The Neftegorsk (Sakhalin Island) 1995 earthquake: a rare interplate event

Summary The Neftegorsk 1995 May 27 earthquake (MS = 7.6) occurred in the northern part of Sakhalin Island in a region considered to be a fairly inactive plate boundary between the North American and Eurasian plates. Coseismic surface ruptures are associated with the Upper Piltoun fault, a secondary feature joining the Sakhalin–Hokkaido and Middle Sakhalin faults, which are the main tectonic elements of the region. Observations obtained during a field experiment that included a local seismic network, neotectonics and geodetic measurements are combined with the analysis of satellite images and a broad-band body wave inversion. A complex source model consisting of four branches is proposed. The two main branches, oriented N–S, show right-lateral strike-slip motion. The rupture nucleates between these two segments and propagates bilaterally. We estimate a total rupture length of 46 km, a width of 12 km, and an average slip value of 3.9 m. However, the maximum observed value of surface slip is 8.1 m, an unusually large value for an event of this magnitude. This earthquake supports the model of a North American plate rotating clockwise with respect to Eurasia.