Short-Term and Long-Term Variations in Seismic Velocity at Shallow Depths of the Overriding Plate West of the Japan Trench

Long-term and short-term temporal variations in seismic velocity have been investigated using seismic interferometry, and significant short-term changes are typically recognized as velocity reductions caused by large earthquakes. In this study, using a dense network of ocean bottom seismometers (Seafloor Observation Network for Earthquakes and Tsunamis along the Japan Trench: S-net), we measured temporal variations in seismic velocity at the top of the overriding plate in the region between the Japanese Islands and the Japan Trench. We detected abrupt velocity reductions due to the nearby Mw 7-class earthquakes in 2016 and 2021, and gradual velocity increases mainly observed near the trench. The sudden velocity reductions due to the shallow earthquake in 2016 within the overriding plate are large above the hypocenter, while those due to the three deep earthquakes in 2021 are large near the trench, although these earthquakes occurred far from the trench. The three earthquakes are located at the top of and within the subducting oceanic crust of the Pacific Plate. The spatial distributions of the velocity reductions for the four earthquakes are consistent with those of the peak ground velocities (PGVs). This indicates that for the three deep earthquakes, seismic waves trapped within the descending Pacific Plate reach to the trench, and the trapped waves induce large PGVs, and hence substantially reducing the velocity. Long-term variations observed near the trench are possibly caused by the recovery process of coseismic velocity reductions due to large earthquakes and/or fluid migration induced by the tectonic stress field in the region.