ALBERT

Description: SUMMARY This study images upper-mantle structure beneath different tectonic and geomorphological provinces in southern Scandinavia by P -wave traveltime tomography based on teleseismic events. We present results using integrated data from several individual projects (CALAS, MAGNUS, SCANLIPS, CENMOVE and Tor) with a total of 202 temporary seismological stations deployed in southern Norway, southern Sweden, Denmark and the northernmost part of Germany. These stations, together with 18 permanent stations, yield a high density data coverage and enable presentation of the first high resolution 3D seismic velocity model for the upper mantle for this region, which includes the entire northern part of the prominent Tornquist Zone and the Southern Scandes Mountains. P -wave arrival time residuals of up to ±1 s are observed indicating large seismic velocity contrasts at depths. Relative regional as well as absolute global tomographic inversion is carried out and consistently show upper-mantle velocity variations relative to the ak135 global reference model of up to ±2–3 per cent corresponding to P -wave velocity differences of 0.4–0.5 km s –1 from depths of about 100 km to more than 300 km. High upper-mantle velocities are observed to great depth to the east in Baltic Shield areas of southwestern Sweden suggesting the existence of a deep lithosphere keel. Lower velocities are found to the west and southwest beneath the Danish and North German sedimentary basins and in most of southern Norway. A well defined, generally narrow and deep boundary is observed between areas of contrasting upper-mantle seismic velocity. In the southern part of the study area, this boundary is localized along and east of the Sorgenfrei–Tornquist Zone. It seems to follow the eastern boundary of a zone of significant Late Carboniferous–Permian volcanic activity from southwestern Sweden to the Oslo Graben area. To the north, it crosses shield units, Caledonides as well as areas of high topography. Supported by independent results of surface wave studies, we interpret this velocity boundary as a first order lithosphere boundary representing the southwestern edge of thick shield lithosphere. In basin areas to the southwest, low upper-mantle velocities are associated with asthenosphere beneath thinned lithosphere and velocity contrasts are likely to arise mainly from temperature differences. To the north structural and geodynamic relations are more complex and both temperature and compositional differences may play a part. Reduced upper-mantle velocity beneath southern Norway also seems, despite relatively low heat flow, to be associated with areas of thinned lithosphere, pointing towards increased temperatures and reduced density in the upper mantle. This feature extends over large areas and seems not directly correlated to the shorter wavelength high topography of the Scandes Mountains, but may contribute with some isostatic buoyancy on a regional scale. For this northern area, there is no obvious geodynamic explanation to reduced upper-mantle velocity. A number of candidates are available including deep transient thermal influence from basin areas to the southwest.

Description: SUMMARY From the S- wave data collected along a 270-km-long profile spanning the Kunlun mountains in NE Tibet, 14595 Sg phase arrivals and 21 SmS phase arrivals were utilized to derive a whole-crustal S velocity model and, together with a previously derived P velocity model, a Poisson's ratio (σ) model beneath the profile. The final tomogram for the upper 10–15 km of the crust reveals the lower velocities associated with the predominantly Neogene-Quaternary sediments of the Qaidam basin to the north and the higher velocities associated with the predominantly Palaeozoic and Mesozoic upper crustal sequences of the Songpan-Ganzi terrane and Kunlun mountains to the south. This study finds no evidence that the Kunlun mountains are involved in large-scale northward overriding of the Qaidam basin along a shallow south-dipping thrust. The σ in the upper 10–15 km of the crust are often lower than 0.25, indicating a preponderance of quartz-rich rocks in the upper crust beneath the profile. Below 10–15 km depth, the remainder of the crust down to the Moho has an average σ of 0.24 beneath the Songpan-Ganzi terrane and Kunlun mountains and 0.25 below the Qaidam basin. These low σ are similar to other low σ found along other profiles in the northeastern part of the plateau. Assuming an isotropic situation and no significant variation in σ between 10–15 km depth and the Moho, then the lower crust between 25–30 km depth below sea level and the Moho with P velocities varying from 6.6 km s −1 at the top to around 6.9 km s −1 at the base and σ of 0.24–0.25 should comprise intermediate granulites in the upper part transitioning to granulite facies metapelites in the lower part. As the pre-Cenozoic Qaidam basin crust has probably not lost any of its lower crust during the present Himalayan orogenic cycle in the Cenozoic and only has a σ of 0.245–0.25, then it appears that the pre-Cenozoic Qaidam basin crust involved in the collision is more felsic and thus weaker and more easily deformable than normal continental crust with a global average σ of 0.265–0.27 and the Tarim and Sichuan basin crusts. This situation then probably facilitates the collision and promotes the formation of new high plateau crust at the NE margin of Tibet. South of the Qaidam basin, the crust of the Songpan-Ganzi terrane and Kunlun mountains has an even lower average crustal σ of 0.23–0.24 and is thus presumably even weaker and more easily deformable than the crust beneath the Qaidam basin. This then supports the hypothesis of Karplus et al. that ‘the high Tibetan Plateau may be thickening northward into south Qaidam as its weak, thickened lower crust is injected beneath stronger Qaidam crust'.

Description: SUMMARY Rapid estimation of earthquake rupture propagation is essential to declare an early warning for tsunami-generating earthquakes. An increasing number of seismological methods have been developed to determine rupture parameters, such as length, velocity and propagation direction, especially since the occurrence of the Sumatra–Andaman earthquake that resulted in a devastating tsunami in the Indian Ocean region. Here, we present a new method to follow the rupture process in near real time by a polarization analysis of local and regional P phases that permits a faster determination of rupture properties than using teleseismic records. The new technique has the capability to provide detailed information in less than 10 min. Originally, the method stems from a single-station earthquake location method and is expanded here to monitor P -phase polarization variations through time. As the earthquake source moves away from the hypocentre, the backazimuth of an incoming P phase is expected to change accordingly. With polarization analysis we may be able to monitor the temporal change in P -wave backazimuth to follow the rupture process in near real time. Three component P phases are scanned to determine the azimuthal variation as a function of time. The backazimuth of a moving rupture front is determined by the first eigenvector of the covariance matrix. The linearity of the particle motion is used as a measure of the quality of the data. Seismic stations at local and regional distances ( ) are used. We tested the new method with a theoretical simulation and observed seismograms of the Sumatra–Andaman earthquake (2004 December 26, M w = 9.3), and we were able to follow the rupture for the first 200 s. For larger ruptures, stations at more than 30° epicentral distances would be required. The method is also successfully applied to the Wenchuan earthquake (2008 May 12, M w = 8.0).

Description: P-to-S converted teleseismic waves recorded by temporary broadband networks across Tibet show a north-dipping interface that begins 50 kilometers north of the Zangbo suture at the depth of the Moho (80 kilometers) and extends to a depth of 200 kilometers beneath the Bangong suture. Under northern Tibet a segmented south-dipping structure was imaged. These observations suggest a different form of detachment of the Indian and Asian lithospheric mantles caused by differences in their composition and buoyancy.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Kosarev -- Kind -- Sobolev -- Yuan -- Hanka -- Oreshin -- New York, N.Y. -- Science. 1999 Feb 26;283(5406):1306-1309.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Institute of the Physics of the Earth, Russian Academy of Sciences, B. Gruzinskaya 10, 128810 Moscow, Russia. GeoForschungsZentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germany. Freie Universitat Berlin, Geophysik, Malteser Strasse 74-100, 12249 Berlin, Germany.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/10037597" target="_blank"〉PubMed〈/a〉

Description: Seismic data from central Tibet have been combined to image the subsurface structure and understand the evolution of the collision of India and Eurasia. The 410- and 660-kilometer mantle discontinuities are sharply defined, implying a lack of a subducting slab beneath the plateau. The discontinuities appear slightly deeper beneath northern Tibet, implying that the average temperature of the mantle above the transition zone is about 300 degrees C hotter in the north than in the south. There is a prominent south-dipping converter in the uppermost mantle beneath northern Tibet that might represent the top of the Eurasian mantle lithosphere underthrusting the northern margin of the plateau.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Kind, R -- Yuan, X -- Saul, J -- Nelson, D -- Sobolev, S V -- Mechie, J -- Zhao, W -- Kosarev, G -- Ni, J -- Achauer, U -- Jiang, M -- New York, N.Y. -- Science. 2002 Nov 8;298(5596):1219-21.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉GeoForschungsZentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germany. kind@gfz-potsdam.de〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/12424374" target="_blank"〉PubMed〈/a〉

Description: Earthquake data collected by the INDEPTH-II Passive-Source Experiment show that there is a substantial south to north variation in the velocity structure of the crust beneath southern Tibet. North of the Zangbo suture, beneath the southern Lhasa block, a midcrustal low-velocity zone is revealed by inversion of receiver functions, Rayleigh-wave phase velocities, and modeling of the radial component of teleseismic P-waveforms. Conversely, to the south beneath the Tethyan Himalaya, no low-velocity zone was observed. The presence of the midcrustal low-velocity zone in the north implies that a partially molten layer is in the middle crust beneath the northern Yadong-Gulu rift and possibly much of southern Tibet.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Kind -- Ni -- Zhao -- Wu -- Yuan -- Sandvol -- Reese -- Nabelek -- Hearn -- New York, N.Y. -- Science. 1996 Dec 6;274(5293):1692-4.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉R. Kind and Xiaohui Yuan, GeoForschungsZentrum Potsdam, 14473 Potsdam, Germany. James Ni, Jianxin Wu, C. Reese, T. Hearn, Department of Physics, New Mexico State University, Las Cruces, NM 88003, USA. Wenjin Zhao, Chinese Academy of Geological Sciences, Beijing, China Lianshe Zhao, Institute for Geophysics, University of Texas at Austin, Austin, TX 78759, USA. E. Sandvol, Department of Geological Sciences, Cornell University, Ithaca, NY 14853, USA. J. Nabelek, College of Oceanography, Oregon State University, Corvalis, OR 97331, USA.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/8939854" target="_blank"〉PubMed〈/a〉

Description: INDEPTH geophysical and geological observations imply that a partially molten midcrustal layer exists beneath southern Tibet. This partially molten layer has been produced by crustal thickening and behaves as a fluid on the time scale of Himalayan deformation. It is confined on the south by the structurally imbricated Indian crust underlying the Tethyan and High Himalaya and is underlain, apparently, by a stiff Indian mantle lid. The results suggest that during Neogene time the underthrusting Indian crust has acted as a plunger, displacing the molten middle crust to the north while at the same time contributing to this layer by melting and ductile flow. Viewed broadly, the Neogene evolution of the Himalaya is essentially a record of the southward extrusion of the partially molten middle crust underlying southern Tibet.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Nelson -- Zhao -- Brown -- Kuo -- Che -- Liu -- Klemperer -- Makovsky -- Meissner -- Mechie -- Kind -- Wenzel -- Ni -- Nabelek -- Leshou -- Tan -- Wei -- Jones -- Booker -- Unsworth -- Kidd -- Hauck -- Alsdorf -- Ross -- Cogan -- Wu -- Sandvol -- Edwards -- New York, N.Y. -- Science. 1996 Dec 6;274(5293):1684-8.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉K. D. Nelson, M. Cogan, C. Wu, Department of Earth Sciences, Syracuse University, Syracuse, NY 13244, USA. W. Zhao, J. Che, X. Liu, Chinese Academy of Geological Sciences, Beijing 100037, China. L. D. Brown, M. Hauck, D. Alsdorf, A. Ross, Institute for the Study of the Continents, Cornell University, Ithaca, NY 14853, USA. J. Kuo, Lamont Doherty Geological Observatory, Palisades, NY, 10964, USA. S. L. Klemperer and Y. Makovsky, Department of Geophysics, Stanford University, Stanford, CA 94305, USA. R. Meissner, Institut fur Geophysik, Christian-Albrechts-Universitaet zu Kiel, 24098 Kiel, Germany. J. Mechie and R. Kind, GeoForschungsZentrum Potsdam (GFZ), 14473 Potsdam, Germany. F. Wenzel, Geophysikalisches Institut, Universitaet Karlsruhe, 76187 Karlsruhe, Germany. J. Ni and E. Sandvol, Department of Physics, New Mexico State University, Las Cruces, NM 88003, USA. J. Nabelek, College of Oceanography, Oregon State University, Corvallis, OR 97331, USA. L. Chen, H. Tan, W. Wei, China University of Geosciences, Beijing, China. A. G. Jones, Geological Survey of Canada, 1 Observatory Crescent, Ottawa, Ontario, Canada. J. Booker and M. Unsworth, Geophysics Program, University of Washington, Seattle, WA 98195, USA. W. S. F. Kidd and M. Edwards, Department of Geosciences, SUNY-Albany, Albany, NY 12222, USA〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/8939851" target="_blank"〉PubMed〈/a〉

Description: Global Seismic Network data were used to image upper-mantle seismic discontinuities. Stacks of phases that precede the PP phase, thought to be underside reflections from the upper-mantle discontinuities at depths of 410 and 660 kilometers, show that the reflection from 410 kilometers is present, but the reflection from 660 kilometers is not observed. A continuous Lame's constant lambda and seismic parameter at the 660-kilometer discontinuity explain the missing underside P reflections and lead to a P-wave velocity jump of only 2 percent, whereas the S-wave velocity and density remain unchanged with respect to previous global models. The model deemphasizes the role of Lame's constant lambda with regard to the shear modulus and constrains the mineralogical composition across the discontinuity.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Estabrook -- Kind -- New York, N.Y. -- Science. 1996 Nov 15;274(5290):1179-82.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉GeoForschungsZentrum Potsdam, Telegrafenberg, D- 14473 Potsdam, Germany.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/8895464" target="_blank"〉PubMed〈/a〉

Description: 〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Kind, Rainer -- Yuan, Xiaohui -- New York, N.Y. -- Science. 2010 Sep 17;329(5998):1479-80. doi: 10.1126/science.1191620.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Deutsches GeoForschungsZentrum, Telegrafenberg, 14473 Potsdam, Germany. kind@gfz-potsdam.de〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/20847259" target="_blank"〉PubMed〈/a〉