Spatio-temporal evolution of postseismic slip associated with the 2007 Niigataken Chuetsu- oki Earthquake (M6.8) as inferred from GPS data

The Mid Niigata Prefecture, which is called Chuetsu district, locates in the strain accumulated zone named Niigata-Kobe Tectonic Zone (NKTZ). The crust is contracting along NW-SE direction, and must be compressed in same direction around Chuetsu district. Historically, several damaging earthquakes have occurred reflecting this regional stress state in this area, for instance, the Mid Niigata Prefecture Earthquake in 2004 (M6.8) occurred on 23 October 2004. The Niigataken Chuetsu-Oki Earthquake in 2007 (M6.8) occurred on 16 July 2007 in this area, about 40 km northwest to the hypocenter of the Mid Niigata Prefecture Earthquake in 2004. The focal mechanisms determined by seismic wave analyses clarified that reverse faults whose strikes direct NE-SW generated this earthquake. we report the results of a GPS network near the epicenter of the Niigataken Chuetsu-Oki Earthquake in 2007 that includes GEONET's sites and GPS sites established by the Japanese UNiversity COnsortium of GPS research (JUNCO). We present the spatio-temporal evolution of the postseismic slip estimated by an inversion analysis based on observed GPS data. The result shows that postseismic slip occurred in and around the co-seismic fault rather than at downdip extension, and it decayed to insignificant level within two weeks after the main shock had occurred. Next, we developed and applied new time dependent inversion method in which the length of each time step is variable in order to express the process of decay of the postseismic slip more accurately because we can not change the length of time step in an analysis as far as we use the method formulated by Yagi and Kikuchi (2003). The whole analysis period can be optimally splitted based on ABIC, however it is necessary to calculate ABICs for all splitting cases. We overcame the difficulty of prodigious requirements for calculation cost by utilizing a genetic algorithm. We will present the resultant postseismic slip evolution estimated by the new inversion method.