Tohoku megathrust earthquakes and structural heterogeneity revealed by S-net and Hi-net data

Abstract:

Large earthquakes take place frequently in the Tohoku megathrust zone off Northeast Japan, in particular, the 2011 Tohoku-oki earthquake (Mw 9.0) was one of the greatest events recorded by modern instruments. This Mw 9.0 event produced a huge tsunami and caused great damage to the local society and infrastructure. However, the mechanism and rupture processes of the megathrust earthquakes in Tohoku are still a matter of controversy due to the lack of permanent near-field observations. Recently, we have used high-quality arrival-time data of local earthquakes recorded at 480 permanent stations of the Japanese Kiban seismic network on land and 120 OBS (ocean bottom seismometer) stations of the S-net that is deployed on the Pacific ocean floor off Eastern Japan. To conduct tomographic imaging of the NE Japan arc, we selected well-located local earthquakes beneath the Kiban seismic network and suboceanic events that were recorded and located by the S-net. From the high-quality vertical-component seismograms recorded by the S-net, we picked P-wave arrival times generated by the local offshore events with a picking accuracy of 0.05-0.15 s. Only arrival times of the events with well-located hypocenters (uncertainty < 3 km) are used in the tomographic inversion. The tomographic method of Zhao et al. (1992, 2011) is used to determine a high-resolution 3-D velocity model of the Tohoku subduction zone. The well-determined geometries of the Conrad and Moho discontinuities and the upper boundary of the subducting Pacific slab are considered in the velocity model. The general pattern of the obtained tomographic model is similar to that of previous models (e.g., Zhao et al., 2011; Liu and Zhao, 2018), but our new model reveals a low-velocity (low-V) anomaly near the Japan Trench off Miyagi and Sanriku. The mainshock hypocenter of the great 2011 Tohoku-oki earthquake is located at the boundary between a down-dip high-velocity anomaly and the up-dip low-V anomaly. The slow anomaly at the shallow depths near the Japan Trench may reflect low-rigidity materials that are close to the free surface, resulting in large slips, weak high-frequency radiation, and the high tsunami on March 11, 2011. Our new tomographic model can explain not only the large slips near the trench but also weak high-frequency radiation from the shallow rupture areas.