Horizontal motions and the generation of strong earthquakes in the North Sakhalin interiors

The recent geodynamics of Sakhalin Island is best described by the convergence of the Eurasian and North American (Sea of Okhotsk) lithospheric plates, which is manifested in the high seismic activity of the island. In North Sakhalin, the plate boundary is thought to correspond to a system of roughly N-S-trending faults, which belong to the North Sakhalin deep fault, and the Upper-Piltun fault; the latter was ruptured by the 1995 M 7.2 Neftegorsk earthquake. This study first confirmed that the stationary motion of the Sea of Okhotsk plate is retarded on this fault to form with time a series of drag folds and stress field anomalies. The latter are released during the subsequent (in a 400⦒o 1000-year period) strong earthquakes by seismic sliding on the flanks of the Upper Piltun fault. The 2003–2006 GPS observations revealed the free state of this fault zone with relative slip rates of 5–6 mm/yr.