Frictional heterogeneity due to geothermal anomaly near volcanoes: Physical modeling for the 2008 Iwate-Miyagi Nairiku, Japan, earthquake

The Mw 6.9 Iwate-Miyagi Nairiku earthquake on June 14 occurred near a volcanic front in northern Honshu, provides us a unique opportunity to physically concern the effect of geothermal structures on the stressing conditions of inland earthquakes. The observed lower limit of the aftershock distribution, reflecting the base of the brittle seismogenic zone, appears to be the deepest near the epicentral location of the hypocenter, near the middle of the estimated main-shock rupture area, and to become gradually shallower to south and steeply to north along strike. It is thought that the geothermal gradient determines the transition depth of the brittle upper crust and the ductile lower crust. There are quaternary active volcanoes near the ends of the rupture area, in which low S-wave velocity anomalies are also situated deep down More over, these aftershocks and coseismic slip estimated by seismic and geodetic inversions appear to be distributed keeping away from such low velocity zones. It should be fare to consider that these high temperature zones make the thermal anomaly that course the along-strike variation in the basement depth of the seismogenic zone. In this study we try to explain several characteristic phenomena in the aftershock distributions and the coseismic dynamic rupture process in terms of a basic physical model called the "gdeep slip"h model (e.g. Scholz, 2002; Tse and Rice, 1986), consisting of an unstable frictional fault in the upper crust and an underneath localized ductile shear zone in the lower crust. In order to consider the geothermal anomaly along the fault, we assume a dipping fault embed in the 3-D half space with horizontal variation in the frictional property related to the brittle-ductile transition determined by the geothermal structure. As for the computation, we develop the current model including the quasi-static stressing process and dynamic rupture process based on the boundary integral equation method (BIEM). Regarding the treatment of the free surface in BIEM, while the quasi-static process is modeled by an existing analytical expression (Okada, 1992), in the dynamic process, the surface is numerically treated as a virtual crack with the zero traction condition. Referring the aftershock distribution, we assume a non-uniform depth distribution in the brittle-ductile transition along strike. With the simulation, we find the rupture is nucleated at the deepest portion in the brittle zone and propagated mainly toward south. The slip also tends to be concentrated in the shallow depth as the rupture propagates to south. These calculation results seem to reproduce the overall observed characteristics of this event.