Stress Variation in North China before and after the 2011 Tohhoku-Oki Earthquake: Results and Implications

We determine the focal mechanism solutions (FMS) of earthquakes located in two different regions in North China: Northeast China (NEC) and Central North China Craton (CNCC), during 2009-2013 using the generalized Cut and Paste method (gCAP, Zhu & Ben-Zion, 2013). Moreover, we also employ gCAP to obtain the centroid depths of those earthquakes. Broadband waveforms from 298 stations including NECESSArray (2009-2011), NECsaids (2010-2013) and permanent records from CEA (2009-2012) are used to invert for the focal mechanism solutions. We employ the gCAP method to obtain not only similar results to the GCMT solutions for moderate earthquakes, but more importantly reliable FMS for events at the level of Mw≈3.0. We also use the FMS to invert for the stress field in both regions using the software SATSI (Hardebeck & Michael, 2006) and compare the results before and after the March 11th, 2011 Tohoku-Oki earthquake, to better understand how this great event influenced the stress fields in NEC and CNCC. Results show that the overall compression in E-W direction is lowered in both regions after the March 11th, 2011 Tohoku-Oki earthquake. However, the back azimuths of these two regions are quite different, which leads to distinct influences from the March 11th, 2011 Tohoku-Oki earthquake, therefore the changes in seismicity and FMS in those two regions also differ from each other.