The Shikotan Earthquake of October 4, 1994 Lithospheric earthquake

A large Mw--8.2 earthquake occurred off Shikotan Is., one of the Kurile Is., on October 4, 1994. We inverted 32 body-wave records to determine the rupture pattern using an iterative deconvolution method. The mechanisms of the subevents were allowed to vary during rupture. The source parameters obtained are: the location of the initial break = (43.48øN, 147.40øE); the centroid depth = 56 km; (strike, dip, rake) = (49 ø, 75 ø, 125 ø) for the total source; the seismic moment Mo = 2.6x1021 Nm (Mw--8.2); source time duration T = 42 s; the average rupture velocity v = 2.5 km/s. We also determined the mechanism using long-period Love and Rayleigh waves from 14 stations. The solution for a finite source distributed over a depth range from 0 to 90 km is (strike, dip, rake) = (54 ø, 76 ø, 129 ø) with Mo = 2.3x1021 Nm, in good agreement with that from body waves. Referring to the extent of the aftershock area and the subevent distribution, we estimated the fault area S•120x60krn 2, the average slip D=5.6m, and the stress drop Ac•--11 MPa. We computed synthetic waveforms as well as static displacements using either the steep or the low-angle plane as the fault plane, and found that the steep-dip fault model fits the data better. Our result (the mechanism, large centroid depth, high stress drop) strongly suggests that the 1994 Shikotan earthquake is a lithospheric earthquake: an intra-plate event that ruptures through a substantial part of the subducting oceanic lithosphere. This type of lithospheric earthquake is relatively common.