Abstract
The east coast of the Tohoku district, Japan has a high seismicity, including aftershocks of the 2011 M9 Tohoku earthquake. We analyzed 1142 earthquakes with 4.4 le M_{W} le 5.0 that occurred in 2003 through 2018 and obtained spatio-temporal pattern of stress drop on the Pacific Plate that subducts beneath the Okhotsk Plate. Here we show that small earthquakes at edges of a region with a large slip during the 2011 Tohoku earthquake had high values of stress drop, indicating that the areas had a high frictional strength and suppressed the coseismic slip of the 2011 Tohoku earthquake. In addition, stress drops of small earthquakes in some of the areas likely decreased after the 2011 Tohoku earthquake. This indicates that the frictional strength decreased at the areas due to the following aftershocks of the 2011 Tohoku earthquake, consistent with a high aftershock activity. This also supports that the frictional properties on a subducting plate interface can be monitored by stress drops of small earthquakes, as pointed out by some previous studies.
Highlights
Tectonics and characteristics of earthquakes off the east coast of the Tohoku district, Japan Numerous large earthquakes as well as the huge 2011 Tohoku earthquake with MW 9.0 have been observed off the east coast of the Tohoku district, Japan, associated with the subduction of the Pacific Plate beneath the Okhotsk Plate at a rate of 80–100 mm/year (DeMets et al 1990)
Both of the results derived from P and S waves indicate the quite similar pattern of spatial heterogeneity on stress drop (Fig. 3e, f ), suggesting that the frictional properties on the Pacific plate are heterogeneous in space
This would indicate a gradual weakening of the shear strength due to the stress concentration associated with the coseismic slip during the 2011 Tohoku earthquake (Ohnaka and Shen 1999)
Summary
Tectonics and characteristics of earthquakes off the east coast of the Tohoku district, Japan Numerous large earthquakes as well as the huge 2011 Tohoku earthquake with MW 9.0 have been observed off the east coast of the Tohoku district, Japan, associated with the subduction of the Pacific Plate beneath the Okhotsk Plate at a rate of 80–100 mm/year (DeMets et al 1990). Nishikawa et al (2019) analyzed waveforms of slow earthquakes observed by the new S-net ocean-bottom seismic network and investigated their spatial distribution along the Japan Trench They found that the area that ruptured during the 2011 Tohoku earthquake was bounded by areas that have large numbers of slow earthquakes. Baba et al (2020) detected very low frequency earthquakes (VLFEs) off the Hokkaido and Tohoku Pacific coasts by a matched-filter technique They pointed out that their spatial distribution is consistent with the afterslip of the 2003 Tokachi-Oki earthquake ( MW 8.0). They found that the VLFE activity inside a large coseismic slip area of the 2011 Tohoku earthquake was low thereafter, whereas outside the area, VLFE activity increased after the 2011 Tohoku earthquake These results suggest that there is significant spatial heterogeneity of the frictional properties on the plate interface in this region
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
