Detailed Crustal Strain Distribution Around Hypocentral Regions of Large Earthquakes and Its Temporal Change

An earthquake is a release process of strain energy stored in the crust. Investigation of stress and strain around the source region of large earthquakes is important in understanding their generation mechanism. Based on the dense GPS observation in Japan, for example, Sagiya et al. (2000) found a good agreement between the large strain rate areas, and the source regions of historical large earthquakes. On the other hand, Sagiya et el. (2002) reported that the 2000 Western-Tottori earthquake (M7.3) occurred in a small strain rate area. Therefore, it is not so straightforward to say that a large earthquake is apt to occur in a large strain rate area. However, the discussion of Sagiya et al. (2002) was based on a smoothed strain rate field by averaging surrounding area for several tens of kilometers. In this study, we estimate the detailed strain distribution and its temporal change around the hypocentral region. We calculate a strain rate for unit triangle of a triangulation network composed of the continuous GPS sites. In the hypocentral region of the 2000 Western-Tottori earthquake, we find that a unit triangle corresponding to the hypocenter has anomalously large (0.3ppm/yr) NW-SE compression. Furthermore, the amplitude of this compressional strain was decreasing toward the occurrence of the Western-Tottori earthquake. From detailed analysis of temporal changes in strain components using the state-space modeling, a significant decrease starts about 300 days before the main shock. As another example, the source region of the 2004 Niigata-Chuetsu earthquake (M6.8), showed a similar tendency of decreasing strain rate toward the main shock. These results imply the next two possibilities. First, an area with large strain rate may become a source region of future large earthquakes. Second, around the future source region of large earthquakes, the strain rate may decrease gradually toward the earthquake occurrence. If these are recognized as a common character for large earthquakes, there is a possibility that we can specify place and time of future large earthquakes.