Seismic source mechanism of the tsunami near the Sofu Seamount on October 8, 2023

Abstract

On October 8, 2023, between 18:50 and 22:00, a cluster of earthquakes occurred near Sofu Seamount, and a tsunami advisory was issued for the surrounding areas. Previous studies showed that the main source of this tsunami was an uplift near the Sofu Seamount and the tsunami could be simulated by the summation of the multiple sources over time. To clarify the seismic mechanism of this volcanic activity, we performed the seismic waveform inversion and cross-correlation analysis. We obtained the source-time function of the 13 repeated earthquakes and found that the source mechanism of the tsunamigenic event was a force symmetric about the vertical axis, such as single-force, isotropic, or vertical compensated-linear-vector-dipole (CLVD) mechanisms. We also found that there were low-frequency harmonic signals between earthquakes, which do not come with clear body waves. We interpreted these signals as a caldera collapse and resonance of the magma reservoir. The repeated earthquakes can be interpreted as the vertical movement of the rock column, supplying magma to the magma chamber and causing the uplift of the caldera floor that excited tsunamis. The harmonic signals between earthquakes may be the resonance at the magma reservoir associated with the magma migration. We assumed that the edifice subsided slowly between earthquakes and repeated this inflation-deflation cycle along with the earthquakes.