A Strain Event Related to Aftershock Activity Following the 2003 Tokachi-oki (M8.0) Earthquake

On September 25, 2003 at 19:50 (UTC) a great thrust earthquake occurred off Tokachi (Tokachi-oki), the junction of the Kuril and the Japan trenches; many aftershocks were recorded. For aftershocks with magnitude >4.0, the dominant active period was from 25th September to 11th October. Almost all the aftershocks are thrust faults on the plate boundary. They did not occur in the large stress-drop zone, but surrounding the main shock (Ito et al., 2004). The pattern suggests that many aftershocks on the plate boundary were triggered by stress increase due to non-uniform rupture process of the main shock (Ito et al., 2004). A Sacks-Evertson borehole strain meter is located 105 km from the epicenter of the main shock at azimuth 300 degrees. The sensor is at a depth of 110m in a borehole at Urakawa Seismological Observatory (KMU) of Hokkaido University in the southern part of the Hidaka Mountains. The strain data showed rapid contraction during the period of high seismic activity with the strain data closely resembling the plot of cumulative number of aftershocks. Furthermore, daily displacement values from a temporary GPS survey (Takahashi et al., 2004) show similar changes. We examine possible mechanisms for the relation between the aftershock activity and the deformation changes observed during the beginning of the aftershock sequence.