# Geophysical Research Letters®

## RESEARCH LETTER

10.1029/2022GL101086

**Key Points:**

* Unusual tsunamis were caused by two $M_w$ 5.8 volcanic earthquakes in 2009 and 2017 near Curtis and Cheeseman Islands in the Kermadec Arc

* By using tsunami and seismic data of the 2017 event, we suggest that a trapdoor faulting event occurred at a submerged resurgent caldera

* Trapdoor faulting events at calderas had abnormally large slips, possibly due to the fault-reservoir interaction in calderas

**Supporting Information:**

Supporting Information may be found in the online version of this article.

**Correspondence to:**

O. Sandanbata, osm3@bosai.go.jp

**Citation:**

Sandanbata, O., Watada, S., Satake, K., Kanamori, H., & Rivera, L. (2023). Two volcanic tsunami events caused by trapdoor faulting at a submerged caldera near Curtis and Cheeseman Islands in the Kermadec Arc. *Geophysical Research Letters*, *50*, e2022GL101086. https://doi.org/10.1029/2022GL101086

Received 31 AUG 2022

Accepted 17 MAR 2023

**Author Contributions:**

**Conceptualization:** Osamu Sandanbata

**Data curation:** Osamu Sandanbata

**Formal analysis:** Osamu Sandanbata

**Funding acquisition:** Osamu Sandanbata, Shingo Watada

**Investigation:** Osamu Sandanbata, Hiroo Kanamori

**Methodology:** Osamu Sandanbata, Shingo Watada, Kenji Satake, Hiroo Kanamori, Luis Rivera

**Resources:** Luis Rivera

**Software:** Osamu Sandanbata, Hiroo Kanamori, Luis Rivera

**Supervision:** Kenji Satake

© 2023. The Authors.

This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

<figure><img src="image_1.png" /></figure>

## Two Volcanic Tsunami Events Caused by Trapdoor Faulting at a Submerged Caldera Near Curtis and Cheeseman Islands in the Kermadec Arc

Osamu Sandanbata¹,² ID, Shingo Watada¹ ID, Kenji Satake¹ ID, Hiroo Kanamori³, and Luis Rivera⁴

¹Earthquake Research Institute, The University of Tokyo, Tokyo, Japan, ²National Research Institute for Earth Science and Disaster Resilience, Ibaraki, Japan, ³Seismological Laboratory, California Institute of Technology, Pasadena, CA, USA,

⁴CNRS, ITES UMR 7063, Université de Strasbourg, Strasbourg, France

**Abstract** Two submarine earthquakes ($M_w$ 5.8) occurred near volcanic islands, Curtis and Cheeseman, in the Kermadec Arc in 2009 and 2017. Following both earthquakes, similar tsunamis with wave heights of about a meter, larger than expected from their moderate seismic magnitudes, were observed by coastal tide gauges. We investigate the source mechanism for both earthquakes by analyzing tsunami and seismic data of the 2017 event. Tsunami waveform analysis indicates that the earthquake uplifted a submerged caldera around the islands. Combined analysis of tsunami and seismic data suggests that trapdoor faulting, or sudden intra-caldera fault slip interacted with magma reservoir deformation, occurred due to magma overpressure, possibly in association with caldera resurgence. The earthquake scaling relationship for trapdoor faulting events at three calderas deviates from that for regular earthquakes, possibly due to the fault-reservoir interaction in calderas.

**Plain Language Summary** Most tsunamis are generated by large earthquakes with seismic magnitudes $M > \sim 7$, but two moderate-sized earthquakes with only $M$ 5.8 near volcanic islands, north of New Zealand, caused unusual tsunamis with a maximum wave height of about a meter. In this study, we examine the unusual source mechanism of the volcanic earthquakes that excited unexpected tsunamis. By analyzing the records of tsunamis and seismic waves from the earthquakes, we suggest that the inside of a curved fault system beneath a submerged volcano with a caldera structure suddenly moved upward, together with a large intra-caldera fault slip and a volume increase of a shallow magma reservoir. Overpressure created by magma accumulation beneath the submarine caldera recurrently induces meter-scale tsunamis without significant ground motions, calling for attention to tsunami hazards from submarine calderas.

## 1. Introduction

On 17 February 2009 and 8 December 2017 (UTC), peculiar tsunamis were recorded at coastal tide gauges for tsunami monitoring in New Zealand, after shallow earthquakes with moment magnitudes of $M_w$ 5.8. Both earthquakes occurred near Curtis and Cheeseman Islands, volcanic islands in the Kermadec Arc (Figure 1a; Table S1 in the Supporting Information S1); we hereafter call them Curtis earthquakes. The maximum tsunami wave heights from the 2009 and 2017 earthquakes were about 50 cm (LOTT) and 80 cm (RFRT), respectively (Figure 1b), much larger than expected for their seismic magnitudes (Abe, 1981). The tsunami waveforms from these earthquakes were similar at three stations, LOTT, NCPT, and TAUT, and their moment tensors were similarly dominated by the compensated-linear-vector-dipole (CLVD) component with a nearly vertical tension-axis. Such vertical-T CLVD earthquakes are often observed in active volcanic contexts (Sandanbata, Kanamori, et al., 2021; Shuler, Nettles, et al., 2013). These indicate that a similar volcanic phenomenon near the islands repeatedly caused the tsunamis.

The mechanism of tsunamigenic vertical-T CLVD earthquakes has been controversial. For the Curtis earthquakes, Gusman et al. (2020) analyzed tsunami data at tide gauges and broad-band (20–500 s) seismic data and proposed a model involving two sources at different depths. They suggested that a hydrofracturing source at a depth of ~1.5 km in the crust generated tsunamis without significant seismic radiations, and a deeper source at ~10 km related to fluid flow radiated seismic waves. Similar events were observed at Sumisu caldera, south of Japan, for which various models were proposed: hydrofracturing of magma-water interaction (Kanamori et al., 1993; Satake & Kanamori, 1991), ring-faulting (Ekström, 1994), shallow tensile crack (Fukao et al., 2018). Recently,
**Validation:** Osamu Sandanbata, Hiroo Kanamori, Luis Rivera

**Visualization:** Osamu Sandanbata

**Writing – original draft:** Osamu Sandanbata

**Writing – review & editing:** Osamu Sandanbata, Shingo Watada, Kenji Satake, Hiroo Kanamori, Luis Rivera

Sandanbata et al. (2022) showed that the trapdoor faulting mechanism explains both tsunami and long-period seismic data from an event at Sumisu caldera.

We investigate the source mechanism of the 2017 Curtis earthquakes using an approach similar to that used by Sandanbata et al. (2022). After estimating the initial sea surface displaced by the earthquake using tsunami data, we explore a unified source model that explains both tsunami and long-period seismic data from the earthquake. We then propose a source mechanism and discuss submarine volcanism and its atypical earthquake scaling relationship.

## 2. Geological Background

Curtis and Cheeseman Islands are the emergent portions of a submarine dacitic volcano containing a crater with fumaroles activity (Doyle et al., 1979; Global Volcanism Program, 2013; Smith et al., 1988). Doyle et al. (1979) reported 18-m uplift during the 200 years before their survey on/around the islands, implying active magma supply. Beneath the islands, a cone structure is surrounded by a circular bathymetric depression with a maximum water depth of ~400 m, indicating a submerged caldera with a size of 7 × 6 km (Figure 2a), as mentioned in Global Volcanism Program (2013); we hereafter call it *Curtis caldera*.

## 3. Data

### 3.1. Tsunami Data

We use four coastal tide gauge records at RBCT, RFRT, LOTT, and GBIT stations from the 2017 earthquake, with a sampling rate of one sample/minute (Figure 1b). We extract the tsunami signals from the raw record by removing tidal trends approximated as low-order polynomial functions. The signals have dominant periods from 200 to 500 s. We do not use records at NCPT and TAUT, because of poor signal-to-noise ratios.

### 3.2. Long-Period Seismic Data

We use long-period seismic waveform data from the 2017 earthquake, recorded on LH or BH channels at seismic stations (network codes: IU, NZ, AU, and G). We select 33 good-quality records after data screening, remove the instrument response from the seismograms to obtain displacement records, and apply a causal fourth-order Butterworth band-pass filter with corner frequencies at 0.005 and 0.0125 Hz (period: 80–200 s); these procedures are performed with the W-phase package (e.g., Kanamori & Rivera, 2008).

## 4. Source Modeling of the 2017 Earthquake

### 4.1. Vertical Sea-Surface Displacement Model

We start our modeling by estimating the vertical sea-surface displacement due to the 2017 earthquake with the tsunami data (see Text S1 in the Supporting Information S1, for details). We first simulate dispersive tsunami waveforms at the tide gauges with the JAGURS code (Baba et al., 2015). We delay the waveforms at two distant stations, LOTT and GBIT, to incorporate the effects of seawater compressibility and Earth's elasticity (Ho et al., 2017; Sandanbata, Watada, et al., 2021; Watada et al., 2014). We then perform a tsunami waveform inversion to reconstruct the vertical sea-surface displacement.

The obtained model (Figure 2b) involves a large uplift of ~1.3 m localized on the western caldera floor, a peripheral subsidence of ~1.3 m to the south of the uplift, and another meter-scale uplift to the southeast. This model explains the tsunami data well (Figure 2c). To examine the model robustness, we estimate a sea-surface *uplift* model without subsidence by imposing the non-negative condition on the inversion. In the obtained uplift model (Figure 2d), a similar localized uplift is estimated over the western part of the caldera but with a larger amplitude of ~2.4 m. In this case, the uplift to the southeast seen in Figure 2b disappears. The two sea-surface displacement models with and without subsidence are different, but the tsunami waveform fits are comparable (Figures 2c and 2e). These results suggest that the tsunami was excited mainly by an uplift localized in the western caldera floor, which is supported by our resolution tests (see Text S1 in the Supporting Information S1).

Given the inferred deformation localized on one side of the caldera, the *trapdoor faulting* mechanism (Figure 3a) seems a good candidate to explain the source process of this unusual event. This mechanism, driven by magma
<figure><img src="image_2.png" /><figcaption>**Figure 1.** Curtis earthquakes. (a) Earthquake centroids (red star) and tide gauges (triangles), with an inset satellite image obtained from Google Earth. (b) Tsunami waveforms at tide gauges. Black and blue lines represent waveforms from the 2009 and 2017 earthquakes, respectively.</figcaption></figure>

pressure in a reservoir, was first proposed for seismic events at a subaerial caldera of Sierra Negra, Galapagos (e.g., Amelung et al., 2000; Zheng et al., 2022), and recently found at Sumisu caldera in oceans (Sandanbata et al., 2022). The trapdoor faulting is a sudden slip of a part of an intra-caldera fault involving an asymmetric deformation of a crack-like reservoir. This mechanism can cause a large asymmetric caldera uplift with a vertical-T CLVD earthquake, as observed from the Curtis earthquake.

## 4.2. Dislocation Model of the Fault-Crack Composite Source

We explore an earthquake source model by using both the tsunami and long-period seismic data, following Sandanbata et al. (2022) (see Text S2 in the Supporting Information S1, for details). First, we assume a fault-crack composite source system, or an inward-dipping partial ring fault connected to a horizontal crack at a depth of 3 km in the crust (Figures 3b and S1 in the Supporting Information S1). We fix the partial ring fault on the western side of the caldera (Figure 3b), given the estimated sea-surface uplift (Figure 2a); the dip angle is varied from 65 to 85°. Second, by a tsunami waveform inversion, we determine dislocations of the source system, that is, reverse slips of the ring fault and vertical opening/closure of the horizontal crack, to obtain source models.
<figure><img src="image_3.png" /><figcaption>**Figure 2.** Vertical sea-surface displacement models for the 2017 earthquake. (a) Bathymetry around the islands, showing a caldera-like depression (white dashed line). Black dashed lines represent 1-m and 2-m contours of the sea-surface uplift model (shown in d). (b) Model estimated when uplift and subsidence are allowed, and (c) synthetic tsunami waveforms (red) and observation (black) (d) Model estimated when only uplift is allowed, and (e) its synthetic tsunami waveforms. In (b) and (d), contour lines are plotted every 50 m of water depth. In (c) and (e), gray lines represent data lengths for the inversion. Rectangle in (d) indicates the area shown in (a).</figcaption></figure>

We assume a reverse slip on an inward-dipping ring fault because this combination can cause a vertical-T CLVD earthquake that accompanies a caldera uplift (see Figure 1 in Sandanbata, Kanamori, et al., 2021). Here, the vertical component of the slip at the fault bottom is assumed to be the same as the opening of the horizontal crack at its contact. Third, we compute long-period (80–200 s) seismograms from the models and compare them with the seismic data for validation, assuming that Lamé’s constants are $\lambda = 34.2$ GPa and $\mu = 26.6$ GPa.

We then find source models that explain both datasets. The tsunami waveform inversion successfully determines the dislocations of the fault-crack composite source system, as shown by an example model in Figure 3b. The model is expected to uplift the sea surface by ~2 m (Figure 4a), explaining the tsunami data well (Figure 4b). Although we vary the ring-fault dip angle, all the obtained models yield similarly good waveform fit (Figure S2 in the Supporting Information S1) with small waveform misfits (tsunami normalized root-mean-square (NRMS) misfits of 0.60–0.76; Figure 3c), indicating weak constraints on the dip angle.

In contrast, long-period seismic wave amplitudes from the models change significantly depending on the dip angle (Figure S3 in the Supporting Information S1), resulting in large variations of the misfit of seismic waveforms (seismic RMS of 0.86–2.08 µm; Figure 3c). This is because seismic excitation by a shallow ring fault becomes less efficient as the dip is close to vertical (Sandanbata, Kanamori, et al., 2021). Thus, models with dip angles in a narrow range of 75°–81° yield reasonable seismic waveform fits (seismic root-mean-square (RMS) misfits < 1.0 µm; Figure 3c). Figures 4c–4e show moment tensors and long-period seismograms computed from the model shown in Figure 3b, which has a ring-fault dip angle of 78°; this model explains the seismic waveform data well (Figure 4e), yielding the smallest seismic RMS misfit of 0.86 µm (Figure 3c).

We therefore propose the model shown in Figure 3b as a best-fit source model. It presents a reverse slip of 4.4 m at a maximum along the ring fault with a dip angle of 78° and asymmetric opening and closure of the horizontal crack with maximum amounts of 5.4 and 2.9 m, respectively. The crack increases its inner volume by +3.5 × 10⁷ m³. This model causes a sea-surface uplift localized over the western caldera floor with a smaller subsidence outside the fault (Figure 4a). The pattern and amplitude of this sea-surface displacement are different from those estimated in Section 4.1 (Figures 2b and 2d) due to strong constraints from the fault-crack source
<figure><img src="image_4.png" /><figcaption>**Figure 3.** Trapdoor faulting mechanism. (a) Schematic illustration of the mechanism (not scaled). (b) Our best-fit source model for the 2017 earthquake. Red color on the ring fault represents reverse slip. Red and blue colors on the horizontal crack represent opening and closing, respectively. (c) Tsunami and seismic waveform misfits for models with different dip angles.</figcaption></figure>

system, but this model explains the tsunami data well (Figure 4b). Figure S4 in the Supporting Information S1 shows synthetic tsunami waveforms from the horizontal crack and the ring fault separately, demonstrating that both parts contribute to the tsunami generation.

The model's total moment tensor is highly isotropic with $M_w$ 6.24 (Figure 4c; Table S2 in the Supporting Information S1), composed of the horizontal crack ($M_w$ 6.19) with a strong isotropic component and the ring fault ($M_w$ 5.96) with a large vertical-T CLVD component (Figure 4d). In Figure S5 in the Supporting Information S1, we show that only limited components of the ring fault, which constitute a vertical-T CLVD moment tensor, mainly contribute to the seismic radiation, and that the horizontal crack contributes little (see Text S3 in the Supporting Information S1, for details). This explains why vertical-T CLVD solutions were obtained by moment tensor analyses assuming no isotropic component, as seen in the GCMT catalog (Figure 1a).

# 5. Discussion

## 5.1. Trapdoor Faulting Mechanism

We have shown that our source model (Figure 3b) explains both the tsunami and long-period seismic data from the 2017 Curtis earthquake. The asymmetrical motion of the source system is like those estimated for trapdoor
<figure><img src="image_5.png" /><figcaption>**Figure 4.** Model performance of our source model (Figure 3b). (a) Vertical sea-surface displacement. (b) Tsunami waveforms of the synthesis (red) and observation (black). Gray lines represent data lengths used for the inversion. (c) Total moment tensor of the model. (d) Partial moment tensors of the horizontal crack and the ring fault. (e) Long-period seismograms of the synthesis with the model (red; moment tensor in c) and observation (black). Red dots delimitate data length to calculate waveform misfit. In inset panels, red circle and blue star represent the station and earthquake locations, respectively. Network and station names, record component, station azimuth, and epicentral distance are indicated on top.</figcaption></figure>
faulting events at Sumisu and Sierra Negra calderas (e.g., Sandanbata et al., 2022; Zheng et al., 2022). Similarly, we suggest the trapdoor faulting mechanism for the Curtis earthquake (Figure 3a), as follows. Before faulting, the caldera had gradually inflated due to continuous magma influx into the reservoir, and the magma pressurization increased shear stress on an inward-dipping ring fault. At the time of the earthquake, the western fault suddenly slips in a reverse sense due to the critically increased shear stress created by the magma pressure. The fault slip instantaneously moves up the western upper wall of the crack, expanding the crack near the fault. The asymmetrical crack expansion induces magma flow from the eastern to western sides, which results in the closure of the eastern side of the crack with magma depressurization; the crack closure might be alternatively caused by magma mass outflow due to an eruption, but the two possible cases are indistinguishable from our dataset.

During the earthquake, the ring faulting, reservoir deformation, and magma movement occur interactively and almost simultaneously, and this entire process contributes to the generation of tsunami and seismic waves. Thus, we refer to this fault-reservoir interactive process as a trapdoor faulting mechanism; this follows the usage in Sandanbata et al. (2022). We note that, most previous studies (e.g., Amelung et al., 2000; Bell, Hernandez, et al., 2021) used the term, “trapdoor faulting,” to describe only the faulting part.

The similarities between the Curtis earthquakes in 2009 and 2017 suggest that trapdoor faulting events recurred at Curtis caldera. The tsunami waveforms from the earthquakes are almost identical at LOTT, NCPT, and TAUT (Figure 1b). To further investigate the earthquake similarity, we perform deviatoric moment tensor analysis using long-period seismic data (Table S3 in the Supporting Information S1; see Text S4 in the Supporting Information S1). We neglect two moment-tensor components, $M_{r0}$ and $M_{rφ}$, that are indeterminate for such shallow earthquakes to obtain resolvable moment tensors $M_{res}$ as proxies for the ring-fault geometry (Sandanbata, Kanamori, et al., 2021) (Figure S6 in the Supporting Information S1). Similar null-axis directions (NNW-SSE) of $M_{res}$ indicate that ring-fault segments of the two events are oriented similarly, while a more dominant vertical-CLVD component of the 2017 event (larger $k_{CLVD}$; see the definition in Text S4 in the Supporting Information S1) implies that a longer ring fault was ruptured by this event than the 2009 event. Recurrence of trapdoor faulting events was reported for Sumisu and Sierra Negra calderas, being attributed to continuous magma supply causing repeated fault failures (Bell, Hernandez, et al., 2021; Gregg et al., 2022; Sandanbata et al., 2022); this would be the case for Curtis caldera.

In Text S5 in the Supporting Information S1, we demonstrate that different fault-crack composite source models, obtained by assuming a deeper crack, a longer ring fault, or an additional normal faulting component on the eastern side of the caldera, similarly reproduce the datasets. Although this indicates limited source resolution of our analysis based on long-period (>~100 s) waveform data at regional-far field, a similar mechanism in a fault-reservoir composite source system is still inferred from these alternative models; we also emphasize that the fault-crack composite sources explain our datasets clearly better than only a ring fault or a horizontal crack (see Text S6 in the Supporting Information S1, for details). To better constrain the source geometries, near-field data analyses with finer surface and sub-caldera structural information would be required; such analyses will also help to resolve how the fault rupture propagates and the magma-filled reservoir responds to faulting.

Gusman et al. (2020) proposed a different mechanism for these earthquakes to explain tsunami and broadband seismic waveforms. They separately attributed the tsunami generation to a hydrofracturing source at a shallow depth and the vertical-T CLVD seismic radiation to a deeper source. On the other hand, we demonstrated that the trapdoor faulting mechanism beneath the caldera reproduces tsunami and long-period seismic waveforms at the same time. The two mechanisms seem indistinguishable from the low-sampling and limited tsunami data. Given the difficulties in determining shallow source depths (Ekström et al., 2012) and resolving seismic sources that involve volume change at shallow depths (Kawakatsu, 1996), the two mechanisms cannot be distinguished from available seismic data at regional-far field.

## 5.2. Implications for Submarine Caldera Volcanism

The trapdoor faulting events may have occurred in the resurgence process of Curtis caldera. Resurgence is a long-term caldera uplift driven by gradual pressure increase of magma accumulating in a shallow reservoir and facilitated by ring-fault reactivations, as observed in many calderas like Toba, Long Valley, Valles, and Iwo-Jima (Iō Tō). Although most resurgent calderas accompany earthquakes with only seismic magnitudes $M < \sim 3$ (Acocella et al., 2015; references therein), $M \sim 5$ trapdoor faulting events occurred during the resurgence of Sierra Negra
<figure><img src="image_6.png" /><figcaption>**Figure 5.** Earthquake scaling relationship for trapdoor faulting events. Circles represent the maximum slip amount and the seismic magnitude for four trapdoor faulting events at Curtis, Sumisu, and Sierra Negra calderas. The magnitude scale is $m_b$ (UGGS) for the 16 April 2005 event at Sierra Negra, but $M_w$ (the GCMT catalog) for the others. The black line represents the relationship for tectonic earthquakes. See text for details.</figcaption></figure>

caldera (Bell, La Femina, et al., 2021; Gregg et al., 2018, 2022). The significant uplift on/around Curtis Island found by Doyle et al. (1979) may be attributed to resurgence of the submerged caldera. The cone structure within the caldera (Figure 2a) is possibly a *resurgent dome*, as seen within many resurgent calderas (Acocella, 2019; Acocella et al., 2001) and replicated by numerical/analog experiments (Acocella & Funiciello, 1999; Brothelande et al., 2016). Active fumaroles on Curtis Island (Doyle et al., 1979) are common features for resurgent domes (Cole et al., 2005; Molin et al., 2003).

The relationship between trapdoor faulting earthquakes and submarine eruptions at Curtis caldera remains unclear. About a month before the 2009 earthquake, ocean acoustic waves from near the caldera were recorded by a seismic network (Global Volcanism Program, 2013). These signals perhaps indicated submarine eruptions, but their origins are yet to be confirmed.

## 5.3. Earthquake Scaling Relationship for Trapdoor Faulting

The earthquake scaling relationship for trapdoor faulting events at calderas is distinct from that for tectonic earthquakes. Figure 5 shows seismic moment magnitudes and maximum fault slips estimated for four trapdoor faulting events with proposed source models: events on 8 December 2017 at Curtis ($M_w$ 5.8 and 4.4 m [this study]), 2 May 2015 at Sumisu ($M_w$ 5.7 and 6.8 m [Sandanbata et al., 2022]), and 16 April 2005 ($m_b$ 4.6 and 2.0 m [Zheng et al., 2022]; $m_b$ is used because $M_w$ was not reported) and 22 October 2005 ($M_w$ 5.4 and 1.8 m [Yun, 2007]) at Sierra Negra. Compared with the empirical scaling relationship by Wells & Coppersmith (1994), the trapdoor faulting events involved far larger, 10–20 times, fault slips than those

by tectonic earthquakes with similar seismic magnitudes. While seismic estimates of moment magnitudes for trapdoor faulting events are often smaller than geodetic (or tsunami) estimates because of the aseismic slip or small shear stiffness in calderas (e.g., Amelung et al., 2000; Jónsson, 2009), the magnitude discrepancies cannot explain the scaling gap (see Text S7 in the Supporting Information S1). The atypical scaling relationship would indicate that intra-caldera faults accommodate large slips because of their connection to a reservoir, contributing to large tsunamis when they occur underwater (Sandanbata et al., 2022). Hence, to understand how the tsunami size is controlled, it is critical to study the earthquake physics in the fault-reservoir interactive system.

## 6. Conclusions

Both seismic and tsunami data from two peculiar earthquakes near Curtis and Cheeseman Islands are explained well by the trapdoor faulting mechanism within the submerged caldera. The large tsunamis can be attributed to the sudden asymmetric caldera-floor uplift of meters, possibly facilitated by the fault-reservoir interactive system beneath calderas. A long-term resurgence of Curtis caldera, inferred from bathymetric and geological features, suggests its eruption potential and the importance of the volcano monitoring.

## Data Availability Statement

The tide-gauge data are available from Land Information New Zealand (LINZ; https://www.linz.govt.nz/sea/tides/sea-level-data/sea-level-data-downloads). The topography and bathymetry data are available from GEBCO Compilation Group through the British Oceanographic Data Centre (Weatherall et al., 2015; https://www.gebco.net/data_and_products/gridded_bathymetry_data/gebco_30_second_grid/), the National Institute of Water and Atmospheric Research in New Zealand (NIWA; https://niwa.co.nz/our-science/oceans/bathymetry), and LINZ (https://data.linz.govt.nz/). The seismic data are available through the Data Management Center of the Incorporated Research Institutions for Seismology (IRIS)'s Wilber 3 system (https://ds.iris.edu/wilber3/) or IRIS Web Services (https://service.iris.edu/), including the seismic networks of IU (GSN; Albuquerque Seismological Laboratory, 1988), AU (ANSN; H., G., & Geoscience Australia, 2021), NZ (New Zealand National Seismograph Network), and G (GEOSCOPE; IPGP & EOST, 1982). The earthquake information is available from the GCMT
Acknowledgments

We thank Editor, Christian Huber,
and Falk Amelung and an anonymous
reviewer for constructive suggestions.
We thank Zhongwen Zhan for helpful
discussion. This study is funded by the
JSPS (Japan Society for the Promotion
of Science) KAKENHI (JP17J02919,
JP20J01689, and JP19K04034), by the
JST (Japan Science and Technology
Agency) J-RAPID (JPMJJR1805). Osamu
Sandanbata's travel to California Institute
of Technology was supported by Oversea
Internship Program of Earthquake
Research Institute, the University of
Tokyo.

catalog (Ekström et al., 2012; https://www.globalcmt.org/; https://www.globalcmt.org/), or UGSG (https://earth-
quake.usgs.gov/; https://earthquake.usgs.gov/). The W-phase package (Duputel et al., 2012; Hayes et al., 2009;
Kanamori & Rivera, 2008) is available through the website of W Phase source inversion (http://eost.u-strasbg.
fr/wphase/index.html). We plotted focal mechanisms representing moment tensors with a MATLAB code devel-
oped by James Conder (available from https://www.mathworks.com/matlabcentral/fileexchange/61227-focal-
mech-fm-centerx-centery-diam-varargin). The best-fit source model presented in this paper is contained in Data
Set S1, which is available from Zenodo (https://doi.org/10.5281/zenodo.7502680).

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