Seismotectonic deformation and crustal stress pattern have been studied comprehensively in major seismogenic structures of the Kharaulakh sector of the Verkhoyansk fold system and adjacent parts of the Chersky seismotectonic zone. The study focuses on neotectonic structures, deep structure, and systems of active faults, as well as tectonic stress fields inferred by tectonophysical analysis of Late Cenozoic faults and folds. The results, along with geological and geophysical data, reveal main strain directions and structural patterns of crustal stress and strain in the Arctic segment of the Eurasia–North America plate boundary. The area is a junction of mid-ocean and continental structures evolving in a mixed setting of extension, compression, and their various combinations. The rotation pole of the two plates is presumably located near Buor-Khaya Bay. In this case, extension is expected to act currently upon the neotectonic structures north of the bay and compression to control those in the south and southeast. This inference is consistent with the identified zoning of stress and strain in the Kharaulakh sector.
Abstract:The paper presents results of seismogeological studies of active faults bordering the Upper Kerulen basin, one of the largest intermountain basins of the Khentei upland. Morphometric and trenching methods were applied to estimate the main parameters of seismogenic dislocations and associated Holocene palaeoearthquakes (540-2810, 3170-3720, and 7480-9220 years ago). The maximum palaeoearthquake magnitude (7.5) characterizes the seismic capacity of the potential focal area (PFA) confined to the Kerulen fault. The new data show the need to revise the potential seismicity concepts of the southern Khentey area and to make appropriate changes in the general seismic zoning maps. The relationship between the dislocations and the modern topography features, as well as deformation of the subsurface sediments in the studied mine openings give evidence of thrusting under the sub-lateral to northwestern subhorizontal compression.Key words: Khentei; Kerulen seismogenic structure; morphometry; trenching; absolute age of a paleoearthquake Аннотация: Представлены результаты сейсмогеологических исследований активных разломов, ограничи-вающих Верхнекеруленскую впадину -одну из крупнейших межгорных впадин Хэнтейского нагорья. Мор-фометрией и тренчингом определены основные параметры сейсмогенных дислокаций и связанных с ними трех палеоземлетрясений, произошедших в течение голоцена (в интервалах 540-2810, 3170-3720 и 7480-9220 лет назад). Максимальная магнитуда палеоземлетрясений (7.5) характеризует сейсмический потенциал приуроченной к Керуленскому разлому зоны вероятных очагов землетрясений (зоны ВОЗ).
Abstract:In the Pribaikalie and adjacent territories, seismogeological studies have been underway for almost a half of the century and resulted in discovery of more than 70 dislocations of seismic or presumably seismic origin. With commencement of paleoseismic studies, dating of paleo-earthquakes was focused on as an indicator useful for long-term prediction of strong earthquakes. V.P. Solonenko [Solonenko, 1977] distinguished five methods for dating paleoseismogenic deformations, i.e. geological, engineering geological, historico-archeological, dendrochronological and radiocarbon methods. However, ages of the majority of seismic deformations, which were subject to studies at the initial stage of development of seismogeology in Siberia, were defined by methods of relative or correlation age determination.Since the 1980s, studies of seismogenic deformation in the Pribaikalie have been widely conducted with trenching. Mass sampling, followed with radiocarbon analyses and definition of absolute ages of paleo-earthquakes, provided new data on seismic regimes of the territory and rates of and recent displacements along active faults, and enhanced validity of methods of relative dating, in particular morphometry. Capacities of the morphometry method has significantly increased with introduction of laser techniques in surveys and digital processing of 3D relief models.Comprehensive seismogeological studies conducted in the Pribaikalie revealed 43 paleo-events within 16 seismogenic structures. Absolute ages of 18 paleo-events were defined by the radiocarbon age determination method. Judging by their ages, a number of dislocations were related with historical earthquakes which occurred in the 18th and 19 th centuries, yet any reliable data on epicenters of such events are not available. The absolute and relative dating methods allowed us to identify sections in some paleoseismogenic structures by differences in ages of activation and thus provided new data for more accurate definitions of epicenters and magnitudes of the paleo-earthquakes. In some cases, it was revealed that neighboring dislocations of seismogenic structures, which were previously considered independent, had been subject to simultaneous opening.The article presents a new approach to selecting regression equations to estimate paleo-magnitudes with regard to specific geodynamic conditions as well as to levels of available knowledge on seismodislocations and reliability of available data parameters.
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