The Neftegorsk, Sakhalin Island, earthquake of 27 May 1995

Abstract Past seismic catastrophes were unknown in Sakhalin Island before 1995 except those suggested from findings of paleoseismodislocations. 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 (?= 52.8° north; δ= 143.2° 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 pipelines, electric and communication lines, and was accompanied by large-scale surface phenomena within a source area. It was felt all over the Sakhalin Island, as well as over the closest part of the Eurasian continent. Surface fracturing was the most impressive effect of the Neftegorsk earthquake. The 37-km long, right-lateral strike-slip fault, with a strike of north 15° east and a horizontal displacement up to a maximum of 8 m, has been observed from Taxon Mountain at the south to the junction of the Cadylanye and Keniga Rivers at the north. According to the results of a detailed geological survey and study of the aftershocks, the total extent of the source area was - 80 km. Various secondary phenomena have been observed at the Earth's surface, such as landslides, falls, soil liquefaction, mud volcanoes etc. The earthquake was followed by hundreds of aftershocks within the following 1-2 months. Spatially, the earthquake fault coincides with the pre-existing Upper Piltun fault, known earlier from geological studies. Recent high activity of the latter fault has been recognized only after the Neftegorsk event because of findings of traces of significant past dislocations within the fault zone. From a tectonic viewpoint it can be suggested that the Upper Piltun fault is a Riedel-type shear fracture located between two main regional faults: the Gyrgylanye-Dagy fault 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 zone of regional right-lateral shear faulting.     

Possibility of the reconstruction of the real dynamics of the Earth’s crust (The example of the southern regions of Sakhalin and Kuril Islands)

Doklady Earth Sciences -     

Improvements on the magnitude-geographic criterion of tsunami hazard

We suggest supplementing the MLH magnitude with the threshold (M _thr) values of MPV, MSH, and MLH magnitudes (Russian scales), as well as M _S and M _W now in wide international use, for issuing tsunami alerts for hazards emanating from the main tsunamigenic zones of the Pacific Ocean. Relations are given to connect the MLH to these magnitudes. A comparative analysis applied to a catalog of large (M ≥ 6) earthquakes in the North Pacific and to the associated tsunami catalog gave the probabilities of false alerts and unpredicted tsunamis as functions of the threshold magnitude value (M _thr). A two-step decision rule is proposed to issue tsunami alerts due to the tsunamigenic zones situated close to the Far East coast of Russia.     

Deep structure and seismotectonics of the Southern Sea of Okhotsk region along a profile from southern Sakhalin to the southern Kuril Islands

The results of deep seismic profiling through Southern Sakhalin, the southern Sea of Okhotsk, and the Southern Kuril Islands allowed the identification of deep fault zones, the hypocenter locations, the features of the stress state, and the types of seismic dislocations at the earthquake sources. The east side of the fault was upthrown relative to the west side beneath the southern part of the Tatar Strait and Sakhalin Island and led, as a result of multiple thrusting events along the fault over the geologic history, to the rise and 5-8 km displacement of the seismic boundaries. The true uplift of the Greater Kuril arc block was determined using the focal mechanism solutions. The seismoctectonics and present-day dynamics of the crustal blocks were estimated using a detailed joint analysis of the position of the structural boundaries at the seismic section and the seismotectonic movements according to the earthquake focal mechanisms.     

The aftershock process and mechanisms of the May 30, 2004 Kostromskoe earthquake,Sakhalin

The results of the instrumental and macroseismic studies are reported for the tangible earthquake with intensity of up to 5–6 and amplitude of MLH = 4.8 that occurred near the western coast of Sakhalin Island. The main parameters of the Kostromskoe earthquake have been estimated in two versions: (1) based on the data from the local network of digital stations located in southern Sakhalin, and (2) from the complex of local, regional, and global observations. It has been noted that the development of the local network in southern Sakhalin allowed the seismic regime in the earthquake area to be investigated in more detail and the mechanisms of both the individual weak and group events to be derived. The acquired data on the dislocation style of the main shock and aftershocks in the days following the event were used for the geological-tectonic interpretation of the Kostromskoe earthquake.