Effects of the 2011 Mw 9.0 Tohoku-Oki Earthquake on the Locking Characteristics and Seismic Risk of the Yishu Fault Zone in China

To ascertain the 2011 Tohoku-Oki Mw 9.0 earthquake's impact on the stability of the Yishu fault zone, this study inverts the fault locking degree and slip rate defect distribution of the Yishu fault zone using GPS horizontal velocity field data covering the period from 1999 to 2009 and from April 2011 to 2019. This is based on the block negative dislocation model. Combined with the b value and strain field characteristics, the properties of deformation of the Yishu fault zone before and after the earthquake are comprehensively analyzed. The results show that before the 2011 Tohoku-Oki Mw 9.0 earthquake, the locking degree and depth of the northern segment of the Yishu fault zone were higher, while the locking depth of the southern segment of the fault was shallower. The 2011 Tohoku-Oki earthquake produced different coseismic effects on the southern and northern sections of the Yishu fault zone. The extension on the southern section and compression on the northern section caused the strain release in the southern sections of the Yishu fault zone following the earthquake. After it, the regional locking degree of the southern section of the Yishu fault zone was relieved. However, the locking degree of the northern segment of the fault zone was still high and the depth was deep, at about 26 km. In addition, the northern section of the Yishu fault zone was a section with an abnormally low b value and small earthquakes in the northern segment are sparse at present. The coseismic compression of the northern section caused by the 2011 Tohoku-Oki earthquake is conducive to its locking and easily accumulates stress, so it is necessary to pay attention to its seismic risk.