RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 10, ES5003, doi:10.2205/2007ES000268, 2008
[19] The tectonic position of the common earthquake swarm can be revealed in the context of the keyboard block structure of the Earth's crust in active continental margins [Yarotskii, 1974, 1976, 2000]. The structure was formed by the system of transregional northwestern deep faults, transverse in relation to longitudinal regional structures of the region. The faults divide the crust into transcrustal blocks that embrace all three layers. They cut the continental margin from the Pacific coast across land and Sea of Okhotsk and grade into the system of longitudinal dislocations in Mesozoic structures of the Kolyma region. At the southeastern termination the blocks are in tense tectonic regime provided by processes in the continent-ocean transition zone. Certain blocks are in the state of emergence and are manifested in the shoreline configuration as peninsulas, others undergo subsidence (or slow emergence) and are represented by the bays. In transverse section the emerging blocks show up as deep wedges; the submerging, as deep trapeziums. Within the emerging blocks the granite metamorphic layer is presumably decreased with the overwhelming thickness of the basaltic one; in the downfaulted blocks the increased thickness of the granite metamorphic layer is inferred. This conclusion is favored by emerging blocks of the southeastern Kamchatka capes, the Shipunskii, Kronotskii, Kamchatskii Mys, and Ozernyi Peninsulas, and complex of the Il'pinskii, Il'pyrskii, and Govena Peninsulas, which surface is mainly composed of basalts or andesite-basalts with portions of intermediate and acidic volcanites and which are characterized by carbonated ground water. The subsiding blocks are represented by the southeastern Kamchatka bays, Avachinskii and Kronotskii Gulfs, and by the southwestern Litke Strait; liparites, dacites, and rhyolites are mainly distributed on their surface. One further peculiarity of subsided blocks is the occurrence of manganese in geologic complexes of the Avachinskii and Kamchatskii Gulfs, Litke Strait, and in southeastern Koryak bays (Opukhi Bay), as well as the widespread nitric ground waters [Yarotskii, 2000].
[20] In the scheme of the keyboard block structure the discussed territory of the Khailino and Olyutorka earthquakes is associated with the subsiding block of the Olyutorskii Bay. The block is bounded in the southwest by the Paren'-Talov-Tilichiki transverse interblock fault; in the northeast, by the Omolon-Kamensk-Olyutorka fault. The area of the common earthquakes swarm is cut by the fault zone directed from the northwest along the Vetrovayam River through the Village Tilichiki and Skobeleva Harbor towards the southeastern coast of the Govena Peninsula. In the northeastern part of the block the fault goes from the Kamenskoe Village along the Belaya River, across the low reaches of the Naivalvayam River to the confluence of the Vyvenki River with the tributaries Vakhavinitapvayam and Irochivivayam Rivers. Between these faults the Tilichiki, Khailino, Uvalistaya, and Inochvivayam areas occur. The fifth area characterized by one aftershock is located outside the zone of the Omolon-Kamensk-Olyutorka fault, northeastward of it, in the adjacent emerged block of the Olyutorskii Peninsula. It is marked by lifted up margins of the blocks that show subvertical movements in different directions.
[21] The above-cited MTZ records [Moroz, 1987] on deep depressions at the top of the Upper Cretaceous basement within the riftogenic Vyvenki depression, provided the estimate of deep structures in the southeast of the Olyutorskii Bay block. It should be noted that the extent of the Vyvenki depression along the 6-km isopach (long axis) is four times greater than its length at the Upper Cretaceous top, i.e. the depression is widened with depth along both axes. This is just the evidence for trapezium-like deep transverse section of the subsiding block of the Olyutorskii Bay.
[22] The generation of transverse interblock faults was synchronous with initiation of the planetary regmatic net. The continental margin of the western Pacific mobile belt and the southern Cordilleras in the east are characterized by the presence of diagonal jointing net with the longitudinal northeastward and transverse northwestward directions. The distance between interblock faults is determined by the thickness of a deformed body with its extension [Shafranovskii and Plotnikov, 1975]. There is evidence that the Koryak continental margin was formed under stretching conditions. Maximum distances occur on relaxation of geodynamic stress at the boundaries of blocks with thick granite metamorphic layer, since there, according to the keyboard block concept, the greatest crust thickness is inferred, as observed in blocks of the southeastern Kamchatka bays, Avachinskii and Kronotskii Gulfs, southwestern Litke Strait, and Olyutorskii Bay. On relaxation of mechanical stress the faults are generated in all crustal layers of ultimate size, i.e. in structures of localized ideal homogeneous solid material; that is, the systems of transverse dislocations arise, similar to the deep ones but at shorter distances. In the discussed region, in upper structural storeys this system was recorded at the adjacent, west of the Olyutorskii Bay, emerging block of the Il'pinskii, Il'pyrskii, and Govena Peninsulas; it is manifested in the form of northwestern linear stripes of alternating various-amplitude blocks, i.e. local horsts and grabens. The stripes 15-20 km wide are oriented northwest (310-330o) and extend for tens of kilometers. On the adjacent territory southeast of the Khailino and Olyutorka earthquakes swarm, in the Machevna-Anivayam interfluve basin these structures are recorded as a series of linear intrusions stretched northwestward (310-315o). The analogous system of intracrustal faults shown up on the surface also occurs in the Vyvenki River valley in the form of faults separating five areas of the earthquakes epicenter clusters. These are the faults of the Vetvei, Levtyrinyvayam, Ogiranvayam, Navkyrvayam, Kailinovayam, and Maini-Lulovayam River valleys. They are followed on both sides of the Vyvenki River valley for tens of kilometers and some of them are traced by the river network, alteration of geologic complexes, and geophysical fields, and further in the adjacent structural-and-formational zones.
[23] Considering the position of the Khailino and Olyutorka earthquakes' common area in the keyboard block structure of the crust we should note that the northwestern faults of different depth represent planes of horizontal movements of adjacent blocks, which indicates their young age. However, they occurred at least from the Jurassic, so they are renewed and are active up to the present. There is evidence of probable horizontal shifting of adjacent blocks. For instance, the Vyvenki deep fault structure that plays a role of sutural zone, changed its extension from submeridional (40o) in the Anapka River valley to that of 60o in the Vetrovayam-Vetvei interfluve, in the area of the Vyvenki, Vakhavnitvayam, and Inochvivayam rivers confluence. The axis of the Koryak Range is shifted in the Vetrovayam-Vetvei interfluve. Examples of this sort, manifested in gravimetric and magnetic fields, in geomorphology and geology in the southwestern Koryak Highland, are quite common.
[24] The subsiding keyboard block of the Olyutorskii Bay in the active continental margin evidently tends to slip down to the ocean. At the boundary of the zone that corresponds to the Vyvenki deep fault depression representing an axis zone of both earthquakes' area, there arises a possibility of overhanging the oceanic bottom by the block's front in the form of four local blocks defined by peculiarities of Khailino and Olyutorka earthquakes clusters. The fifth block belongs to the adjacent emerging block of the Olyutorskii Peninsula and can be considered as a lifted up portion of two contiguous blocks. The weakest point at the block's termination is the Khailino block area, which is associated with the greatest troughs in the Upper Cretaceous and crystalline basements. It is precisely the area characterized by destructive earthquakes with densely concentrated foreshocks and aftershocks of both earthquakes.
[25] The proposed model of earthquake generation in the Vyvenki River basin at a distance of 65-90 km from the Bering Sea shore agrees with a tense fault tectonic regime in the continent-ocean transition zone of the Earth's crust. Major structural directions of seismic events are represented there by northeastern and northwestern plans. The northeastern plan is representative of the Earth's crust building up process at the active continental margin, with about 31-km-thick crust and relatively thick granite metamorphic layer. The northwestern plan is reflected in deep transverse dislocations that form the keyboard block of the Earth's crust as a whole [Yarotskii, 1976, 2000] and local keyboard blocks (as defined by [Lobkovskii, 1988]) of its upper storeys. The depth of the keyboard block base can be inferred from the depth of the Khailino earthquake [Lander et al., 1991]. According to some estimates, it is 17 km, i.e. an intracrustal depth of decoupling of local keyboard blocks; according to another assessment, it constitutes 25-50 km, that is a transcrustal depth of spalling away of the whole subsiding Olyutorskii Bay block. The small depth (4 km) of the Olyutorka earthquake indicates its association with local keyboard blocks. The earthquake occurred in the region, adjacent on the west to the Khailino area that is characterized by the maximum downwarping of the Upper Cretaceous and crystalline basements.
[26] The earthquakes' initiation scheme inferred from the occurrence of keyboard blocks at the continental margin, was proposed in 1988 [Lobkovskii, 1988] and illuminates the generation of transverse epicenter areas of the Khailino and similar earthquakes. However, this scheme should be supplemented with the mechanism of participation in preparation and in the event, as such, of not single keyboard block but of several ones. This is apparent from the discussed tectonically related position of the Khailino and Olyutorka earthquakes' five epicenter areas considered as blocks. The scheme is also supplemented by the author's concept on keyboard block structure of the Earth's crust, which was formulated and developed since 1974 [Yarotskii, 1974, 1976, 2000]. On the other hand, the conception by [Lobkovskii, 1988] is a particular verification of our concept and their synthesis will provide the better insight into further events in the southwestern Chukchi seismic belt.
[27] The general conclusion of this paper is that the northeastern extension of the Khailino and Olyutorka earthquakes area marks the edge of the continent from which the key of the Olyutorskii Bay subsiding block is breaking off. The decoupling block includes local keys, namely, the blocks of northwestern extension that slide over intracrustal planes at the subsiding block margin. It is possible that sliding of the whole block is characterized by subcrustal earthquakes and that of its keyboard blocks, by the earthquakes in superficial thrust sheets. The development of observation network in the southwestern Koryak Highland will produce the evolving of more perfect models of intracontinental earthquakes.
Citation: 2008), Tectonic position of the Khailino and Olyutorka earthquakes, Russ. J. Earth Sci., 10, ES5003, doi:10.2205/2007ES000268.
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