Rupture process of the 2007 Nevelsk, Sakhalin, MW 6.2 mainshock and MW 5.8 aftershock

An M_w 6.2 earthquake occurred on August 2, 2007 in the Tatar Strait about 15 km away from the city of Nevelsk on Sakhalin Island and it was followed by four consecutive M 5 aftershocks within 8 hours. The mainshock also generated tsunami waves that were recorded by tide gauges located around Hokkaido (Japan) and Russia, which is unusual for such earthquakes of this size and shallow shelf depth. After the earthquakes, uplifts of up to 0.8 m were observed along the coastal area of the Nevelsk town which is close to the aftershock epicenters. These earthquakes raise questions as to how the rupture processes of the moderate-sized earthquakes can cause such a relatively large tsunami and uplift. To understand the spatial and temporal characteristics of the events, we invert teleseismic body waves of the mainshock and the M_w 5.8 event which was the largest aftershock. Teleseismic records with the epicentral distances of 30° ~ 90° are selected. Long period P waves are considered on the vertical component and S waves on the transverse components. We calculated the surface displacement of each event using slip distributions obtained from the inversions to compare with the observed uplifts. We found that asperities of the mainshock distributed along the top of a long and narrow rupture, while the fault plane of the aftershock is approximately square in shape with smaller dimensions than that of the mainshock. The fault model of the mainshock drew reasonable tsunami simulation results and this showed that the mainshock has been associated with one fault segment. The results of surface displacements suggest that the uplifts off the Nevelsk town were more affected by the aftershocks than the mainshock.