Quantifying magma overpressure beneath a submarine caldera: A mechanical modeling approach to tsunamigenic trapdoor faulting near Kita-Ioto Island, Japan

Abstract

Submarine volcano monitoring is vital for assessing volcanic hazards but challenging in remote and inaccessible environments. In the vicinity of Kita-Ioto Island, south of Japan, unusual M~5 non-double-couple volcanic earthquakes exhibited quasi-regular repetition near a submarine caldera. Following the 2008 earthquake, a distant ocean bottom pressure sensor recorded a distinct tsunami signal. In this study, we aim to find a source model of the tsunami-generating earthquake and quantify the pre-seismic magma overpressure within the caldera’s magma reservoir. Based on the earthquake’s atypical focal mechanism and efficient tsunami generation, we hypothesize that submarine trapdoor faulting occurred due to highly pressurized magma. To investigate this hypothesis, we establish a mechanical earthquake model that links pre-seismic magma overpressure to the size of the resulting trapdoor faulting, by considering stress interaction between a ring-fault system and a reservoir of the caldera. The model reproduces the observed tsunami waveform data. Our estimates indicate trapdoor faulting with large fault slip occurred in the critically stressed submarine caldera accommodating pre-seismic magma overpressure of ~10 MPa. The model infers that the earthquake partially reduced magma overpressure by 10–20%, indicating that the magmatic system maintained high stress levels even after the earthquake. Due to limited data, uncertainties persist, and alternative source geometries of trapdoor faulting could lead to estimate variations. These results suggest that magmatic systems beneath calderas are influenced much by intra-caldera fault systems. Monitoring and investigation of volcanic tsunamis and earthquakes help to obtain quantitative insights into submarine volcanism hidden under the ocean.

Enigmatic Tsunami Waves Amplified by Repetitive Source Events Near Sofugan Volcano, Japan

“…Another series of abnormal tsunamis have recurrently taken place, almost every 10 years, due to M w 5.4–5.8 volcanic earthquakes at another submarine caldera, Sumisu Caldera (Figure 1c) (Kanamori et al., 1993; Satake & Kanamori, 1991), for which a submarine trapdoor faulting in the inflating caldera was proposed (Sandanbata et al., 2022); the same mechanism has been recently proposed for tsunamigenic earthquakes at two other submarine calderas: Curtis Caldera, New Zealand (Sandanbata et al., 2023), and Kita‐Ioto Caldera, Japan (Sandanbata & Saito, 2024). These submarine trapdoor faulting events are tsunamigenic but exhibit atypically inefficient seismic excitation (Sandanbata et al., 2021, 2022).…”

“…However, the Izu‐Bonin region hosts tens of volcano islands and submarine volcanoes, and active back‐arc rift systems (Kodaira et al., 2007), implying various types of potential tsunami hazards. As discussed above, M > 5 trapdoor faulting earthquakes can cause notable tsunamis from submarine calderas (Sandanbata et al., 2022; Sandanbata & Saito, 2024). An M w 6.4 normal faulting earthquake on 24 October 2006 in a region between Sofugan volcano and Nichiyo Seamount (Figure 1c) also generated about 10‐cm tsunamis (Japan Meteorological Agency, 2023).…”

Accelerating Seafloor Uplift of Submarine Caldera Near Sofugan Volcano, Japan, Resolved by Distant Tsunami Recordings

“…Αt active volcanic calderas, various mechanisms can cause significant vertical deformation capable of exciting tsunamis (e.g., Acocella, 2007; Cole et al., 2005; Lipman, 1997). Although we cannot determine the exact process at the caldera due to the limitation of the data set we used, a possible candidate to explain the seafloor uplift is a trapdoor faulting, or sudden slip of the intra‐caldera ring fault caused by overpressurization of its underlying magma reservoir, which can cause sudden uplift of the caldera (Sandanbata et al., 2022; Sandanbata & Saito, 2024; Zheng et al., 2022). It has been often reported that submarine trapdoor faulting excites significant tsunamis, which have larger amplitudes than those expected from their seismic magnitudes (e.g., Fukao et al., 2018; Sandanbata et al., 2022; Sandanbata et al., 2023).…”

Quantifying Magma Overpressure Beneath a Submarine Caldera: A Mechanical Modeling Approach to Tsunamigenic Trapdoor Faulting Near Kita‐Ioto Island, Japan

“…Focal mechanisms representing moment tensors are plotted with a MATLAB code of focalmech (Conder, 2019). The data of the source model proposed for the main results in this study (Figure 3) can be obtained from an open-access repository of Zenodo (Sandanbata & Saito, 2023).…”