TY - JOUR A1 - Bauer, Klaus A1 - Moeck, Inga A1 - Norden, Ben A1 - Schulze, Alexander A1 - Weber, Michael H. A1 - Wirth, Holger T1 - Tomographic P wave velocity and vertical velocity gradient structure across the geothermal site Groß Schoenebeck (NE German Basin) : relationship to lithology, salt tectonics, and thermal regime N2 - Seismic wide-angle data were collected along a 40-km-long profile centered at the geothermal research well GrSk 3/90 in the Northeast German Basin. Tomographic inversion of travel time data provided a velocity and a vertical velocity gradient model, indicative of Cenozoic to Pre-Permian sediments. Wide-angle reflections are modeled and interpreted as top Zechstein and top Pre-Permian. Changes in velocity gradients are interpreted as the transition from mechanical to chemical compaction at 2-3 km depth, and localized salt structures are imaged, suggesting a previously unknown salt pillow in the southern part of the seismic profile. The Zechstein salt shows decreased velocities in the adjacent salt pillows compared to the salt lows, which is confirmed by sonic log data. This decrease in velocity could be explained by the mobilization of less dense salt, which moved and formed the salt pillows, whereas the denser salt remained in place at the salt lows. We interpret a narrow subvertical low-velocity zone under the salt pillow at GrSk 3/ 90 as a fault in the deep Permian to Pre-Permian. This WNW-ESE trending fault influenced the location of the salt tectonics and led to the formation of a fault-bounded graben in the Rotliegend sandstones with optimal mechanical conditions for geothermal production. Thermal modeling showed that salt pillows are related to chimney effects, a decrease in temperature, and increasing velocity. The assumed variations in salt lithology, density, and strain must thus be even higher to compensate for the temperature effect. Y1 - 2010 UR - http://www.agu.org/journals/jb/ U6 - https://doi.org/10.1029/2009jb006895 SN - 0148-0227 ER - TY - JOUR A1 - Muksin, Umar A1 - Haberland, Christian A1 - Bauer, Klaus A1 - Weber, Michael H. T1 - Three-dimensional upper crustal structure of the geothermal system in Tarutung (North Sumatra, Indonesia) revealed by seismic attenuation tomography JF - Geophysical journal international N2 - The geothermal potential in Tarutung is controlled by both the Sumatra Fault system and young arc volcanism. In this study we use the spatial distribution of seismic attenuation, calculated from local earthquake recordings, to image the 3-D seismic attenuation of the area and relate it with the temperature anomalies and the fluid distribution of the subsurface. A temporary seismic network of 42 stations was deployed around Tarutung and Sarulla (south of Tarutung) for a period of 10 months starting in 2011 May. Within this period, the network recorded 2586 local events. A high-quality subset of 229 events recorded by at least 10 stations was used for the attenuation inversion (tomography). Path-average attenuation (t(p)*) was calculated by using a spectral inversion method. The spread function, the contour lines of the model resolution matrix and the recovery test results show that our 3-D attenuation model (Q(p)) has good resolution around the Tarutung Basin and along the Sarulla graben. High attenuation (low Q(p)) related to the geothermal system is found in the northeast of the Tarutung Basin suggesting fluid pathways from below the Sumatra Fault. The upper part of the studied geothermal system in the Tarutung district seems to be mainly controlled by the fault structure rather than by magmatic activities. In the southwest of the Tarutung Basin, the high attenuation zone is associated with the Martimbang volcano. In the Sarulla region, a low-Q(p) anomaly is found along the graben within the vicinity of the Hopong caldera. KW - Seismicity and tectonics KW - Body waves KW - Seismic attenuation KW - Seismic tomography Y1 - 2013 U6 - https://doi.org/10.1093/gji/ggt383 SN - 0956-540X SN - 1365-246X VL - 195 IS - 3 SP - 2037 EP - 2049 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Petrunin, Alexey G. A1 - Rioseco, Ernesto Meneses A1 - Sobolev, Stephan Vladimir A1 - Weber, Michael H. T1 - Thermomechanical model reconciles contradictory geophysical observations at the Dead Sea Basin JF - Geochemistry, geophysics, geosystems N2 - The Dead Sea Transform (DST) comprises a boundary between the African and Arabian plates. During the last 15-20 m.y. more than 100 km of left lateral transform displacement has been accumulated on the DST and about 10 km thick Dead Sea Basin (DSB) was formed in the central part of the DST. Widespread igneous activity since some 20 Ma ago and especially in the last 5 m.y., thin (60-80 km) lithosphere constrained by seismic data and absence of seismicity below the Moho, seem to be quite natural for this tectonically active plate boundary. However, surface heat flow values of less than 50-60 mW/m(2) and deep seismicity in the lower crust (deeper than 20 km) reported for this region are apparently inconsistent with the tectonic settings specific for an active continental plate boundary and with the crustal structure of the DSB. To address these inconsistencies which comprise what we call the "DST heat-flow paradox," we have developed a numerical model that assumes an erosion of initially thick and cold lithosphere just before or during the active faulting at the DST. The optimal initial conditions for the model are defined using transient thermal analysis. From the results of our numerical experiments we conclude that the entire set of observations for the DSB can be explained within the classical pull-apart model assuming that the lithosphere has been thermally eroded at about 20 Ma and the uppermost mantle in the region have relatively weak rheology consistent with experimental data for wet olivine or pyroxenite. KW - heat flow KW - pull-apart basin KW - tectonophysics KW - thermomechanical modeling KW - transform fault Y1 - 2012 U6 - https://doi.org/10.1029/2011GC003929 SN - 1525-2027 VL - 13 IS - 8 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Paschke, Marco A1 - Stiller, Manfred A1 - Ryberg, Trond A1 - Weber, Michael H. T1 - The shallow P-velocity structure of the southern Dead Sea basin derived from near-vertical incidence reflection seismic data in project DESIRE JF - Geophysical journal international N2 - As a part of the DEad Sea Integrated REsearch (DESIRE) project a near-vertical incidence reflection (NVR) experiment with a profile length of 122 km was completed in spring 2006. The profile crossed the southern Dead Sea basin (DSB), a pull-apart basin due to the strike-slip motion along the Dead Sea Transform (DST). The DST with a total displacement of 107 km since about 18 Ma is part of a left-lateral fault system which connects the spreading centre in the Red Sea with the Taurus collision zone in Turkey over a distance of about 1100 km. The seismic experiment comprises 972 source locations and 1045 receiver locations. Each source was recorded by similar to 180 active receivers and a field data set with 175 000 traces was created. From this data set, 124 444 P-wave first-break traveltimes have been picked. With these traveltimes a tomographic inversion was carried out, resulting in a 2-D P-wave velocity model with a rms error of 20.9 ms. This model is dominated by a low-velocity region associated with the DSB. Within the DSB, the model shows clearly the position of the Lisan salt diapir, identified by a high-velocity zone. A further feature is an unexpected laterally low-velocity zone with P-velocities of 3 km s1 embedded in regions with 4 km s1 in the shallow part on the west side of the DSB. Another observation is an anticlinal structure west of the DSB interpretated to the related Syrian arc fold belt. KW - Tomography KW - Controlled source seismology KW - Transform faults Y1 - 2012 U6 - https://doi.org/10.1111/j.1365-246X.2011.05270.x SN - 0956-540X VL - 188 IS - 2 SP - 524 EP - 534 PB - Wiley-Blackwell CY - Malden ER - TY - JOUR A1 - Mohsen, Ayman A1 - Asch, Günter A1 - Kind, Rainer A1 - Mechie, James A1 - Weber, Michael H. T1 - The lithosphere-asthenosphere boundary in the eastern part of the Dead Sea Basin (DSB) from S-to-P receiver functions JF - Arabian journal of geosciences N2 - 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 (similar to 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. KW - Dead Sea basin KW - S receiver functions KW - Lithosphere Y1 - 2013 U6 - https://doi.org/10.1007/s12517-011-0503-4 SN - 1866-7511 SN - 1866-7538 VL - 6 IS - 7 SP - 2343 EP - 2350 PB - Springer CY - Heidelberg ER - TY - JOUR A1 - Budweg, Martin A1 - Bock, Günter A1 - Weber, Michael H. T1 - The Eifel Plume : imaged with converted seismic waves N2 - Receiver functions (RF) are used to investigate the upper mantle structure beneath the Eifel, the youngest volcanic area of Central Europe. Data from 96 teleseismic events recorded by 242 seismological stations from permanent and a temporary network has been analysed. The temporary network operated from 1997 November to 1998 June and covered an area of approximately 400 x 250 km(2) centred on the Eifel volcanic fields. The average Moho depth in the Eifel is approximately 30 km, thinning to ca. 28 km under the Eifel volcanic fields. RF images suggest the existence of a low velocity zone at about 60-90 km depth under the West Eifel. This observation is supported by P- and S-wave tomographic results and absorption (but the array aperture limits the resolution of the tomographic methods to the upper 400 km). There are also indications for a zone of elevated velocities at around 200 km depth, again in agreement with S-wave and absorption tomographic results. This anomaly is not visible in P-wave tomography and could be due to S-wave anisotropy. The RF anomalies at the Moho, at 60-90 km, and near 200 km depth have a lateral extent of about 100 km. The 410 km discontinuity under the Eifel is depressed by 15-25 km, which could be explained by a maximum temperature increase of +200 degrees C to +300 degrees C. In the 3-D RF image of the Eifel Plume we also notice two additional currently unexplained conversions between 410 and 550 km depth. They could represent remnants of previous subduction or anomalies due to delayed phase changes. The lateral extent of these conversions and the depression of the 410 km discontinuity is about 200 km. The 660 km discontinuity does not show any depth deviation from its expected value. Our observations are consistent with interpretation in terms of an upper mantle plume but they do not rule out connections to processes at larger depth Y1 - 2006 UR - http://onlinelibrary.wiley.com/journal/10.1111/(ISSN)1365-246X U6 - https://doi.org/10.1111/j.1365-246X.2005.02778.x SN - 0956-540X ER - TY - JOUR A1 - Mechie, James A1 - Ben-Avraham, Zvi A1 - Weber, Michael H. A1 - Götze, Hans-Jürgen A1 - Koulakov, Ivan A1 - Mohsen, A. A1 - Stiller, M. T1 - The distribution of Moho depths beneath the Arabian plate and margins JF - TECTONOPHYSICS N2 - In this study three new maps of Moho depths beneath the Arabian plate and margins are presented. The first map is based on the combined gravity model, EIGEN 06C, which includes data from satellite missions and ground-based studies, and thus covers the whole region between 31 degrees E and 60 inverted perpendicular E and between 12 degrees N and 36 degrees N. The second map is based on seismological and ground-based gravity data while the third map is based only on seismological data. Both these maps show gaps due to lack of data coverage especially in the interior of the Arabian plate. Beneath the interior of the Arabian plate the Moho lies between 32 and 45 km depth below sea level. There is a tendency for higher Pn and Sn velocities beneath the northeastern parts of the plate interior with respect to the southwestern parts of the plate interior. Across the northern, destructive margin with the Eurasian plate, the Moho depths increase to over 50 km beneath the Zagros mountains. Across the conservative western margin, the Dead Sea Transform (DST). Moho depths decrease from almost 40 km beneath the highlands east of the DST to about 21-23 km under the southeastern Mediterranean Sea. This decrease seems to be modulated by a slight depression in the Moho beneath the southern DST. The constructive southwestern and southeastern margins of the Arabian plate also show the Moho shallowing from the plate interior towards the plate boundaries. A comparison of the abruptness of the Moho shallowing between the margins of the Arabian plate, the conjugate African margin at 26 degrees N and several Atlantic margins shows a complex picture and suggests that the abruptness of the Moho shallowing may reflect fundamental differences in the original structure of the margins. (C) 2012 Elsevier B.V. All rights reserved. KW - Moho depths KW - Arabian plate KW - Red Sea KW - Velocity models KW - Receiver functions KW - Satellite gravity data Y1 - 2013 U6 - https://doi.org/10.1016/j.tecto.2012.11.015 SN - 0040-1951 SN - 1879-3266 VL - 609 SP - 234 EP - 249 PB - ELSEVIER SCIENCE BV CY - AMSTERDAM ER - TY - JOUR A1 - Scherbaum, Frank A1 - Weber, Michael H. A1 - Borm, G. T1 - The deep seismological lab in the KTB borehole: Status 1999 Y1 - 2000 ER - TY - JOUR A1 - Weber, Michael H. A1 - Abu-Ayyash, Khalil A1 - Abueladas, Abdel-Rahman A1 - Agnon, Amotz A1 - Al-Amoush, H. A1 - Babeyko, Andrey A1 - Bartov, Yosef A1 - Baumann, M. A1 - Ben-Avraham, Zvi A1 - Bock, Günter A1 - Bribach, Jens A1 - El-Kelani, R. A1 - Forster, A. A1 - Förster, Hans-Jürgen A1 - Frieslander, U. A1 - Garfunkel, Zvi A1 - Grunewald, Steffen A1 - Gotze, Hans-Jürgen A1 - Haak, Volker A1 - Haberland, Christian A1 - Hassouneh, Mohammed A1 - Helwig, S. A1 - Hofstetter, Alfons A1 - Jackel, K. H. A1 - Kesten, Dagmar A1 - Kind, Rainer A1 - Maercklin, Nils A1 - Mechie, James A1 - Mohsen, Amjad A1 - Neubauer, F. M. A1 - Oberhänsli, Roland A1 - Qabbani, I. A1 - Ritter, O. A1 - Rumpker, G. A1 - Rybakov, M. A1 - Ryberg, Trond A1 - Scherbaum, Frank A1 - Schmidt, J. A1 - Schulze, A. A1 - Sobolev, Stephan Vladimir A1 - Stiller, M. A1 - Th, T1 - The crustal structure of the Dead Sea Transform N2 - To address one of the central questions of plate tectonics-How do large transform systems work and what are their typical features?-seismic investigations across the Dead Sea Transform (DST), the boundary between the African and Arabian plates in the Middle East, were conducted for the first time. A major component of these investigations was a combined reflection/ refraction survey across the territories of Palestine, Israel and Jordan. The main results of this study are: (1) The seismic basement is offset by 3-5 km under the DST, (2) The DST cuts through the entire crust, broadening in the lower crust, (3) Strong lower crustal reflectors are imaged only on one side of the DST, (4) The seismic velocity sections show a steady increase in the depth of the crust-mantle transition (Moho) from 26 km at the Mediterranean to 39 km under the Jordan highlands, with only a small but visible, asymmetric topography of the Moho under the DST. These observations can be linked to the left-lateral movement of 105 km of the two plates in the last 17 Myr, accompanied by strong deformation within a narrow zone cutting through the entire crust. Comparing the DST and the San Andreas Fault (SAF) system, a strong asymmetry in subhorizontal lower crustal reflectors and a deep reaching deformation zone both occur around the DST and the SAF. The fact that such lower crustal reflectors and deep deformation zones are observed in such different transform systems suggests that these structures are possibly fundamental features of large transform plate boundaries Y1 - 2004 ER - TY - JOUR A1 - Fromm, T. A1 - Planert, Lars A1 - Jokat, Wilfried A1 - Ryberg, Trond A1 - Behrmann, Jan H. A1 - Weber, Michael H. A1 - Haberland, Christian T1 - South Atlantic opening: A plume-induced breakup? JF - Geology N2 - Upwelling hot mantle plumes are thought to disintegrate continental lithosphere and are considered to be drivers of active continental breakup. The formation of the Walvis Ridge during the opening of the South Atlantic is related to a putative plume-induced breakup. We investigated the crustal structure of the Walvis Ridge (southeast Atlantic Ocean) at its intersection with the continental margin and searched for anomalies related to the possible plume head. The overall structure we identify suggests that no broad plume head existed during opening of the South Atlantic and anomalous mantle melting occurred only locally. We therefore question the importance of a plume head as a driver of continental breakup and further speculate that the hotspot was present before the rifting, leaving a track of kimberlites in the African craton. Y1 - 2015 U6 - https://doi.org/10.1130/G36936.1 SN - 0091-7613 SN - 1943-2682 VL - 43 IS - 10 SP - 931 EP - 934 PB - American Institute of Physics CY - Boulder ER - TY - JOUR A1 - Thomas, Ch. A1 - Weber, Michael H. A1 - Wicks, Chuck A1 - Scherbaum, Frank T1 - Small scatterers in the lower mantle observed at German broadband arrays Y1 - 1999 ER - TY - JOUR A1 - Stankiewicz, Jacek A1 - Munoz, G. A1 - Ritter, Oliver A1 - Bedrosian, Paul A. A1 - Ryberg, Trond A1 - Weckmann, Ute A1 - Weber, Michael H. T1 - Shallow lithological structure across the Dead Sea Transform derived from geophysical experiments JF - Geochemistry, geophysics, geosystems N2 - In the framework of the DEad SEa Rift Transect (DESERT) project a 150 km magnetotelluric profile consisting of 154 sites was carried out across the Dead Sea Transform. The resistivity model presented shows conductive structures in the western section of the study area terminating abruptly at the Arava Fault. For a more detailed analysis we performed a joint interpretation of the resistivity model with a P wave velocity model from a partially coincident seismic experiment. The technique used is a statistical correlation of resistivity and velocity values in parameter space. Regions of high probability of a coexisting pair of values for the two parameters are mapped back into the spatial domain, illustrating the geographical location of lithological classes. In this study, four regions of enhanced probability have been identified, and are remapped as four lithological classes. This technique confirms the Arava Fault marks the boundary of a highly conductive lithological class down to a depth of similar to 3 km. That the fault acts as an impermeable barrier to fluid flow is unusual for large fault zone, which often exhibit a fault zone characterized by high conductivity and low seismic velocity. At greater depths it is possible to resolve the Precambrian basement into two classes characterized by vastly different resistivity values but similar seismic velocities. The boundary between these classes is approximately coincident with the Al Quweira Fault, with higher resistivities observed east of the fault. This is interpreted as evidence for the original deformation along the DST originally taking place at the Al Quweira Fault, before being shifted to the Arava Fault. KW - magnetotellurics KW - seismic tomography Y1 - 2011 U6 - https://doi.org/10.1029/2011GC003678 SN - 1525-2027 VL - 12 IS - 3-4 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Weber, Michael H. A1 - Wicks, Charles A1 - Le Stunff, Yves A1 - Romanowicz, Barbara A1 - Krüger, Frank T1 - Seismic evidence for a steeply dipping reflector-stagnant slab in the mantle transition zone JF - Geophysical journal international N2 - Studies of seismic tomography have been highly successful at imaging the deep structure of subduction zones. In a study complementary to these tomographic studies, we use array seismology and reflected waves to image a stagnant slab in the mantle transition zone. Using P and S (SH) waves we find a steeply dipping reflector centred at ca. 400 km depth and ca. 550 km west of the present Mariana subduction zone (at 20N, 140E). The discovery of this anomaly in tomography and independently in array seismology (this paper) helps in understanding the evolution of the Mariana margin. The reflector/stagnant slab may be the remains of the hypothetical North New Guinea Plate, which was theorized to have subducted ca. 50 Ma. KW - Mantle processes KW - Composition of the mantle KW - Seismic tomography KW - Wave scattering and diffraction KW - Subduction zone processes KW - Volcanic arc processes KW - Continental margins: convergent KW - Pacific Ocean Y1 - 2015 U6 - https://doi.org/10.1093/gji/ggu438 SN - 0956-540X SN - 1365-246X VL - 200 IS - 2 SP - 1235 EP - 1251 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Weber, Michael H. A1 - Zetsche, F. A1 - Ryberg, Trond A1 - Schulze, A. A1 - Spangenberg, Erik A1 - Huenges, Ernst T1 - Seismic detection limits of small, deep, man-made reflectors : a test at a geothermal site in northern Germany N2 - A key question for the development of geothermal plants is the seismic detection and monitoring of fluid injections at several kilometers depth. The detection and monitoring limits are controlled by several parameters, for example, the strength of seismic sources, number of receivers, vertical stacking, and noise conditions. For a known reference reflector at 2.66 km depth at a geothermal site in northern Germany the results of a simple surface seismic experiment were therefore combined with numerical forward modeling for different injection scenarios at 3.8 km depth. The underlying idea is that changes of reflectivity from the injection at 3.8 km must be larger than the variance of the measurements to be observable. Assuming that the injection at 3.8 km depth would produce a subhorizontal disklike target with a fracture porosity of 2% or 5% (the critical porosity) the water injection volume has to be at least 443 and 115 m(3), respectively, to be detectable from the surface. If the injection on the other hand does not create subhorizontal but subvertical pathways or only reduces the seismic velocities via the increased pore pressure in the immediate vicinity of the bore hole, the injection is undetectable from the surface. The most promising approach is therefore to move sources and/or receivers closer to the target, that is, the use of borehole instrumentation Y1 - 2005 SN - 0037-1106 ER - TY - THES A1 - Weber, Michael H. T1 - Robotic telescopes & Doppler imaging : measuring differential rotation on long-period active stars N2 - Auf der Sonne sind viele Phänomene zu sehen die mit der solaren magnetischen Aktivität zusammenhängen. Das dafür zuständige Magnetfeld wird durch einen Dynamo erzeugt, der sich vermutlich am Boden der Konvektionszone in der sogenannten Tachocline befindet. Angetrieben wird der Dynamo teils von der differenziellen Rotation, teils von den magnetischen Turbulenzen in der Konvektionszone. Die differentielle Rotation kann an der Sonnenoberfläche durch beobachten der Sonnenfleckbewegungen gemessen werden.Um einen größeren Parameterraum zum Testen von Dynamotheorien zu erhalten, kann man diese Messungen auch auf andere Sterne ausdehnen. Das primäre Problem dabei ist, dass die Oberflächen von Sternen nicht direkt beobachtet werden können. Indirekt kann man dies jedoch mit Hilfe der Doppler-imaging Methode erreichen, die die Doppler-Verbreitung der Spektrallinien von schnell rotierenden Sternen benützt. Um jedoch ein Bild der Sternoberfläche zu erhalten, bedarf es vieler hochaufgelöster spektroskopischer Beobachtungen, die gleichmäßig über eine Sternrotation verteilt sein müssen. Für Sterne mit langen Rotationsperioden sind diese Beobachtungen nur schwierig durchzuführen. Das neue robotische Observatorium STELLA adressiert dieses Problem und bietet eine auf Dopplerimaging abgestimmte Ablaufplanung der Beobachtungen an. Dies wird solche Beobachtungen nicht nur leichter durchführbar machen, sondern auch effektiver gestalten.Als Vorschau welche Ergebnisse mit STELLA erwartet werden können dient eine Studie an sieben Sternen die allesamt eine lange (zwischen sieben und 25 Tagen) Rotationsperiode haben. Alle Sterne zeigen differentielle Rotation, allerdings sind die Messfehler aufgrund der nicht zufriedenstellenden Datenqualität von gleicher Größenordnung wie die Ergebnisse, ein Problem das bei STELLA nicht auftreten wird. Um die Konsistenz der Ergebnisse zu prüfen wurde wenn möglich sowohl eine Kreuzkorrelationsanalyse als auch die sheared-image Methode angewandt. Vier von diesen sieben Sternen weisen eine differentielle Rotation in umgekehrter Richtung auf als auf der Sonne zu sehen ist. Die restlichen drei Sterne weisen schwache, aber in der Richtung sonnenähnliche differentielle Rotation auf.Abschließend werden diese neuen Messungen mit bereits publizierten Werten kombiniert, und die so erhaltenen Daten auf Korrelationen zwischen differentieller Rotation, Rotationsperiode, Evolutionsstaus, Spektraltyp und Vorhandensein eines Doppelsterns überprüft. Alle Sterne zusammen zeigen eine signifikante Korrelation zwischen dem Betrag der differenziellen Rotation und der Rotationsperiode. Unterscheidet man zwischen den Richtungen der differentiellen Rotation, so bleibt nur eine Korrelation der Sterne mit antisolarem Verhalten. Darüberhinaus zeigt sich auch, dass Doppelsterne schwächer differentiell rotieren. N2 - The sun shows a wide variety of magnetic-activity related phenomena. The magnetic field responsible for this is generated by a dynamo process which is believed to operate in the tachocline, which is located at the bottom of the convection zone. This dynamo is driven in part by differential rotation and in part by magnetic turbulences in the convection zone. The surface differential rotation, one key ingredient of dynamo theory, can be measured by tracing sunspot positions.To extend the parameter space for dynamo theories, one can extend these measurements to other stars than the sun. The primary obstacle in this endeavor is the lack of resolved surface images on other stars. This can be overcome by the Doppler imaging technique, which uses the rotation-induced Doppler-broadening of spectral lines to compute the surface distribution of a physical parameter like temperature. To obtain the surface image of a star, high-resolution spectroscopic observations, evenly distributed over one stellar rotation period are needed. This turns out to be quite complicated for long period stars. The upcoming robotic observatory STELLA addresses this problem with a dedicated scheduling routine, which is tailored for Doppler imaging targets. This will make observations for Doppler imaging not only easier, but also more efficient.As a preview of what can be done with STELLA, we present results of a Doppler imaging study of seven stars, all of which show evidence for differential rotation, but unfortunately the errors are of the same order of magnitude as the measurements due to unsatisfactory data quality, something that will not happen on STELLA. Both, cross-correlation analysis and the sheared image technique where used to double check the results if possible. For four of these stars, weak anti-solar differential rotation was found in a sense that the pole rotates faster than the equator, for the other three stars weak differential rotation in the same direction as on the sun was found.Finally, these new measurements along with other published measurements of differential rotation using Doppler imaging, were analyzed for correlations with stellar evolution, binarity, and rotation period. The total sample of stars show a significant correlation with rotation period, but if separated into antisolar and solar type behavior, only the subsample showing anti-solar differential rotation shows this correlation. Additionally, there is evidence for binary stars showing less differential rotation as single stars, as is suggested by theory. All other parameter combinations fail to deliver any results due to the still small sample of stars available. T2 - Robotic telescopes & Doppler imaging : measuring differential rotation on long-period active stars KW - Teleskop KW - Robotik KW - Differentielle Rotation KW - Tomographie KW - Sternoberfläche KW - Sternaktivität KW - telescope KW - robotic KW - differential rotation KW - tomography KW - stellar surface KW - stellar activity Y1 - 2004 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-0001834 ER - TY - JOUR A1 - Weber, Michael H. A1 - Wicks, Chuck A1 - Krüger, Frank A1 - Jahnke, Gunnar A1 - Baumann, M. T1 - Reply to comment on "Asymmetric radiation of seismic waves from an atoll : Nuclear tests in French Polynesia" by A. Douglas Y1 - 2000 ER - TY - JOUR A1 - Gassner, Alexandra A1 - Thomas, Christine A1 - Krüger, Frank A1 - Weber, Michael H. T1 - Probing the core-mantle boundary beneath Europe and Western Eurasia: A detailed study using PcP JF - Physics of the earth and planetary interiors N2 - We use PcP (the core reflected P phase) recordings of deep earthquakes and nuclear explosions from the Grafenberg (Germany) and NORSAR (Norway) arrays to investigate the core-mantle boundary region beneath Europe and western Eurasia. We find evidence for a previously unknown ultra-low velocity zone 600 km south-east of Moscow, located at the edge of a middle-size low shear- velocity region imaged in seismic tomography that is located beneath the Volga river region. The observed amplitude variations of PcP can be modelled by velocity reductions of P and S-waves of -5% and -15%, respectively, with a density increase of +15%. Travel time delays of pre-and postcursors are indicating a thickness of about 13 km for this ultra-low velocity region (ULVZ). However, our modelling also reveals highly ambiguous amplitude variations of PcP and a reflection off the top of the anomaly for various ULVZs and topography models. Accordingly, large velocity contrasts of up to -10% in V-P and -20% in Vs cannot be excluded. In general, the whole Volga river region shows a complex pattern of PcP amplitudes caused most likely by CMB undulations. Further PcP probes beneath Paris, Kiev and northern Italy indicate likely normal CMB conditions, whereas the samples below Finland and the Hungary-Slovakia border yield strongly amplified PcP signals suggesting strong CMB topography effects. We evaluate the amplitude behaviour of PcP as a function of distance and several ULVZ models using the 1D reflectivity and the 2D Gauss beam method. The influence of the velocity and density perturbations is analysed as well as the anomaly thickness, the dominant period of the source wavelet and interface topographies. Strong variation of the PcP amplitude are obtained as a function of distance and of the impedance contrast. We also consider two types of topographies: undulations atop the CMB in the presence of flat ULVZs and vice versa. Where a broad range of CMB topography dimensions lead to large PcP amplitude variations, only large ULVZ undulations generate significant amplitude scattering. Consequently, this indicates that topography effects of anomalies may mask the true medium parameters as well as the ULVZ thickness. Moreover, there might be a possibility of misinterpreting the precursor as PcP, in particular for thin ULVZs. (C) 2015 Elsevier B.V. All rights reserved. KW - Core-mantle boundary KW - Ultra-low velocity zones KW - Seismology KW - Amplitude and waveform analysis of PcP KW - Earthquakes KW - Nuclear explosions Y1 - 2015 U6 - https://doi.org/10.1016/j.pepi.2015.06.007 SN - 0031-9201 SN - 1872-7395 VL - 246 SP - 9 EP - 24 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Weber, Michael H. A1 - Helwig, S. L. A1 - Bauer, Klaus A1 - Haberland, Christian A1 - Koch, Olaf A1 - Ryberg, T. A1 - Maercklin, N. A1 - Ritter, O. A1 - Schulze, A. T1 - Near-surface properties of an active fault derived by joint interpretation of different geophysical methods - the Arava/Araba Fault in the Middle East JF - Near surface geophysics N2 - The motion of tectonic plates is accommodated at fault zones. One of the unanswered questions about fault zones relates to the role they play in controlling shallow and local hydrology. This study focuses on the Arava/Araba Fault (AF) zone, the southern portion of the Dead Sea Transform (DST) in the Middle East. We combine seismic and electromagnetic methods (EM) to image the geometry and map the petro-physical properties and water occurrence in the top 100 m of this active fault. For three profiles, P-velocity and resistivity images were derived independently. Using a neural network cluster analysis three classes with similar P-velocity and resistivities could then be determined from these images. These classes correspond to spatial domains of specific material and wetness. The first class occurs primarily east of the fault consisting of 'wet' sand (dunes) and brecciated sediments, whereas the second class composed of similar material located west of the fault is 'dry'. The third class lies at depth below ca. 50 m and is composed of highly deformed and weathered Precambrian rocks that constitute the multi-branch fault zone of the AF at this location. The combination of two independent measurements like seismics and EM linked by a stringent mathematical approach has thus shown the potential to delineate the interplay of lithology and water near active faults. Y1 - 2012 U6 - https://doi.org/10.3997/1873-0604.2012031 SN - 1569-4445 VL - 10 IS - 5 SP - 381 EP - 390 PB - European Association of Geoscientists & Engineers CY - Houten ER - TY - JOUR A1 - Braeuer, B. A1 - Asch, Günter A1 - Hofstetter, Rami A1 - Haberland, Christian A1 - Jaser, Darweesh A1 - El-Kelani, Radwan J.. A1 - Weber, Michael H. T1 - Microseismicity distribution in the southern Dead Sea basin and its implications on the structure of the basin JF - Geophysical journal international N2 - While the Dead Sea basin has been studied for a long time, the available knowledge about the detailed seismicity distribution in the area, as well as the deeper structure of the basin, is limited. Therefore, within the framework of the international project DESIRE (DEad Sea Integrated REsearch project), a dense temporary local seismological network was operated in the southern Dead Sea area. We use 530 local earthquakes, having all together 26 730 P- and S-arrival times for a simultaneous inversion of 1-D velocity models, station corrections and precise earthquake locations. Jackknife tests suggest an accuracy of the derived hypocentre locations of about 1 km. Thus, the result is the first clear image of the absolute distribution of the microseismicity of the area, especially in depth. The seismicity is concentrated in the upper crust down to 20 km depth while the lower limit of the seismicity is reached at 31 km depth. The seismic events at the eastern boundary fault (EBF) in the southern part of the study area represent the northward transform motion of the Arabian Plate along the Dead Sea Transform. North of the Boqeq fault the seismic activity represents the transfer of the motion in the pull-apart basin from the eastern to the western boundary. We find that from the surface downward the seismic events are tracing the boundary faults of the basin. The western boundary is mapped down to 12 km depth while the EBF reaches about 17 km depth, forming an asymmetric basin. One fifth of the data set is related to a specific cluster in time and space, which occurred in 2007 February at the western border fault. This cluster is aligned vertically, that is, it is perpendicular to the direction of the dominating left-lateral strike-slip movement at the main transform fault. KW - Seismicity and tectonics KW - Continental tectonics: strike-slip and transform KW - Asia Y1 - 2012 U6 - https://doi.org/10.1111/j.1365-246X.2011.05318.x SN - 0956-540X VL - 188 IS - 3 SP - 873 EP - 878 PB - Wiley-Blackwell CY - Malden ER - TY - JOUR A1 - Vales, Dina A1 - Dias, Nuno A. A1 - Rio, Ines A1 - Matias, Luis A1 - Silveira, Graca A1 - Madeira, Jose A1 - Weber, Michael H. A1 - Carrilho, Fernando A1 - Haberland, Christian T1 - Intraplate seismicity across the Cape Verde swell: A contribution from a temporary seismic network JF - Tectonophysics : international journal of geotectonics and the geology and physics of the interior of the earth N2 - We present an analysis and characterization of the regional seismicity recorded by a temporary broadband seismic network deployed in the Cape Verde archipelago between November 2007 and September 2008. The detection of earthquakes was based on spectrograms, allowing the discrimination from low-frequency volcanic signals, resulting in 358 events of which 265 were located, the magnitudes usually being smaller than 3. For the location, a new 1-D P-velocity model was derived for the region showing a crust consistent with an oceanic crustal structure. The seismicity is located mostly offshore the westernmost and geologically youngest areas of the archipelago, near the islands of Santo Antao and Sao Vicente in the NW and Brava and Fogo in the SW. The SW cluster has a lower occurrence rate and corresponds to seismicity concentrated mainly along an alignment between Brava and the Cadamosto seamount presenting normal faulting mechanisms. The existence of the NW cluster, located offshore SW of Santo Antao, was so far unknown and concentrates around a recently recognized submarine cone field; this cluster presents focal depths extending from the crust to the upper mantle and suggests volcanic unrest No evident temporal behaviour could be perceived, although the events tend to occur in bursts of activity lasting a few days. In this recording period, no significant activity was detected at Fogo volcano, the most active volcanic edifice in Cape Verde. The seismicity characteristics point mainly to a volcanic origin. The correlation of the recorded seismicity with active volcanic structures agrees with the tendency for a westward migration of volcanic activity in the archipelago as indicated by the geologic record. (C) 2014 Elsevier B.V. All rights reserved. KW - Intraplate seismicity KW - Clustering KW - Local magnitude scale KW - Active volcanism KW - Cape Verde KW - Atlantic Ocean Y1 - 2014 U6 - https://doi.org/10.1016/j.tecto.2014.09.014 SN - 0040-1951 SN - 1879-3266 VL - 636 SP - 325 EP - 337 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Maercklin, Nils A1 - Haberland, Christian A1 - Ryberg, Trond A1 - Weber, Michael H. A1 - Bartov, Yosef T1 - Imaging the Dead Sea Transform with scattered seismic waves N2 - With controlled seismic sources and specifically designed receiver arrays, we image a subvertical boundary between two lithological blocks at the Arava Fault (AF) in the Middle East. The AF is the main strike-slip fault of the Dead Sea Transform (DST) in the segment between the Dead Sea and the Red Sea. Our imaging (migration) method is based on array beamforming and coherence analysis of P to P scattered seismic phases. We use a 1-D background velocity model and the direct P arrival as a reference phase. Careful resolution testing is necessary, because the target volume is irregularly sampled by rays. A spread function describing energy dispersion at localized point scatterers and synthetic calculations for large planar structures provides estimates of the resolution of the images. We resolve a 7 km long steeply dipping reflector offset roughly 1 km from the surface trace of the AF. The reflector can be imaged from about 1 km down to 4 km depth. Previous and ongoing studies in this region have shown a strong contrast across the fault: low seismic velocities and electrical resistivities to the west and high velocities and resistivities to the east of it. We therefore suggest that the imaged reflector marks the contrast between young sedimentary fill in the west and Precambrian rocks in the east. If correct, the boundary between the two blocks is offset about 1 km east of the current surface trace of the AF Y1 - 2004 SN - 0956-540X ER - TY - JOUR A1 - Bräuer, B. A1 - Asch, Günter A1 - Hofstetter, Rami A1 - Haberland, Christian A1 - Jaser, D. A1 - El-Kelani, R. A1 - Weber, Michael H. T1 - High-resolution local earthquake tomography of the southern Dead Sea area JF - Geophysical journal international N2 - Local earthquake data from a dense temporary seismological network in the southern Dead Sea area have been analysed within the project DESIRE (Dead Sea Integrated Research Project). Local earthquakes are used for the first precise image of the distribution of the P-wave velocity and the vP/vS ratios. 65 stations registered 655 local events within 18 months of observation time. A subset of 530 well-locatable events with 26 730 P- and S-arrival times was used to calculate a tomographic model for the vP and vP/vS distribution. Since the study area is at first-order 2-D, a gradual approach was chosen, which compromised a 2-D inversion followed by a 3-D inversion. The sedimentary basin fill is clearly imaged through high vP/vS ratios and low vP. The basin fill shows an asymmetric structure with average depth of 7 km at the western boundary and depth between 10 and 14 km at the eastern boundary. This asymmetry is reflected by the vertical strike-slip eastern border fault, and the normal faulting at the western boundary, caused by the transtensional deformation within the last 5 Myr. Within the basin fill the Lisan salt diapir is imaged through low vP/vS ratios, reflecting its low fluid content. The extensions were determined to 12 km in EW and 17 km in NS direction while its depth is 56 km. The thickness of the pre-basin sediments below the basin fill cannot be derived from the tomography datait is estimated to less than 3 km from former investigations. Below the basin, down to 18 km depth very low P-wave velocities and low vP/vS ratios are observedmost likely caused by fluids from the surrounding crust or the upper mantle. KW - Seismic tomography KW - Continental margins: transform KW - Continental tectonics: strike-slip and transform Y1 - 2012 U6 - https://doi.org/10.1111/j.1365-246X.2012.05668.x SN - 0956-540X VL - 191 IS - 3 SP - 881 EP - 897 PB - Wiley-Blackwell CY - Hoboken ER - TY - JOUR A1 - Vinnik, L. A1 - Kiselev, S. A1 - Weber, Michael H. A1 - Oreshin, S. A1 - Makeyeva, L. T1 - Frozen and active seismic anisotropy beneath southern Africa JF - Geophysical research letters N2 - P receiver functions from 23 stations of the SASE experiment in southern Africa are inverted simultaneously with SKS waveforms for azimuthal anisotropy in the upper mantle. Our analysis resolves the long-standing issue of depth dependence and origins of anisotropy beneath southern Africa. In the uppermost mantle we observe anisotropy with a nearly E-W fast direction, parallel to the trend of the Limpopo belt. This anisotropy may be frozen since the Archean. At a depth of 160 km the fast direction of anisotropy changes to 40 degrees and becomes close to the recent plate motion direction. This transition is nearly coincident in depth with activation of dominant glide systems in olivine and with a pronounced change in other properties of the upper mantle. Another large change in the fast direction of anisotropy corresponds to the previously found low-S-velocity layer atop the 410-km discontinuity. Citation: Vinnik, L., S. Kiselev, M. Weber, S. Oreshin, and L. Makeyeva (2012), Frozen and active seismic anisotropy beneath southern Africa, Geophys. Res. Lett., 39, L08301, doi: 10.1029/2012GL051326. Y1 - 2012 U6 - https://doi.org/10.1029/2012GL051326 SN - 0094-8276 VL - 39 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Scherbaum, Frank A1 - Krüger, Frank A1 - Weber, Michael H. T1 - Double beam imaging : mapping lower mantle heterogeneities using combinations of source and receiver arrays Y1 - 1997 ER - TY - JOUR A1 - Braeuer, Benjamin A1 - Asch, Günter A1 - Hofstetter, Rami A1 - Haberland, Christian A1 - Jaser, D. A1 - El-Kelani, R. A1 - Weber, Michael H. T1 - Detailed seismicity analysis revealing the dynamics of the southern Dead Sea area JF - Journal of seismology N2 - Within the framework of the international DESIRE (DEad Sea Integrated REsearch) project, a dense temporary local seismological network was operated in the southern Dead Sea area. During 18 recording months, 648 events were detected. Based on an already published tomography study clustering, focal mechanisms, statistics and the distribution of the microseismicity in relation to the velocity models from the tomography are analysed. The determined b value of 0.74 leads to a relatively high risk of large earthquakes compared to the moderate microseismic activity. The distribution of the seismicity indicates an asymmetric basin with a vertical strike-slip fault forming the eastern boundary of the basin, and an inclined western boundary, made up of strike-slip and normal faults. Furthermore, significant differences between the area north and south of the Bokek fault were observed. South of the Bokek fault, the western boundary is inactive while the entire seismicity occurs on the eastern boundary and below the basin-fill sediments. The largest events occurred here, and their focal mechanisms represent the northwards transform motion of the Arabian plate along the Dead Sea Transform. The vertical extension of the spatial and temporal cluster from February 2007 is interpreted as being related to the locking of the region around the Bokek fault. North of the Bokek fault similar seismic activity occurs on both boundaries most notably within the basin-fill sediments, displaying mainly small events with strike-slip mechanism and normal faulting in EW direction. Therefore, we suggest that the Bokek fault forms the border between the single transform fault and the pull-apart basin with two active border faults. KW - Dead Sea basin KW - Microseismicity KW - Cluster KW - Pull-apart basin KW - Asymmetric basin KW - Transform fault Y1 - 2014 U6 - https://doi.org/10.1007/s10950-014-9441-4 SN - 1383-4649 SN - 1573-157X VL - 18 IS - 4 SP - 731 EP - 748 PB - Springer CY - Dordrecht ER - TY - JOUR A1 - Muksin, Umar A1 - Haberland, Christian A1 - Nukman, Mochamad A1 - Bauer, Klaus A1 - Weber, Michael H. T1 - Detailed fault structure of the Tarutung Pull-Apart Basin in Sumatra, Indonesia, derived from local earthquake data JF - Journal of Asian earth sciences N2 - The Tarutung Basin is located at a right step-over in the northern central segment of the dextral strike-slip Sumatran Fault System (SFS). Details of the fault structure along the Tarutung Basin are derived from the relocations of seismicity as well as from focal mechanism and structural geology. The seismicity distribution derived by a 3D inversion for hypocenter relocation is clustered according to a fault-like seismicity distribution. The seismicity is relocated with a double-difference technique (HYPODD) involving the waveform cross-correlations. We used 46,904 and 3191 arrival differences obtained from catalogue data and cross-correlation analysis, respectively. Focal mechanisms of events were analyzed by applying a grid search method (HASH code). Although there is no significant shift of the hypocenters (10.8 m in average) and centroids (167 m in average), the application of the double difference relocation sharpens the earthquake distribution. The earthquake lineation reflects the fault system, the extensional duplex fault system, and the negative flower structure within the Tarutung Basin. The focal mechanisms of events at the edge of the basin are dominantly of strike-slip type representing the dextral strike-slip Sumatran Fault System. The almost north south striking normal fault events along extensional zones beneath the basin correlate with the maximum principal stress direction which is the direction of the Indo-Australian plate motion. The extensional zones form an en-echelon pattern indicated by the presence of strike-slip faults striking NE SW to NW SE events. The detailed characteristics of the fault system derived from the seismological study are also corroborated by structural geology at the surface. (C) 2014 Elsevier Ltd. All rights reserved. KW - Focal mechanism KW - Seismicity KW - Structural geology KW - Extensional duplex KW - Flower structure KW - Sumatran fault KW - Pull-Apart Basin Y1 - 2014 U6 - https://doi.org/10.1016/j.jseaes.2014.09.009 SN - 1367-9120 SN - 1878-5786 VL - 96 SP - 123 EP - 131 PB - Elsevier CY - Oxford ER - TY - JOUR A1 - Wolbern, I A1 - Jacob, A. W. B. A1 - Blake, T. A. A1 - Kind, Rainer A1 - Li, X A1 - Yuan, X. H A1 - Duennebier, F A1 - Weber, Michael H. T1 - Deep origin of the Hawaiian tilted plume conduit derived from receiver functions N2 - We employ P to S converted waveforms to investigate effects of the hot mantle plume on seismic discontinuities of the crust and upper mantle. We observe the Moho at depths between 13 and 17 km, regionally covered by a strong shallow intracrustal converted phase. Coherent phases on the transverse component indicate either dipping interfaces, 3- D heterogeneities or lower crustal anisotropy. We find anomalies related to discontinuities in the upper mantle down to the transition zone evidently related to the hot mantle plume. Lithospheric thinning is confirmed in greater detail than previously reported by Li et al., and we determine the dimensions of the low-velocity zone within the asthenosphere with greater accuracy. Our study mainly focuses on the temperature-pressure dependent discontinuities of the upper mantle transition zone. Effects of the hot diapir on the depths of mineral phase transitions are verified at both major interfaces at 410 and 660 km. We determine a plume radius of about 200 km at the 660 km discontinuity with a core zone of about 120 km radius. The plume conduit is located southwest of Big Island. A conduit tilted in the northeast direction is required in the upper mantle to explain the observations. The determined positions of deflections of the discontinuities support the hypothesis of decoupled upper and lower mantle convection Y1 - 2006 UR - http://onlinelibrary.wiley.com/doi/10.1111/j.1365-246X.2006.03036.x/full U6 - https://doi.org/10.1111/j.1365-246X.2006.03036.x ER - TY - JOUR A1 - Stankiewicz, Jacek A1 - Weber, Michael H. A1 - Mohsen, Ayman A1 - Hofstetter, Rami T1 - Dead Sea Basin imaged by ambient seismic noise tomography JF - Pure and applied geophysics N2 - In the framework of the Dead Sea Integrated Research project (DESIRE), 59 seismological stations were deployed in the region of the Dead Sea Basin. Twenty of these stations recorded data of sufficiently high quality between May and September 2007 to be used for ambient seismic noise analysis. Empirical Green's functions are extracted from cross-correlations of long term recordings. These functions are dominated by Rayleigh waves, whose group velocities can be measured in the frequency range from 0.1 to 0.5 Hz. Analysis of positive and negative correlation lags of the Green's functions makes it possible to identify the direction of the source of the incoming energy. Signals with frequencies higher than 0.2 Hz originate from the Mediterranean Sea, while low frequencies arrive from the direction of the Red Sea. Travel times of the extracted Rayleigh waves were measured between station pairs for different frequencies, and tomographically inverted to provide independent velocity models. Four such 2D models were computed for a set of frequencies, all corresponding to different sampling depths, and thus together giving an indication of the velocity variations in 3D extending to a depth of 10 km. The results show low velocities in the Dead Sea Basin, consistent with previous studies suggesting up to 8 km of recent sedimentary infill in the Basin. The complex structure of the western margin of the Basin is also observed, with sedimentary infill present to depths not exceeding 5 km west of the southern part of the Dead Sea. The high velocities associated with the Lisan salt diapir are also observed down to a depth of similar to 5 km. The reliability of the results is confirmed by checkerboard recovery tests. KW - Dead Sea Basin KW - ambient noise KW - tomography Y1 - 2012 U6 - https://doi.org/10.1007/s00024-011-0350-y SN - 0033-4553 VL - 169 IS - 4 SP - 615 EP - 623 PB - Springer CY - Basel ER - TY - JOUR A1 - Heit, Benjamin A1 - Yuan, Xiaohui A1 - Weber, Michael H. A1 - Geissler, Wolfram H. A1 - Jokat, Wilfried A1 - Lushetile, Bufelo A1 - Hoffmann, Karl-Heinz T1 - Crustal thickness and V-p/V-s ratio in NW Namibia from receiver functions: Evidence for magmatic underplating due to mantle plume-crust interaction JF - Geophysical research letters N2 - A seismological network was operated at the junction of the aseismic Walvis Ridge with the northwestern Namibian coast. We mapped crustal thickness and bulk V-p/V-s ratio by the H-k analysis of receiver functions. In the Damara Belt, the crustal thickness is similar to 35km with a V-p/V-s ratio of <1.75. The crust is similar to 30km thick at the coast in the Kaoko Belt. Strong variations in crustal thickness and V-p/V-s ratios are found at the landfall of the Walvis Ridge. Here and at similar to 150km northeast of the coast, the crustal thickness increases dramatically reaching 44km and the V-p/V-s ratios are extremely high (similar to 1.89). These anomalies are interpreted as magmatic underplating produced by the mantle plume during the breakup of Gondwana. The area affected by the plume is smaller than 300km in diameter, possibly ruling out the existence of a large plume head under the continent during the breakup. KW - crustal thickness KW - V-p KW - V-s ratios KW - magmatic underplating KW - Walvis Ridge KW - continental breakup Y1 - 2015 U6 - https://doi.org/10.1002/2015GL063704 SN - 0094-8276 SN - 1944-8007 VL - 42 IS - 9 SP - 3330 EP - 3337 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Mechie, James A1 - Abu-Ayyash, Khalil A1 - Ben-Avraham, Zvi A1 - El-Kelani, Radwan A1 - Qabbani, Isam A1 - Weber, Michael H. T1 - Crustal structure of the southern Dead Sea basin derived from project DESIRE wide-angle seismic data N2 - 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. Y1 - 2009 UR - http://onlinelibrary.wiley.com/journal/10.1111/(ISSN)1365-246X U6 - https://doi.org/10.1111/j.1365-246X.2009.04161.x SN - 0956-540X ER - TY - JOUR A1 - Mohsen, Amjad A1 - Asch, Günter A1 - Mechie, James A1 - Kind, Rainer A1 - Hofstetter, Rami A1 - Weber, Michael H. A1 - Stiller, M. A1 - Abu-Ayyash, Khalil T1 - Crustal structure of the Dead Sea Basin (DSB) from a receiver function analysis JF - Geophysical journal international N2 - 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. KW - Transform faults KW - Crustal structure Y1 - 2011 U6 - https://doi.org/10.1111/j.1365-246X.2010.04853.x SN - 0956-540X VL - 184 IS - 1 SP - 463 EP - 476 PB - Wiley-Blackwell CY - Malden ER - TY - JOUR A1 - Ryberg, Trond A1 - Haberland, Christian A1 - Haberlau, Thomas A1 - Weber, Michael H. A1 - Bauer, Klaus A1 - Behrmann, Jan H. A1 - Jokat, Wilfried T1 - Crustal structure of northwest Namibia: Evidence for plume-rift-continent interaction JF - Geology N2 - The causes for the formation of large igneous provinces and hotspot trails are still a matter of considerable dispute. Seismic tomography and other studies suggest that hot mantle material rising from the core-mantle boundary (CMB) might play a significant role in the formation of such hotspot trails. An important area to verify this concept is the South Atlantic region, with hotspot trails that spatially coincide with one of the largest low-velocity regions at the CMB, the African large low shear-wave velocity province. The Walvis Ridge started to form during the separation of the South American and African continents at ca. 130 Ma as a consequence of Gondwana breakup. Here, we present the first deep-seismic sounding images of the crustal structure from the landfall area of the Walvis Ridge at the Namibian coast to constrain processes of plume-lithosphere interaction and the formation of continental flood basalts (Parana and Etendeka continental flood basalts) and associated intrusive rocks. Our study identified a narrow region (<100 km) of high-seismic-velocity anomalies in the middle and lower crust, which we interpret as a massive mafic intrusion into the northern Namibian continental crust. Seismic crustal reflection imaging shows a flat Moho as well as reflectors connecting the high-velocity body with shallow crustal structures that we speculate to mark potential feeder channels of the Etendeka continental flood basalt. We suggest that the observed massive but localized mafic intrusion into the lower crust results from similar-sized variations in the lithosphere (i.e., lithosphere thickness or preexisting structures). Y1 - 2015 U6 - https://doi.org/10.1130/G36768.1 SN - 0091-7613 SN - 1943-2682 VL - 43 IS - 8 SP - 739 EP - 742 PB - American Institute of Physics CY - Boulder ER - TY - JOUR A1 - Mechie, James A1 - Abu-Ayyash, Khalil A1 - Ben-Avraham, Zvi A1 - El-Kelani, R. A1 - Mohsen, Amjad A1 - Rumpker, Georg A1 - Saul, J. A1 - Weber, Michael H. T1 - Crustal shear velocity structure across the Dead Sea Transform from two-dimensional modelling of DESERT project explosion seismic data N2 - An analysis of the shear (S) waves recorded during the wide-angle reflection/refraction (WRR) experiment as part of the DESERT project crossing the Dead Sea Transform (DST) reveals average crustal S-wave velocities of 3.3-3.5 km s(-1) beneath the WRR profile. Together with average crustal P-wave velocities of 5.8-6.1 km s(-1) from an already published study this provides average crustal Poisson's ratios of 0.26-0.27 (V-p/V-s = 1.76-1.78) below the profile. The top two layers consisting predominantly of sedimentary rocks have S- wave velocities of 1.8-2.7kms(-1) and Poisson's ratios of 0.25-0.31 (V-p/V-s = 1.73-1.91). Beneath these two layers the seismic basement has average S- wave velocities of around 3.6 km s(-1) east of the DST and about 3.7 km s(-1) west of the DST and Poisson's ratios of 0.24-0.25 (V-p/V-s = 1.71-1.73). The lower crust has an average S-wave velocity of about 3.75 km s(-1) and an average Poisson's ratio of around 0.27 (V-p/V-s = 1.78). No Sn phase refracted through the uppermost mantle was observed. The results provide for the first time information from controlled source data on the crustal S-wave velocity structure for the region west of the DST in Israel and Palestine and agree with earlier results for the region east of the DST in the Jordanian highlands. A shear wave splitting study using SKS waves has found evidence for crustal anisotropy beneath the WRR profile while a receiver function study has found evidence for a lower crustal, high S-wave velocity layer east of the DST below the profile. Although no evidence was found in the S-wave data for either feature, the S-wave data are not incompatible with crustal anisotropy being present as the WRR profile only lies 30 degrees off the proposed symmetry axis of the anisotropy where the difference in the two S-wave velocities is still very small. In the case of the lower crustal, high S-wave velocity layer, if the velocity change at the top of this layer comprises a small first-order discontinuity underlain by a 2 km thick transition zone, instead of just a large first-order discontinuity, then both the receiver function data and the WRR data presented here can be satisfied. Finally, the S-wave velocities and Poisson's ratios which have been derived in this study are typical of continental crust and do not require extensional processes to explain them Y1 - 2005 ER - TY - JOUR A1 - Maercklin, Nils A1 - Bedrosian, Paul A. A1 - Haberland, Christian A1 - Ritter, O. A1 - Ryberg, Trond A1 - Weber, Michael H. A1 - Weckmann, Ute T1 - Characterizing a large shear-zone with seismic and magnetotelluric methods : the case of the Dead Sea Transform N2 - Seismic tomography, imaging of seismic scatterers, and magnetotelluric soundings reveal a sharp lithologic contrast along a similar to 10 km long segment of the Arava Fault (AF), a prominent fault of the southern Dead Sea Transform (DST) in the Middle East. Low seismic velocities and resistivities occur on its western side and higher values east of it, and the boundary between the two units coincides partly with a seismic scattering image. At 1 - 4 km depth the boundary is offset to the east of the AF surface trace, suggesting that at least two fault strands exist, and that slip occurred on multiple strands throughout the margin's history. A westward fault jump, possibly associated with straightening of a fault bend, explains both our observations and the narrow fault zone observed by others Y1 - 2005 SN - 0094-8276 ER - TY - JOUR A1 - Vinnik, Lev A1 - Silveira, Graca A1 - Kiselev, Sergei A1 - Farra, Veronique A1 - Weber, Michael H. A1 - Stutzmann, Eleonore T1 - Cape verde hotspot from the upper crust to the top of the lower mantle JF - Earth & planetary science letters N2 - We investigate the crust, upper mantle and mantle transition zone of the Cape Verde hotspot by using seismic P and S receiver functions from several tens of local seismograph stations. We find a strong discontinuity at a depth of similar to 10 km underlain by a similar to 15-km thick layer with a high (similar to 1.9) Vp/Vs velocity ratio. We interpret this discontinuity and the underlying layer as the fossil Moho, inherited from the pre-hotspot era, and the plume-related magmatic underplate. Our uppermost-mantle models are very different from those previously obtained for this region: our S velocity is much lower and there are no indications of low densities. Contrary to previously published arguments for the standard transition zone thickness our data indicate that this thickness under the Cape Verde islands is up to similar to 30 km less than in the ambient mantle. This reduction is a combined effect of a depression of the 410-km discontinuity and an uplift of the 660-km discontinuity. The uplift is in contrast to laboratory data and some seismic data on a negligible dependence of depth of the 660-km discontinuity on temperature in hotspots. A large negative pressure-temperature slope which is suggested by our data implies that the 660-km discontinuity may resist passage of the plume. Our data reveal beneath the islands a reduction of S velocity of a few percent between 470-km and 510-km depths. The low velocity layer in the upper transition zone under the Cape Verde archipelago is very similar to that previously found under the Azores and a few other hotspots. In the literature there are reports on a regional 520-km discontinuity, the impedance of which is too large to be explained by the known phase transitions. Our observations suggest that the 520-km discontinuity may present the base of the low-velocity layer in the transition zone. KW - hotspot KW - plume KW - crust KW - upper mantle KW - mantle transition zone KW - receiver function Y1 - 2012 U6 - https://doi.org/10.1016/j.epsl.2011.12.017 SN - 0012-821X VL - 319 IS - 4 SP - 259 EP - 268 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Weber, Michael H. A1 - Wicks, Chuck A1 - Krüger, Frank A1 - Jahnke, Gunnar A1 - Schlittenhardt, Jörg T1 - Asymmetric radiation of seismic waves from an atoll : nuclear tests in French Polynesia Y1 - 1998 ER - TY - JOUR A1 - Weber, Michael H. A1 - Abu-Ayyash, Khalil A1 - Abueladas, Abdel-Rahman A1 - Agnon, Amotz A1 - Alasonati-Tašárová, Zuzana A1 - Al-Zubi, Hashim A1 - Babeyko, Andrey A1 - Bartov, Yuval A1 - Bauer, Klaus A1 - Becken, Michael A1 - Bedrosian, Paul A. A1 - Ben-Avraham, Zvi A1 - Bock, Günter A1 - Bohnhoff, Marco A1 - Bribach, Jens A1 - Dulski, Peter A1 - Ebbing, Joerg A1 - El-Kelani, Radwan J. A1 - Foerster, Andrea A1 - Förster, Hans-Jürgen A1 - Frieslander, Uri A1 - Garfunkel, Zvi A1 - Götze, Hans-Jürgen A1 - Haak, Volker A1 - Haberland, Christian A1 - Hassouneh, Mohammed A1 - Helwig, Stefan L. A1 - Hofstetter, Alfons A1 - Hoffmann-Rothe, Arne A1 - Jaeckel, Karl-Heinz A1 - Janssen, Christoph A1 - Jaser, Darweesh A1 - Kesten, Dagmar A1 - Khatib, Mohammed Ghiath A1 - Kind, Rainer A1 - Koch, Olaf A1 - Koulakov, Ivan A1 - Laske, Maria Gabi A1 - Maercklin, Nils T1 - Anatomy of the Dead Sea transform from lithospheric to microscopic scale N2 - Fault zones are the locations where motion of tectonic plates, often associated with earthquakes, is accommodated. Despite a rapid increase in the understanding of faults in the last decades, our knowledge of their geometry, petrophysical properties, and controlling processes remains incomplete. The central questions addressed here in our study of the Dead Sea Transform (DST) in the Middle East are as follows: (1) What are the structure and kinematics of a large fault zone? (2) What controls its structure and kinematics? (3) How does the DST compare to other plate boundary fault zones? The DST has accommodated a total of 105 km of left-lateral transform motion between the African and Arabian plates since early Miocene (similar to 20 Ma). The DST segment between the Dead Sea and the Red Sea, called the Arava/Araba Fault (AF), is studied here using a multidisciplinary and multiscale approach from the mu m to the plate tectonic scale. We observe that under the DST a narrow, subvertical zone cuts through crust and lithosphere. First, from west to east the crustal thickness increases smoothly from 26 to 39 km, and a subhorizontal lower crustal reflector is detected east of the AF. Second, several faults exist in the upper crust in a 40 km wide zone centered on the AF, but none have kilometer-size zones of decreased seismic velocities or zones of high electrical conductivities in the upper crust expected for large damage zones. Third, the AF is the main branch of the DST system, even though it has accommodated only a part (up to 60 km) of the overall 105 km of sinistral plate motion. Fourth, the AF acts as a barrier to fluids to a depth of 4 km, and the lithology changes abruptly across it. Fifth, in the top few hundred meters of the AF a locally transpressional regime is observed in a 100-300 m wide zone of deformed and displaced material, bordered by subparallel faults forming a positive flower structure. Other segments of the AF have a transtensional character with small pull-aparts along them. The damage zones of the individual faults are only 5-20 m wide at this depth range. Sixth, two areas on the AF show mesoscale to microscale faulting and veining in limestone sequences with faulting depths between 2 and 5 km. Seventh, fluids in the AF are carried downward into the fault zone. Only a minor fraction of fluids is derived from ascending hydrothermal fluids. However, we found that on the kilometer scale the AF does not act as an important fluid conduit. Most of these findings are corroborated using thermomechanical modeling where shear deformation in the upper crust is localized in one or two major faults; at larger depth, shear deformation occurs in a 20-40 km wide zone with a mechanically weak decoupling zone extending subvertically through the entire lithosphere. Y1 - 2009 UR - http://www.agu.org/journals/rg/ U6 - https://doi.org/10.1029/2008rg000264 SN - 8755-1209 ER - TY - JOUR A1 - Thomas, Ch. A1 - Igel, Heiner A1 - Weber, Michael H. A1 - Scherbaum, Frank T1 - Acoustic simulation of P-wave propagation in a heterogeneous spherical earth : numerical method and application to precursor waves to PKPdf Y1 - 2000 ER - TY - JOUR A1 - Mohsen, Amjad A1 - Hofstetter, Rami A1 - Bock, Günter A1 - Kind, Rainer A1 - Weber, Michael H. A1 - Wylegalla, Kurt A1 - Rumpker, Georg T1 - A receiver function study across the Dead Sea Transform N2 - We report on a receiver function study of the crust and upper mantle within DESERT, a multidisciplinary geophysical project to study the lithosphere across the Dead Sea Transform (DST). A temporary seismic network was operated on both sides of the DST between 2000 April and 2001 June. The depth of the Moho increases smoothly from about 30 to 34-38 km towards the east across the DST, with significant north-south variations east of the DST. These Moho depth estimates from receiver functions are consistent with results from steep-and wide-angle controlled-source techniques. Steep-angle reflections and receiver functions reveal an additional discontinuity in the lower crust, but only east of the DST. This leads to the conclusion that the internal crustal structure east and west of the DST is different. The P to S converted phases from both discontinuities at 410 and 660 km are delayed by 2 s with respect to the IASP91 global reference model. This would indicate that the transition zone is consistent with the global average, but the upper mantle above 410 km is 3-4 per cent slower than the standard earth model Y1 - 2005 SN - 0956-540X ER -