Aseismic and seismic slip around hypocenter of the giant Off-Tokachi earthquakes

1. Introduction On September 11, 2008, a large thrust earthquake with magnitude (JMA) of 7.1 occurred off Tokachi, southeastern Hokkaido, Japan, located near hypocenter of the giant 1952 and 2003 Off-Tokachi earthquakes. It should be noted that the very-low frequency (VLF) earthquakes were detected in/around source area of 2008 event. To understand the stress accumulation process around hypocenter of the giant earthquakes, we estimated the spatio-temporal aseimsic distribution along inter-plate boundary using the repeating earthquake. Then, combining the co-seismic fault motions, we examined the distribution of dislocation along plate boundary at various stages. 2. Data and method We used waveform data of the high-sensitivity seismograph network of Japan (Hi-net) operated by the National Research Institute for Earth Science and Disaster Prevention (NIED) from October 2000 to July 2009 to find the repeating earthquakes and analyze the interplate slip history near the hypocenter of the 2008 event.. The method used to identify the repeating earthquakes is the same as that applied by Matsubara et al. (2005). The repeating earthquake information is then analyzed to infer the aseismic slip distribution on the plate boundary. We also used distant body waves observed at FDSN and GSN networks stations (IRIS-DMC archive) to clarify the coseismic slip of the 2008 event. 3. Result Near the coseismic region of the 2008 event, 80 repeating earthquakes, clustered in 22 groups, have been identified. The aseismic slip rate was accelerated just after the 2003 event and the slip amount reached approximately 25 cm at the end of October 2004. We interpret this aseismic slip acceleration as afterslip due to the 2003 earthquake. From November 2004 to September 2008, the slip rate is significantly accelerated compared to a “normal”, time-decaying afterslip background. The slip rate increased in November 2004 (Period 1), June 2005 (Period 2), March 2006 (Period 3) and February 2007 (Period 4). Two moderate earthquakes with moment magnitudes of 6.3 and 6.2, respectively, occurred at approximately 50km from the target region on November 11,2004 (Period 1) and on February 17, 2007 (Period 4), respectively, however, in the other two periods (i.e., Period 2 and 3), there was no significant large earthquake near the target region. Before the Periods 2 and 3, the VLF earthquake activity (Asano et al, 2008) became high. The VLF earthquake activity may accelerate the slip rate at the plate boundary. 4. Conclusion The 2008 event broke the plate interface on the southeastern (shallower) side of the coseismic region of the 2003 interplate event. Afterslip of the 2003 event decreased at the end October 2004. After that, four significant accelerations of slip rate were observed. Two accelerations were accompanied by two moderate events (Mw ~ 6.2), while the other two accelerations were accompanied by high seismicity of very-low frequency earthquakes.