Seismotectonic study of the Fergana region (Southern Kyrgyzstan): distribution and kinematics of local seismicity

We present new seismicity and focal-mechanism data for the Fergana basin and surrounding mountain belts in western Kyrgyzstan from a temporary local seismic network. A total of 210 crustal earthquakes with hypocentral depths shallower than 25 km were observed during a 12-month period in 2009/2010. The hypocenter distribution indicates a complex net of seismically active structures. The seismicity derived in this study is mainly concentrated at the edges of the Fergana basin, whereas the observed rate of seismicity within the basin is low. The seismicity at the dominant tectonic feature of the region, the Talas-Fergana fault, is likewise low, so the fault seems to be inactive or locked. To estimate the uncertainties of earthquake locations derived in this study, a strong explosion with known origin time and location is used as a ground truth calibration event which suggests a horizontal and vertical accuracy of about 1 km for our relocations. We derived 35 focal mechanisms using first motion polarities and retrieved a set of nine moment tensor solutions for earthquakes with moment magnitude (Mw) ranging from 3.3 to 4.9 by waveform inversion. The solutions reveal both thrust and strike-slip mechanisms compatible with a NW-SE direction of compression for the Fergana region. Two previously unknown tectonic structures in the Fergana region could be identified, both featuring strike-slip kinematics. The combined analysis of the results derived in this study allowed a detailed insight into the currently active tectonic structures and their kinematics where little information had previously been available

The 2008 Nura earthquake sequence at the Pamir-Tian Shan collision zone, southern Kyrgyzstan

We analyzed the 5 October 2008, Mw 6.6 Nura earthquake, which occurred in the border triangle between Kyrgyzstan, Tajikistan, and China, and its aftershock series based on locally recorded seismic data. More than 3000 aftershocks were detected and located, using a double-difference technique and a regional 3-D velocity model. Moment tensors for the main event and the 42 largest aftershocks were determined by full-waveform inversion of long-period displacement seismograms. The Nura main shock was a shallow (∼3.4 km deep) reverse faulting event and occurred on an approximately east striking rupture plane situated east of the Alai Valley, along the Pamir Frontal Thrust of the Trans Alai Range, the leading edge of the Pamir Thrust System. Its presumed rupture plane dips steeply (∼59°) southward. The aftershocks constitute several distinct clusters that can be attributed to the activation of an array of individual faults including the one that was presumably broken by the main shock. Background seismicity occurred mainly further south, behind the crest of the Trans Alai Range, in an approximately east trending zone of dextral transpressional motion in the interior of the Pamir Thrust System. We show that nearly all reactivated structures lie in regions that experienced an increase in Coulomb stress due to the main shock rupture. The Nura earthquake sequence indicates slip partitioning between north-south shortening that creates large earthquakes along the Pamir Frontal Thrust, and lateral movement in the interior of the Pamir Thrust System.This research was funded by DFG bundle TIPAGE (PAK 443), the CAME project bundle TIPTIMON funded by the German Federal Ministry of Education and Research (support code 03G0809), and GFZ. We acknowledge funding for the Earthquake Task Force deployment by GFZ and the Hannover Rück reinsurance company

Соотношение напряженного состояния земной коры, сейсмичности и оползневой активности (на примере Ферганской впадины, Тянь‐Шань)

The impacts of seismicity on the landslide activity in Kyrgyzstan have been in the focus of our study since 2010 [Kalmetyeva et al., 2010]. As the study progressed [Kalmetyeva, Moldobekov, 2012, 2013; Kalmetyeva et al., 2013, 2014], the initial problem statement has been revised as follows: do earthquake influence the occurrence of land‐ slides, and, if so, what is the mechanism of this influence. This paper presents the results of detailed analysis of the distribution of earthquakes and landslides in space and time in correlation with focal mechanisms (azimuth and plunge of the principal compression stress axis) of earthquakes occurred in the Fergana basin and its mountainous frame. These are grounds to conclude that the landslide activity is mainly influenced by the response of the internal structure of the upper crust to local stresses. The mechanism of influence of strong earthquakes on the landslide activity is redistribution of local stresses, which results from partial release of regional stresses due to rupturing in the source zones of strong earthquakes. Using this concept of the landslide activity, a methodology of landslide‐hazard mapping is the goal of our future studies aimed at the following: (1) zoning of the study area with respect to the features of the internal structure of the upper crust, (2) geological, geophysical and seismological studies of the state of stresses in the study area, (3) instrumental monitoring of landslides movements in the zones that differ in the structure of the upper crust, and (4) analysis of preparation and consequences of past strong earthquakes that took place in the study area in comparison with the landslide activity.Наши исследования были начаты в 2010 году [Kalmetyeva et al., 2010] с целью изучения влияния сейсмичности на оползневую активность в Кыргызстане. Однако в процессе исследований [Kalmetyeva, Moldobekov, 2012, 2013; Kalmetyeva et al., 2013, 2014] формулировка поставленной изначально задачи несколько изменилась, а именно: влияют ли землетрясения на возникновение оползней и, если да, то каков механизм этого воздействия. Настоящее сообщение является завершающим в решении поставленной задачи. Приводятся результаты более детального рассмотрения пространственно‐временного распределения оползней и землетрясений в сопоставлении с данными о механизмах очагов (азимут и угол погружения главной оси напряжения сжатия Р) этих землетрясений для территории Ферганской впадины и ее горного обрамления. Проведенный анализ позволяет сделать предположение, что решающее значение в оползневой активности имеет реакция внутренней структуры земной коры на локальные напряжения. Механизм воздействия сильных землетрясений на активизацию оползней состоит в перераспределении локальных напряжений, которое происходит в результате высвобождения некоторой части региональных напряжений, связанных с образованием разрыва в очаге сильного землетрясения. Такое представление об оползневой активности показывает, что основные направления исследований по разработке методики составления карт оползневой опасности должны состоять в следующем: районирование территории по внутренней структуре верхних частей земной коры (1), установка пунктов инструментальных наблюдений за движением оползней на участках территории с различающейся структурой верхних частей земной коры (2), изучение напряженного состояния территории методами геологии, геофизики, сейсмологии (3), анализ подготовки и последствий прошлых сильных землетрясений в сопоставлении с оползневой активностью (4)

Evidence for subduction of Asian continental lithosphere under the Pamir from lithospheric imaging

Abstract HKT-ISTP 2013 A

Images of crustal and mantle structure across the northern margin of the Tibet-Pamir plateau

Abstract HKT-ISTP 2013 A

Correlation of the state of crustal stresses, seismicity and landslide activity (Fergana basin, Tien Shan)

The impacts of seismicity on the landslide activity in Kyrgyzstan have been in the focus of our study since 2010 [Kalmetyeva et al., 2010]. As the study progressed [Kalmetyeva, Moldobekov, 2012, 2013; Kalmetyeva et al., 2013, 2014], the initial problem statement has been revised as follows: do earthquake influence the occurrence of land‐ slides, and, if so, what is the mechanism of this influence. This paper presents the results of detailed analysis of the distribution of earthquakes and landslides in space and time in correlation with focal mechanisms (azimuth and plunge of the principal compression stress axis) of earthquakes occurred in the Fergana basin and its mountainous frame. These are grounds to conclude that the landslide activity is mainly influenced by the response of the internal structure of the upper crust to local stresses. The mechanism of influence of strong earthquakes on the landslide activity is redistribution of local stresses, which results from partial release of regional stresses due to rupturing in the source zones of strong earthquakes. Using this concept of the landslide activity, a methodology of landslide‐hazard mapping is the goal of our future studies aimed at the following: (1) zoning of the study area with respect to the features of the internal structure of the upper crust, (2) geological, geophysical and seismological studies of the state of stresses in the study area, (3) instrumental monitoring of landslides movements in the zones that differ in the structure of the upper crust, and (4) analysis of preparation and consequences of past strong earthquakes that took place in the study area in comparison with the landslide activity

Geometry of the Pamir-Hindu Kush intermediate-depth earthquake zone from local seismic data

We present new seismicity images based on a two-year seismic deployment in the Pamir and SW Tien Shan. A total of 9532 earthquakes were detected, located, and rigorously assessed in a multistage automatic procedure utilizing state-of-the-art picking algo

Crustal and uppermost mantle velocity structure along a profile across the Pamir and southern Tien Shan as derived from project TIPAGE wide-angle seismic data

Utilizing seismic refraction/wide-angle reflection data from 11 approximately in-line earthquakes, 2-D P- and S-velocity models and a Poisson's ratio model of the crust and uppermost mantle beneath the southern Tien Shan and the Pamir have been derived along the 400-km long main profile of the TIPAGE (TIen shan-PAmir GEodynamic program) project. These models show that the crustal thickness varies from about 65.5 km close to the southern end of the profile beneath the South Pamir through about 73.6 km under Lake Karakul in the North Pamir, to about 57.7 km, 50 km south of the northern end of the profile in the southern Tien Shan. Average crustal P velocities are low with respect to the global average, varying from 6.26 to 6.30 km s -1. The average crustal S velocity varies from 3.54 to 3.70 km s -1 along the profile and thus average crustal Poisson's ratio (σ) varies from 0.23 beneath the central Pamir in the south central part of the profile to 0.265 towards the northern end of the profile beneath the southern Tien Shan. The main layer of the upper crust extending from about 2 km below the Earth's surface to 27 km depth below sea level (b.s.l.) has average P velocities of about 6.05-6.1 km s -1, except beneath the south central part of the profile where they decrease to around 5.95 km s -1. This is in contrast to the S velocities which range from 3.4 to 3.6 km s -1 and exhibit the highest values of 3.55-3.6 km s -1 where the P velocity is lowest. Thus, σ for the main layer of the upper crust is 0.26 beneath the profile except beneath the south central part of the profile where it decreases to 0.22. The low value of 0.22 for σ under the central Pamir, the along-strike equivalent of the Qiangtang terrane in Tibet, is similar to that within the corresponding layer beneath the northern Lhasa and southern Qiangtang terranes in central Tibet and is indicative of felsic rocks rich in quartz in the α state. The lower crust below 27 km b.s.l. has P velocities ranging from 6.1 km s -1 at the top to 7.1 km s -1 at the base. Further, σ for this layer is 0.27-0.28 towards the northern end of the profile but is low at about 0.24 beneath the central and southern parts of the profile, which is similar to the situation found in the northeast Tibetan plateau. The low values can be explained by felsic schists and gneisses in the upper part of the lower crust transitioning to granulite-facies and possibly also eclogite-facies metapelites in the lower part. Within the uppermost mantle, the average P velocity is about 8.10-8.15 km s -1 and σ is about 0.26. Assuming an isotropic situation, then a relatively cool (700-800°C) uppermost mantle beneath the profile is indicated. This would in turn indicate an intact mantle lid beneath the profile. An upper mantle reflector dipping from 104 km b.s.l., 120 km from the southern end of the profile to 86 km b.s.l., 155 km from the northern end of the profile has also been identified. The preferred model presented here for the crustal and lithospheric mantle structure beneath the Pamir calls for nearly horizontal underthrusting of relatively cool Indian mantle lithosphere, the leading edge of which is outlined by the Pamir seismic zone. This cool Indian mantle lithosphere is overlain by significantly shortening, warm Asian crust. The Moho trough that is a feature seen beneath some other orogenic belts, for example the Alps and the Urals, beneath the northern Pamir may mark the southern tip of the actively underthrusting Tien Shan crust along the Main Pamir thrust

Seismic imaging of subducting continental lower crust beneath the Pamir

Exhumation of ultra-high pressure metamorphic rocks testifies that the continental crust can subduct to significant depth into the mantle despite its buoyancy. However, direct observation of ongoing subduction of continental crust is rare. The Pamir is r