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- Title
Short‐Term and Long‐Term Variations in Seismic Velocity at Shallow Depths of the Overriding Plate West of the Japan Trench.
- Authors
Tonegawa, T.; Takagi, R.; Sawazaki, K.; Shiomi, K.
- Abstract
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. Plain Language Summary: Seismic interferometry is a powerful tool for estimating temporal variations in seismic velocity. We applied this technique to the continuous records observed at a dense network of permanent ocean bottom seismometers deployed around the Japan Trench and examined temporal variations in seismic velocity at the shallow depths of the overriding plate. As a result, we observed sudden velocity reductions associated with the nearby Mw 7‐class earthquakes in 2016 and 2021. The region with large velocity reductions in the 2016 earthquake is above its hypocenter, while large reductions in the cases of the three 2021 earthquakes are observed near the trench, although their epicenters are distant from the trench. The depth of the 2016 earthquake is 25 km and those of the three 2021 earthquakes are 50–60 km. These results indicate that seismic waves trapped within the subducting Pacific Plate in the case of deep earthquakes reach the trench and induce large sudden reductions due to strong seafloor motions. Long‐term variations in seismic velocity were also observed near the trench, which are caused by the recovery process of earthquakes and/or fluid migration induced by the tectonic stress field. Key Points: We examined temporal variations in seismic velocity at shallow depths of the overriding plate around the Japan TrenchSeismic waves trapped within the subducting plate cause velocity reductions far from the epicentersLong‐term variations are caused by the recovery processes of coseismic velocity reductions and/or fluid migration due to the tectonic stress field
- Subjects
JAPAN; SEISMIC wave velocity; SURFACE waves (Seismic waves); TSUNAMIS; TRENCHES; SEISMIC waves; OCEAN bottom; OCEANIC crust; EARTHQUAKES
- Publication
Journal of Geophysical Research. Solid Earth, 2023, Vol 128, Issue 1, p1
- ISSN
2169-9313
- Publication type
Article
- DOI
10.1029/2022JB025262