Focal mechanisms and stress field in the Atotsugawa fault area, central Honshu, Japan

We have determined 151 high quality focal mechanism solutions for earthquakes that occurred between January 2005 and December 2006 in and around the Atotsugawa fault area in central Honshu, Japan. We used P-wave first motion polarity data observed by a dense temporary seismic observation conducted in the area by the Japanese University Group. The types of obtained focal mechanism solutions are predominantly strike-slip, however, some earthquakes exhibit reverse-and normal-fault type focal mechanisms. Without regard to faulting type, the averaged directions of compressional (P) and extensional (T) axes are rather uniform, N70 degrees W and N16 degrees E, respectively. We found that not a small number of normal-fault type earthquakes occurred in a small cluster near the central part of Atotsugawa fault, and in a very short period from the end of March to the beginning of April in 2005. In order to estimate stress field around the fault, we applied a stress tensor inversion method to the focal mechanism solutions. Both the maximum principal stress (sigma(1)) and the minimum principal stress (sigma(3)) axes are almost horizontal and trend N72 degrees W to N77 degrees W and N14 degrees E to N20 degrees E, respectively. The direction of sigma(1) and the fault trace form an angle of 43 degrees-48 degrees. It is clear that the sigma(1) axis is neither perpendicular nor parallel to the Atotsugawa fault, indicating strong coupling of fault. The stress tensor inversion also suggests local extensional stress field at a deep (>8 km) central part of Atotsugawa fault. We present a hypothesis that the extensional stress is caused locally in a transition zone from the fluid-rich aseismic creep zone to the seismogenic zone that is interpreted as an asperity ruptured by the 1858 Hietsu Earthquake (M = 7.0).