CHARACTERISTICS OF MODERN HORIZONTAL MOVEMENTS IN THE CENTRAL SECTOR OF THE GREATER CAUCASUS ACCORDING TO GPS OBSERVATIONS DATA AND THEIR RELATIONSHIP WITH THE NEWEST TECTONICS AND DEEP STRUCTURE OF THE CRUST

В статье освещены результаты впервые выполненных GPS-измерений вдоль геодезического профиля, пересекающего все основные геологические структуры в Осетинском регионе Большого Кавказа. Интерпретация результатов измерений выполнена в сопоставлении с результатами неотектонических исследований и данными о глубинном строении. Максимальное уменьшение современных скоростей поперечного сжатия зафиксировано в области наивысшего подъема к земной поверхности низкодобротного объема земной коры, оказывающего распирающее действие на все горное сооружение. Наличие значимых поперечных смещений поверхности отражает сдвиговую составляющую по крупнейшим разломам кавказского простирания. Полностью подтвердилось прогнозированное ранее существенное снижение скорости горизонтальных движений по окончании афтершокового процесса катастрофического Рачинского землетрясения 1991 г. на южном склоне Большого Кавказа. The article highlights the results of the first performed GPS-measurements along the geodetic profile crossing all the main geological structures in the Ossetian region of the greater Caucasus.The interpretation of the measurement results was performed in comparison with the results of neotectonic studies and data on the deep structure.The maximum decrease in the current transverse velocity of compression rates is recorded in the area of the highest rise to the earth's surface of the low quality volume of the earth's crust, which has a bursting effect on the entire mountain structure.The presence of significant transverse displacements of the surface reflects the shear component along the largest faults of the Caucasian strike.The previously predicted significant decrease in the rate of horizontal movements at the end of the aftershock process of the catastrophic Racha earthquake of 1991 on the southern slope of the greater Caucasus was fully confirmed.

MACROSEISMIC EVIDENCE OF SEISMIC EVENTS CAUSED BY INFLUENCE OF GROUND CONDITIONS AND FORMATION OF MAPS OF SEISMIC MICROZONATION

Проявление каждого ощутимого землетрясения несет в себе уникальную информацию о его реальном проявлении на исследуемой территории. Поэтому сбор и анализ всех имеющихся данных о проявлении сейсмически событий на исследуемой территории является важным по оценке сейсмической опасности. В работе рассмотрено проявление наиболее сильного землетрясения, произошедшего в современное время – Рачинского землетрясения 29 апреля 1991 г. на территории г. Амбролаури. Показано усиление сейсмического эффекта, обусловленного влиянием грунтовых условий. Рассмотрено проявление слабого землетрясения 2005 г. на территории г. Владикавказа и выполнено сопоставление с действующей картой сейсмического микрорайонирования. Использование современных информационных технологий позволило по признакам проявления слабых воздействий выделить отдельные участки, в которых сейсмический эффект может быть обусловлен индивидуальными локальными особенностями грунтовых условий, а также спектральными особенностями воздействия и состояния застройки Macroseismic evidence of each significant earthquake carries a unique information about its actual impact on the investigated territory. Therefore, the collection and analysis of all available data on the occurrence of seismic events in the study area is important in assessing seismic hazard. The paper shows the manifestation of the strongest earthquake that occurred in modern times – the Racha earthquake on April 29, 1991 in the territory of Ambrolauri city. The increase in the seismic effect due to the influence of ground conditions is shown. The manifestation of a weak earthquake in 2005 in the territory of the city of Vladikavkaz is considered and a comparison is made with the current seismic microzoning map. The use of modern information technologies made it possible, on the basis of manifestations of weak effects, to single out individual areas in which the seismic effect can be caused by individual local features of ground conditions, as well as the spectral features of the impact and building conditions

TECTONIC POSITION, SISMOLOGICAL AND GEOLOGICAL MANIFESTATION OF 7 SEPTEMBER 2009 ONI?II EARTHQUAKE FOCUS ON THE SOUTHERN SLOPES OF THE GREAT CAUCASUS

В статье приведены сейсмологические и сейсмотектонические материалы о главном толчке и афтершоках Онийского-II землетрясения 7 сентября 2009 г. с Мs = 5,8 на южном склоне Большого Кавказа. Положение облака эпицентров основного толчка и афтершоков совпадает с северной ветвью очаговой зоны Рачинского землетрясения 29.04.1991 г. с МS = 7,0, I0 = 7-8. Глубина гипоцентра основного толчка составляет 8?15 км. В качестве действующей в очаге принята пологая плоскость, погружающаяся в север – северо-восточном направлении. Тип подвижки по такой плоскости – надвиг с компонентами правостороннего сдвига. Сейсмодислокации носили вторичный, гравитационный характер. Результаты палеосейсмологические исследований, проведенных в восточной части эпицентральной области, Рачинского землетрясения, показали, что в этом сейсмической очаге и раньше происходили сильне сейсмические толчки. Согласно полученным данням возраст предыдущего сильного землетрясения в Рача-Джавской зоне (т. е. до 1991 г.) – около 2000 лет назад. Еще одно, болем древнее событие произошло около 6000 лет назад. Период повторяемости сильних землетрясений, подобных катастрофе 1991 г., таким образом, составляет в среднем 2000-3000 лет. The article provides seismological and seismotectonic materials about the main shock and aftershocks of the Oni-II earthquake of 7 September 2009, with MS = 5,8 on the South slope of the Greater Caucasus. The position of the cloud of epicenters of the main shock and aftershocks coincides with the northern branch of the focal zone of 29.04.1991 Racha earthquake, MS = 7,0, I0 = 7?8. The focal depth of the main shock is 8 to 15 km. As the active in the focus adopted the sloping plane, plunging to the North – North-East direction. Type progress on such a plane – thrust with component of right-lateral strike-slip. Seismodislocationswere of secondary gravitational nature. The results of paleoseismological studies conducted in the Eastern part of the epicentral area of the Racha earthquake, showed that this seismic source the strong seismic shocks happened before. According to the obtained data, the age of the previous strong earthquake in the Racha – Dzhava zone (i.e., before 1991) – about 2000 years ago. Another, more ancient event occurred about 6,000 years ago. The recurrence period of strong earthquakes, similar to the disaster of 1991, thus, is an average of 2000?3000 years.

InSAR observations of the 2009 Racha earthquake, Georgia

Central Georgia is an area strongly affected by earthquake and landslide hazards. On 29 April 1991 a major earthquake (Mw = 7.0) struck the Racha region in Georgia, followed by aftershocks and significant afterslip. The same region was hit by another major event (Mw = 6.0) on 7 September 2009. The aim of the study reported here was to utilize interferometric synthetic aperture radar (InSAR) data to improve knowledge about the spatial pattern of deformation due to the 2009 earthquake. There were no actual earthquake observations by InSAR in Georgia. We considered all available SAR data images from different space agencies. However, due to the long wavelength and the frequent acquisitions, only the multi-temporal ALOS L-band SAR data allowed us to produce interferograms spanning the 2009 earthquake. We detected a local uplift around 10 cm (along the line-of-sight propagation) in the interferogram near the earthquake's epicenter, whereas evidence of surface ruptures could not be found in the field along the active thrust fault. We simulated a deformation signal which could be created by the 2009 Racha earthquake on the basis of local seismic records and by using an elastic dislocation model. We compared our modeled fault surface of the September 2009 with the April 1991 Racha earthquake fault surfaces and identify the same fault or a sub-parallel fault of the same system as the origin. The patch that was active in 2009 is just adjacent to the 1991 patch, indicating a possible mainly westward propagation direction, with important implications for future earthquake hazards.

InSAR observations of the 2009 Racha earthquake, the Republic Georgia

Central Georgia is an area strongly affected by earthquake and landslide hazards. On 29 April 1991 a major earthquake (Mw = 7.0) struck the Racha region in the republic Georgia, followed by aftershocks and significant afterslip. The same region was hit by another major event (Mw = 6.0) on 7 September 2009. The aim of the study reported here was to utilize geodetic data as synthetic aperture radar interferometry (InSAR) to improve a knowledge about the spatial pattern of deformation due to the earthquakes in the seismic active central Georgia. There were no actual earthquake observations by InSAR in Georgia. We used the multi-temporal ALOS L-band InSAR data to produce interferograms spanning times before and after the 2009 earthquake. We detected a local uplift around 10 cm in the interferogram near the earthquake's epicenter whereas evidence of surface ruptures could not be found in the field along the active thrust fault. We simulated a deformation signal which could be created by the 2009 Racha earthquake on the basis of local seismic records and by using an elastic dislocation model. The observed InSAR deformation is in good agreement with our model. We compared our modeled fault surface of the September 2009 with the April 1991 Racha earthquake fault surfaces, and identify the same fault or a sub-parallel fault of the same system as the origin. The patch that was active in 2009 is just adjacent to the 1991 patch, indicating a possible mainly westward propagation direction, with important implications for future earthquake hazards.