V043-04
Unexpectedly large tsunamis generated by submarine volcanic earthquakes: Evidence of trapdoor faulting at a submarine caldera

Wednesday, 16 December 2020: 11:42
Virtual
Osamu Sandanbata1,2, Shingo Watada1, Kenji Satake3, Hiroo Kanamori4, Luis A Rivera5 and Zhongwen Zhan6, (1)Earthquake Research Institute, University of Tokyo, Tokyo, Japan, (2)NIED National Research Institute for Earth Science and Disaster Prevention, Tsukuba, Japan, (3)The University of Tokyo, Earthquake Research Institute, Tokyo, Japan, (4)California Institute of Technology, Seismological Laboratory, Pasadena, CA, United States, (5)Université de Strasbourg-CNRS, Institut de Physique du Globe de Strasbourg, UMR-7516, Strasbourg, France, (6)California Institute of Technology, Pasadena, CA, United States
Recorded Presentation
Abstract:
Almost every decade, anomalous volcanic earthquakes repeatedly occurred at submarine caldera with around 8.0 km of diameter near Torishima Island in Japan, which are often called Torishima earthquakes (e.g. Fukao et al., 2018, Sci. Adv.). Despite their intermediate-class seismic magnitudes Mw 5–6, all the Torishima earthquakes generated larger tsunamis than expected from the seismic magnitudes; for example, the 2015 Torishima earthquake (Mw 5.7 on 2 May 2005 in UTC; GCMT) caused tsunamis with a peak amplitude of ~60-cm at Hachijojima Island. Considering their volcanic origins and efficient tsunami excitations, they are regarded as volcanic tsunami earthquakes. We aim to determine the physical mechanism of volcanic tsunami earthquakes, which has been unclear so far.

In order to explore a kinematic source model of the 2015 Torishima earthquake, we analyzed its tsunami records of 24 ocean-bottom-pressure gauges (epicentral distance: 90–1,430 km) and long-period (60–250 s) seismic records of stations from F-net and the Global Seismographic Network (epicentral distance: 1­–27°). Consequently, we successfully obtain a kinematic source model explaining both of the records quantitatively. The model consists of a 3.2­-m thrust slip on an inwardly down-dipping ring fault extending partially along the caldera rim, and asymmetric opening (2.3 m) and closing (3.2­ m) of a sub-caldera horizontal crack lying at 2.3 km of depth. We interpret the model as an abrupt thrust slip on the partial ring fault induced by highly pressurized magma inside the crack, which may be equivalent to the trapdoor faulting mechanism that had been observed geodetically at Sierra Negra caldera, Galápagos (e.g. Jónsson, 2009, Tectonophysics). We also discuss how the earthquakes repeated and generated unexpectedly large tsunamis.

In conclusion, our kinematic source modeling study provided an evidence of trapdoor faulting phenomenon at a submarine caldera for the first time. We revealed active volcanism at a submarine volcano without any in-situ observation network only by remote observations of tsunami and seismic waveforms from a Mw 5–6 earthquake. Such investigations will broaden and deepen our scope to unrevealed phenomena at submarine volcanoes.

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