ALBERT

Description: The Dead Sea Transform (DST) is a major left-lateral strike-slip fault that accommodates the relative motion between the African and Arabian plates, connecting a region of extension in the Red Sea to the Taurus collision zone in Turkey over a length of about 1100 km. The Dead Sea Basin (DSB) is one of the largest basins along the DST. The DSB is a morphotectonic depression along the DST, divided into a northern and a southern sub-basin, separated by the Lisan salt diapir. We report on a receiver function study of the crust within the multidisciplinary geophysical project, DEad Sea Integrated REsearch (DESIRE), to study the crustal structure of the DSB. A temporary seismic network was operated on both sides of the DSB between 2006 October and 2008 April. The aperture of the network is approximately 60 km in the E—W direction crossing the DSB on the Lisan peninsula and about 100 km in the N—S direction. Analysis of receiver functions from the DESIRE temporary network indicates that Moho depths vary between 30 and 38 km beneath the area. These Moho depth estimates are consistent with results of near-vertical incidence and wide-angle controlled-source techniques. Receiver functions reveal an additional discontinuity in the lower crust, but only in the DSB and west of it. This leads to the conclusion that the internal crustal structure east and west of the DSB is different at the present-day. However, if the 107 km left-lateral movement along the DST is taken into account, then the region beneath the DESIRE array where no lower crustal discontinuity is observed would have lain about 18 Ma ago immediately adjacent to the region under the previous DESERT array west of the DST where no lower crustal discontinuity is recognized.

Description: The territory of Lithuania and adjacent areas of the East European Craton have always been considered a region of low seismicity. Two recent earthquakes with magnitudes of more than 5 in the Kaliningrad District (Russian Federation) on 21 September 2004 motivated re-evaluation of the seismic hazard in Lithuania and adjacent territories. A new opportunity to study seismicity in the region is provided by the PASSEQ (Pasive Seismic Experiment) project that aimed to study the lithosphere–asthenosphere structure around the Trans-European Suture Zone. Twenty-six seismic stations of the PASSEQ temporary seismic array were installed in the territory of Lithuania. The stations recorded a number of local and regional seismic events originating from Lithuania and adjacent areas. This data can be used to answer the question of whether there exist seismically active tectonic zones in Lithuania that could be potentially hazardous for critical industrial facilities. Therefore, the aim of this paper is to find any natural tectonic seismic events in Lithuania and to obtain more general view of seismicity in the region. In order to do this, we make a manual review of the continuous data recorded by the PASSEQ seismic stations in Lithuania. From the good quality data, we select and relocate 45 local seismic events using the well-known LocSAT and VELEST location algortithms. In order to discriminate between possible natural events, underwater explosions and on-shore blasts, we analyse spatial distribution of epicenters and temporal distribution of origin times and perform both visual analysis of waveforms and spectral analysis of recordings. We show that the relocated seismic events can be grouped into five clusters (groups) according to their epicenter coordinates and origin and that several seismic events might be of tectonic origin. We also show that several events from the off-shore region in the Baltic Sea (at the coasts of the Kaliningrad District of the Russian Federation) are non-volcanic tremors, although the origin of these tremor-type events is not clear.

Description: Clear S-to-P converted waves from the crust–mantle boundary (Moho) and lithosphere–asthenosphere boundary (LAB) have been observed on the eastern part of the Dead Sea Basin (DSB), and are used for the determination of the depth of the Moho and the LAB. A temporary network consisting of 18 seismic broad-band stations was operated in the DSB region as part of the DEad Sea Integrated REsearch project for 1.5 years beginning in September 2006. The obtained Moho depth (∼35 km) from S-to-P receiver functions agrees well with the results from P-to-S receiver functions and other geophysical data. The thickness of the lithosphere on the eastern part of the DSB is about 75 km. The results obtained here support and confirm previous studies, based on xenolith data, geodynamic modeling, heat flow observations, and S-to-P receiver functions. Therefore, the lithosphere on the eastern part of the DSB and along Wadi Araba has been thinned in the Late Cenozoic, following rifting and spreading of the Red Sea. The thinning of the lithosphere occurred without a concomitant change in the crustal thickness and thus an upwelling of the asthenosphere in the study area is invoked as the cause of the lithosphere thinning.

Description: Based on a 2 year seismic record from a local network, we characterize the deformation of the seismogenic crust of the Pamir in the northwestern part of the India‐Asia collision zone. We located more than 6000 upper crustal earthquakes in a regional 3‐D velocity model. For 132 of these events, we determined source mechanisms, mostly through full waveform moment tensor inversion of locally and regionally recorded seismograms. We also produced a new and comprehensive neotectonic map of the Pamir, which we relate to the seismic deformation. Along Pamir's northern margin, where GPS measurements show significant shortening, we find thrust and dextral strike‐slip faulting along west to northwest trending planes, indicating slip partitioning between northward thrusting and westward extrusion. An active, north‐northeast trending, sinistral transtensional fault system dissects the Pamir's interior, connecting the lakes Karakul and Sarez, and extends by distributed faulting into the Hindu Kush of Afghanistan. East of this lineament, the Pamir moves northward en bloc, showing little seismicity and internal deformation. The western Pamir exhibits a higher amount of seismic deformation; sinistral strike‐slip faulting on northeast trending or conjugate planes and normal faulting indicate east‐west extension and north‐south shortening. We explain this deformation pattern by the gravitational collapse of the western Pamir Plateau margin and the lateral extrusion of Pamir rocks into the Tajik‐Afghan depression, where it causes thin‐skinned shortening of basin sediments above an evaporitic décollement. Superposition of Pamir's bulk northward movement and collapse and westward extrusion of its western flank causes the gradual change of surface velocity orientations from north‐northwest to due west observed by GPS geodesy. The distributed shear deformation of the western Pamir and the activation of the Sarez‐Karakul fault system may ultimately be caused by the northeastward propagation of India's western transform margin into Asia, thereby linking deformation in the Pamir all the way to the Chaman fault in the south in Afghanistan.

Description: The northeastern boundary of the Tibetan high plateau is marked by a 2 km topographic drop and a coincident rapid change in crustal thickness. Surface tectonics are dominated by the Kunlun strike‐slip fault system and adjacent Kunlun concealed thrust. The main objective of the current study is to map lateral variations of seismic anisotropy parameters in this region along the linear INDEPTH IV array in order to investigate the link between surface and internal deformation in the context of crust and mantle structure. To achieve this aim, we performed Minimum‐Transverse‐Energy based SKS splitting measurements using 23 stations of the INDEPTH IV array deployed across the northeastern margin of Tibet. Average fast polarization directions and splitting time delays are obtained by averaging stacked misfit surfaces of all analyzed events at each station. The agreement of fast directions with the strikes of major active strike‐slip faults and strike‐slip focal mechanisms, but not with fossil structures such as the Jinsha suture, implies that the anisotropy records lithospheric petrofabric formed by recent deformation within the lithosphere rather than representing frozen‐in anisotropy or shear within the asthenosphere due to absolute plate motion. The distribution of large splitting delays throughout the northern plateau suggests that deformation is distributed rather than focused onto narrow shear zones associated with the Kunlun strike‐slip faults. The drop in splitting delays toward the Qaidam is then a natural consequence of the much lower degree of deformation there.