[S-SS03] Seismological advances in the ocean

[SSS03-06] Evidence of a shear zone inside the Philippine Sea plate slab in Suruga Bay, provided by long term OBS observations.

*Yannis Panayotopoulos¹、Hisatoshi Baba²、Takahito Nishimiya³ (1.Association for the Development of Earthquake Prediction、2.Tokai Univershity、3.Meteorological Research Institute, Japan Meteorological Agency)

キーワード：Ocean Bottom Seismometers, Seismicity , Suruga Bay, Nankai Trough , Shear Zone, Tectonics

Suruga Bay lies along the Tokai segment of the Nankai-trough, which last ruptured in the 1854 M8.4 Ansei-Tokai Earthquake, making it a potential locus for the next major earthquake on the Nankai-trough. Moderate size earthquakes have occurred in the Suruga Trough axial region (M6.5 in 2009 and M6.2 in 2011) that resulted in a prominent excitation of seismicity. Unfortunately, our ability to accurately locate these events is hampered by a sparse routine seismic observation network around the bay. In order to improve location accuracy, a total of 2 sets of 7 OBSs were deployed inside Suruga Bay, 6 from the spring of 2017, and an additional OBS from spring 2019. By combining the data from our OBS stations with that of existing land-based stations surrounding Suruga Bay, we were able to accurately relocate the observed seismic activity inside the Philippine Sea plate (PHS). We observed and analyzed a total of 755 events reported in the JMA unified catalogue that occurred within a 25 km radius of the OBS stations. We combined our data with that of routine network stations in a 50 km radius around the Bay and manually picked the P- and S- wave arrivals. We improved the location accuracy of hypocenter determinations by using a pseudo-3D location scheme that assigned a different 1D velocity structure to each station depending on the geological characteristics of the area upon which it sits, and additionally by solving iteratively to estimate station corrections from the calculated average RMS station delays. The relocated hypocenters appear to occur inside a narrow 2-5 km continuous zone that starts on the east side of the bay at approximately 10 km depth and deepens towards the west at an angle of 20°~25°. Analysis of the focal mechanisms of the observed events shows that the P-axis inside Suruga Bay is almost exclusively oriented in a northerly direction, in contrast to the northwest subduction direction of the PHS, which suggests that that the stress field in the area is not favorably oriented for thrust faulting on the Tokai segment. We propose the existence a shear zone subparallel to the subduction direction of the PHS with a trend approximately 35° counterclockwise from north. The net movement of Izu peninsula observed in the GNSS data can be explained by the presence of a zone on which strain partitioning occurs between its northern and southern parts. This shear deformation zone acts as a region of strain release and explains the apparent difference in the angle of movement of the Izu peninsula and the angle of convergence of the PHS. This zone could have important implications in the case of a future Tokai earthquake because it could function as a buffer that
would impede the fault rupture process.