo and Kong 2001 ). However, existing studies on Hachijojima Island, an inhabited volcanic island in Japan, have been limited in terms of subsurface structure thus far (Hagiwara and Watanabe 2019 ; Kanke et al. 2021 ). This study will contribute to expanding seismic tomography studies on volcanoes in Japan, specifically Hachijojima Island. Thus, it will gain insights into volcanic eruption mechanisms and implement effective volcanic disaster mitigation measures on this island. Hachijojima, located in the group of Izu Islands (Fig. 1 ), is known for being one of the largest volcanic islands within the volcanic front of the Izu-Bonin-Mariana Arc that stretches over a distance of approximately 2800 km from Japan to Guam. The island is situated approximately 287 km south of Tokyo and covers an area of about 63 km 2 . Hachijojima region consists of two nearby islands, namely the main island of Hachijojima and smaller island known as Hachijokojima which is a part of the Hachijojima volcanic system. The main island is home to two volcanoes i.e., Nishiyama (854 m) and Higashiyama (701 m). These volcanoes are classified as stratovolcanoes built up mainly by their basaltic lavas and pyroclastic materials (Suga 1993 , 1994 ). The topographic evidence suggests that Higashiyama volcano formed prior to Nishiyama (Sugihara 1998 ). The Higashiyama volcano's eruption began roughly 100,000 years ago and subsided around 4000 years ago. On the other hand, the Nishiyama volcano underwent volcanic activity approximately 10,000 years ago with the most recent eruption of this volcano likely occurred in 1605 (Ishizuka and Geshi 2018 ). Despite the fact that there have not been any magmatic eruptions since the last known eruption, the magmatic system is still regarded as being active. Previous studies have examined the underground magmatic plumbing system of Hachijojima, utilizing various approaches such as petrology and geochemistry. In a recent study by Oiwa et al. ( 2023 ), it has been discovered that the magma plumbing system is comprised of two chambers: a mid-crustal magma chamber located at depths ranging from 9 to 12 km, and a deeper magma chamber which likely serves as the source of primitive magma. It is worth noting that earthquake swarms observed in 2002 were possibly triggered by injections of primitive magma into the mid-crustal magma chamber. In addition, Ishizuka et al. ( 2008 ) have also conducted a comprehensive study on the magma plumbing system using various methods such as petrography, geochemistry, and marine geophysical surveys. The bathymetry map near Hachijojima revealed the presence of other submarine volcanic chains extending from Nishiyama, namely the Hachijo NW chain in the northwest and the NE edifices in the northeast slope of Nishiyama. The magma for the Hachijo NW chain is believed to have been transported over a significant distance in the middle to lower crust due to regional extensional stress conditions surrounding the arc volcano. On the other hand, it is likely that magma for the NE edifices was laterally transferred from the shallower magma chamber beneath the Nishiyama. Hagiwara and Watanabe ( 2019 ) conducted seismic tomography utilizing seismic data from 2003 to 2018 obtained through the Izu Islands volcanic observation system. They identified earthquakes occurring within a middle-velocity region surrounding a high-velocity zone located at a depth of 10–14 km. It was inferred that the high-velocity zone beneath Nishiyama volcano is associated with relatively cool body. They proposed that the low-velocity zones correspond to hot magmatic substances while the high-velocity zones are linked to solidified volcanic structures. Another recent study was conducted by Kanke et al. ( 2021 ) to estimate the velocity image through travel-time tomography, despite a significantly limited number of seismic events available. The findings of this research revealed a three-dimensional velocity representation beneath Nishiyama. Seismic observations were conducted on Hachijojima Island in 2019 and 2021 to collect earthquake data for this seismic tomography study (Kusumo et al. 2025 ). The seismic data collected was used in this study to analyze the subsurface structure beneath Nishiyama, which experienced recent seismic activity characterized by an earthquake swarm and ground deformation observed in 2002 (Kimata et al. 2004 ; Kumagai et al. 2003 ). The primary objective is to investigate the subsurface structure of Hachijojima Island, with a particular emphasis on Nishiyama, to enhance understanding of its magmatic system. By providing new insights into the island’s subsurface velocity structure, our study contributes to advancements in the field of seismic tomography and volcanic hazard assessment. 2. Data and Method 2.1. Data Tokyo Metropolitan University has conducted temporary seismic observation twice in 2019 and 2021 in collaboration with Nagoya University (Kusumo et al. 2025 ). The geological conditions of Hachijojima Island presented difficulties for conducting seismic observations because of persistent and intense winds and waves. Consequently, the acquired seismic data frequently exhibited strong noise, leading to challenges in precisely identifying and establishing the arrival times of P- and S-waves, especially for events with lower magnitudes. A total of 179 earthquake events were identified, consisting of 119 events occurred in 2019 and the remaining 60 in 2021, forming a dataset ready for use in this study. In order to locate the initial hypocenters, stations from neighboring islands were also used. Nevertheless, specifically for seismic tomography purpose in this study, we utilized only the data observed from 55 stations installed on Hachijojima Island, encompassing both temporary and permanent stations (Fig. 2). 2.2. Method Double difference (DD) tomography (Zhang and Thurber 2003 , 2006 ) was utilized in this study to obtain the three-dimensional subsurface velocity structure below Hachijojima Island. The absolute and differential arrival times are inverted simultaneously in this method. The DD tomography applies the concept of the double-difference method to precisely determine the hypocenter location from Waldhauser and Ellsworth ( 2000 ). The DD method is based on the idea that if the separation distance between two earthquakes' hypocenters is relatively small compared to the event-station distance, their waveforms and ray paths will exhibit similarities. Thus, any difference in travel time observed at one station can be accurately attributed to the spatial separation between the events. This is acceptable because the discrepancies arise from similar origins, with only minor variations in regions where there are differences in ray paths at the source. The ray path and travel time were calculated using the pseudo-bending ray-tracing method (Um and Thurber 1987 ). This method is commonly used to estimate subsurface structures in volcanic regions with significant velocity perturbations. The inversion was conducted using the LSQR technique by (Paige and Saunders 1982 ). The study employed a two-step seismic tomography inversion process in order to construct a robust initial reference velocity model. The first step utilized a coarser and uniform grid size to generate a 3D velocity model, which was then utilized as the initial model for the second tomography inversion. This approach ensured that the initial velocity model was reasonably accurate. The first step employed a grid size of 2 km x 2 km x 4 km. The second step utilized a similar grid size, but with a finer resolution of 1 km x 1 km x 2 km around the shallow area of Nishiyama. The finer grid in the z-direction was applied to a depth of 4 km only. The specific details of the grid used in the second step and its differences from the first step can be found in Fig. 3 . Besides the grid size and initial velocity model, we also consider another factor i.e. damping. The tomography inversion stability depends on the damping value used. The stability of the inversion process in DD tomography is measured using Condition Number Damping (CND) values, which are determined by the ratio of the largest to smallest eigenvalue. Generally, the CND values between 40 to 80 indicate a stable inversion (Waldhauser and Ellsworth 2000 ). In our analysis, we created an L curve for each step that represents the trade-off between data variance and model variance while testing damping values ranging from 100 to 500. Figure 4 shows the L-curve, indicating an ideal damping value for the first step and second step. Ultimately, we applied damping value of 150 in the first step and 180 in the second step of tomography inversion. 2.3. Initial Velocity Model For the first step tomography inversion, one-dimensional velocity analysis was conducted using the Velest program, in order to acquire an initial reference velocity model for three-dimensional local earthquake tomography. This program, developed by Kissling et al. ( 1994 ), performs simultaneous inversion processes that yield minimum 1-D seismic velocities for P- and S-waves, along with hypocentral parameters. In this stage, we adopted a set of 1-D velocity of P-wave from the WIN System, which was used to locate the hypocenter previously, to establish the initial velocity model in Velest. This model was subject to random perturbations up to a depth of 40 km and then used as input for 1-D seismic velocity inversions. The resulting inverted models were evaluated based on their travel time residuals, with only those exhibiting small residuals being considered acceptable. From these accepted results, an average is computed to obtain the final velocity model Fig. 5 . This ultimate reference velocity model eventually serves as input for first step DD tomography. Furthermore, the 3D velocity model obtained from the first step was subsequently used as the initial reference velocity model for the second step of DD tomography. This two-step method was intended to yield a reasonably accurate velocity model for the objective area. 2.4. Resolution Test Resolution test was conducted to evaluate the reliability of the 3-D seismic velocity model obtained from the inversion process. We carried out the Checkerboard Resolution Test (CRT) and Derivative Weight Sum (DWS) for this purpose. To initiate CRT, we generated a synthetic velocity model that incorporated a regular pattern of high and low velocity perturbations resembling a checkerboard pattern (Lévěque et al. 1993 ). In this study we used alternating positive and negative perturbations of ± 10%. The initial velocity model for CRT was a 1-D velocity model that is similar to the initial velocity model used in the first step inversion of real data. Using this synthetic velocity model, along with the earthquake hypocenters and stations from the earthquake catalogue dataset of both 2019 and 2021, we generated synthetic wave travel time data. The event pair, a crucial step in the DD method, is then identified from the obtained synthetic data. The inversion was performed subsequently using the same procedure and parameters as used in the inversion of actual observation data. This resolution test aimed to evaluate spatially the region that can be well interpreted. A region that displays a pattern like the input velocity model indicates that it is well resolved and can be properly interpreted. Meanwhile, DWS is similar to the number of ray paths that pass through a particular grid node (Toomey & Foulger 1989 ). It calculates the density of rays around the grid node with a certain weight. Previous studies have indicated that the DWS values align closely with the diagonal elements of the resolution matrix and possible smearing as indicated by the spread function. A high DWS value represents a zone that has high resolution and has a low smearing effect (Hauksson & Shearer 2006 ; Rietbrock 2001 ). Figure 6 and Fig. 7 illustrates the resolution test analysis of this study. 3. Result Based on the results of the CRT, the inversion method effectively resolved the Nishiyama region’s subsurface structure down to a depth of approximately 8 km (Fig. 6 ). At greater depths, improved resolution was observed primarily in the northwestern area of Hachijojima Island. This pattern aligns with the Derivative Weight Sum (DWS) results, where high DWS values (indicated in blue in Fig. 7 ) are concentrated beneath Nishiyama in the shallow region, gradually shifting toward the northwestern part of the island at greater depths. This pattern is likely influenced by the concentration of earthquake activity near the northern edge of the rift margin (Kusumo et al. 2025 ), which results in ray paths predominantly originating from the northwest. Areas with low DWS values (non-blue regions) are considered unresolved. The application of DD tomography produced two primary outcomes: a refined velocity structure image and improved earthquake event relocation. As shown in Fig. 8 , the relocated earthquake events from the 2019 and 2021 datasets indicate significant improvements in RMS residuals. The RMS residuals are more tightly clustered around zero after DD relocation, indicating improved consistency in the travel time residuals. Before DD relocation, 21,378 data points (77.89%) fall within the range of -0.25 to 0.25 seconds, while after DD relocation, 23,145 data points (84.33%) fall within the same range. The improvement of the DD tomography inversion during the second step, which employed a finer grid size in the objective area, is illustrated in Fig. 9 . The RMS error initially decreased slightly from iteration 1 to 3, followed by a substantial reduction after the 4th iteration. This convergence indicates improved model stability and accuracy, with a final RMS value of 0.1673 s at the last iteration. Given the limited data availability in this study, we approach the interpretation of our results with caution. Nevertheless, the velocity structure obtained still provides meaningful insights. The interpolated velocity values were visualized as a three-dimensional velocity cube, enabling further interpretation. Horizontal sections of the P- and S-wave velocity structure, created at depths from 0 to 8 km with 2 km intervals (Fig. 10 ), reveal a distinct low-velocity region extending from the southeastern part of Nishiyama to the central part of Hachijojima Island, primarily at the surface (0 km depth). This low-velocity region is further emphasized in Fig. 11 along the A-A’ slice. Additionally, a high-velocity anomaly was identified below Nishiyama at approximately 4 km depth, extending vertically from deeper regions (Fig. 12 , slice B-B’). Despite the limitations in data quantity, these findings offer valuable insights into the subsurface magmatic processes beneath Hachijojima Island. The identified velocity anomalies provide crucial information for understanding the region’s magmatic system and may contribute to improved volcanic hazard assessment. 4. Discussion The limited availability of recorded seismic data posed a challenge in this tomographic study of Hachijojima Island. This limitation is evident in the checkerboard resolution test (CRT) results, which indicate that the inversion can reliably resolve structures in the Nishiyama region only to a depth of approximately 8 km. However, the tomography successfully identifies deeper structures, extending down to 20 km in the northwestern region, where the majority of the earthquakes were observed. This result highlights the influence of earthquake distribution on resolution quality, with ray paths predominantly concentrated in this northwestern area. Notably, the obtained velocity structure aligns with the geological and gravity cross-section analysis conducted by NEDO (1992), as illustrated in Fig. 11 . A distinct low-velocity zone near the surface between Nishiyama and Higashiyama (marked by the black dashed box) is interpreted as the top layer of the Neogene stratum, predominantly composed of volcanic sediment deposits. The interpreted faults identified in this study are consistent with those depicted in the geological cross-section. Additionally, the structure characterized by low P-wave velocity values in the northwestern portion of Nishiyama may indicate a fault or fractured zone. Our findings also corroborate previous research conducted by Kanke et al. ( 2021 ), further supporting the robustness of the velocity model. A prominent high-velocity P-wave anomaly was identified beneath Nishiyama, extending vertically from deeper regions to approximately 4 km depth (Fig. 12 ). This feature is interpreted as a solidified magma pathway from past volcanic activity, likely representing the remnants of a preferential magma flow channel. The combination of high P-wave velocities, elevated Vp/Vs ratios, and reduced S-wave velocities in this region suggests the possible presence of fluid content (Koulakov et al. 2018 ; Koulakov and Shapiro 2021 ). Our tomographic results at deeper regions (Fig. 13 ) provide significant insights into the underlying magma plumbing system beneath Hachijojima. These findings align well with previously proposed magma transport models by Ishizuka et al. ( 2008 ) and Oiwa et al. ( 2023 ). The tomographic model reveals a lateral magma transport pathway situated at a depth of approximately 10–20 km in the northwestern region (zone I), where low P-wave velocity perturbations and elevated Vp/Vs ratios coincide with the location of deep earthquake activity recorded in 2002 and 2020, suggesting the presence of partial melt or high-temperature, volatile-rich magma. This observation supports Ishizuka et al. ( 2008 ), who inferred a lateral magma transport pathway at similar depths based on hypocenter distributions and petrological constraints. The alignment between these results indicates that basaltic magma from a deeper mantle source was likely transported laterally through this pathway. In addition, a low P-wave velocity perturbation coupled with high Vp/Vs ratios at depths of approximately 8–12 km beneath Nishiyama (zone II) is interpreted as a mid-crustal magma reservoir. This feature is consistent with the concept of magma differentiation, where ascending magma undergoes compositional changes as it cools and crystallizes (Oiwa et al. 2023 ). The presence of this magma chamber suggests that it may be intermittently supplied by deeper sources. The earthquake swarms observed in 2002 in this region may have been triggered by the injection of magma into this chamber, suggesting that this mid-crustal magma storage system remains active. Furthermore, the anomalies observed at approximately 5 km depths beneath Nishiyama (zone III) are consistent with conceptual model of pre-eruption where the crystal fractionation and storage processes described by Oiwa et al. ( 2023 ). This anomaly is characterized by decreased P-wave velocity and increased Vp/Vs ratios, which may correspond to an H₂O-saturated magma accumulation zone, corroborating the interpretation presented by Oiwa et al. ( 2023 ). This feature is interpreted as a region where temporary magma stalling and plagioclase overgrowth occur prior to an eruption. The presence of a high Vp/Vs ratio at this depth (slice B-B’) reinforces this interpretation. This anomaly may represent a partially crystallized magma pathway from the past that still contains residual fluids, suggesting this zone is not completely solidified, meaning it could still play a role in magma migration or remobilization. These findings support the interpretation of a complex, multi-stage magma evolution process beneath Nishiyama, characterized by periodic injections of primitive basalt, magma differentiation, and pre-eruptive magma storage. The consistency between our seismic results and these conceptual models strengthens the interpretation of a dynamic magma plumbing system beneath Nishiyama, controlled by both deep lateral magma transport and crustal-level magma differentiation. 5. Conclusion We have successfully conducted three-dimensional seismic tomography to analyze the subsurface structures beneath Hachijojima Island, Japan. The 3-D velocity structure of seismic P-wave and S-wave was determined using DD tomography method. The seismic data was collected from two seismic observations in 2019 and 2021. Seismic tomography was performed using a combination of 46 temporary stations and 9 permanent stations. Despite the limited number of earthquake events due to the volcano’s inactivity, the dataset remains valuable for understanding the island’s subsurface characteristics. Resolution tests indicate that the Nishiyama area and its northwestern region exhibits satisfactory resolution up to a depth of 8 km and 20 km, respectively. The tomographic results identified a prominent low-velocity region in the shallow zone between Nishiyama and Higashiyama, interpreted as volcanic sediment deposits. In contrast, a distinct high-velocity anomaly was observed beneath Nishiyama, indicating solidified magma that likely represents a past volcanic conduit. The high P-wave velocities and Vp/Vs ratios further suggest the possible presence of fluids in this region. At greater depths, the northwestern region is characterized by low P-wave velocity perturbations and high Vp/Vs ratios, indicating the presence of a mid-crustal magma chamber at approximately 8–12 km depth. This chamber appears to be connected to a lateral magmatic transport network extending to depths of ~ 10–20 km. Additionally, a shallow anomaly at ~ 5 km depth beneath Nishiyama is interpreted as an H₂O-saturated magma accumulation zone, where magma stalling and plagioclase overgrowth may occur prior to an eruption. This feature may represent a partially crystallized magma pathway that retains residual fluids, suggesting its potential role in future magma migration or remobilization. These findings provide valuable insights into the magmatic processes beneath Hachijojima Island, improving our understanding of the island’s volcanic system Declarations Ethics approval and consent to participate The authors declare that this paper does not involve ethical issues. Consent to publish All authors have read and agreed to the version of the manuscript. Competing Interests The authors declares that there is no conflict of interest. Funding This research was supported by the Tokyo Metropolitan Government. Author Contribution Each author has contributed to this paper. Conceptualization: A.W.K and Y.O.; Data Collection: H.A., T.W., and Y.O.; Data Analysis: A.W.K., H.A., and Y.O.; Interpretation of Results: A.W.K., H.A., and Y.O.; Writing – original draft: A.W.K.; Writing – review and editing: H.A., T.W., and Y.O.; Supervision: H.A. and Y.O.. Acknowledgement We express our sincere appreciation to the entities and individuals whose contributions were vital to the completion of this research. The Hachijo Branch Office of the Tokyo Metropolitan Government and the Hachijo Town provided crucial observational support, and we extend our gratitude to the private companies and individuals who facilitated our study. We also offer special appreciation to Dr. Tsutomu Ochiai, Assistant Professor at Kanagawa University, for his valuable insights and guidance. In addition, we acknowledge the invaluable data provided by The National Research Institute for Earth Science and Disaster Resilience, The Japan Meteorological Agency, and the Tokyo Metropolitan Government. 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Earth, Planets and Space , 66 (1). https://doi.org/10.1186/1880-5981-66-114 Paige, C. C., & Saunders, M. A. (1982). LSQR: An Algorithm for Sparse Linear Equations and Sparse Least Squares. ACM Transactions on Mathematical Software (TOMS) , 8 (1), 43–71. https://doi.org/10.1145/355984.355989 Rietbrock, A. (2001). P wave attenuation structure in the fault area of the 1995 Kobe earthquake. Journal of Geophysical Research: Solid Earth , 106 (B3), 4141–4154. https://doi.org/10.1029/2000jb900234 Sudo, Y., & Kong, L. (2001). Three-dimensional seismic velocity structure beneath Aso Volcano, Kyushu, Japan. Bulletin of Volcanology , 63 (5), 326–344. https://doi.org/10.1007/s004450100145 Suga, K. (1993). The Latest Activities of Higashiyama Volcano and Development of Nishiyama Volcano, Hachijojima during Past 10,000 Years. Bulletin of the Volcanological Society of Japan , 38 , 115-127 (in Japanese). https://doi.org/https://doi.org/10.18940/kazan.38.4_115 Suga, K. (1994). Volcanic History of Higashiyama, Hachijojima. Bulletin of the Volcanological Society of Japan , 39 , 13-24 (in Japanese). https://doi.org/https://doi.org/10.18940/kazan.39.1_13 Sugihara, S. (1998). Tephrochronological Study of Higashiyama Volcano at Hachijojima, Izu Islands. Journal of Geography , 107 (3), 390-420 (in Japanese). https://doi.org/https://doi.org/10.5026/jgeography.107.3_390 Toomey, D. R., & Foulger, G. R. (1989). Tomographic inversion of local earthquake data from the Hengill- Grensdalur central volcano complex, Iceland. Journal of Geophysical Research , 94 (B12). https://doi.org/10.1029/jb094ib12p17497 Um, J., & Thurber, C. (1987). A fast algorithm for two-point seismic ray tracing. Bulletin of the Seismological Society of America , 77 (3), 972–986. https://doi.org/10.1785/BSSA0770030972 Waldhauser, F., & Ellsworth, W. L. (2000). A Double-Difference Earthquake Location Algorithm : Method and Application to the Northern Hayward Fault , California. Bulletin of the Seismological Society of America , 90 (6), 1353–1368. https://doi.org/https://doi.org/10.1785/0120000006 Widiyantoro, S., Ramdhan, M., Métaxian, J. P., Cummins, P. R., Martel, C., Erdmann, S., Nugraha, A. D., Budi-Santoso, A., Laurin, A., & Fahmi, A. A. (2018). Seismic imaging and petrology explain highly explosive eruptions of Merapi Volcano, Indonesia. Scientific Reports , 8 (1). https://doi.org/10.1038/s41598-018-31293-w Yukutake, Y., Abe, Y., Honda, R., & Sakai, S. (2021). Magma Reservoir and Magmatic Feeding System Beneath Hakone Volcano, Central Japan, Revealed by Highly Resolved Velocity Structure. Journal of Geophysical Research: Solid Earth , 126 (4). https://doi.org/10.1029/2020JB021236 Zhang, H., & Thurber, C. (2003). Double-Difference Tomography: The Method and Its Application to the Hayward Fault, California. Bulletin of the Seismological Society of America , 93 (5), 1875–1889. https://doi.org/10.1785/0120020190 Zhang, H., & Thurber, C. (2006). Development and applications of double-difference seismic tomography. Pure and Applied Geophysics , 163 (2–3), 373–403. https://doi.org/10.1007/s00024-005-0021-y Additional Declarations No competing interests reported. Cite Share Download PDF Status: Published Journal Publication published 01 Jul, 2025 Read the published version in Journal of Seismology → Version 1 posted Editorial decision: Revision requested 21 May, 2025 Reviews received at journal 20 May, 2025 Reviewers agreed at journal 11 May, 2025 Reviews received at journal 22 Apr, 2025 Reviewers agreed at journal 01 Apr, 2025 Reviewers invited by journal 20 Mar, 2025 Editor assigned by journal 13 Mar, 2025 Submission checks completed at journal 13 Mar, 2025 First submitted to journal 10 Mar, 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-6195617","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":433189466,"identity":"83eff5ca-36f0-4605-825a-084014145ae3","order_by":0,"name":"Adrianto Widi Kusumo","email":"data:image/png;base64,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","orcid":"","institution":"Tokyo Metropolitan University","correspondingAuthor":true,"prefix":"","firstName":"Adrianto","middleName":"Widi","lastName":"Kusumo","suffix":""},{"id":433189467,"identity":"fb50ee58-f36f-4e42-a833-419d65e7337f","order_by":1,"name":"Hiroyuki Azuma","email":"","orcid":"","institution":"Tokyo Metropolitan University","correspondingAuthor":false,"prefix":"","firstName":"Hiroyuki","middleName":"","lastName":"Azuma","suffix":""},{"id":433189468,"identity":"3847f81d-9ec4-4e83-bba4-7ce3296b8e49","order_by":2,"name":"Toshiki Watanabe","email":"","orcid":"","institution":"Nagoya University","correspondingAuthor":false,"prefix":"","firstName":"Toshiki","middleName":"","lastName":"Watanabe","suffix":""},{"id":433189469,"identity":"02f4a484-2724-456c-83e8-4826d4f3aa35","order_by":3,"name":"Yoshiya Oda","email":"","orcid":"","institution":"Tokyo Metropolitan University","correspondingAuthor":false,"prefix":"","firstName":"Yoshiya","middleName":"","lastName":"Oda","suffix":""}],"badges":[],"createdAt":"2025-03-10 12:38:32","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6195617/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6195617/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s10950-025-10309-9","type":"published","date":"2025-07-01T15:58:34+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":79578425,"identity":"74719806-48f8-436a-a5e2-ad3882e317b9","added_by":"auto","created_at":"2025-03-31 11:34:06","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":84402,"visible":true,"origin":"","legend":"\u003cp\u003eMap showing the detailed location of Hachijojima Island. The island is located in the volcanic front of the Izu-Bonin-Mariana Arc, approximately 287 km south of Tokyo, Japan\u003c/p\u003e","description":"","filename":"1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6195617/v1/554bb409de8f23b2fab29234.jpg"},{"id":79575999,"identity":"4edf1221-9296-4bb3-a665-bd8c738ec28f","added_by":"auto","created_at":"2025-03-31 11:18:06","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":33764,"visible":true,"origin":"","legend":"\u003cp\u003eStation distribution network that was used in this study, includes a total of 46 temporary stations (green) that were installed for a duration of 7 months in each period, along with 9 permanent stations (orange) operated by JMA, Hi-net, F-net, and Tokyo Metropolitan Government. Blue dashed box shows the objective region, Nishiyama\u003c/p\u003e","description":"","filename":"2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6195617/v1/cc478a71dfa2de5ef0ead8b4.jpg"},{"id":79577539,"identity":"d36fee75-d8fa-42e2-8979-3a1eb71d0522","added_by":"auto","created_at":"2025-03-31 11:26:06","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":83278,"visible":true,"origin":"","legend":"\u003cp\u003eConfiguration of 3-D grid nodes model for first step (a) and second step (b) of tomographic inversion. First step utilized 2 km x 2 km x 4 km uniform grid size. Meanwhile, second step used 1 km x 1 km x 2 km grid size in objective area, Nishiyama\u003c/p\u003e","description":"","filename":"3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6195617/v1/1c762fa2817448b23f786896.jpg"},{"id":79576005,"identity":"7fff8e4c-fd8c-4d8b-9b0b-f5db8068d7e7","added_by":"auto","created_at":"2025-03-31 11:18:06","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":42797,"visible":true,"origin":"","legend":"\u003cp\u003eL curve (trade off) between model variance and data variance of (a) first step and (b) second step tomographic inversion to determine the optimal damping value. This includes taking into account the Condition Number Damping as part of the selection process\u003c/p\u003e","description":"","filename":"4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6195617/v1/d02ef4cfbb36b7ae313e93f5.jpg"},{"id":79576003,"identity":"d2aa3446-6ac5-4a1e-955f-b658a71b85f1","added_by":"auto","created_at":"2025-03-31 11:18:06","extension":"jpg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":35300,"visible":true,"origin":"","legend":"\u003cp\u003e1-D velocity analysis to derive initial reference velocity for 3-D seismic tomography. Initial velocity model (blue), randomly perturbed velocity model (gray), and the inverted velocity model (black) are displayed on the left image. Accepted results from inverted velocity model (black) and final velocity model (red) are shown on the right image\u003c/p\u003e","description":"","filename":"5.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6195617/v1/3f1e8d9b12bed7d425a0a7cc.jpg"},{"id":79576025,"identity":"0ec14918-bccb-432b-9642-7d0b1e89070c","added_by":"auto","created_at":"2025-03-31 11:18:07","extension":"jpg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":166563,"visible":true,"origin":"","legend":"\u003cp\u003eHorizontal cross section of Checkerboard Resolution Test (CRT) of Vp (a) and Vs (b). The section covers depths of 0, 2, 4, 8, 12, 16 and 20 km based on the grid node usage. The dots represent the grid node used in this study\u003c/p\u003e","description":"","filename":"6.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6195617/v1/64ecd94eac06a5517e8158b7.jpg"},{"id":79576015,"identity":"fd27fc27-1182-4c7c-bdb8-43605a13bf66","added_by":"auto","created_at":"2025-03-31 11:18:06","extension":"jpg","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":162555,"visible":true,"origin":"","legend":"\u003cp\u003eHorizontal cross section of Derivative Weight Sum (DWS) for Vp (a) and Vs (b). The section covers depths of 0, 2, 4, 8, 12, 16 and 20 km based on the grid node usage. The dots represent the grid node used in this study\u003c/p\u003e","description":"","filename":"7.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6195617/v1/dec17bc45f3f5a5f876efb78.jpg"},{"id":79576026,"identity":"e58387ba-b89b-48bb-8b6d-b3d59e814103","added_by":"auto","created_at":"2025-03-31 11:18:07","extension":"jpg","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":98118,"visible":true,"origin":"","legend":"\u003cp\u003eInitial and relocated 2019-2021 events obtained from the second step of DD tomography inversion simultaneously (a) and their RMS residuals improvement (b)\u003c/p\u003e","description":"","filename":"8.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6195617/v1/a286667198262687b8cbe890.jpg"},{"id":79576028,"identity":"43eb5584-3bd4-4532-a3b1-d3873a44afd3","added_by":"auto","created_at":"2025-03-31 11:18:07","extension":"jpg","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":17295,"visible":true,"origin":"","legend":"\u003cp\u003ePlot of RMS residual of the second step tomography inversion\u003c/p\u003e","description":"","filename":"9.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6195617/v1/8068164a24d27b13d010426d.jpg"},{"id":79577552,"identity":"b31ee175-03a1-4c16-8112-757307e02537","added_by":"auto","created_at":"2025-03-31 11:26:07","extension":"jpg","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":82468,"visible":true,"origin":"","legend":"\u003cp\u003eHorizontal cross section of Vp (a) and Vs (b) obtained from the second step of DD tomography. Images only display the region with good resolution, indicated by high DWS value and resolved CRT pattern\u003c/p\u003e","description":"","filename":"10.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6195617/v1/311dbe0ba3f282dfd9c97275.jpg"},{"id":79576036,"identity":"08616110-da3c-45fd-9d40-f491d066c26f","added_by":"auto","created_at":"2025-03-31 11:18:07","extension":"jpg","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":124735,"visible":true,"origin":"","legend":"\u003cp\u003eComparison of geological (a) and gravity analysis (b) cross section from NEDO (1992) with obtained P-wave velocity structure (c) at A-A’ slice, which is oriented in the NW-SE direction,. Low velocity region is indicated in the black dashed box at A-A’ slice. Red dashed lines are interpreted faults\u003c/p\u003e","description":"","filename":"11.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6195617/v1/34f79f0b08d34c6fffeebced.jpg"},{"id":79576007,"identity":"3846034e-83a3-4e2c-8f53-0f2aaa9af642","added_by":"auto","created_at":"2025-03-31 11:18:06","extension":"jpg","order_by":12,"title":"Figure 12","display":"","copyAsset":false,"role":"figure","size":48684,"visible":true,"origin":"","legend":"\u003cp\u003eVertical cross sections of Vp (a), Vs (b), and Vp/Vs ratio (c) obtained from DD tomography. The B-B' section is oriented in the N-S direction. The region of interest is denoted by the black dashed box\u003c/p\u003e","description":"","filename":"12.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6195617/v1/c0b9f7019ba7ea1ca9c81444.jpg"},{"id":79577554,"identity":"8a068eeb-2718-4013-98e0-f3c732172f5d","added_by":"auto","created_at":"2025-03-31 11:26:07","extension":"jpg","order_by":13,"title":"Figure 13","display":"","copyAsset":false,"role":"figure","size":125028,"visible":true,"origin":"","legend":"\u003cp\u003eVertical cross section C-C’ which is oriented NW-SE direction at deeper region showing Vp perturbation (a) and Vp/Vs ratio (b) and its interesting regions shown by dashed box and ellipse. Red circles and turquoise circles represent the earthquake events used in this study and occurred in 2002, respectively. The 2002 earthquake events are provided by the Japan Meteorological Agency (2024). Schematic model of magmatic plumbing system is illustrated in (c)\u003c/p\u003e","description":"","filename":"13.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6195617/v1/9a85eec6fb3f1246a094a7f5.jpg"},{"id":86180321,"identity":"b2b16f99-66f3-46db-abf2-6044ad1eb4d5","added_by":"auto","created_at":"2025-07-07 16:22:12","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1637022,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6195617/v1/6493d702-828a-4b1a-a56c-473bc4df1cb2.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Seismic Tomography for Subsurface Structures Imaging beneath Hachijojima Volcanic Island, Izu-Bonin Arc, Japan","fulltext":[{"header":"Article Highlights","content":"\u003cul\u003e\n \u003cli\u003eSeismic observations using a dense array within volcanic front of the Izu-Bonin Arc, Japan.\u003c/li\u003e\n \u003cli\u003eSubsurface structures imaging beneath Hachijojima Island, Japan, using seismic tomography.\u003c/li\u003e\n \u003cli\u003e3D tomography revealed the magmatic plumbing system beneath the volcanic island.\u003c/li\u003e\n\u003c/ul\u003e"},{"header":"1. Introduction","content":"\u003cp\u003eSeismic tomography is a widely used geophysical method for investigating the subsurface of the Earth, particularly in volcanic areas. It allows us to obtain detailed information about the geometry, location, and dimensions of magmatic systems (Ardianto et al. \u003cspan class=\"CitationRef\"\u003e2021\u003c/span\u003e; Lees \u003cspan class=\"CitationRef\"\u003e2007\u003c/span\u003e). Additionally, another parameter appears to be the Vp/Vs ratio can provide insights for fluid and melt content evaluation (Koulakov and Shapiro \u003cspan class=\"CitationRef\"\u003e2021\u003c/span\u003e). Seismic tomography has been successfully applied to study volcanoes around the world (Bushenkova et al. \u003cspan class=\"CitationRef\"\u003e2023\u003c/span\u003e; Komzeleva et al. \u003cspan class=\"CitationRef\"\u003e2021\u003c/span\u003e; Koulakov et al. \u003cspan class=\"CitationRef\"\u003e2023\u003c/span\u003e; Nugraha et al. \u003cspan class=\"CitationRef\"\u003e2019\u003c/span\u003e; Widiyantoro et al. \u003cspan class=\"CitationRef\"\u003e2018\u003c/span\u003e) and particularly in Japan, such as Hakone volcano (Yukutake et al. \u003cspan class=\"CitationRef\"\u003e2021\u003c/span\u003e), Naruko volcano (Okada et al. \u003cspan class=\"CitationRef\"\u003e2014\u003c/span\u003e), Sakurajima volcano (Alanis et al. \u003cspan class=\"CitationRef\"\u003e2012\u003c/span\u003e), Onikobe volcanic area (Nakajima and Hasegawa \u003cspan class=\"CitationRef\"\u003e2003\u003c/span\u003e), and Aso volcano (Sudo and Kong \u003cspan class=\"CitationRef\"\u003e2001\u003c/span\u003e). However, existing studies on Hachijojima Island, an inhabited volcanic island in Japan, have been limited in terms of subsurface structure thus far (Hagiwara and Watanabe \u003cspan class=\"CitationRef\"\u003e2019\u003c/span\u003e; Kanke et al. \u003cspan class=\"CitationRef\"\u003e2021\u003c/span\u003e). This study will contribute to expanding seismic tomography studies on volcanoes in Japan, specifically Hachijojima Island. Thus, it will gain insights into volcanic eruption mechanisms and implement effective volcanic disaster mitigation measures on this island.\u003c/p\u003e\n\u003cp\u003eHachijojima, located in the group of Izu Islands (Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e), is known for being one of the largest volcanic islands within the volcanic front of the Izu-Bonin-Mariana Arc that stretches over a distance of approximately 2800 km from Japan to Guam. The island is situated approximately 287 km south of Tokyo and covers an area of about 63 km\u003csup\u003e2\u003c/sup\u003e. Hachijojima region consists of two nearby islands, namely the main island of Hachijojima and smaller island known as Hachijokojima which is a part of the Hachijojima volcanic system. The main island is home to two volcanoes i.e., Nishiyama (854 m) and Higashiyama (701 m). These volcanoes are classified as stratovolcanoes built up mainly by their basaltic lavas and pyroclastic materials (Suga \u003cspan class=\"CitationRef\"\u003e1993\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e1994\u003c/span\u003e). The topographic evidence suggests that Higashiyama volcano formed prior to Nishiyama (Sugihara \u003cspan class=\"CitationRef\"\u003e1998\u003c/span\u003e). The Higashiyama volcano\u0026apos;s eruption began roughly 100,000 years ago and subsided around 4000 years ago. On the other hand, the Nishiyama volcano underwent volcanic activity approximately 10,000 years ago with the most recent eruption of this volcano likely occurred in 1605 (Ishizuka and Geshi \u003cspan class=\"CitationRef\"\u003e2018\u003c/span\u003e). Despite the fact that there have not been any magmatic eruptions since the last known eruption, the magmatic system is still regarded as being active.\u003c/p\u003e\n\u003cp\u003ePrevious studies have examined the underground magmatic plumbing system of Hachijojima, utilizing various approaches such as petrology and geochemistry. In a recent study by Oiwa et al. (\u003cspan class=\"CitationRef\"\u003e2023\u003c/span\u003e), it has been discovered that the magma plumbing system is comprised of two chambers: a mid-crustal magma chamber located at depths ranging from 9 to 12 km, and a deeper magma chamber which likely serves as the source of primitive magma. It is worth noting that earthquake swarms observed in 2002 were possibly triggered by injections of primitive magma into the mid-crustal magma chamber. In addition, Ishizuka et al. (\u003cspan class=\"CitationRef\"\u003e2008\u003c/span\u003e) have also conducted a comprehensive study on the magma plumbing system using various methods such as petrography, geochemistry, and marine geophysical surveys. The bathymetry map near Hachijojima revealed the presence of other submarine volcanic chains extending from Nishiyama, namely the Hachijo NW chain in the northwest and the NE edifices in the northeast slope of Nishiyama. The magma for the Hachijo NW chain is believed to have been transported over a significant distance in the middle to lower crust due to regional extensional stress conditions surrounding the arc volcano. On the other hand, it is likely that magma for the NE edifices was laterally transferred from the shallower magma chamber beneath the Nishiyama.\u003c/p\u003e\n\u003cp\u003eHagiwara and Watanabe (\u003cspan class=\"CitationRef\"\u003e2019\u003c/span\u003e) conducted seismic tomography utilizing seismic data from 2003 to 2018 obtained through the Izu Islands volcanic observation system. They identified earthquakes occurring within a middle-velocity region surrounding a high-velocity zone located at a depth of 10\u0026ndash;14 km. It was inferred that the high-velocity zone beneath Nishiyama volcano is associated with relatively cool body. They proposed that the low-velocity zones correspond to hot magmatic substances while the high-velocity zones are linked to solidified volcanic structures. Another recent study was conducted by Kanke et al. (\u003cspan class=\"CitationRef\"\u003e2021\u003c/span\u003e) to estimate the velocity image through travel-time tomography, despite a significantly limited number of seismic events available. The findings of this research revealed a three-dimensional velocity representation beneath Nishiyama.\u003c/p\u003e\n\u003cp\u003eSeismic observations were conducted on Hachijojima Island in 2019 and 2021 to collect earthquake data for this seismic tomography study (Kusumo et al. \u003cspan class=\"CitationRef\"\u003e2025\u003c/span\u003e). The seismic data collected was used in this study to analyze the subsurface structure beneath Nishiyama, which experienced recent seismic activity characterized by an earthquake swarm and ground deformation observed in 2002 (Kimata et al. \u003cspan class=\"CitationRef\"\u003e2004\u003c/span\u003e; Kumagai et al. \u003cspan class=\"CitationRef\"\u003e2003\u003c/span\u003e). The primary objective is to investigate the subsurface structure of Hachijojima Island, with a particular emphasis on Nishiyama, to enhance understanding of its magmatic system. By providing new insights into the island\u0026rsquo;s subsurface velocity structure, our study contributes to advancements in the field of seismic tomography and volcanic hazard assessment.\u003c/p\u003e"},{"header":"2. Data and Method","content":"\u003cdiv id=\"Sec2\" class=\"Section2\"\u003e\n \u003ch2\u003e2.1. Data\u003c/h2\u003e\n \u003cp\u003eTokyo Metropolitan University has conducted temporary seismic observation twice in 2019 and 2021 in collaboration with Nagoya University (Kusumo et al. \u003cspan class=\"CitationRef\"\u003e2025\u003c/span\u003e). The geological conditions of Hachijojima Island presented difficulties for conducting seismic observations because of persistent and intense winds and waves. Consequently, the acquired seismic data frequently exhibited strong noise, leading to challenges in precisely identifying and establishing the arrival times of P- and S-waves, especially for events with lower magnitudes. A total of 179 earthquake events were identified, consisting of 119 events occurred in 2019 and the remaining 60 in 2021, forming a dataset ready for use in this study. In order to locate the initial hypocenters, stations from neighboring islands were also used. Nevertheless, specifically for seismic tomography purpose in this study, we utilized only the data observed from 55 stations installed on Hachijojima Island, encompassing both temporary and permanent stations (Fig. 2).\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\n \u003ch2\u003e2.2. Method\u003c/h2\u003e\n \u003cp\u003eDouble difference (DD) tomography (Zhang and Thurber \u003cspan class=\"CitationRef\"\u003e2003\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e2006\u003c/span\u003e) was utilized in this study to obtain the three-dimensional subsurface velocity structure below Hachijojima Island. The absolute and differential arrival times are inverted simultaneously in this method. The DD tomography applies the concept of the double-difference method to precisely determine the hypocenter location from Waldhauser and Ellsworth (\u003cspan class=\"CitationRef\"\u003e2000\u003c/span\u003e). The DD method is based on the idea that if the separation distance between two earthquakes\u0026apos; hypocenters is relatively small compared to the event-station distance, their waveforms and ray paths will exhibit similarities. Thus, any difference in travel time observed at one station can be accurately attributed to the spatial separation between the events. This is acceptable because the discrepancies arise from similar origins, with only minor variations in regions where there are differences in ray paths at the source. The ray path and travel time were calculated using the pseudo-bending ray-tracing method (Um and Thurber \u003cspan class=\"CitationRef\"\u003e1987\u003c/span\u003e). This method is commonly used to estimate subsurface structures in volcanic regions with significant velocity perturbations. The inversion was conducted using the LSQR technique by (Paige and Saunders \u003cspan class=\"CitationRef\"\u003e1982\u003c/span\u003e).\u003c/p\u003e\n \u003cp\u003eThe study employed a two-step seismic tomography inversion process in order to construct a robust initial reference velocity model. The first step utilized a coarser and uniform grid size to generate a 3D velocity model, which was then utilized as the initial model for the second tomography inversion. This approach ensured that the initial velocity model was reasonably accurate. The first step employed a grid size of 2 km x 2 km x 4 km. The second step utilized a similar grid size, but with a finer resolution of 1 km x 1 km x 2 km around the shallow area of Nishiyama. The finer grid in the z-direction was applied to a depth of 4 km only. The specific details of the grid used in the second step and its differences from the first step can be found in Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e. Besides the grid size and initial velocity model, we also consider another factor i.e. damping. The tomography inversion stability depends on the damping value used. The stability of the inversion process in DD tomography is measured using Condition Number Damping (CND) values, which are determined by the ratio of the largest to smallest eigenvalue. Generally, the CND values between 40 to 80 indicate a stable inversion (Waldhauser and Ellsworth \u003cspan class=\"CitationRef\"\u003e2000\u003c/span\u003e). In our analysis, we created an L curve for each step that represents the trade-off between data variance and model variance while testing damping values ranging from 100 to 500. Figure \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e shows the L-curve, indicating an ideal damping value for the first step and second step. Ultimately, we applied damping value of 150 in the first step and 180 in the second step of tomography inversion.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\n \u003ch2\u003e2.3. Initial Velocity Model\u003c/h2\u003e\n \u003cp\u003eFor the first step tomography inversion, one-dimensional velocity analysis was conducted using the Velest program, in order to acquire an initial reference velocity model for three-dimensional local earthquake tomography. This program, developed by Kissling et al. (\u003cspan class=\"CitationRef\"\u003e1994\u003c/span\u003e), performs simultaneous inversion processes that yield minimum 1-D seismic velocities for P- and S-waves, along with hypocentral parameters. In this stage, we adopted a set of 1-D velocity of P-wave from the WIN System, which was used to locate the hypocenter previously, to establish the initial velocity model in Velest. This model was subject to random perturbations up to a depth of 40 km and then used as input for 1-D seismic velocity inversions. The resulting inverted models were evaluated based on their travel time residuals, with only those exhibiting small residuals being considered acceptable. From these accepted results, an average is computed to obtain the final velocity model Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e. This ultimate reference velocity model eventually serves as input for first step DD tomography. Furthermore, the 3D velocity model obtained from the first step was subsequently used as the initial reference velocity model for the second step of DD tomography. This two-step method was intended to yield a reasonably accurate velocity model for the objective area.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\n \u003ch2\u003e2.4. Resolution Test\u003c/h2\u003e\n \u003cp\u003eResolution test was conducted to evaluate the reliability of the 3-D seismic velocity model obtained from the inversion process. We carried out the Checkerboard Resolution Test (CRT) and Derivative Weight Sum (DWS) for this purpose. To initiate CRT, we generated a synthetic velocity model that incorporated a regular pattern of high and low velocity perturbations resembling a checkerboard pattern (L\u0026eacute;věque et al. \u003cspan class=\"CitationRef\"\u003e1993\u003c/span\u003e). In this study we used alternating positive and negative perturbations of \u0026plusmn;\u0026thinsp;10%. The initial velocity model for CRT was a 1-D velocity model that is similar to the initial velocity model used in the first step inversion of real data. Using this synthetic velocity model, along with the earthquake hypocenters and stations from the earthquake catalogue dataset of both 2019 and 2021, we generated synthetic wave travel time data. The event pair, a crucial step in the DD method, is then identified from the obtained synthetic data. The inversion was performed subsequently using the same procedure and parameters as used in the inversion of actual observation data. This resolution test aimed to evaluate spatially the region that can be well interpreted. A region that displays a pattern like the input velocity model indicates that it is well resolved and can be properly interpreted. Meanwhile, DWS is similar to the number of ray paths that pass through a particular grid node (Toomey \u0026amp; Foulger \u003cspan class=\"CitationRef\"\u003e1989\u003c/span\u003e). It calculates the density of rays around the grid node with a certain weight. Previous studies have indicated that the DWS values align closely with the diagonal elements of the resolution matrix and possible smearing as indicated by the spread function. A high DWS value represents a zone that has high resolution and has a low smearing effect (Hauksson \u0026amp; Shearer \u003cspan class=\"CitationRef\"\u003e2006\u003c/span\u003e; Rietbrock \u003cspan class=\"CitationRef\"\u003e2001\u003c/span\u003e). Figure \u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003e and Fig. \u003cspan class=\"InternalRef\"\u003e7\u003c/span\u003e illustrates the resolution test analysis of this study.\u003c/p\u003e\n\u003c/div\u003e"},{"header":"3. Result","content":"\u003cp\u003eBased on the results of the CRT, the inversion method effectively resolved the Nishiyama region\u0026rsquo;s subsurface structure down to a depth of approximately 8 km (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e6\u003c/span\u003e). At greater depths, improved resolution was observed primarily in the northwestern area of Hachijojima Island. This pattern aligns with the Derivative Weight Sum (DWS) results, where high DWS values (indicated in blue in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e7\u003c/span\u003e) are concentrated beneath Nishiyama in the shallow region, gradually shifting toward the northwestern part of the island at greater depths. This pattern is likely influenced by the concentration of earthquake activity near the northern edge of the rift margin (Kusumo et al. \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2025\u003c/span\u003e), which results in ray paths predominantly originating from the northwest. Areas with low DWS values (non-blue regions) are considered unresolved.\u003c/p\u003e \u003cp\u003eThe application of DD tomography produced two primary outcomes: a refined velocity structure image and improved earthquake event relocation. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e8\u003c/span\u003e, the relocated earthquake events from the 2019 and 2021 datasets indicate significant improvements in RMS residuals. The RMS residuals are more tightly clustered around zero after DD relocation, indicating improved consistency in the travel time residuals. Before DD relocation, 21,378 data points (77.89%) fall within the range of -0.25 to 0.25 seconds, while after DD relocation, 23,145 data points (84.33%) fall within the same range.\u003c/p\u003e \u003cp\u003eThe improvement of the DD tomography inversion during the second step, which employed a finer grid size in the objective area, is illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e9\u003c/span\u003e. The RMS error initially decreased slightly from iteration 1 to 3, followed by a substantial reduction after the 4th iteration. This convergence indicates improved model stability and accuracy, with a final RMS value of 0.1673 s at the last iteration.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eGiven the limited data availability in this study, we approach the interpretation of our results with caution. Nevertheless, the velocity structure obtained still provides meaningful insights. The interpolated velocity values were visualized as a three-dimensional velocity cube, enabling further interpretation. Horizontal sections of the P- and S-wave velocity structure, created at depths from 0 to 8 km with 2 km intervals (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e10\u003c/span\u003e), reveal a distinct low-velocity region extending from the southeastern part of Nishiyama to the central part of Hachijojima Island, primarily at the surface (0 km depth).\u003c/p\u003e \u003cp\u003eThis low-velocity region is further emphasized in Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e11\u003c/span\u003e along the A-A\u0026rsquo; slice. Additionally, a high-velocity anomaly was identified below Nishiyama at approximately 4 km depth, extending vertically from deeper regions (Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e12\u003c/span\u003e, slice B-B\u0026rsquo;). Despite the limitations in data quantity, these findings offer valuable insights into the subsurface magmatic processes beneath Hachijojima Island. The identified velocity anomalies provide crucial information for understanding the region\u0026rsquo;s magmatic system and may contribute to improved volcanic hazard assessment.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"4. Discussion","content":"\u003cp\u003eThe limited availability of recorded seismic data posed a challenge in this tomographic study of Hachijojima Island. This limitation is evident in the checkerboard resolution test (CRT) results, which indicate that the inversion can reliably resolve structures in the Nishiyama region only to a depth of approximately 8 km. However, the tomography successfully identifies deeper structures, extending down to 20 km in the northwestern region, where the majority of the earthquakes were observed. This result highlights the influence of earthquake distribution on resolution quality, with ray paths predominantly concentrated in this northwestern area.\u003c/p\u003e \u003cp\u003eNotably, the obtained velocity structure aligns with the geological and gravity cross-section analysis conducted by NEDO (1992), as illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e11\u003c/span\u003e. A distinct low-velocity zone near the surface between Nishiyama and Higashiyama (marked by the black dashed box) is interpreted as the top layer of the Neogene stratum, predominantly composed of volcanic sediment deposits. The interpreted faults identified in this study are consistent with those depicted in the geological cross-section. Additionally, the structure characterized by low P-wave velocity values in the northwestern portion of Nishiyama may indicate a fault or fractured zone.\u003c/p\u003e \u003cp\u003eOur findings also corroborate previous research conducted by Kanke et al. (\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2021\u003c/span\u003e), further supporting the robustness of the velocity model. A prominent high-velocity P-wave anomaly was identified beneath Nishiyama, extending vertically from deeper regions to approximately 4 km depth (Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e12\u003c/span\u003e). This feature is interpreted as a solidified magma pathway from past volcanic activity, likely representing the remnants of a preferential magma flow channel. The combination of high P-wave velocities, elevated Vp/Vs ratios, and reduced S-wave velocities in this region suggests the possible presence of fluid content (Koulakov et al. \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Koulakov and Shapiro \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2021\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eOur tomographic results at deeper regions (Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e13\u003c/span\u003e) provide significant insights into the underlying magma plumbing system beneath Hachijojima. These findings align well with previously proposed magma transport models by Ishizuka et al. (\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2008\u003c/span\u003e) and Oiwa et al. (\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). The tomographic model reveals a lateral magma transport pathway situated at a depth of approximately 10\u0026ndash;20 km in the northwestern region (zone I), where low P-wave velocity perturbations and elevated Vp/Vs ratios coincide with the location of deep earthquake activity recorded in 2002 and 2020, suggesting the presence of partial melt or high-temperature, volatile-rich magma. This observation supports Ishizuka et al. (\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2008\u003c/span\u003e), who inferred a lateral magma transport pathway at similar depths based on hypocenter distributions and petrological constraints. The alignment between these results indicates that basaltic magma from a deeper mantle source was likely transported laterally through this pathway.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIn addition, a low P-wave velocity perturbation coupled with high Vp/Vs ratios at depths of approximately 8\u0026ndash;12 km beneath Nishiyama (zone II) is interpreted as a mid-crustal magma reservoir. This feature is consistent with the concept of magma differentiation, where ascending magma undergoes compositional changes as it cools and crystallizes (Oiwa et al. \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). The presence of this magma chamber suggests that it may be intermittently supplied by deeper sources. The earthquake swarms observed in 2002 in this region may have been triggered by the injection of magma into this chamber, suggesting that this mid-crustal magma storage system remains active.\u003c/p\u003e \u003cp\u003eFurthermore, the anomalies observed at approximately 5 km depths beneath Nishiyama (zone III) are consistent with conceptual model of pre-eruption where the crystal fractionation and storage processes described by Oiwa et al. (\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). This anomaly is characterized by decreased P-wave velocity and increased Vp/Vs ratios, which may correspond to an H₂O-saturated magma accumulation zone, corroborating the interpretation presented by Oiwa et al. (\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). This feature is interpreted as a region where temporary magma stalling and plagioclase overgrowth occur prior to an eruption. The presence of a high Vp/Vs ratio at this depth (slice B-B\u0026rsquo;) reinforces this interpretation. This anomaly may represent a partially crystallized magma pathway from the past that still contains residual fluids, suggesting this zone is not completely solidified, meaning it could still play a role in magma migration or remobilization. These findings support the interpretation of a complex, multi-stage magma evolution process beneath Nishiyama, characterized by periodic injections of primitive basalt, magma differentiation, and pre-eruptive magma storage. The consistency between our seismic results and these conceptual models strengthens the interpretation of a dynamic magma plumbing system beneath Nishiyama, controlled by both deep lateral magma transport and crustal-level magma differentiation.\u003c/p\u003e"},{"header":"5. Conclusion","content":"\u003cp\u003eWe have successfully conducted three-dimensional seismic tomography to analyze the subsurface structures beneath Hachijojima Island, Japan. The 3-D velocity structure of seismic P-wave and S-wave was determined using DD tomography method. The seismic data was collected from two seismic observations in 2019 and 2021. Seismic tomography was performed using a combination of 46 temporary stations and 9 permanent stations. Despite the limited number of earthquake events due to the volcano\u0026rsquo;s inactivity, the dataset remains valuable for understanding the island\u0026rsquo;s subsurface characteristics.\u003c/p\u003e \u003cp\u003eResolution tests indicate that the Nishiyama area and its northwestern region exhibits satisfactory resolution up to a depth of 8 km and 20 km, respectively. The tomographic results identified a prominent low-velocity region in the shallow zone between Nishiyama and Higashiyama, interpreted as volcanic sediment deposits. In contrast, a distinct high-velocity anomaly was observed beneath Nishiyama, indicating solidified magma that likely represents a past volcanic conduit. The high P-wave velocities and Vp/Vs ratios further suggest the possible presence of fluids in this region.\u003c/p\u003e \u003cp\u003eAt greater depths, the northwestern region is characterized by low P-wave velocity perturbations and high Vp/Vs ratios, indicating the presence of a mid-crustal magma chamber at approximately 8\u0026ndash;12 km depth. This chamber appears to be connected to a lateral magmatic transport network extending to depths of ~\u0026thinsp;10\u0026ndash;20 km. Additionally, a shallow anomaly at ~\u0026thinsp;5 km depth beneath Nishiyama is interpreted as an H₂O-saturated magma accumulation zone, where magma stalling and plagioclase overgrowth may occur prior to an eruption. This feature may represent a partially crystallized magma pathway that retains residual fluids, suggesting its potential role in future magma migration or remobilization. These findings provide valuable insights into the magmatic processes beneath Hachijojima Island, improving our understanding of the island\u0026rsquo;s volcanic system\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e \u003cstrong\u003eEthics approval and consent to participate\u003c/strong\u003e \u003cp\u003eThe authors declare that this paper does not involve ethical issues.\u003c/p\u003e \u003c/p\u003e \u003cp\u003e \u003cstrong\u003eConsent to publish\u003c/strong\u003e \u003cp\u003eAll authors have read and agreed to the version of the manuscript.\u003c/p\u003e \u003c/p\u003e \u003cp\u003e \u003cstrong\u003eCompeting Interests\u003c/strong\u003e \u003cp\u003eThe authors declares that there is no conflict of interest.\u003c/p\u003e \u003c/p\u003e\u003ch2\u003eFunding\u003c/h2\u003e \u003cp\u003eThis research was supported by the Tokyo Metropolitan Government.\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eEach author has contributed to this paper. Conceptualization: A.W.K and Y.O.; Data Collection: H.A., T.W., and Y.O.; Data Analysis: A.W.K., H.A., and Y.O.; Interpretation of Results: A.W.K., H.A., and Y.O.; Writing \u0026ndash; original draft: A.W.K.; Writing \u0026ndash; review and editing: H.A., T.W., and Y.O.; Supervision: H.A. and Y.O..\u003c/p\u003e\u003ch2\u003eAcknowledgement\u003c/h2\u003e\u003cp\u003eWe express our sincere appreciation to the entities and individuals whose contributions were vital to the completion of this research. The Hachijo Branch Office of the Tokyo Metropolitan Government and the Hachijo Town provided crucial observational support, and we extend our gratitude to the private companies and individuals who facilitated our study. We also offer special appreciation to Dr. Tsutomu Ochiai, Assistant Professor at Kanagawa University, for his valuable insights and guidance. In addition, we acknowledge the invaluable data provided by The National Research Institute for Earth Science and Disaster Resilience, The Japan Meteorological Agency, and the Tokyo Metropolitan Government. We also thank the students of Tokyo Metropolitan University and Nagoya University for their cooperation during the observation process.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eAlanis, P. K., Miyamachi, H., Yakiwara, H., Goto, K., Kobayashi, R., Tameguri, T., \u0026amp; Iguchi, M. (2012). Seismic Velocity Structure of the Crust Beneath the Aira Caldera in Southern Kyushu by Tomography of Travel Times of Local Earthquake Data. \u003cem\u003eBulletin of the Volcanological Society of Japan\u003c/em\u003e, \u003cem\u003e57\u003c/em\u003e(4), 227\u0026ndash;234. https://doi.org/https://doi.org/10.18940/kazan.57.4_227\u003c/li\u003e\n\u003cli\u003eArdianto, A., Nugraha, A. D., Afif, H., Syahbana, D. K., Sahara, D. P., Zulfakriza, Z., Widiyantoro, S., Priyono, A., Rosalia, S., Saepuloh, A., Kasbani, K., Muttaqy, F., Rahsetyo, P. P., Priambodo, I. C., \u0026amp; Martanto, M. (2021). Imaging the Subsurface Structure of Mount Agung in Bali (Indonesia) Using Volcano-Tectonic (VT) Earthquake Tomography. \u003cem\u003eFrontiers in Earth Science\u003c/em\u003e, \u003cem\u003e9\u003c/em\u003e. https://doi.org/10.3389/feart.2021.619587\u003c/li\u003e\n\u003cli\u003eBushenkova, N., Koulakov, I., Bergal-Kuvikas, O., Shapiro, N., Gordeev, E. I., Chebrov, D. V., Abkadyrov, I., Jakovlev, A., Stupina, T., Novgorodova, A., Droznina, S., \u0026amp; Huang, H. H. (2023). 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(2006). Development and applications of double-difference seismic tomography. \u003cem\u003ePure and Applied Geophysics\u003c/em\u003e, \u003cem\u003e163\u003c/em\u003e(2\u0026ndash;3), 373\u0026ndash;403. https://doi.org/10.1007/s00024-005-0021-y\u003c/li\u003e\n\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":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"journal-of-seismology","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"jose","sideBox":"Learn more about [Journal of Seismology](http://link.springer.com/journal/10950)","snPcode":"10950","submissionUrl":"https://submission.nature.com/new-submission/10950/3","title":"Journal of Seismology","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Hachijojima, seismic tomography, double-difference, magmatic plumbing system, volcano seismology","lastPublishedDoi":"10.21203/rs.3.rs-6195617/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6195617/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eWe present a seismic tomography study of the subsurface structure beneath Hachijojima Island, one of the volcanic fronts in the Izu-Bonin Arc, Japan. Seismic observations were conducted over two 7-month periods in 2019 and 2021, utilizing 55 densely installed stations on the island. During these periods, a total of 179 local earthquakes were recorded \u0026mdash; 119 in 2019 and 60 in 2021 \u0026mdash; resulting in 4671 P-wave arrival times and 3927 S-wave arrival times. The 3-D tomography, derived using the double-difference technique, revealed a shallow low-velocity region between the island\u0026rsquo;s two main volcanoes, Nishiyama and Higashiyama, suggesting the presence of volcanic sediments near the surface. Additionally, a high-velocity anomaly was identified at a depth of 4\u0026ndash;5 km, extending vertically from deeper regions beneath Nishiyama. This feature is interpreted as a magma pathway from past volcanic activity, with high P-wave velocities and elevated Vp/Vs ratios indicating possible fluid presence. At greater depths, low P-wave velocity perturbations and elevated Vp/Vs ratios suggest a magmatic plumbing system comprising a mid-crustal magma chamber at approximately 8\u0026ndash;12 km depth and lateral magmatic pathways at 10\u0026ndash;20 km depth. Furthermore, a distinct zone characterized by reduced P-wave velocity and increased Vp/Vs is interpreted as a shallow magma chamber with H₂O-saturated magma accumulation. These findings provide valuable insights into the subsurface magmatic processes beneath Hachijojima Island, which are crucial for improving volcanic hazard assessment.\u003c/p\u003e","manuscriptTitle":"Seismic Tomography for Subsurface Structures Imaging beneath Hachijojima Volcanic Island, Izu-Bonin Arc, Japan","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-03-31 11:18:01","doi":"10.21203/rs.3.rs-6195617/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2025-05-21T15:36:36+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-05-20T13:36:37+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"306591062434437333587949528803604848547","date":"2025-05-12T03:13:13+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-04-22T15:07:50+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"319909080839360658113500073980893093686","date":"2025-04-02T02:16:49+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-03-20T14:26:53+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-03-13T18:00:34+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-03-13T17:57:31+00:00","index":"","fulltext":""},{"type":"submitted","content":"Journal of Seismology","date":"2025-03-10T12:34:04+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"journal-of-seismology","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"jose","sideBox":"Learn more about [Journal of Seismology](http://link.springer.com/journal/10950)","snPcode":"10950","submissionUrl":"https://submission.nature.com/new-submission/10950/3","title":"Journal of Seismology","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"8ab6a047-a93b-426e-ab70-fb706c6b5507","owner":[],"postedDate":"March 31st, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2025-07-07T16:17:35+00:00","versionOfRecord":{"articleIdentity":"rs-6195617","link":"https://doi.org/10.1007/s10950-025-10309-9","journal":{"identity":"journal-of-seismology","isVorOnly":false,"title":"Journal of Seismology"},"publishedOn":"2025-07-01 15:58:34","publishedOnDateReadable":"July 1st, 2025"},"versionCreatedAt":"2025-03-31 11:18:01","video":"","vorDoi":"10.1007/s10950-025-10309-9","vorDoiUrl":"https://doi.org/10.1007/s10950-025-10309-9","workflowStages":[]},"version":"v1","identity":"rs-6195617","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6195617","identity":"rs-6195617","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}