Recent change of stress field in North China derived from station composite focal mechanisms

Two great earthquakes occurred on driving-force boundary of North China tectonic block: the 12 May 2008 Wenchuan M8.0 earthquake & the 11 March 2011 Tohoku M9.0 earthquake. Based on the event observatory report from 2008 to 2014 in North China, which are provided by China Earthquake Networks Center, we pick first motion polarities at the station, and calculate composite focal mechanism for it. From these results, we firstly discussed the faulting model of regional tectonic stress in North China, and found that it agrees with conclusions given in geology. Then we defined R to be a weighted summation of inconsistencies between observed and calculated polarities, and found that variations of R value with regions might reflect some characteristics in local seismicity. Taking Tohuku earthquake as a demarcation line, two 3-year periods are separated. Comparing results from the former periods and the latter, we found that stress fields in the middle-east part of North China become more concentrate after 2011, and Tancheng—Lujiang fault zone and Yanshan—Bohai seismic zone are obviously affected by the Tohoku earthquake. We finally found that, the descend of station inconsistencies ratio R is corresponding to regional middle-strong or remarkable event, which implies that these changes might reflect the seismogenic process of strong shocks.