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: We describe results of an active-source seismology experiment across the Chilean subduction zone at 38.2◦S. The seismic sections clearly show the subducted Nazca plate with varying reflectivity. Below the coast the plate interface occurs at 25 km depth as the sharp lower boundary of a 2–5 km thick, highly reflective region, which we interpret as the subduction channel, that is, a zone of subducted material with a velocity gradient with respect to the upper and lower plate. Further downdip along the seismogenic coupling zone the reflectivity decreases in the area of the presumed 1960 Valdivia hypocentre. The plate interface itself can be traced further down to depths of 50–60 km below the Central Valley. We observe strong reflectivity at the plate interface as well as in the continental mantle wedge. The sections also show a segmented forearc crust in the overriding South American plate. Major features in the accretionary wedge, such as the Lanalhue fault zone, can be identified. At the eastern end of the profile a bright west-dipping reflector lies perpendicular to the plate interface and may be linked to the volcanic arc.

Description: As part of the DEad Sea Integrated REsearch project (DESIRE) a 235 km long seismic wide-angle reflection/refraction (WRR) profile was completed in spring 2006 across the Dead Sea Transform (DST) in the region of the southern Dead Sea basin (DSB). The DST with a total of about 107 km multi-stage left-lateral shear since about 18 Ma ago, accommodates the movement between the Arabian and African plates. It connects the spreading centre in the Red Sea with the Taurus collision zone in Turkey over a length of about 1 100 km. With a sedimentary infill of about 10 km in places, the southern DSB is the largest pull-apart basin along the DST and one of the largest pull-apart basins on Earth. The WRR measurements comprised 11 shots recorded by 200 three-component and 400 one-component instruments spaced 300 m to 1.2 km apart along the whole length of the E–W trending profile. Models of the P-wave velocity structure derived from the WRR data show that the sedimentary infill associated with the formation of the southern DSB is about 8.5 km thick beneath the profile. With around an additional 2 km of older sediments, the depth to the seismic basement beneath the southern DSB is about 11 km below sea level beneath the profile. Seismic refraction data from an earlier experiment suggest that the seismic basement continues to deepen to a maximum depth of about 14 km, about 10 km south of the DESIRE profile. In contrast, the interfaces below about 20 km depth, including the top of the lower crust and the Moho, probably show less than 3 km variation in depth beneath the profile as it crosses the southern DSB. Thus the Dead Sea pull-apart basin may be essentially an upper crustal feature with upper crustal extension associated with the left-lateral motion along the DST. The boundary between the upper and lower crust at about 20 km depth might act as a decoupling zone. Below this boundary the two plates move past each other in what is essentially a shearing motion. Thermo-mechanical modelling of the DSB supports such a scenario. As the DESIRE seismic profile crosses the DST about 100 km north of where the DESERT seismic profile crosses the DST, it has been possible to construct a crustal cross-section of the region before the 107 km left-lateral shear on the DST occurred.

Description: SKS and SKKS shear waves recorded on the INDEPTH III seismic array deployed in central Tibet during 1998 - 1999 have been analysed for the direction and extent of seismic polarization anisotropy. The 400-km-long NNW trending array extended south to north, from the central Lhasa terrane, across the Karakoram-Jiali fault system and Banggong-Nujiang suture to the central Qiangtang terrane. Substantial splitting with delay times from 1 to 2 s, and fast directions varying from E-W to NE-SW, was observed for stations in the Qiangtang trerrane and northernmost Lhasa terrane. No detectable splitting was observed for stations located farther south in the central Lahsa terrane. The change in shear wave splitting characteristics occurs at 32°N, approximately coincident with the transcurrent Karakoram-Jiali fault system but ~40 km south of the surface trace of the Banggong-Nujiang suture. This location is also near the southernmost edge of a region of high Sn attenuation and low upper mantle velocities found in previous studies. The transition between no measured splitting and strong anisotropy (2.2 s delay time) is exceptionally sharp (=15 km), suggesting a large crustal contribution to the measured splitting. The E-W to NE-SW fast directions are broadly similar to the fast directions observed farther east along the Yadong-Golmud highway, suggesting that no large-scale change in anisotropic properties occurs in the east-west direction. However, in detail, fast directions and delay times vary over lateral distances of ~100 km in both the N-S and E-W direction by as much as 40° and 0.5-1 s, respectively. The onset of measurable splitting at 32° N most likely marks the northern limit of the underthrusting Indian lithosphere, which is characterized by negligible polarization anisotropy. Taken in conjunction with decades of geophysical and geological observations in Tibet, the new anisotropy measurements are consistent with a model where hot and weak upper mantle beneath northern Tibet is being squeezed and sheared between the advancing Indian lithosphere to the south and the Tsaidam and Tarim Lithospheres to the north and west, resulting in eastward flow and possibly thickening and subsequent detachment due to gravitational instability. In northern Tibet, crustal deformation clearly follows this large-scale deformation pattern.

Description: On 12 May, a great earthquake (Ms=8.0) on the Longmenshan thrust fault rumbled through Chinas Sichuan province, killing more than 69,000 people and injuring 374,000. The Longmenshan thrust is part of the eastern border of the Tibetan Plateau, but it is not the plateaus only restless margin. An even larger earthquake (Ms=8.1) on the Kunlun fault shook northeastern Tibet in 2001, fortunately in a sparsely populated area. These massive quakes underscore the importance of understanding the tectonic response of Asia to collision by India. The International Deep Profiling of Tibet and the Himalaya (INDEPTH) program explores the dynamics of the India-Asia collision. Though many past geophysical studies have focused on the Himalayas and the southern Tibetan Plateau, the INDEPTH IV project examines the deep structure of the northeastern margin of the Tibetan Plateau.