QUICK AFTERSHOCK RELOCATION OF THE 1994 SHIKOTAN EARTHQUAKE AND ITS FAULT PLANES

The 1994 Shikotan earthquake of Mw 8.2 occurred near the western Kurile trench where the Pacific plate is subducting beneath the North American plate. Although its mechanism is thrust-type; it is not a typical low-angle thrust event with a nodal plane parallel to the plate boundary that occurs at the plate boundary. In order to determine which of the two nodal planes of the main shock was the fault plane, we relocated the main shock and all aftershocks with M>5.3 using a modified joint hypocenter determination method. Earthquakes on the northwestern side of the aftershock area including the main shock are located along a plane dipping east-southeastward with a dip of 70 degrees. This plane is about 80 to 150 km long and 70 km wide. The main shock Occurred at the north-northeastern bottom of this plane, which is almost parallel to one of the two nodal planes of the main shock determined by Harvard University, which has a strike of N51 degrees E and a dip of 76 degrees. Thus we conclude that this plane is the fault plane, that the fault broke at the northeastern bottom of the fault plane and that the rupture propagated to the surface and also to the southeast. In addition to the aftershocks aligned with this fault plane, there is a secondary alignment, which is subparallel to the main fault plane and about 50 km east, where the largest aftershock of Mw 7.3 occurred on Oct. 9. Since Several aftershocks occurred on this secondary fault plane on the same day as the main shock, it seems reasonable to suppose that this secondary fault plane was generated together with the main fault plane at the time of the main shock.