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Physical Geography of Northern Eurasia

Tectonics and Geology of Northern Eurasia

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Seismic Activity

Within Northern Eurasia, zones of high seismic activity occur along the southern boundaries of the territory of the FSU and coincide with regions of the modern young dissected mountain relief, which formed in Pliocene to Quaternary time (Bune and Gorshkov, 1980) (Figure 1.16).

Fig. 1.16 Seismic activity in Northern Eurasia

Within the Alpine fold belt these are, primarily, the Crimean mountains, the Caucasus, Kopetdagh, and Pamir (i.e., the outer areas of the belt). They formed as a result of the collision of the Eurasian Plate with the African-Arabian and Indian Plates together with the closing of the Tethys Ocean in the west. Over the course of the past hundred years catastrophic earthquakes have occurred (e.g., Krasnovodsk in 1895 in the western spurs of the Kopetdagh and Ashgabat in 1989 and 1948). The last one caused the total destruction of the capital of Turkmenistan, Ashgabat; and more than 100 000 lives were lost. In 1966, the capital of Uzbekistan, Tashkent, was destroyed by an earthquake (see below). In the Caucasus, destructive earthquakes have repeatedly been registered: in 1902 in Shemakha, Azerbaijan; in 1926 in Leninakan (now Gyumri), Armenia; in 1931, in Zangezur; in 1970, in Dagestan; and in 1988 in Spitak, Armenia, when over 2 5 000 people were killed and hundreds of thousands were injured. The earthquake foci in the Crimea, Caucasus, and Kopetdagh are located at a depth of 10-30 km, within the earth's crust. They are rarely deeper and only in the region of Grozny are deep foci of 100-120 km recognized.

The mountain chains of northern, middle, and southern Tien-Shan are also characterized by high seismicity; they join the thick mountain junction of the Pamir. The Tien-Shan belongs to the degrading mountain systems, which formed during the late Cenozoic at the site of the Epi-Paleozoic fold area, a part of the Urals-Okhotsk belt, characterized in the Mesozoic and most of the Cenozoic period by levelled, flat relief. In the mountain ridges zone of Tien-Shan and the Pamir junction, in particular, strong earthquakes occur.

Eastwards, the belt of higher seismicity spreads into the southern regions of Kazakhstan, Altay, and Eastern Sayan, which are characterized by the contrasted relief and high gradients of modern tectonic movements.

These mountain ridges, with altitudes of 2000-2500 m, some up to 4000 m, also formed in the late Cenozoic at the site of the Paleozoic fold systems and belong to the degrading mountains. The area of high seismicity extends along Lake Baikal and the whole of the western Transbaikalia, representing the mid-mountain country with dissected relief and deep rifts, within the limits of which Lake Baikal proper is located. Intensive modern tectonic movements, connected with the continuing formation of the Baikal rift, cause strong earthquakes. Thus, across the whole of Inner Asia, coinciding with the areas of recent late Cenozoic orogenesis, the belt of increased dissipated seismicity extends, connected with the crushing of the Eurasian continental lithospheric plate by its continuing collision with the African-Arabian and Indian lithospheric plates.

Vast areas of the western part of the Pacific fold belt within the limits of the Verkhoyansk, Chersky and Momsky mountains, Sette-Daban Ridge, Kolyma uplift and others are also active seismic regions. The earthquakes are registered within the limits of the Laptev Sea, where the epicentre zone serves as an extension of the spreading of the Gakkel Ridge. The earthquakes occur under extension. The Chersky zone is the most seismic-ally active, where the earthquakes are related to the compression that occurs along the boundary of the converging Eurasian and North American Plates. The eastern regions of Northern Eurasia within the limits of Kamchatka, the Kuril islands, and Sakhalin are characterized by very high seismisity. Along the eastern shore of Kamchatka and the Kuril islands the inclined seismofocal Benioff zone is well traced up to a depth of 500-600 km, and results from ocean litho-sphere subduction beneath the continent at the rate of 4-7 cm a year. On the ocean floor, the downsinking of the ocean lithosphere is expressed by the deep-sea Kuril-Kamchatka trough, which begins near the Komandorskiye islands, and joins the Aleutian deep-sea trough almost at a right angle. At this latitude the northernmost active volcano, Shiveluch on Kamchatka, is located. Earthquakes, occurring in the ocean, cause tidal waves which break on the shore and cause great damage. The Benioff zone stretches southwards along the Japanese islands. The Sakhalin island is seismically active; the January 1996 catastrophic earthquake in the north of the peninsula is indicative of it. The earthquake in Neftegorsk levelled the town and caused great human loss. The littoral zone of Sikhote-Alin and south of the Maritime Province are also seismically hazardous.

The vast areas of the East European and Siberian platforms, as well as the Urals and West Siberian Plate, are practically aseismic. Only within some of the regions, such as the littoral of the White Sea and the middle course of the Kama river, are weak earthquakes registered. The latter is probably of technogenic origin, caused by the filling of a large reservoir or by the pumping out of oil and gas from a number of deposits on the Volga-Urals anticline.

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