L. P. Imaeva

Seismicity of the Bering Strait region: Evidence for a Bering block

Newly compiled Russian and U.S. seismological data support an independent Bering block in motion relative to the North American plate. This motion is likely to be driven by the westward extrusion of southwestern Alaska, resulting from compression in southern Alaska due to subduction of the Pacific plate and terrane accretion. Seismicity extends from central Alaska, through the Bering Strait, and into Chukotka. In eastern Chukotka several southwest trends are evident, some of which continue through the Koryak Highlands to Kamchatka. The seismicity outlines the Bering block, which includes most of the Bering Sea, Chukchi Peninsula, Seward Peninsula, and parts of western Alaska. Focal mechanisms, young basaltic volcanism, and normal faults in western Alaska and Chukotka indicate that the Bering Strait is under northeast-southwest extension. This, in conjunction with thrust faulting in the Koryak Highlands, indicates that the Bering block is rotating clockwise relative to the North American plate.

Explosion Contamination of the Northeast Siberian Seismicity Catalog: Implications for Natural Earthquake Distributions and the Location of the Tanlu Fault in Russia

Russian regional seismicity catalogs, including those in the annual “Earthquakes of the USSR,” are contaminated by industrial explosions. In northeastern Russia, explosions occur in tin, coal, and gold mines, as well as in the construction of roads, railways, and dams. Most seismically recorded mining- and construction-related explosions have magnitudes of about 2.0 and occur during local daytime. In addition, explosions in placer mining areas are concentrated from midwinter to early spring, when frozen placers are broken up for the summer processing season. We analyzed the temporal variation of over 87,000 events occurring in northeast Russia using a newly compiled seismicity catalog to identify areas where there may be explosion contamination. Areas with temporal biases indicative of mining or other explosions include the Yana River delta and Chukotka (placers), the southern Amur district (coal mining), the trace of the Baikal–Amur railroad (construction), Lazo (quarry), the south Yakutian gold fields, and the Kolyma gold belt. The locations, and estimates of the level, of explosion contamination of the catalog suggest that the natural seismicity may be lower, and not as diffuse, along the plate boundaries in northeastern Russia than previously thought. Use of only nighttime events from the seismicity catalog, which should have a minimum of explosions, helps to clarify the extension of the Tanlu fault into Russia and may ultimately help elaborate tectonics in other areas of eastern Russia.

Seismotectonic analysis of the Yana-Indigirka segment of the Chersky zone

Kinematic types of seismogenerating structures, morphotectonic features of the present-day topography, and the corresponding tectonic stress fields derived from the focal mechanisms of local earthquake and the types of Late Cenozoic folding and faulting are analyzed to create a model of current geodynamics in the Yana-Indigirka segment of the Chersky seismotectonic zone. The kinematic environment in the Yana-Indigirka segment of the Chersky seismotectonic zone is a result of the interaction of conjugate differently oriented strike-slip displacements under transpression conditions (a compression with strike-slip). It is possible to reconstruct these conditions, if we assume that the Kolyma-Omolon block (superterrain) located in the frontal part of the North American Plate played the role of an active indenter during the approach of the North American and Eurasian Plates.

Structural dynamic analysis of the epicentral zone of the Ilin-Tas earthquake (Feb 14, 2013, Ms = 6.9)

The paper presents the results of analysis of tectonics, modern relief morphology, active faults, and types of Cenozoic deformations in the epicentral area of the Ilin-Tas (Ms = 6.9) earthquake, one of the strongest events registered at the boundary of the Eurasian and North American lithospheric plates in northeast Russia. Geological, tectonic, and geophysical characteristics of the Yana-Indigirka segment of the Chersky seismotectonic zone are studied. The methods of investigation were elaborated at the Institute of the Earth’s Physics, RAS (Moscow), and were adapted to conducting seismotectonic work on the territory of northeast Russia. The results of instrumental observations are summarized, and manifestations of strong seismic events are discussed. Description is given of the structural–tectonic setting in which the Andrei-Tas seismic maximum developed. It originated under the effect of the Kolyma-Omolon terrane (indentor) which “intruded” into the Chersky seismotectonic zone on the side of the North American plate, thus leading to the formation of major seismoactive structures in the frontal Ilin-Tas folded zone. The indentor moved in NE-SW direction, which is consistent with the orientation of the major axis of isoseism ellipses (azimuth 50–85°) constructed from observation of macroseismic effects of the Uyandina (Ms = 5.6), Andrei-Tas (Ms = 6.1), and Ilin-Tas (Ms = 6.9) earthquakes.

The Stress–Strain State of Recent Structures in the Northeastern Sector of the Russian Arctic Region

Complex research to determine the stress–strain state of the Earth’s crust and the types of seismotectonic destruction for the northeastern sector of the Russian Arctic was conducted. The principles of regional ranking of neotectonic structures were developed according to the activity of geodynamic processes, and argumentation for their class differentiation is presented. The structural-tectonic position, the parameters of the deep structure, the system of active faults, and the tectonic stress fields, calculated on the basis of both tectonophysical analysis of discontinuous and folded late Cenozoic deformations and seismological data, were analyzed. This complex of investigations made it possible to determine the directions of the main axes of deformations of the stress–strain state of the Earth’s crust and to reveal the regularity in the change of tectonic regimes.

Seismotectonic studies of the pleistoseist area of the Ms = 6.9 Ilin-Tass earthquake in northeast Yakutia

The complex seismotectonic studies of the pleistoseist area of the Ilin-Tas earthquake (Ms = 6.9), one of the strongest seismic events ever recorded by the regional seismic network in northeastern Russia, are carried out. The structural tectonic position, morphotectonic features of present-day topography, active faults, and types of Cenozoic deformations of the epicentral zone are analyzed. The data of the instrumental observations are summarized, and the manifestations of the strong seismic events in the Yana–Indigirka segment of the Cherskii seismotectonic zone are considered. The explanation is suggested for the dynamical tectonic setting responsible for the Andrei-Tas seismic maximum. This setting is created by the influence of the Kolyma–Omolon indenter, which intrudes into the Cherskii seismotectonic zone from the region of the North American lithospheric plate and forms the main seismogenic structures of the Yana–Indigirka segment in the frontal zone (the Ilin-Tas anticlinorium). The highest seismic potential is noted in the Andrei- Tas block—the focus of the main tectonic impacts from the Kolyma–Omolon superterrane. The general trend of this block coincides with the orientation of the major axis of isoseismal ellipses (azimuth 50°–85°), which were determined from the observations of macroseismic effects on the ground after the Uyandina (Ms = 5.6), Andrei-Tas (Ms = 6.1), and Ilin-Tas (Ms = 6.9) earthquakes.

Seismotectonic destruction of the Earth’s crust in the northeastern flank of the Baikal Rift Zone

481 The peculiarities of the geodynamic development and the mechanism of formation of morphotectonic structures in the northeastern flank of the Baikal Rift Zone (BRZ) remain poorly studied due to the lack of actual data on the fracture structure, fields of tectonic stresses, and geophysical characteristics of the medium. This study is aimed to distinguish the mod ern structural tectonic map and types of seismotec tonic destruction in the most active seismic segment of the BRZ (its northeastern flank) and the dynamically conjugated system of the seismic generating structures of the West Stanovoi tectonic block.

Formation dynamics of fault-block structures in the eastern segment of the Baikal-Stanovoi seismic belt

The combined analysis of geological-geophysical characteristics of active faults, morphotectonic features of the relief, and data of satellite measurements and on seismicity in the southeastern Aldan-Stanovoi block (the eastern flank of the Olekma-Stanovoi seismotectonic zone) made it possible to reconstruct the kinematic development of recent falt-block structures with the defining conjugate systems of main seismogenerating faults located in the zone of dynamic influence of three major lithospheric plates: Eurasian, Amur, and Sea of Okhotsk.

Transpressional structures of the eastern segment of Aldan-Stanovoi block

According to seismotectonic analysis, the territory of the AldanoStanovoi block is the least understood, and, actually, it has not been studied with the use of structuralkinematic models of the modern tectonic process. Analysis of the geological-geophysical char� acteristics of active faults, morphotectonic peculiari� ties of relief, the data set of satellite measurements, and seismic activity made it possible to determine the kinematic plan of development of modern faultblock structures and determine the systems of the main seis� mogenerating faults, located in the contact zone of dynamic affection of three large lithosphere plates: Eurasian, Amur, and Okhotomorsk.