The Shikotan Earthquake of October 4, 1994: Lithospheric earthquake

A large M_w=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 = (43.48°N, 147.40°E); the centroid depth = 56 km; (strike, dip, rake) = (49°, 75°, 125°) for the total source; the seismic moment M_o = 2.6×10^(21) Nm (M_w=8.2); source time duration T = 42 s; the average rupture velocity ν = 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°, 76°, 129°) with Mo = 2.3×10^(21) 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 = 120 × 60 km², the average slip D=5.6m, and the stress drop Δσ=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.