V. S. Imaev

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.

Extrusion tectonics of the Okhotsk Plate, northeast Asia

Based on focal mechanism, geologic, and lineament data, faulting in the Chersky Range, northeast Asia, changes from right-lateral transpression between North America and Eurasia to left-lateral strike-slip between North America and the Okhotsk plate. The location of the Okhotsk-Eurasia boundary is poorly constrained; available data suggest this boundary is right-lateral. The Okhotsk plate is being compressed and extruded from between North America and Eurasia resulting in high levels of microseismic activity along the southern flanks of the Chersky Range.

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.

The August 27, 2008, MW = 6.3, Kultuk earthquake effects in the Nearepicenter zone: Macroseismic survey results

In this work, the macroseismic effects of the Kultuk earthquake (MW = 6.3), which took place on August 27, 2008 in the southwestern closure of the Baikal Lake, are under consideration. The intensity of shocks in inhabited localities located in the epicentral zone reached 7–8 points on the MSK-64 scale. The earthquake was named after the local settlement of Kultuk, which was the mostly damaged area by the earthquake. The considered seismic event caused significant material damage (about 250 million rubles according to preliminary estimates). In inhabited localities of Southern Pribaikalie thousands of cases of damaged ovens and chimneys were registered. Some buildings were highly damaged and, accordingly, they are unfit for further use. The earthquake was the cause of numerous rockslides, rockfalls, and landslides on steep natural and artificial slopes. A macroseismic survey allowed us to establish the asymmetrical distribution of the intensity of shocks relative to the epicenter.

Paleoseismicity of the Pribaikalie

Results of paleoseismogeological investigations, including trenching carried out over 20 years, are shown. Absolute 14C dates of buried soils and a list of paleoseismic dislocations and paleoevents dated by different methods are also given. Unexpected results obtained for the paleoearthquake magnitude and kinematics of dislocations are discussed.

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.

Tectonic position and geological manifestations of the Mogod (Central Mongolia), January 5, 1967, earthquake (a view after 40 years)

The earthquake source, reaching the surface in the form of an extended system of faults, encompassed the N-S and NW-SE planes of two large faults near their juncture zone. A revised seismotectonic study of the system of coseismic ruptures performed after many years revealed a complex structure of primary coseismic ruptures in the juncture area of fault branches of different directions. In addition to the two major faults, the juncture zone consists of intersecting or parallel branches of both structural directions. The trench study and detailed mapping of the shallow structure of the seismic rupture characterizes it as a right-lateral-thrust fault on the N-S branch and a strike-slip-reverse fault on the NW-SE branch. Results of our paleoseismogeological study indicate that equally strong earthquakes are likely to have occurred in the same seismic source in the past (about 8000 and 160 years ago).

Recent structures and tectonic regimes of the stress–strain state of the Earth’s crust in the northeastern sector of the Russian Arctic region

A comprehensive investigation aimed at determining seismotectonic types of destruction and the stress–strain state of the Earth’s crust in the main seismogenerating structures of the Arctic–Asian seismic belt is conducted for the territory of the northeastern sector of the Russian Arctic region. Based on the degree of activity of geodynamical processes, the regional principles for ranking neotectonic structures are elaborated, and neotectonic zoning is carried out based on the substantiated differentiation of the corresponding classes. Within the limits of the Laptev Sea, Kharaulakh, and Lena-Anabar segments, we analyzed I the structural–tectonic position of the most recent structures, II the deep structure parameters, III the parameters of the active fault system, and IV the parameters of the tectonic stress field, as revealed from tectonophysical analysis of Late Cenozoic fault and fold deformations. Based on the seismological data, the mean seismotectonic deformation tensors are calculated to determine, in combination with geological and geophysical data, the orientations of the principal stress axes and to reveal the structural–tectonic regularity for tectonic regimes of the stress–strain state of the Earth’s crust in the Arctic sector of the boundary between the Eurasian and North American lithospheric 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.

Change in the Physical Properties of Rocks Resulting from Quarry Blasts in South Yakutia

In order to estimate the influence of seismic effects from quarry blasts on the state of rock massif by means of electrotomography, magnetometry, and seismological observations, the variations in the physical properties of rocks before and after massive blasts have been studied. Experimental monitoring along the geophysical profile directed across one of the regional faults crossing the quarry field of the Neryungri open-pit coal mine has revealed no significant changes in the magnetic field after blasts. The revealed features include an increase in the electrical resistivity of rocks in the blast-directed wall of the tectonic fault and a decrease in the fault zone; the change in electrical resistivity in the hanging wall is insignificant. It has been found that the change in physical properties of the rock massif is determined mostly by the state (frozen or thawed) of the rocks, rather than by the power of the blast.