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

The Neftegorsk 1995 May 27 earthquake (M-S = 7.6) occurred in the northern part of Sakhalin Island in a region considered to be a fairly inactive plat e boundary between the North American and Eurasian plates. Coseismic surfac e ruptures are associated with the Upper Piltoun fault, a secondary feature joining the Sakhalin-Hokkaido and Middle Sakhalin faults, which are the ma in tectonic elements of the region. Observations obtained during a field ex periment that included a local seismic network, neotectonics and geodetic m easurements are combined with the analysis of satellite images and a broad- band body wave inversion. A complex source model consisting of four branche s is proposed. The two main branches, oriented N-S, show right-lateral stri ke-slip motion. The rupture nucleates between these two segments and propag ates bilaterally. We estimate a total rupture length of 46 km, a width of 1 2 km, and an average slip value of 3.9 m. However, the maximum observed val ue 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 rot ating clockwise with respect to Eurasia.