Full-spectrum similarity of repeating trapdoor faulting

Osamu SANDANBATA1& ShunsukeTAKEMURA1

1. Earthquake Research Institute, The University of Tokyo


Sandanbata, O., & Takemura, S. (2025). Full‐spectrum similarity of repeating trapdoor faulting. Geophysical Research Letters, 52(24), e2025GL119615.　https://doi.org/10.1029/2025gl119615 


　At Sumisu Caldera, a unique fault motion known as trapdoor faulting occurs repeatedly about every 10 years, each time generating earthquakes ofMw5.4–5.7accompanied by tsunamis (see Explanatory Article). During these earthquakes driven by sub-caldera magma pressure, the fault slips abruptly, causing significant uplift of the caldera, which in turn triggers tsunami generation. In this study, we compared seismic and tsunami waveforms over a broad period range(0.2–500s) for multiple earthquakes that occurred at Sumisu Caldera. We found that the three largest events exhibit highly consistent waveforms (Fig.1). This strong similarity indicates that the same largeseismic fault patch (asperity) ruptures during each event, and that its location and rupture process remain stable over decades. In contrast, smaller similar earthquakes show waveform differences, likely reflecting partial rupture of the main patch or activity of surrounding smaller patches. Furthermore, waveform comparisons of trapdoor faulting earthquakes at other submarine calderas also revealed high similarity. These observations suggest that, as at Sumisu Caldera, specific fault segments repeatedly rupture. However, recurrence intervals and earthquake magnitudes vary among calderas, reflecting differences in fault structure and frictional properties. Such studies provide important insights into the frequency and magnitude of recurring earthquakes and tsunamis at submarine calderas, as well as the mechanical state of the caldera.

ExplanatoryArticle: https://www.eri.u-tokyo.ac.jp/en/research/5323/

Fig.1. Comparison of waveforms for repeating earthquakes atSumisuCaldera. The 2015 earthquake (black) is used as a reference, andthe 1996 (orange) and 2024 (blue) events are compared across the following dataand periodbands:(a) tsunami waveforms (B0:100–500 s; no data for 1996), (b–e) vertical-component seismic waveforms (B1–B3: 50–200s,20–50s,and10–20s). Waveforms arevertically offset for each observation point. Below each station name, color-coded cross-correlationcoefficients with the referenceearthquake are shown (values closer to 1 indicate higher waveform similarity).(e) Location of Sumisu Caldera (red triangle) and thepositions of the tsunami gauges (blue: DONET) and seismometers (orange: F-net) used in thisstudy.(f) Schematic illustration of thering-fault structure and magma reservoir beneath Sumisu Caldera. Long-term pressurization of the magma eventually causes rupture ofthe same segment of the fault at intervals of approximately ten years, resulting in the repeated observation of remarkably similar tsunamiand seismic waveforms.