Afterslip of the 14. August 2001 Earthquake (MW 6.4) Occurring Near the Northern Asperity of the 1968 Tokachi-oki Earthquake off the Pacific Coast of Northern Honshu, Japan

Abstract

We investigated the rupture process of 2001 August 14 earthquake (MJ 6.2) that occurred at the eastern tip of a seismic gap off the Pacific coast of Aomori, northern Honshu, Japan. An average slip of 11cm on a fault size of 25×35km^2(MW=6.4) was estimated as the coseismic slip by inverting waveform data recorded by the K-Net. A detailed analysis of daily site positions of GPS stations from GEONET allowed us to find post-seismic horizontal displacements of several millimeters with a decaying-time constant of 2.2 months. The post-seismic deformation was explained by an afterslip that took place mainly on the northern periphery of the northern asperity of the 1968 Tokachi-oki earthquake (Mw 8.2). The seismic moment of the afterslip is estimated to be about three times greater (MW 6.7) than that of the coseismic slip. We infer that the afterslip has increased the earthquakegenerating stress in the northern asperity that has been considered a seismic gap since the occurrence of the 1994 Sanriku-haruka-oki earthquake (Mw 7.6). This slow event was not accompanied by any salient seismic activity