THE NEFTEGORSK, SAKHALIN ISLAND, EARTHQUAKE OF 27 MAY 1995

Past seismic catastrophes were unknown in Sakhalin Island before 1995 except tho se suggested from findings of paleoseismo dislocations. The first time that; dwellers have experienced such a catastrophe in the Sakhalin Island history was on 27 May 1995. The devastating Neftegorsk earthquake occurred in Northern Sakhalin (phi = 52.8 degrees north; l ambda = 143.2 degrees east; H = 18 km; M-s = 7.2), killed almost 2000 people in the small city of Neftegorsk, caused damage and destruction of buildings, bridges, railways and roads, breakage of oil and gas pip elines, electric and communication lines, and was accompanied by large -scale surface phenomena within a source area. It was felt all over th e Sakhalin Island, as well as over the closest part of the Eurasian co ntinent. Surface fracturing was the most impressive effect of the Neft egorsk earthquake. The 37-km long, light-lateral strike-slip fault, wi th a strike of north 15 degrees east and a horizontal displacement up to a maximum of 8 m, has been observed from Taxon Mountain at the sout h to the junction of the Cadylanye and Keniga Rivers at the north. Acc ording to the results of a detailed geological survey and study of the aftershocks, the total extent of the source area was similar to 80 km . Various secondary phenomena have been observed at the Earth's surfac e, such as landslides, falls, soil liquefaction, mud volcanoes etc. Th e earthquake was followed by hundreds of aftershocks within the follow ing 1-2 months. Spatially, the earthquake fault coincides with the pre -existing Upper Piltun fault, known earlier from geological studies. R ecent high activity of the latter fault has been recognized only after the Neftegorsk event because of findings of traces of significant pas t dislocations within the fault zone. From a tectonic viewpoint it can be suggested that the Upper Piltun fault is a Riedel-type shear fract ure located between two main regional faults: the Gyrgylanye-Dagy faul t at the west and the Piltun-Ekhaby fault at the east. Therefore, its present activity, expressed by the destructive Neftegorsk earthquake, seems to be explained by a long strain accumulation within a broad zon e of regional light-lateral shear faulting.