Shallow volcanic earthquakes in the Owakudani geothermal area, Hakone volcano, Japan

Shallow volcanic earthquakes in the Owakudani geothermal area, Hakone volcano, 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 Shallow volcanic earthquakes in the Owakudani geothermal area, Hakone volcano, Japan Ryo Kurihara, Yutaka Nagaoka, Ryou Honda, Kazuhiro Itadera This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7304798/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 29 Dec, 2025 Read the published version in Earth, Planets and Space → Version 1 posted 5 You are reading this latest preprint version Abstract Hakone volcano, located in central Japan, produced a small phreatic eruption in June 2015. Although seismic activity in the Hakone region is generally low, the area experiences episodic earthquake swarms approximately once every few years. In the volcano, many small earthquakes were observed at very shallow depths near Owakudani geothermal area from May to July 2022. In contrast to regular volcano-tectonic earthquakes, these events had no clear P- and S-wave arrivals. In this study, we sought to identify these earthquakes in continuous data and locate their hypocenters. We identified 11,016 earthquakes with similar waveforms between 2014 and 2023 using the matched-filter technique. Many earthquakes occurred in 2015, when the phreatic eruption occurred; however, the shallow seismicity was also active in 2020–2022 at a time when no other volcanic activity including volcano-tectonic earthquakes and crustal deformation occurred. The earthquakes were sometimes triggered by volcanic activity and sometimes occurred ambiently. The hypocenters of the earthquakes were located based on amplitude source location method around the Owakudani geothermal area at depths of − 1 to 0 km below sea level, close to the surface. The hypocenters are located close to the crack that opened around the time of the 2015 phreatic eruption and close to fissure consisting of older craters. Given the waveforms, locations, and timing of the earthquakes, we infer that they were caused by the movement of fluid and volcanic gas near the surface. Volcanic earthquake Hakone volcano Matched-filter technique Phreatic eruption Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 1. Introduction Many types of volcanic earthquakes associated with the movement of magma occur in volcanic regions, including volcano-tectonic and low-frequency earthquakes and volcanic tremor (cf. Chouet and Matoza 2013 ). Hakone volcano is located approximately 100 km west of Tokyo, the capital of Japan, and attracts many tourists due to its hot springs and scenic landscapes. Therefore, it is important to understand volcanic activity during both periods of volcanic unrest and quiescence, because even a small eruption would affect the residents and economy of the Hakone area. There has not been an eruption producing tephra at Hakone in the last 800 years (cf. Kobayashi et al. 2006 ), however, volcanic unrest, including earthquake swarms, crustal deformation, and hydrothermal activity, occurs every few years since 2001. In 2015, a volcanic unrest occurred, with a small phreatic eruption (cf. Mannen et al. 2018 ). During the eruption, blowouts of steam wells and ash emissions were observed around the Owakudani geothermal area, and the most prominent activities were VT earthquakes occurring at depths of 0–5km. Although no significant VT earthquake activity or crustal deformation was observed between May and July 2022, numerous microearthquakes with unclear P- and S-wave onsets—distinct from typical VT earthquakes—were detected around the Owakudani geothermal area. Given that the hypocenters of the earthquakes appear to be located beneath the fumarolic area and do not correlate with the overall volcanic activity of Hakone Volcano, it is inferred that they are associated with subsurface fluids or the structural characteristics of the fumarolic zone. Most of these earthquakes were not identified using conventional methods. These earthquakes may be keys that reveal volcanic activity during periods of volcanic unrest and quiescence periods as well as the structure of the geothermal area. We investigated the seismicity since 2014 and determined the hypocenters of the earthquakes to better understand the volcanic activity and shallow structure of the geothermal area. 2. Waveform characteristics of the target earthquakes The Hot Spring Research Institute of Kanagawa Prefecture has deployed seismic stations around Hakone volcano to monitor volcanic activity. In addition, the Japan Meteorological Agency (JMA) and National Research Institute for Earth Science and Disaster Resilience (NIED) have deployed observation stations (Fig. 1 ). These stations define a dense observation network for volcanic earthquakes at Hakone, especially around the Owakudani geothermal area. Figure 2 shows an example of the waveforms of a target earthquake at Owakudani. The P- and S-wave arrivals of the earthquake are less clear than those of regular volcano-tectonic earthquakes (Fig. 3 ). The target earthquakes are characterized by (1) no clear P-wave arrivals; (2) larger amplitudes at OWD than those at the other stations, which suggests a shallow hypocenter; (3) flat peaks in amplitude (i.e., a long duration for the peak amplitude); (4) coda amplitudes that decreases quickly; and (5) sometimes multiple peaks that suggest repeated earthquakes. The waveforms in Fig. 2 have three peaks, which suggests that three earthquakes occurred over an interval of a few seconds. The dominant frequencies of these earthquakes are 10–30 Hz (Fig. 4 ), which is similar to that of volcano-tectonic earthquakes at Hakone, suggesting that they are not low-frequency earthquakes. The amplitudes of this earthquake recorded by stations around the Owakudani geothermal area (e.g., OWD and OWJ) are large; however, we cannot find a signal at the KIN station (Fig. 2 ), 4 km from the Owakudani geothermal area, which suggests that this earthquake occurred at a very shallow depth near Owakudani. Such earthquakes are rarely listed in earthquake catalogs, as they are only recorded by a few stations near Owakudani; however, we identified many earthquakes with similar waveforms in the continuous data from May to July 2022, when we first identified these earthquakes. We describe how we identify these earthquakes in section 3 , and how we locate their hypocenters in section 4 . 3 Detection method and results We used the matched-filter technique (Gibbons and Ringdal 2006 ) to identify the target earthquakes. 122 template earthquakes were selected by visual check and detected large amplitude earthquakes detected simplified matched filter technique. We used four stations (OWD, KOM, KZY, and KZR; Fig. 1 ) to identify the target earthquakes. We applied a 4–16 Hz band pass filter to waveforms. The length of the template was 4 s, and the detection period was 10 y, from 2014 to 2023. The threshold for detection was defined as when the sum of the correlation coefficients of the three components of the four stations was > 4.0. We identified 11,016 earthquakes that matched the template earthquakes. Although the identified earthquakes include a few VT earthquakes with clear P-wave arrivals, noise was rarely misidentified as an earthquake. Even if some events with low signal-to-noise ratios were detected, weak signals from these events were recorded at some stations with high signal-to-noise ratios (examples of these waveforms are shown in Figures. S1–S5). Swarms of the shallow earthquakes detected around the time of the 2015 phreatic eruption, during the 2019 unrest, and during 2020–2022 (Fig. 5 a), while swarms of VT earthquakes were observed in only 2015 and 2019. Looking closer at the activity in 2022, some episodic activity can be observed in the swarm, with intervals of one week to one month (Fig. 6 ). 4 Method and results of location determination We cannot determine the hypocenters of the very shallow earthquakes by picking arrival phases, given the lack of clear P- and S-wave arrivals; consequently, we employed the amplitude source location (ASL) method (e.g., Battaglia and Aki 2003 ) using the nine seismic stations indicated by blue and white triangles in Fig. 1 . We applied a 4–16 Hz bandpass filter to all seismic waveform data. The 122 earthquakes with high signal-to-noise ratios, which were used as template earthquakes, were relocated. At station i , the amplitude of an earthquake can be express as $$\:A\left(i\right)={A}_{\text{s}\text{o}\text{u}\text{r}\text{c}\text{e}}\text{*}\frac{\text{e}\text{x}\text{p}(-\frac{\pi\:f}{Q{V}_{\text{s}}}r)}{r}\text{*}S\left(i\right)$$ 1 , where A source is the amplitude at the source, Q is the attenuation parameter, r is the distance from the hypocenter, f is the frequency of the seismic wave, and V s is the S-wave velocity. S (i) is the site amplification factor, which must be estimated at each station to apply the ASL method. We calculated the site amplification factors for all stations using the coda-normalization method (Sato and Fehler 1998 ) and the coda wave of 143 earthquakes with magnitudes of 3.0–6.0 in the Japan Meteorological Agency (JMA) catalog that occurred 20–100 km from the Owakudani geothermal area (Figure S6). The TNM station, which is located on the outer rim of Hakone Caldera and has low noise levels, was selected as the reference point. A 5 s window starting at the twice of the S-wave travel times from origin time at the TNM station was selected for the time of coda waves at all stations, and the coda amplitude was defined as the root mean square (RMS) amplitude during this window. The site amplification factor at each station was calculated as the mean ratio of the coda amplitude of each target earthquake to that at the TNM station. 17 earthquakes with signal-to-noise ratios of < 2 at the TNM station were removed from target earthquakes, where the noise amplitude was defined as the RMS amplitude of a 1 min window before the P-wave onset of the target earthquake. Then the number of earthquakes used in the analysis was 126 (Figure S6). While the earthquakes with a signal-to-noise ratio at each station except TNM of < 2 were removed from the mean for each station. We use a Q value of 100 in Eq. ( 1 ) based on the study of attenuation tomography (Kashiwagi et al. 2020 ). Since we use a 4–16 Hz bandpass filter, we use a value of 10 Hz for f . We use a value of 3.0 km/s for V s , based on a velocity model estimated using seismic tomography (Yukutake et al. 2015 ). We then estimated the locations of the hypocenters of the shallow earthquakes using the ASL method and the site amplification factor for each station. The hypocenter locations can be estimated by minimizing the error between the calculated ( A cal ) and observed ( A obs ) amplitudes. We defined the error function as $$\:\:\text{E}\text{r}\text{r}\:=\:\sum\:_{i=1}^{n}{\left({A}_{cal}(x,y,z,i)-{A}_{obs}\left(i\right)\right)}^{2}$$ 2 , where i is the station and n is the total number of stations (i.e., 9). A cal is calculated from the Eq. ( 1 ). A source in the Eq. ( 1 ) is estimated based on the equation \(\:{A}_{\text{s}\text{o}\text{u}\text{r}\text{c}\text{e}}=\frac{1}{n}\sum\:_{i=1}^{n}\frac{{A}_{obs}\left(i\right)}{\frac{\text{e}\text{x}\text{p}(-\frac{\pi\:f}{Q{V}_{\text{s}}}r)}{r}\text{*}S\left(i\right)}\) (3), i.e., \(\:{A}_{\text{s}\text{o}\text{u}\text{r}\text{c}\text{e}}\) is estimated by average value of each station for each assumed hypocenter. We minimized the error function (2) using a grid search over longitudes of 138.920° E–139.120° E and latitudes of 35.144° N–35.344° N with an interval of 0.001°, and over depths of − 1 to 9 km below sea level with an interval of 0.1 km. The hypocenters showed a shallow and localized distribution beneath the Owakudani geothermal area (Fig. 7 ), and the 2015 events occurred to the northeast of those recorded after 2016. Although we cannot use the OWJ, SMYB, and SOZ stations to locate the 2015 events, as they were installed after 2015, excluding these stations in the hypocenter determination for earthquakes after 2016 does not significantly affect the results (Figure S7). The hypocenters after 2016 are aligned almost parallel to the crack that opened during the 2015 phreatic eruption, although the precision of ASL methods is low and the apparent alignment of hypocenters may be affected by the distribution of stations. 5 Discussion The waveforms of the shallow earthquakes differ from those of the regular volcano-tectonic earthquakes, as they do not show clear P- and S-wave arrivals, have relatively long durations, and have multiple peaks in a single sequence, which suggests that they occur repeatedly over short intervals and have a different mechanism to the regular volcano-tectonic earthquakes. Shallow volcanic earthquakes except volcano-tectonic earthquakes have been observed at many volcanoes. The characteristics of these waveforms and hypocenter distributions closely resemble those of the so-called “B-type earthquakes” described in earlier studies (Minakami 1960 ). BH-type earthquakes at Sakurajima (Iguchi 1993 ), Asama (Maeda et al. 2019 ), and Tarumae (Aoyama et al. 2004 ) volcanoes, and non-double couple (NDC) earthquakes at Unzen volcano (Hashimoto et al. 2020 ) were the typical similar earthquakes. BH-type earthquakes at Sakurajima and Asama have dominant frequencies of 4–7 Hz, lower than those of the shallow earthquakes at Hakone. The BH-type earthquakes are thought to be triggered directly by magmatic activity (e.g., magmatic intrusion); however, no ascending magma has been observed near the surface at Hakone, and the shallow earthquakes at Owakudani are unlikely to be BH-type earthquakes. The BH earthquakes at Tarumae have a dominant frequency of 10–17 Hz (Aoyama et al. 2004 ), and their waveforms are similar to those of the shallow earthquakes at Hakone. The BH earthquakes at Tarumae are very shallow and occur near the lava dome at the summit. The NDC earthquakes at Unzen (Hashimoto et al. 2020 ) also have similar source locations and characteristics to the shallow earthquakes at Hakone. The Unzen NDC earthquakes occur at depths corresponding to the pre-eruption ground surface level prior to the 1990–1995 eruptions. This suggests that these earthquakes are caused by collapses of small vapor-filled voids within the lava dome. However, the initial P-wave motion of the Hakone earthquakes is less clear that that of the Unzen earthquakes, and it is unlikely that they have the same source mechanism. The mechanism responsible for the shallow Hakone earthquakes may be different from that of similar earthquakes at other volcanoes. We interpret the mechanisms responsible for the earthquakes at Hakone based on their spatial and temporal distribution. These earthquakes occur in the very shallow part of the geothermal area at Hakone. In Owakudani, there are steam wells that produced hot springs, which reach a maximum depth of ~ 500 m. In the depths, the hypocenters of the shallow earthquakes were estimated. Then, this indicates that the earthquakes occur around the geothermal area, suggesting that they are associated with hydrothermal fluid or steam in shallow underground. Most of these earthquakes occurred during 2015 and 2019–2022. A phreatic eruption occurred in 2015 along with other volcanic activity, including crustal deformation, an increase in other types of volcanic earthquakes, and steam well blowouts. However, when these earthquakes occurred in 2022, no other volcanic activity was observed; therefore, the occurrence of these earthquakes is not necessarily associated with large-scale volcanic activity, but reflects local conditions. Although the hypocenter locations are not particularly precise, they are located near the Owakudani geothermal area, close to the crack that appeared at the 2015 phreatic eruption (Honda et al. 2018 ); therefore, it is possible that the shallow earthquakes were triggered by the injection of groundwater and volcanic gas into the existing crack (Fig. 8 ). In this area, fissure including a chain of craters were observed by air survey and high-resistivity structure were observed by controlled-source audio-frequency magneto-telluric survey (Mannen et al. 2025 ). In addition, change of volcanic gas composition were observed in Owakudani and Kamiyu geothermal area, which locate 500 meters north of Owakudani in 2023 (Toyama et al. 2024 ). The shallow earthquakes may correspond the underground structures. Although the number of shallow earthquakes has been low since 2023, these earthquakes could potentially be associated with more-dangerous activity (e.g., the explosive release of volcanic gas); therefore, it is important that a denser observation network is installed near the geothermal area to monitor the seismicity and to better understand the underlying physical mechanisms. 6 Conclusions We investigated the occurrence and distribution of shallow earthquakes with no clear P- and S-wave arrivals near the Owakudani geothermal area at Hakone volcano. The number of shallow earthquakes increased during the period of volcanic unrest in 2015, and also between 2020 and 2022, when volcanic unrest was not observed. The hypocenters of the earthquakes are concentrated near the Owakudani geothermal area, with likely depths of ≤ 1 km. The earthquakes occurred near the crack that opened at the time of the 2015 phreatic eruption. The earthquakes may have been triggered by the injection of groundwater and volcanic gas into the crack. Abbreviations MFT: matched filter technique JMA: Japan Meteorological Agency NIED: National institute of earth science and disaster resilience Hi-net: High Sensitivity Seismograph Network of Japan GEONET: GNSS Earth Observation Network System Declarations Ethics approval and consent to participate Not applicable Consent for publication Not applicable Availability of data and materials Hypocenter locations of the template events and catalog based on the matched-filter technique are provided in the supplementary material. We used seismic data from the National Research Institute for Earth Science and Disaster Resilience (2019), the Hot Springs Research Institute of Kanagawa Prefecture, and the Japan Meteorological Agency. All seismic data used in this study are available from the NIED Hi-net website (http://www.hinet.bosai.go.jp). Competing interests The authors declare that they have no competing interests. Funding This work was supported by a Japan Society for the Promotion of Science KAKENHI grant (22K14113). Authors’ contributions RK designed the study, analyzed the data, and wrote the paper. The other authors provided advice and discussion. Acknowledgments We used Generic Mapping Tools to produce the figures (Wessel and Smith 1998) and used Hi-net seismic data (http://www.hinet.bosai.go.jp) from (National Research Institute for Earth Science and Disaster Resilience 2019) and JMA’s unified earthquake catalog (http://www.jma.go.jp). The catalog of the shallow earthquakes constructed by this study are available in supporting material. References Aoyama H, Ohshima H, Suzuki A, Maekawa T (2004) Recent seismic activities at active volcanoes in Hokkaido -Tarumaesan-. 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(Part 1). Bulletin of the earthquake research institute 38:497–544 National Research Institute for Earth Science and Disaster Resilience (2019) NIED Hi-net. National Research Institute for Earth Science and Disaster Resilience. https://doi.org/10.17598/nied.0003 Sagiya T (2014) A decade of GEONET: 1994–2003 —The continuous GPS observation in Japan and its impact on earthquake studies—. Earth Planet Sp 56:xxix–xli. https://doi.org/10.1186/BF03353077 Sato H, Fehler M (1998) Seismic wave propagation and scattering in the heterogeneous earth. Springer Verlag and AIP press, New York 1–308 Toyama K, Mannen K, Ninomiya R, Miyashita Y (2024) Results of Periodic Survey of Volcanic Gases and Hot Springs around Owakudani, Hakone Volcano, in 2023. “Kansoku-dayori” of the Hot Springs Research Institute of Kanagawa prefecture 89-92 (in Japanese) Wessel P, Smith WHF (1998) New, improved version of generic mapping tools released. Eos, Transactions American Geophysical Union 79:579–579. https://doi.org/10.1029/98EO00426 Yukutake Y, Honda R, Harada M, et al (2015) A magma-hydrothermal system beneath Hakone volcano, central Japan, revealed by highly resolved velocity structures. J Geophys Res Solid Earth 120:3293–3308. https://doi.org/10.1002/2014JB011856 Supplementary Files additionalfigures.docx catalogshallowearthquake.dat graphicalabst.jpg Cite Share Download PDF Status: Published Journal Publication published 29 Dec, 2025 Read the published version in Earth, Planets and Space → Version 1 posted Editorial decision: Major Revision 21 Sep, 2025 Reviewers agreed at journal 22 Aug, 2025 Reviewers invited by journal 20 Aug, 2025 Editor assigned by journal 07 Aug, 2025 First submitted to journal 05 Aug, 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-7304798","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":502868634,"identity":"24a934bd-022a-40df-b77c-19b4a18e981f","order_by":0,"name":"Ryo Kurihara","email":"data:image/png;base64,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","orcid":"https://orcid.org/0000-0003-2954-8486","institution":"Kanagawa-ken","correspondingAuthor":true,"prefix":"","firstName":"Ryo","middleName":"","lastName":"Kurihara","suffix":""},{"id":502868635,"identity":"0b75d36e-04ac-4359-a96c-b2834fea0453","order_by":1,"name":"Yutaka Nagaoka","email":"","orcid":"","institution":"Kanagawa-ken","correspondingAuthor":false,"prefix":"","firstName":"Yutaka","middleName":"","lastName":"Nagaoka","suffix":""},{"id":502868636,"identity":"9330c83f-6952-4b9d-93e0-d8844cd220fe","order_by":2,"name":"Ryou Honda","email":"","orcid":"","institution":"Kanagawa-ken","correspondingAuthor":false,"prefix":"","firstName":"Ryou","middleName":"","lastName":"Honda","suffix":""},{"id":502868637,"identity":"8432d4cb-ec85-4be5-817c-0ae596e1f128","order_by":3,"name":"Kazuhiro Itadera","email":"","orcid":"","institution":"Kanagawa-ken","correspondingAuthor":false,"prefix":"","firstName":"Kazuhiro","middleName":"","lastName":"Itadera","suffix":""}],"badges":[],"createdAt":"2025-08-06 02:15:33","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-7304798/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-7304798/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1186/s40623-025-02341-3","type":"published","date":"2025-12-29T15:58:30+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":90078715,"identity":"e6310e9a-3239-4856-bff4-87843eaa0911","added_by":"auto","created_at":"2025-08-28 08:30:12","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":698185,"visible":true,"origin":"","legend":"\u003cp\u003eDistribution of observation stations used in this study. White inverted triangles indicate stations used solely for determining hypocenters, blue inverted triangles indicate stations used for both hypocenter determination and earthquake detection, and black inverted triangles indicate stations that are not used in our analyses, but that are used to check the waveforms visually. Red diamonds indicate the Susono2 and Odawara GNSS stations of the GNSS Earth Observation Network System (GEONET; Sagiya 2014),operated by the Geospatial Information Authority of Japan.\u003c/p\u003e","description":"","filename":"Figure1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7304798/v1/bfc377d211cc454c06a69736.jpg"},{"id":90078720,"identity":"7bd05bb6-70e1-4b78-b747-79f079c0c5bd","added_by":"auto","created_at":"2025-08-28 08:30:12","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":524159,"visible":true,"origin":"","legend":"\u003cp\u003eWaveforms of a target earthquake from Owakudani, which occurred at 11:50:00 (Japan Standard Time) on 24 May 2022. No bandpass filter has been applied to the waveforms. The waveforms are normalized by the maximum velocity written in the right of the waveforms.\u003c/p\u003e","description":"","filename":"Figure2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7304798/v1/61dcfc0ba0b41acdc7a92b87.jpg"},{"id":90078714,"identity":"e5632fad-a92f-487c-ac1a-9c9da1da6d82","added_by":"auto","created_at":"2025-08-28 08:30:12","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":376896,"visible":true,"origin":"","legend":"\u003cp\u003eWaveforms of a volcano-tectonic earthquake that occurred at 22:21:23 on 22 July 2019, with a magnitude of 0.7. The hypocenter was located 1 km southeast of Owakudani at a depth of 4.4 km.\u003c/p\u003e","description":"","filename":"Figure3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7304798/v1/0b4a00bb2f1a5565c6f36547.jpg"},{"id":90078865,"identity":"c5b65aca-f38e-44d6-adf6-5c4a5863c60f","added_by":"auto","created_at":"2025-08-28 08:38:12","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":692195,"visible":true,"origin":"","legend":"\u003cp\u003e(a–c) Spectrum of an earthquake that occurred at 11:50 on 24 May 2022, (Japan Standard Time) recorded at the OWD station, and (d) the raw waveform. (a) East–west, (b) north–south, and (c) vertical components.\u003c/p\u003e","description":"","filename":"Figure4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7304798/v1/6f61cf582d7d5b0ee88c79fe.jpg"},{"id":90078718,"identity":"1ab9153c-52fe-4a56-ae19-1a3560c07311","added_by":"auto","created_at":"2025-08-28 08:30:12","extension":"jpg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":955140,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Cumulative number of very shallow earthquakes after 2014 (red line) and maximum amplitude at the OWD station (green dots). (b) Cumulative number of volcanic earthquakes (red line) and their magnitudes based on the earthquake catalog of Hot Springs Research Institute of Kanagawa Prefecture (blue dots). (c) Change in the baseline between the Susono2 and Odawara GNSS stations (Figure 3).\u003c/p\u003e","description":"","filename":"Figure5.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7304798/v1/dcee4cfe8553605dbe30a5f8.jpg"},{"id":90078863,"identity":"13f5b696-375e-47e1-8bef-0311d70cc5de","added_by":"auto","created_at":"2025-08-28 08:38:12","extension":"jpg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":577920,"visible":true,"origin":"","legend":"\u003cp\u003eEnlarged view of Figure 5, focusing on the year 2022.\u003c/p\u003e","description":"","filename":"Figure6.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7304798/v1/70a74b8820f25143a3642740.jpg"},{"id":90078723,"identity":"f6f791a1-d289-4e58-a2c0-34cd73238276","added_by":"auto","created_at":"2025-08-28 08:30:12","extension":"jpg","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":305389,"visible":true,"origin":"","legend":"\u003cp\u003eDistribution of hypocenters of shallow earthquakes estimated using the ASL method. Red, yellow, green, and blue stars are earthquakes that occurred during 2015, 2016–2021, 2022, and 2023, respectively. The dashed line indicates the crack that opened during the 2015 eruption (Honda et al. 2018).\u003c/p\u003e","description":"","filename":"Figure7.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7304798/v1/78d72cdb73d150dd63a78b11.jpg"},{"id":90078864,"identity":"e1a1dc5f-271b-42e6-854f-781abb8e53ba","added_by":"auto","created_at":"2025-08-28 08:38:12","extension":"jpg","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":182128,"visible":true,"origin":"","legend":"\u003cp\u003eConceptual model of the shallow earthquakes. Yellow and red stars indicate the hypocenters of the shallow earthquakes.\u003c/p\u003e","description":"","filename":"Figure8.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7304798/v1/4c8baebba7a27cc13b7d8a5b.jpg"},{"id":99545380,"identity":"47402776-56c6-4f40-ae87-b1231127fcbe","added_by":"auto","created_at":"2026-01-05 16:06:45","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":4755164,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7304798/v1/823f5312-13b2-4d0e-b814-26b50b723846.pdf"},{"id":90078728,"identity":"2c57c976-69ff-4d84-8f6a-0f0d5d0e2f35","added_by":"auto","created_at":"2025-08-28 08:30:12","extension":"docx","order_by":12,"title":"","display":"","copyAsset":false,"role":"supplement","size":3584004,"visible":true,"origin":"","legend":"","description":"","filename":"additionalfigures.docx","url":"https://assets-eu.researchsquare.com/files/rs-7304798/v1/831516dd6b942656e05a1928.docx"},{"id":90079747,"identity":"d1810f64-ba13-458d-979e-2f99661dd75e","added_by":"auto","created_at":"2025-08-28 08:46:12","extension":"dat","order_by":13,"title":"","display":"","copyAsset":false,"role":"supplement","size":803672,"visible":true,"origin":"","legend":"","description":"","filename":"catalogshallowearthquake.dat","url":"https://assets-eu.researchsquare.com/files/rs-7304798/v1/99306ca39ac29448bfb85e49.dat"},{"id":90078730,"identity":"e28fbb72-1ba9-4a94-8136-04397345bebe","added_by":"auto","created_at":"2025-08-28 08:30:12","extension":"jpg","order_by":14,"title":"","display":"","copyAsset":false,"role":"supplement","size":1569284,"visible":true,"origin":"","legend":"","description":"","filename":"graphicalabst.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7304798/v1/844e40a3c32d54f9789559a4.jpg"}],"financialInterests":"","formattedTitle":"Shallow volcanic earthquakes in the Owakudani geothermal area, Hakone volcano, Japan","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eMany types of volcanic earthquakes associated with the movement of magma occur in volcanic regions, including volcano-tectonic and low-frequency earthquakes and volcanic tremor (cf. Chouet and Matoza \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2013\u003c/span\u003e). Hakone volcano is located approximately 100 km west of Tokyo, the capital of Japan, and attracts many tourists due to its hot springs and scenic landscapes. Therefore, it is important to understand volcanic activity during both periods of volcanic unrest and quiescence, because even a small eruption would affect the residents and economy of the Hakone area.\u003c/p\u003e\u003cp\u003eThere has not been an eruption producing tephra at Hakone in the last 800 years (cf. Kobayashi et al. \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2006\u003c/span\u003e), however, volcanic unrest, including earthquake swarms, crustal deformation, and hydrothermal activity, occurs every few years since 2001. In 2015, a volcanic unrest occurred, with a small phreatic eruption (cf. Mannen et al. \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). During the eruption, blowouts of steam wells and ash emissions were observed around the Owakudani geothermal area, and the most prominent activities were VT earthquakes occurring at depths of 0\u0026ndash;5km.\u003c/p\u003e\u003cp\u003eAlthough no significant VT earthquake activity or crustal deformation was observed between May and July 2022, numerous microearthquakes with unclear P- and S-wave onsets\u0026mdash;distinct from typical VT earthquakes\u0026mdash;were detected around the Owakudani geothermal area. Given that the hypocenters of the earthquakes appear to be located beneath the fumarolic area and do not correlate with the overall volcanic activity of Hakone Volcano, it is inferred that they are associated with subsurface fluids or the structural characteristics of the fumarolic zone. Most of these earthquakes were not identified using conventional methods. These earthquakes may be keys that reveal volcanic activity during periods of volcanic unrest and quiescence periods as well as the structure of the geothermal area. We investigated the seismicity since 2014 and determined the hypocenters of the earthquakes to better understand the volcanic activity and shallow structure of the geothermal area.\u003c/p\u003e"},{"header":"2. Waveform characteristics of the target earthquakes","content":"\u003cp\u003eThe Hot Spring Research Institute of Kanagawa Prefecture has deployed seismic stations around Hakone volcano to monitor volcanic activity. In addition, the Japan Meteorological Agency (JMA) and National Research Institute for Earth Science and Disaster Resilience (NIED) have deployed observation stations (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). These stations define a dense observation network for volcanic earthquakes at Hakone, especially around the Owakudani geothermal area.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eFigure \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e shows an example of the waveforms of a target earthquake at Owakudani. The P- and S-wave arrivals of the earthquake are less clear than those of regular volcano-tectonic earthquakes (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). The target earthquakes are characterized by (1) no clear P-wave arrivals; (2) larger amplitudes at OWD than those at the other stations, which suggests a shallow hypocenter; (3) flat peaks in amplitude (i.e., a long duration for the peak amplitude); (4) coda amplitudes that decreases quickly; and (5) sometimes multiple peaks that suggest repeated earthquakes. The waveforms in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e have three peaks, which suggests that three earthquakes occurred over an interval of a few seconds. The dominant frequencies of these earthquakes are 10\u0026ndash;30 Hz (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e), which is similar to that of volcano-tectonic earthquakes at Hakone, suggesting that they are not low-frequency earthquakes.\u003c/p\u003e\u003cp\u003eThe amplitudes of this earthquake recorded by stations around the Owakudani geothermal area (e.g., OWD and OWJ) are large; however, we cannot find a signal at the KIN station (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e), 4 km from the Owakudani geothermal area, which suggests that this earthquake occurred at a very shallow depth near Owakudani. Such earthquakes are rarely listed in earthquake catalogs, as they are only recorded by a few stations near Owakudani; however, we identified many earthquakes with similar waveforms in the continuous data from May to July 2022, when we first identified these earthquakes. We describe how we identify these earthquakes in section \u003cspan refid=\"Sec3\" class=\"InternalRef\"\u003e3\u003c/span\u003e, and how we locate their hypocenters in section \u003cspan refid=\"Sec4\" class=\"InternalRef\"\u003e4\u003c/span\u003e.\u003c/p\u003e"},{"header":"3 Detection method and results","content":"\u003cp\u003eWe used the matched-filter technique (Gibbons and Ringdal \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2006\u003c/span\u003e) to identify the target earthquakes. 122 template earthquakes were selected by visual check and detected large amplitude earthquakes detected simplified matched filter technique. We used four stations (OWD, KOM, KZY, and KZR; Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) to identify the target earthquakes. We applied a 4\u0026ndash;16 Hz band pass filter to waveforms. The length of the template was 4 s, and the detection period was 10 y, from 2014 to 2023. The threshold for detection was defined as when the sum of the correlation coefficients of the three components of the four stations was \u0026gt;\u0026thinsp;4.0.\u003c/p\u003e\u003cp\u003eWe identified 11,016 earthquakes that matched the template earthquakes. Although the identified earthquakes include a few VT earthquakes with clear P-wave arrivals, noise was rarely misidentified as an earthquake. Even if some events with low signal-to-noise ratios were detected, weak signals from these events were recorded at some stations with high signal-to-noise ratios (examples of these waveforms are shown in Figures. S1\u0026ndash;S5).\u003c/p\u003e\u003cp\u003eSwarms of the shallow earthquakes detected around the time of the 2015 phreatic eruption, during the 2019 unrest, and during 2020\u0026ndash;2022 (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea), while swarms of VT earthquakes were observed in only 2015 and 2019. Looking closer at the activity in 2022, some episodic activity can be observed in the swarm, with intervals of one week to one month (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e"},{"header":"4 Method and results of location determination","content":"\u003cp\u003eWe cannot determine the hypocenters of the very shallow earthquakes by picking arrival phases, given the lack of clear P- and S-wave arrivals; consequently, we employed the amplitude source location (ASL) method (e.g., Battaglia and Aki \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2003\u003c/span\u003e) using the nine seismic stations indicated by blue and white triangles in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. We applied a 4\u0026ndash;16 Hz bandpass filter to all seismic waveform data. The 122 earthquakes with high signal-to-noise ratios, which were used as template earthquakes, were relocated.\u003c/p\u003e\u003cp\u003eAt station \u003cem\u003ei\u003c/em\u003e, the amplitude of an earthquake can be express as\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$\\:A\\left(i\\right)={A}_{\\text{s}\\text{o}\\text{u}\\text{r}\\text{c}\\text{e}}\\text{*}\\frac{\\text{e}\\text{x}\\text{p}(-\\frac{\\pi\\:f}{Q{V}_{\\text{s}}}r)}{r}\\text{*}S\\left(i\\right)$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e,\u003c/p\u003e\u003cp\u003ewhere \u003cem\u003eA\u003c/em\u003e\u003csub\u003esource\u003c/sub\u003e is the amplitude at the source, \u003cem\u003eQ\u003c/em\u003e is the attenuation parameter, \u003cem\u003er\u003c/em\u003e is the distance from the hypocenter, \u003cem\u003ef\u003c/em\u003e is the frequency of the seismic wave, and \u003cem\u003eV\u003c/em\u003e\u003csub\u003es\u003c/sub\u003e is the S-wave velocity. \u003cem\u003eS\u003c/em\u003e(i) is the site amplification factor, which must be estimated at each station to apply the ASL method.\u003c/p\u003e\u003cp\u003eWe calculated the site amplification factors for all stations using the coda-normalization method (Sato and Fehler \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e1998\u003c/span\u003e) and the coda wave of 143 earthquakes with magnitudes of 3.0\u0026ndash;6.0 in the Japan Meteorological Agency (JMA) catalog that occurred 20\u0026ndash;100 km from the Owakudani geothermal area (Figure S6). The TNM station, which is located on the outer rim of Hakone Caldera and has low noise levels, was selected as the reference point. A 5 s window starting at the twice of the S-wave travel times from origin time at the TNM station was selected for the time of coda waves at all stations, and the coda amplitude was defined as the root mean square (RMS) amplitude during this window. The site amplification factor at each station was calculated as the mean ratio of the coda amplitude of each target earthquake to that at the TNM station. 17 earthquakes with signal-to-noise ratios of \u0026lt;\u0026thinsp;2 at the TNM station were removed from target earthquakes, where the noise amplitude was defined as the RMS amplitude of a 1 min window before the P-wave onset of the target earthquake. Then the number of earthquakes used in the analysis was 126 (Figure S6). While the earthquakes with a signal-to-noise ratio at each station except TNM of \u0026lt;\u0026thinsp;2 were removed from the mean for each station.\u003c/p\u003e\u003cp\u003eWe use a Q value of 100 in Eq.\u0026nbsp;(\u003cspan refid=\"Equ1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) based on the study of attenuation tomography (Kashiwagi et al. \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Since we use a 4\u0026ndash;16 Hz bandpass filter, we use a value of 10 Hz for \u003cem\u003ef\u003c/em\u003e. We use a value of 3.0 km/s for \u003cem\u003eV\u003c/em\u003e\u003csub\u003es\u003c/sub\u003e, based on a velocity model estimated using seismic tomography (Yukutake et al. \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2015\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eWe then estimated the locations of the hypocenters of the shallow earthquakes using the ASL method and the site amplification factor for each station. The hypocenter locations can be estimated by minimizing the error between the calculated (\u003cem\u003eA\u003c/em\u003e\u003csub\u003ecal\u003c/sub\u003e) and observed (\u003cem\u003eA\u003c/em\u003e\u003csub\u003eobs\u003c/sub\u003e) amplitudes. We defined the error function as\u003cdiv id=\"Equ2\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ2\" name=\"EquationSource\"\u003e\n$$\\:\\:\\text{E}\\text{r}\\text{r}\\:=\\:\\sum\\:_{i=1}^{n}{\\left({A}_{cal}(x,y,z,i)-{A}_{obs}\\left(i\\right)\\right)}^{2}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e2\u003c/div\u003e\u003c/div\u003e,\u003c/p\u003e\u003cp\u003ewhere \u003cem\u003ei\u003c/em\u003e is the station and \u003cem\u003en\u003c/em\u003e is the total number of stations (i.e., 9). \u003cem\u003eA\u003c/em\u003e\u003csub\u003ecal\u003c/sub\u003e is calculated from the Eq.\u0026nbsp;(\u003cspan refid=\"Equ1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). \u003cspan type=\"ItalicUnderline\" class=\"ItalicUnderline\" name=\"Emphasis\"\u003eA\u003c/span\u003e\u003csub\u003esource\u003c/sub\u003e in the Eq.\u0026nbsp;(\u003cspan refid=\"Equ1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) is estimated based on the equation\u003c/p\u003e\u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{A}_{\\text{s}\\text{o}\\text{u}\\text{r}\\text{c}\\text{e}}=\\frac{1}{n}\\sum\\:_{i=1}^{n}\\frac{{A}_{obs}\\left(i\\right)}{\\frac{\\text{e}\\text{x}\\text{p}(-\\frac{\\pi\\:f}{Q{V}_{\\text{s}}}r)}{r}\\text{*}S\\left(i\\right)}\\)\u003c/span\u003e\u003c/span\u003e (3),\u003c/p\u003e\u003cp\u003ei.e., \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{A}_{\\text{s}\\text{o}\\text{u}\\text{r}\\text{c}\\text{e}}\\)\u003c/span\u003e\u003c/span\u003e is estimated by average value of each station for each assumed hypocenter. We minimized the error function (2) using a grid search over longitudes of 138.920\u0026deg; E\u0026ndash;139.120\u0026deg; E and latitudes of 35.144\u0026deg; N\u0026ndash;35.344\u0026deg; N with an interval of 0.001\u0026deg;, and over depths of \u0026minus;\u0026thinsp;1 to 9 km below sea level with an interval of 0.1 km.\u003c/p\u003e\u003cp\u003eThe hypocenters showed a shallow and localized distribution beneath the Owakudani geothermal area (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e), and the 2015 events occurred to the northeast of those recorded after 2016. Although we cannot use the OWJ, SMYB, and SOZ stations to locate the 2015 events, as they were installed after 2015, excluding these stations in the hypocenter determination for earthquakes after 2016 does not significantly affect the results (Figure S7). The hypocenters after 2016 are aligned almost parallel to the crack that opened during the 2015 phreatic eruption, although the precision of ASL methods is low and the apparent alignment of hypocenters may be affected by the distribution of stations.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e"},{"header":"5 Discussion","content":"\u003cp\u003eThe waveforms of the shallow earthquakes differ from those of the regular volcano-tectonic earthquakes, as they do not show clear P- and S-wave arrivals, have relatively long durations, and have multiple peaks in a single sequence, which suggests that they occur repeatedly over short intervals and have a different mechanism to the regular volcano-tectonic earthquakes.\u003c/p\u003e\u003cp\u003eShallow volcanic earthquakes except volcano-tectonic earthquakes have been observed at many volcanoes. The characteristics of these waveforms and hypocenter distributions closely resemble those of the so-called \u0026ldquo;B-type earthquakes\u0026rdquo; described in earlier studies (Minakami \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e1960\u003c/span\u003e). BH-type earthquakes at Sakurajima (Iguchi \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e1993\u003c/span\u003e), Asama (Maeda et al. \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2019\u003c/span\u003e), and Tarumae (Aoyama et al. \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2004\u003c/span\u003e) volcanoes, and non-double couple (NDC) earthquakes at Unzen volcano (Hashimoto et al. \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2020\u003c/span\u003e) were the typical similar earthquakes. BH-type earthquakes at Sakurajima and Asama have dominant frequencies of 4\u0026ndash;7 Hz, lower than those of the shallow earthquakes at Hakone. The BH-type earthquakes are thought to be triggered directly by magmatic activity (e.g., magmatic intrusion); however, no ascending magma has been observed near the surface at Hakone, and the shallow earthquakes at Owakudani are unlikely to be BH-type earthquakes.\u003c/p\u003e\u003cp\u003eThe BH earthquakes at Tarumae have a dominant frequency of 10\u0026ndash;17 Hz (Aoyama et al. \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2004\u003c/span\u003e), and their waveforms are similar to those of the shallow earthquakes at Hakone. The BH earthquakes at Tarumae are very shallow and occur near the lava dome at the summit. The NDC earthquakes at Unzen (Hashimoto et al. \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2020\u003c/span\u003e) also have similar source locations and characteristics to the shallow earthquakes at Hakone. The Unzen NDC earthquakes occur at depths corresponding to the pre-eruption ground surface level prior to the 1990\u0026ndash;1995 eruptions. This suggests that these earthquakes are caused by collapses of small vapor-filled voids within the lava dome. However, the initial P-wave motion of the Hakone earthquakes is less clear that that of the Unzen earthquakes, and it is unlikely that they have the same source mechanism.\u003c/p\u003e\u003cp\u003eThe mechanism responsible for the shallow Hakone earthquakes may be different from that of similar earthquakes at other volcanoes. We interpret the mechanisms responsible for the earthquakes at Hakone based on their spatial and temporal distribution. These earthquakes occur in the very shallow part of the geothermal area at Hakone. In Owakudani, there are steam wells that produced hot springs, which reach a maximum depth of ~\u0026thinsp;500 m. In the depths, the hypocenters of the shallow earthquakes were estimated. Then, this indicates that the earthquakes occur around the geothermal area, suggesting that they are associated with hydrothermal fluid or steam in shallow underground. Most of these earthquakes occurred during 2015 and 2019\u0026ndash;2022. A phreatic eruption occurred in 2015 along with other volcanic activity, including crustal deformation, an increase in other types of volcanic earthquakes, and steam well blowouts. However, when these earthquakes occurred in 2022, no other volcanic activity was observed; therefore, the occurrence of these earthquakes is not necessarily associated with large-scale volcanic activity, but reflects local conditions.\u003c/p\u003e\u003cp\u003eAlthough the hypocenter locations are not particularly precise, they are located near the Owakudani geothermal area, close to the crack that appeared at the 2015 phreatic eruption (Honda et al. \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2018\u003c/span\u003e); therefore, it is possible that the shallow earthquakes were triggered by the injection of groundwater and volcanic gas into the existing crack (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e). In this area, fissure including a chain of craters were observed by air survey and high-resistivity structure were observed by controlled-source audio-frequency magneto-telluric survey (Mannen et al. \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). In addition, change of volcanic gas composition were observed in Owakudani and Kamiyu geothermal area, which locate 500 meters north of Owakudani in 2023 (Toyama et al. \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). The shallow earthquakes may correspond the underground structures. Although the number of shallow earthquakes has been low since 2023, these earthquakes could potentially be associated with more-dangerous activity (e.g., the explosive release of volcanic gas); therefore, it is important that a denser observation network is installed near the geothermal area to monitor the seismicity and to better understand the underlying physical mechanisms.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e"},{"header":"6 Conclusions","content":"\u003cp\u003eWe investigated the occurrence and distribution of shallow earthquakes with no clear P- and S-wave arrivals near the Owakudani geothermal area at Hakone volcano. The number of shallow earthquakes increased during the period of volcanic unrest in 2015, and also between 2020 and 2022, when volcanic unrest was not observed. The hypocenters of the earthquakes are concentrated near the Owakudani geothermal area, with likely depths of \u0026le;\u0026thinsp;1 km. The earthquakes occurred near the crack that opened at the time of the 2015 phreatic eruption. The earthquakes may have been triggered by the injection of groundwater and volcanic gas into the crack.\u003c/p\u003e"},{"header":"Abbreviations","content":"\u003cp\u003eMFT: matched filter technique\u003c/p\u003e\n\u003cp\u003eJMA: Japan Meteorological Agency\u003c/p\u003e\n\u003cp\u003eNIED: National institute of earth science and disaster resilience\u003c/p\u003e\n\u003cp\u003eHi-net: High Sensitivity Seismograph Network of Japan\u003c/p\u003e\n\u003cp\u003eGEONET: GNSS Earth Observation Network System\u003c/p\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\u003eAvailability of data and materials\u003c/p\u003e\n\u003cp\u003eHypocenter locations of the template events and catalog based on the matched-filter technique are provided in the supplementary material. We used seismic data from the National Research Institute for Earth Science and Disaster Resilience (2019), the Hot Springs Research Institute of Kanagawa Prefecture, and the Japan Meteorological Agency. All seismic data used in this study are available from the NIED Hi-net website (http://www.hinet.bosai.go.jp).\u003c/p\u003e\n\u003cp\u003eCompeting interests\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no competing interests.\u003c/p\u003e\n\u003cp\u003eFunding\u003c/p\u003e\n\u003cp\u003eThis work was supported by a Japan Society for the Promotion of Science KAKENHI grant (22K14113).\u003c/p\u003e\n\u003cp\u003eAuthors\u0026rsquo; contributions\u003c/p\u003e\n\u003cp\u003eRK designed the study, analyzed the data, and wrote the paper. The other authors provided advice and discussion.\u003c/p\u003e\n\u003cp\u003eAcknowledgments\u003c/p\u003e\n\u003cp\u003eWe used Generic Mapping Tools to produce the figures (Wessel and Smith 1998) and used Hi-net seismic data (http://www.hinet.bosai.go.jp) from (National Research Institute for Earth Science and Disaster Resilience 2019) and JMA\u0026rsquo;s unified earthquake catalog (http://www.jma.go.jp). The catalog of the shallow earthquakes constructed by this study are available in supporting material.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eAoyama H, Ohshima H, Suzuki A, Maekawa T (2004) Recent seismic activities at active volcanoes in Hokkaido -Tarumaesan-. Geophysical bulletin of Hokkaido University. https://doi.org/10.14943/gbhu.67.111\u003c/li\u003e\n\u003cli\u003eBattaglia J, Aki K (2003) Location of seismic events and eruptive fissures on the Piton de la Fournaise volcano using seismic amplitudes. 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National Research Institute for Earth Science and Disaster Resilience. https://doi.org/10.17598/nied.0003\u003c/li\u003e\n\u003cli\u003eSagiya T (2014) A decade of GEONET: 1994\u0026ndash;2003 \u0026mdash;The continuous GPS observation in Japan and its impact on earthquake studies\u0026mdash;. Earth Planet Sp 56:xxix\u0026ndash;xli. https://doi.org/10.1186/BF03353077\u003c/li\u003e\n\u003cli\u003eSato H, Fehler M (1998) Seismic wave propagation and scattering in the heterogeneous earth. Springer Verlag and AIP press, New York 1\u0026ndash;308\u003c/li\u003e\n\u003cli\u003eToyama K, Mannen K, Ninomiya R, Miyashita Y (2024) Results of Periodic Survey of Volcanic Gases and Hot Springs around Owakudani, Hakone Volcano, in 2023. \u0026ldquo;Kansoku-dayori\u0026rdquo; of the Hot Springs Research Institute of Kanagawa prefecture 89-92 (in Japanese)\u003c/li\u003e\n\u003cli\u003eWessel P, Smith WHF (1998) New, improved version of generic mapping tools released. Eos, Transactions American Geophysical Union 79:579\u0026ndash;579. https://doi.org/10.1029/98EO00426\u003c/li\u003e\n\u003cli\u003eYukutake Y, Honda R, Harada M, et al (2015) A magma-hydrothermal system beneath Hakone volcano, central Japan, revealed by highly resolved velocity structures. J Geophys Res Solid Earth 120:3293\u0026ndash;3308. https://doi.org/10.1002/2014JB011856\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":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":"Volcanic earthquake, Hakone volcano, Matched-filter technique, Phreatic eruption","lastPublishedDoi":"10.21203/rs.3.rs-7304798/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7304798/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eHakone volcano, located in central Japan, produced a small phreatic eruption in June 2015. Although seismic activity in the Hakone region is generally low, the area experiences episodic earthquake swarms approximately once every few years. In the volcano, many small earthquakes were observed at very shallow depths near Owakudani geothermal area from May to July 2022. In contrast to regular volcano-tectonic earthquakes, these events had no clear P- and S-wave arrivals. In this study, we sought to identify these earthquakes in continuous data and locate their hypocenters. We identified 11,016 earthquakes with similar waveforms between 2014 and 2023 using the matched-filter technique. Many earthquakes occurred in 2015, when the phreatic eruption occurred; however, the shallow seismicity was also active in 2020\u0026ndash;2022 at a time when no other volcanic activity including volcano-tectonic earthquakes and crustal deformation occurred. The earthquakes were sometimes triggered by volcanic activity and sometimes occurred ambiently. The hypocenters of the earthquakes were located based on amplitude source location method around the Owakudani geothermal area at depths of \u0026minus;\u0026thinsp;1 to 0 km below sea level, close to the surface. The hypocenters are located close to the crack that opened around the time of the 2015 phreatic eruption and close to fissure consisting of older craters. Given the waveforms, locations, and timing of the earthquakes, we infer that they were caused by the movement of fluid and volcanic gas near the surface.\u003c/p\u003e","manuscriptTitle":"Shallow volcanic earthquakes in the Owakudani geothermal area, Hakone volcano, Japan","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-08-28 08:30:07","doi":"10.21203/rs.3.rs-7304798/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Major Revision","date":"2025-09-22T02:50:43+00:00","index":"","fulltext":""},{"type":"reviewerAgreed","content":"","date":"2025-08-22T05:11:36+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-08-20T04:29:41+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-08-07T18:26:40+00:00","index":"","fulltext":""},{"type":"submitted","content":"Earth, Planets and Space","date":"2025-08-05T22:14:29+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":"3f802b1a-d778-437d-a079-5878162605f9","owner":[],"postedDate":"August 28th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2026-01-05T16:02:04+00:00","versionOfRecord":{"articleIdentity":"rs-7304798","link":"https://doi.org/10.1186/s40623-025-02341-3","journal":{"identity":"earth-planets-and-space","isVorOnly":false,"title":"Earth, Planets and Space"},"publishedOn":"2025-12-29 15:58:30","publishedOnDateReadable":"December 29th, 2025"},"versionCreatedAt":"2025-08-28 08:30:07","video":"","vorDoi":"10.1186/s40623-025-02341-3","vorDoiUrl":"https://doi.org/10.1186/s40623-025-02341-3","workflowStages":[]},"version":"v1","identity":"rs-7304798","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7304798","identity":"rs-7304798","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}