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

The 1994 Shikotan earthquake of Mw 8.2 occurred near the western Kuril e trench where the Pacific plate is subducting beneath the North Ameri can plate. Although its mechanism is thrust-type; it is not a typical low-angle thrust event with a nodal plane parallel to the plate bounda ry that occurs at the plate boundary. In order to determine which of t he two nodal planes of the main shock was the fault plane, we relocate d 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 p lane dipping east-southeastward with a dip of 70 degrees. This plane i s about 80 to 150 km long and 70 km wide. The main shock Occurred at t he north-northeastern bottom of this plane, which is almost parallel t o 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 degree s. 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 rupt ure propagated to the surface and also to the southeast. In addition t o the aftershocks aligned with this fault plane, there is a secondary alignment, which is subparallel to the main fault plane and about 50 k m east, where the largest aftershock of Mw 7.3 occurred on Oct. 9. Sin ce Several aftershocks occurred on this secondary fault plane on the s ame day as the main shock, it seems reasonable to suppose that this se condary fault plane was generated together with the main fault plane a t the time of the main shock.