Ayumu Mizutani , Diego Melgar 

On October 8, 2023, at 21:40 UTC (6:40 on October 9 local time), a tsunami warning was declared for the Izu Islands and southwest Japan. This tsunami was initially considered to be associated with the M4.9 earthquake at 20:25 UTC (5:25 JST). However, we know events of this magnitude are far too small to generate observed tsunamis from coseismic deformation alone. In this study, we analyzed the ocean-bottom pressure records of DONET and S-net, real-time cabled observation networks on the Pacific coast of Japan. We find that the dominant period of this tsunami was relatively short, 250 sec, and that the largest tsunami occurred at 21:13 (6:13 JST) near Sofu-gan volcano. In addition, T waves, or the ocean-acoustic waves, were clearly observed by DONET – we posit these correspond to a vigorous earthquake swarm at the same region of the tsunami source. We formally invert for the tsunami source and find that several tsunami sources with an interval of about 4 min are necessary to reproduce the observed records. These most likely correspond to volcanic eruptions.