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Controls on the deformation pattern (shortening mode and tectonic style) of orogenic forelands during tectonic shortening remain poorly understood. Here, we use high-resolution 2D thermomechanical ...

Giant earthquakes with magnitudes above 8.5 occur only in subduction zones. Despite the developments made in observing large subduction zone earthquakes with geophysical instruments, the factors controlling the maximum size of these earthquakes are still poorly understood. Previous studies have suggested the importance of slab shape, roughness of the plate interface contact, state of the strain in the upper plate, thickness of sediments filling the trenches, and subduction rate. Here, we present 2‐D cross‐scale numerical models of seismic cycles for subduction zones with various geometries, subduction channel friction configurations, and subduction rates. We found that low‐angle subduction and thick sediments in the subduction channel are the necessary conditions for generating giant earthquakes, while the subduction rate has a negligible effect. We suggest that these key parameters determine the maximum magnitude of a subduction earthquake by controlling the seismogenic zone width and smoothness of the subduction interface. This interpretation supports previous studies that are based upon observations and scaling laws. Our modeling results also suggest that low static friction in the sediment‐filled subduction channel results in neutral or moderate compressive deformation in the overriding plate for low‐angle subduction zones hosting giant earthquakes. These modeling results agree well with observations for the largest earthquakes. Based on our models we predict maximum magnitudes of subduction earthquakes worldwide, demonstrating the fit to magnitudes of all giant earthquakes of the 20th and 21st centuries and good agreement with the predictions based on statistical analyses of observations.

The classical Wilson Cycle concept, describing repeated opening and closing of ocean basins, hypothesizes spontaneous conversion of passive continental margins into subduction zones. This process, however, is impeded by the high strength of passive margins, and it has never occurred in Cenozoic times. Here using thermomechanical models, we show that additional forcing, provided by mantle flow, which is induced by neighboring subduction zones and midmantle slab remnants, can convert a passive margin into a subduction zone. Models suggest that this is a long-term process, thus explaining the lack of Cenozoic examples. We speculate that new subduction zones may form in the next few tens of millions of years along the Argentine passive margin and the U.S. East Coast. Mantle suction force can similarly trigger subduction initiation along large oceanic fracture zones. We propose that new subduction zones will preferentially originate where subduction zones were active in the past, thus explaining the remarkable colocation of subduction zones during at least the last 400 Myr.

Subduction is substantially multi-scale process where the stresses are built by long-term tectonic motions, modified by sudden jerky deformations during earthquakes, and then restored by following multiple relaxation processes. Here, we develop a cross-scale thermomechanical model aimed to simulate the subduction process from 1 minute to million years' time scale. The model employs elasticity, nonlinear transient viscous rheology, and rate-and-state friction. It generates spontaneous earthquake sequences and by using an adaptive time-step algorithm, recreates the deformation process as observed naturally during the seismic cycle and multiple seismic cycles. The model predicts that viscosity in the mantle wedge drops by more than three orders of magnitude during the great earthquake with a magnitude above 9. As a result, the surface velocities just an hour or day after the earthquake are controlled by viscoelastic relaxation in the several hundred km of mantle landward of the trench and not by the afterslip localized at the fault as is currently believed. Our model replicates centuries-long seismic cycles exhibited by the greatest earthquakes and is consistent with the postseismic surface displacements recorded after the Great Tohoku Earthquake. We demonstrate that there is no contradiction between extremely low mechanical coupling at the subduction megathrust in South Chile inferred from long-term geodynamic models and appearance of the largest earthquakes, like the Great Chile 1960 Earthquake.

An inclined zone of intermediate-depth seismicity beneath the Pamir orogen in Central Asia has been interpreted as southward subduction of a slab of Asian lithosphere. However, it is not known whether Asian lithosphere subducts intact or only partially. We used arrival times of shallow and intermediate-depth earthquakes, recorded with a temporary (2008–2010) seismic network in this region, to invert for 3D models of seismic velocities in an attempt to answer this question. With local seismicity reaching depths of up to 240 km, the deep structure of the Pamir could be illuminated with high resolution. The resulting velocity models show a north–south contrast in crustal seismic velocities in the Pamir, with very low P velocities (5.7–5.9 km/s at 15–30 km depth), coupled with relatively low vp/vsvp/vs (

We present the GITEWS approach to source modeling for the tsunami early warning in Indonesia. Near-field tsunami implies special requirements to both warning time and details of source characterization. To meet these requirements, we employ geophysical and geological information to predefine a maximum number of rupture parameters. We discretize the tsunamigenic Sunda plate interface into an ordered grid of patches (150×25) and employ the concept of Green's functions for forward and inverse rupture modeling. Rupture Generator, a forward modeling tool, additionally employs different scaling laws and slip shape functions to construct physically reasonable source models using basic seismic information only (magnitude and epicenter location). GITEWS runs a library of semi- and fully-synthetic scenarios to be extensively employed by system testing as well as by warning center personnel teaching and training. Near real-time GPS observations are a very valuable complement to the local tsunami warning system. Their inversion provides quick (within a few minutes on an event) estimation of the earthquake magnitude, rupture position and, in case of sufficient station coverage, details of slip distribution.

The Indonesian archipelago is known for the occurrence of catastrophic earthquake-generated tsunamis along the Sunda Arc. The tsunami hazard associated with submarine landslides however has not been fully addressed. In this paper, we compile the known tsunamigenic events where landslide involvement is certain and summarize the properties of published landslides that were identified with geophysical methods. We depict novel mass movements, found in newly available bathymetry, and determine their key parameters. Using numerical modeling, we compute possible tsunami scenarios. Furthermore, we propose a way of identifying landslide tsunamis using an array of few buoys with bottom pressure units.

Our analysis of new bathymetric data reveals six submarine landslides at the eastern Sunda margin between central Java and Sumba Island, Indonesia. Their volumes range between 1 km³ in the Java fore-arc basin up to 20 km³ at the trench off Sumba and Sumbawa. We estimate the potential hazard of each event by modeling the corresponding tsunami and its run-up on nearby coasts. Four slides are situated remarkably close to the epicenter of the 1977 tsunamigenic Sumba M w = 8.3 earthquake. However, comparison of documented tsunami run-up heights and arrival times with our modeling results neither allows us to confirm nor can we falsify the hypothesis that the earthquake triggered these submarine landslides.

Pull-apart basins are depressions that form as the result of crustal extension along strike-slip systems where the sense of fault stepping or bending coincides with that of fault slip. They are common features of strike-slip systems. We perform a number of numerical thermomechanical experiments to explore how the rheology of the lithosphere influences basin evolution and lithospheric structure beneath the basin. Our modeling shows that basin subsidence results from the competition of extension of the brittle part of the lithosphere, which leads to its subsidence, and of the compensating flow of the deeper ductile part of the lithosphere, which pushes the extended brittle block upwards. The result of this competition is the subsidence rate. Strain partitioning beneath the basin and crustal structures is controlled by (i) the thickness of the brittle layer and basin width, (ii) the magnitude of strike-slip displacement, (iii) the rate of frictional softening of the crust, and (iv) the viscosity of the ductile part of the lithosphere. The thickness of the brittle layer and the viscosity of the underlying ductile part of the lithosphere in turn depend on temperature, composition and material softening. We interpret the modeling results, deducing simple analytical expressions based on the “brittle brick stretching” (BBS) approach, which despite its simplicity describes the structure and evolution of pull-apart basins reasonably well. We also demonstrate that the structure and evolution of the Dead Sea Basin, located at a left step of the Dead Sea Transform in the Middle East, is consistent with a BBS type of deformation with only a minor contribution from compensational flow in the ductile part of the lithosphere. Finally, we show that the formation of a deep narrow pull-apart basin in relatively cold lithosphere, as in the Dead Sea Basin, requires very low friction at major faults (lower than 0.1–0.2). If this condition is not satisfied, strike-slip deformation does not localise and deep basins do not form.

SUMMARY ∼82 000 P and S arrival times from ∼3000 sources recorded by ∼250 seismic stations from the revised ISC catalogue are employed to study a circular area of 6 ◦ radius centred on the Dead Sea. We use the linearized tomographic approach based on the rays constructed in a 1-D spherical velocity model and corrected for the Moho depth variation and relief. All the sources were relocated. As the result of simultaneous iterative inversion we get 3-D P and S velocity anomalies in the crust and uppermost mantle, Moho depth and corrected source parameters. The resulting images fit well with the existing tectonic elements in the study area. In the crust, a narrow P and S low-velocity anomaly marks the position of the Dead Sea Transform (DST) that is interpreted as sediments in the shallower layer and a zone of fractured and deformed rocks in the middle and lower crust. There is a narrow (50 km wide) band of thickening of the crust along the DST in the Arava valley between the Red Sea and Dead Sea and some 100 km north of the Dead Sea. This zone may be associated with the minimum of the lithospheric strength and, therefore, explain the location of the DST in the Arava Valley. The velocity anomalies under the crust and the map of the Moho depth clearly distinguish the oceanic (Levant basin) and continental types of crust (Asia Minor, Zagros, Cyprus and Eratosthenes Mount). Verification of the results takes an important part in this study. Inversions of different starting models and independent processing of data subsets show high robustness of the results. Synthetic tests clearly show the limits of the resolving power of the inversion with the existing data set.

The classic example of the not-well-understood rapid change of tectonic plate motion is the increase and then decrease of the convergence rate between the Nazca and South America plates during the past 25–20 m.y. that coincided with the growth of the Andes Mountains. Currently, the decrease in convergence rate is explained either by the increasing load of the Andes or by the appearance of flat slab segments beneath South America. Here, we present an alternative view based on a thermomechanical self-consistent (gravity driven) model of Nazca plate subduction. We explain the changes in the convergence rate as a natural consequence of the Nazca plate penetration into the transition zone and lower mantle after long-term oblique subduction of the Farallon plate. The model is consistent with seismic tomographic images of the Nazca plate beneath South America. Our model also shows that the presence of the Andes does not significantly affect the convergence rate between the Nazca and South America plates.

The 1989 VSP program at the KTB pilot hole was complemented in 1992 by a standard VSP in the KTB super-deep borehole from 3000 to 6013 m. P-wave velocities oscillate around 5.8 km/s in the upper 3150 m in accordance with sonic log velocities and correlate with paragneisses which prevail in this depth range. At about 3150 m depth, velocities increase to 6.4 km/s correlating with metabasites which dominate in the depth range 3150 to 7500 m. Laboratory measurements and petrophysical modelling provide evidence that the intrinsic velocities were reduced by 5–10% by fracture density and porosity at all depth ranges. Subvertical dip (50–70°) in structures and textures prevail, causing about 10% S-wave anisotropy, indicated by direct observations of S-wave splitting. Pronounced P-wave reflections, accompanied by P- to S-wave conversions and a lack of S-wave reflections, occur in the lower depth range only (3000–6000 m) and correlate with fluid-filled fracture systems. Lithological contrasts (gneiss-amphibolite) play a minor role in generating reflections. The most prominent reflecting elements known from surface profiling and 3D-surveys (i.e. the ‘Franconian Lineament’ reflector at about 7 km, and P-wave reflections at 8.3 km with notably absent associated S-wave reflections) coincide with pronounced anomalies observed in the logging data indicating the presence of major fracture zones.

[1] In many cases the initial stage of continental break-up was and is associated with oblique rifting. That includes break-up in the Southern and Equatorial Atlantic, separation from eastern and western Gondwana as well as many recent rift systems, like Gulf of California, Ethiopia Rift and Dead Sea fault. Using a simple analytic mechanical model and advanced numerical, thermomechanical modeling techniques we investigate the influence of oblique extension on the required tectonic force in a three-dimensional setting. While magmatic processes have been already suggested to affect rift evolution, we show that additional mechanisms emerge due to the three-dimensionality of an extensional system. Focusing on non-magmatic rift settings, we find that oblique extension significantly facilitates the rift process. This is due to the fact that oblique deformation requires less force in order to reach the plastic yield limit than rift-perpendicular extension. The model shows that in the case of two competing non-magmatic rifts, with one perpendicular and one oblique to the direction of extension but otherwise having identical properties, the oblique rift zone is mechanically preferred and thus attracts more strain.

[1] We present a three-dimensional finite element thermomechanical model idealizing the complex deformation processes associated with evolution of the San Andreas Fault system (SAFS) in northern and central California over the past 20 Myr. More specifically, we investigate the mechanisms responsible for the eastward (landward) migrationof the San Andreas plate boundary over time, a process thathas largely determined the evolution and present structure of SAFS. Two possible mechanisms had been previously suggested. One mechanism suggests that the Pacific plate first cools and captures uprising mantle in the slab window, subsequently causing accretion of the continental crustal blocks. An alternative scenario attributes accretion to the capture of plate fragments (microplates) stalled in the ceased Farallon-North America subduction zone. Here we test both these scenarios numerically using a recently developed lithospheric-scale code, SLIM3D, that employs free surface, nonlinear temperature- and stress-dependent elastoviscoplastic rheology and allows for self-generation of faults. Modeling suggests that microplate capture is the primary driving mechanism fortheeastwardmigrationoftheplateboundary,whiletheslabwindowcoolingmechanismaloneisincapable of explaining this phenomenon. We also show that the system evolves to the present day structure of SAFS only if the coefficient of friction at mature faults is low (0.08 for the best fit model). Thus, our model provides an independent constraint supporting the “weak fault in a strong crust” hypothesis for SAFS.

[1] 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/m2 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.

[1] Large submarine landslides can generate dangerous tsunamis. Because of their long-period signal, detection of landslides by common seismological methods is difficult. Here we suggest a method of detecting submarine landslides by using an array of land-based tiltmeters. The displacement of a large volume of sediments during landsliding produces a detectable elastic response of the lithosphere. We propose a technique to calculate this response and to invert for tsunami relevant parameters like slide location, volume, and velocity. We exemplify our method by applying it to the Storegga slide west of Norway and other tsunamigenic landslide events. The parameter which can be most robustly estimated from tiltmeter array measurements is the product of slide volume and its velocity (slide tsunamigenic potential). This parameter also controls the amplitude of the generated tsunami wave. The inversion accuracy of this parameter and the estimated tsunami height near the coast depends on the noise level of tiltmeter measurements, distance of the tiltmeters from the slide, and slide tsunamigenic potential itself. The tsunamigenic potential of the most dangerous slides like Storegga can be estimated well by tiltmeters at the coast if the effective noise level does not exceed 50 nrad.

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.

The present geological setting west of Svalbard closely parallels the situation off mid-Norway after the last glaciation, when crustal unloading by melting of ice induced very large earthquakes. Today, on the modern Svalbard margin, increasing bottom water temperatures are destabilizing marine gas hydrates, which are held in continental margin sediments consisting of interlayered contourite deposits and glacigenic debris flows. Both unloading earthquakes and hydrate failure have been identified as key factors causing several megalandslides off Norway during early Holocene deglaciation. The most prominent event was the Storegga Slide 8200 years B.P. which caused a tsunami up to 23 m high on the Faroe and Shetland islands. Here we show by numerical tsunami modeling that a smaller submarine landslide west of Svalbard, 100 m high and 130 km wide, would cause a tsunami capable of reaching northwest Europe and threatening coastal areas. A tsunami warning system based on tiltmeters would give a warning time of 1–4 h.

[1] Since the devastating earthquake and tsunami in 2004 offshore Sumatra, many source models have been put forward. Recent studies clearly show that modern GPS-processing could achieve high resolving power for slip in near real time, which is crucial for determining tsunami initial conditions, provided accurate GPS-processing and inversion. Here, we propose an inversion technique with improved representation of the subduction zone geometry and physically justified boundary conditions. We show that the discrepancy between the inversion of near- and far field GPS data for the 2004 event, which is often explained by postseismic slip, can be eliminated by using our inversion method and IASP91 earth model. Inverted source models, including versions with splay faulting, are shown to be consistent with satellite altimetry data of offshore tsunami wave height, suggesting that displacement at the splay fault might have been present but was likely a second order process.

We employ high-resolution 2D thermomechanical modelling with complex elasto-visco-plastic rheology to analyse stresses in subducting and overriding plates. The model contains a dynamic subduction channel, defined as a few km thick zone of specific (weak) visco-plastic rheology between the slab and the overriding plate. Our high-resolution model corresponds to the zoom-in of the large-scale thermomechanical model of the Central Andes, with the 4-fold resolution increase. Initial model configuration (material, temperature and stress distributions) was interpolated from the parent model. Boundary conditions including slab pull and push velocities as well as velocity of the overriding plate, were also re-mapped from the large scale model. We demonstrate that, for shallow-dipping overridden slabs, the dominant stress is unbending stress at 50–100 km depth. This stress reaches 1–1.5 GPa and strongly prevails over the slab bending stress near the trench, which is limited by the frictional plastic yield. Unbending creates double (5–10 km distance between maximums) compression zone in the upper part of the slab and a zone of extension some 20–30 km below. Pressure in these zones differs from the lithostatic pressure to as much as plus/minus 0.8 GPa. Details of the distribution and magnitude of bending and unbending stresses in the slab are mainly controlled by the overriding velocity and direction of the slab-pull force. Overpressure in the subduction channel in all models does not exceed 100 MPa. Large over- and under-pressures in the slab, do not, however, lead to significant (more than 10 km) offsets of main dehydration reactions in crust and mantle of the slab.

[1] The 2004 catastrophic Indian Ocean tsunami has strongly emphasized the need for reliable tsunami early warning systems. Another giant tsunamigenic earthquake may occur west of Sumatra, close to the large city of Padang. We demonstrate that the presence of islands between the trench and the Sumatran coast makes earthquake-induced tsunamis especially sensitive to slip distribution on the rupture plane as wave heights at Padang may differ by more than a factor of 5 for earthquakes having the same seismic moment (magnitude) and rupture zone geometry but different slip distribution. Hence reliable prediction of tsunami wave heights for Padang cannot be provided using traditional, earthquake-magnitude-based methods. We show, however, that such a prediction can be issued within 10 minutes of an earthquake by incorporating special types of near-field GPS arrays (‘‘GPS-Shield’’). These arrays measure both vertical and horizontal displacements and can resolve higher order features of the slip distribution on the fault than the seismic moment if placed above the rupture zone or are less than 100 km away of the rupture zone. Stations in the arrays are located as close as possible to the trench and are aligned perpendicular to the trench, i.e., parallel to the expected gradient of surface coseismic displacement. In the case of Sumatra and Java, the GPS-Shield arrays should be placed at Mentawai Islands, located between the trench and Sumatra and directly at the Sumatra and Java western coasts. We demonstrate that the ‘‘GPS-Shield’’ can also be applied to northern Chile, where giant earthquakes may also occur in the near future. Moreover, this concept may be applied globally to many other tsunamigenic active margins where the land is located above or close to seismogenic zones.

We present results of a tomographic inversion of teleseismic data recorded at 48 stations of a temporary network which was installed in the area of the Dead Sea Transform (DST) and operated for 1 yr in the framework of the multidisciplinary DESERT Project. The 3366 teleseismic P and PKP phases from 135 events were hand picked and corrected for surface topography and crustal thickness. The inversion shows pronounced low-velocity anomalies in the crust, beneath the DST, which are consistent with recent results from local-source tomography. These anomalies are likely related to the young sediments and fractured rocks in the fault zone. The deeper the retrieved anomalies are quite weak. Most prominent is the high-velocity strip-like anomaly striking SE-NW. We attribute this anomaly to the inherited heterogeneity of lithospheric structure, with a possible contribution by the shallow Precambrian basement east of the DST and to lower crustal heterogeneity reported in this region by other seismic studies. We do not observe reliable signature of the DST in the upper mantle structure. Some weak indications of low-velocity anomalies in the upper mantle beneath the DST may well result from the down-smearing of the strong upper crustal anomalies. We also see very little topography of the lithosphere-asthenosphere boundary beneath the DST, which would generate significant horizontal velocity variations. These results are consistent with predictions from a recent thereto-mechanical model of the DST. Our tomographic model provides some indication of hot mantle flow from the deeper upper mantle rooted in the region of the Red Sea. However, resolution tests show that this anomaly may well be beyond resolution of the model. (c) 2006 Elsevier B.V. All rights reserved.

SUMMARY About 50 000 P and S arrival times and 25 000 values of t* recorded at seismic arrays operated in the Central Andes between 20°S and 25°S in the time period from 1994 to 1997 have been used for locating more than 1500 deep and crustal earthquakes and creating 3-D P, S velocity and Qp models. The study volume in the reference model is subdivided into three domains: slab, continental crust and mantle wedge. A starting velocity distribution in each domain is set from a priori information: in the crust it is based on the controlled sources seismic studies; in slab and mantle wedge it is defined using relations between P and S velocities, temperature and composition given by mineral physics. Each iteration of tomographic inversion consists of the following steps: (1) absolute location of sources in 3-D velocity model using P and S arrival times; (2) double-difference relocation of the sources and (3) simultaneous determination of P and S velocity anomalies, P and S station corrections and source parameters by inverting one matrix. Velocity parameters are computed in a mesh with the density of nodes proportional to the ray density with double-sided nodes at the domain boundaries. The next iteration is repeated with the updated velocity model and source parameters obtained at the previous step. Different tests aimed at checking the reliability of the obtained velocity models are presented. In addition, we present the results of inversion for Vp and Vp/Vs parameters, which appear to be practically equivalent to Vp and Vs inversion. A separate inversion for Qp has been performed using the ray paths and source locations in the final velocity model. The resulting Vp, Vs and Qp distributions show complicated, essentially 3-D structure in the lithosphere and asthenosphere. P and S velocities appear to be well correlated, suggesting the important role of variations of composition, temperature, water content and degree of partial melting.

The Andes were formed by Cenozoic tectonic shortening of the South American plate margin overriding the subducting Nazca Plate. Using coupled, thermo-mechanical, numerical modeling of the dynamic interaction between subducting and overriding plates, we searched for factors controlling the intensity of the tectonic shortening. From our modeling, constrained by geological and geophysical observations, we infer that the most important factor was fast and accelerating (from 2 to 3 cm yr(hoch)-1) westward drift of the South American Plate, whereas possible changes in the convergence rate were not as important. Other important factors are the crustal structure of the overriding plate and the shear coupling at the plate interface. The model in which the South American Plate has a thick (40-45 km at 35 Ma) crust and relatively high friction coefficient (0.05) at the Nazca-South American plate interface generates more than 300 km of tectonic shortening over the past 35 million years and replicates well the crustal structure and evolution of the high Central Andes, However, modeling does not confirm that possible climate-controlled changes to the sedimentary trench-fill during the last 30 million years might have significantly influenced the upperplate shortening rate. The model with initially thinner (less than 40 km) continental crust and a lower friction coefficient (less than 0.015) results in less than 40 ikm of shortening in the South American Plate, replicating the situation in the Southern Andes. During upper-plate deformation, the processes that cause a reduction in lithospheric strength and an increase in interplate coupling are particularly important. The most significant of these processes appears to be: (1) delamination of the lower crust and mantle lithosphere, driven by gabbro-eclogite transformation in the thickening lower crust, and (2) mechanical failure of the foreland sediments. The modeling demonstrates that delaminating lithosphere interacts with subduction-zone corner flow, influencing both the rate of tectonic shortening and magmatic-arc productivity, and suggests an anti-correlation between these two parameters. Our model also predicts that the down-dip limit of the frictional coupling domain between the Nazca and South American Plates should be ~15-20 km deeper in the Southern Andes (south of 28° S) compared to the high Central Andes, which is consistent with GPS and seismological observations.

SUMMARY The P and S seismic anomalies in the upper mantle beneath the Pamir‐Hindukush collision zone and surrounding areas are investigated using more than 4 × 10 5 traveltimes from 5775 events located in the study region, recorded by the stations of the worldwide seismic network and reported in the ISC catalogue. All available epicentral distance ranges of seismic rays are considered. All sources are relocated using an algorithm, which includes the double-difference method. In addition, source parameters are corrected in the main inversion step, simultaneously with the velocity models. The high reliability of the resulting P-velocity images is confirmed by various tests. The S model provides much smoother patterns, which are, however, generally consistent with P anomalies. Our model shows a complex image of lithospheric plate under the study region. Under SW Hindukush, the northwards subducting Indian plate is imaged throughout the upper mantle and transition zone. Further to the NE, within a 300-km-wide zone the upper part of the plate overturns and then the slab breaks off after it narrows at ∼250 km depth. The tomographic model also provides evidence for the southward subduction of the lithosphere beneath the southern Tien Shan down to at least 250 km depth. To the north of Tarim, we observe a northward-dipping of the Tarim plate down to ∼500 km depth. At the crustal depth our model shows alternating belts of high and low velocities parallel to the main thrusting faults, which indicate complex crustal structure in this region.

The Andes, the world's second highest orogenic belt, were generated by the Cenozoic tectonic shortening of the South American plate margin overriding the subducting Nazca plate. We use a coupled thermomechanical numerical modeling technique to identify factors controlling the intensity of the tectonic shortening. From the modeling, we infer that the most important factor was accelerated westward drift of the South American plate; changes in the subduction rate were less important. Other important factors are crustal structure of the overriding plate and shear coupling at the plates' interface. The model with a thick (40–45 km at 30 Ma) South American crust and relatively high friction coefficient (0.05) at the Nazca–South American interface generates >300 km of tectonic shortening during 30–35 m.y. and replicates the crustal structure and evolution of the high central Andes. The model with an initially thinner (

[1] We have found seismic evidence for the existence of narrow (about 200 km diameter) and hot plumes in the upper mantle above the 410 km discontinuity. They are located mainly under ocean islands, not however, under the central parts of major continents. These results are obtained from comparing globally the observed travel time differences between Ps conversions and SS precursors from the discontinuity at 410 km depth, using published data and new observations from ocean island stations. Comparison of SS precursors and Ps conversions, however, leads also to indications of significantly fewer narrow plumes in the upper mantle transition zone between 410 and 660 km depth, than in the upper mantle above 410 km depth.

The Dead Sea Rift Transect (DESERT 2000) is a multinational and interdisciplinary study of the Dead Sea Rift. The project began fieldwork in February 2000 and the first experiments were successfully completed in May. The seismic, seismological, and magnetotelluric experiments presented here, along with the future electromagnetic, gravity, magnetic, geodynamic, and geological studies, will provide the basic geophysical and geodynamic frame for further geoscientific research. DESERT 2000 should also help address a fundamental question of plate tectonics: How do shear zones work and what controls them?

We have imaged the upper mantle discontinuities in a 30×20° large region at the active continental margin of the Japan subduction zone and neighboring areas, using P-to-S converted phases from teleseismic records of permanent broadband stations. The 410 km discontinuity is detected within ±10 km of its global average position. An interesting exception in its observation is a gap near 135°E, very close to the slab penetration of the 410 km discontinuity and in an area where we have rather high data density. The 660 km discontinuity reaches 700 km depth at two places where it is hit directly by the slab. These data generally show good agreement with tomographic results. Both, the 660 km discontinuity depression and the P-velocity anomalies, suggest about 400–500 K temperature deficit within the slab. However, no downward bending of the 660 km discontinuity is observed in eastern China where the flat lying slab is imaged by tomography. This suggests that the slab does not cool the 660 km discontinuity in this region. Therefore the positive buoyancy required to keep the slab lying flat cannot have been provided by the negative Clapeyron slope of the spinel–perovskite phase transition. Another mechanism is needed, which could possibly be metastable olivine in the cold core of the slab. We have also imaged the shallower portion of the slab down to about 150 km underneath some seismic stations, likely because metastable gabbro is still existing to this depth providing a sufficient velocity contrast. A strong negative converted phase is observed at 150–200 km depth underneath a volcanic region in Japan. If real (observed at one station only), the zone which produces this conversion and which may begin exactly above the slab would require fluid-fed melting.

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.

Observations made in different continental rift systems (European, Red Sea-Gulf of Aden, and East African Rift Systems) were investigated in terms of the influence of different parameters on the style of rifting. Apart from the lithospheric thermal regime at the time of rift initiation, the process of rifting seems to be mainly controlled by the far-field stress regime and the presence or absence of a mantle plume. In a hot lithosphere the low viscosity of the lower crust enables the upper crust to be detached from the mantle and be deformed independently under far-field stresses. Therefore, in western Europe the main rifts could open obliquely to the direction of mantle movement in crustal levels without appreciable extension in the lithospheric mantle. In contrast, the colder lithosphere of Arabia did not allow detachment of crust and mantle. Therefore, despite being in a similar tectonic situation as in western Europe, i.e. rifting in front of an orogen, the whole lithosphere deformed congruently. Rift opening occurred parallel to mantle movement, i.e. parallel to the direction of extensional stress in the lithospheric mantle induced by the pull of the subducting slab at the orogenic front. The forces needed to extend the whole relatively cool Arabian lithosphere could, however, not be produced by slab pull alone. Additional forces and weakening of the lithosphere were produced by the Afar mantle plume. Mantle plumes are generally not able to break very thick cratonic lithosphere but they deflect sidewards when hitting this kind of lithosphere. Warmer (but still relatively cool) lithosphere like in the surroundings of the East African Tanzania craton or in Arabia can, by the buoyancy of a plume, be bent strongly enough to break. As a consequence, long linear rift structures develop with generally high shoulders. The presence of a plume explains thus the position of the East African and Red Sea-Gulf of Aden rifts. Under far-field compression, rifts will open only a small amount, whereas under far-field extension continental break-up may occur. A plume hitting a hot lithosphere may penetrate it without producing long linear rifts. Instead, crustal deformation will be distributed in parallel basins over a wide area with only minor amounts of rift shoulder uplift as has happened in northern Kenya and the French Massif Central.

The Chyulu Hills, a 1.4 Ma B.P. to Holocene volcanic field located about 150 km to the east of the Kenya rift, is one of the few locations on Earth for which detailed geochemical (volcanic rocks), thermobarometric (xenoliths), seismological and gravity data are available. This paper combines these data to achieve an integrated seismic-petrological model for the deep structure of this volcanic field. Results of a wide-angle reflection and refraction experiment reveal an average crustal thickness of 40 km and a thickness of 20 km for the lower crust. Beneath the volcanic field, the crust thickens to about 44 km. In this region a low-velocity body (LVZ) is modelled which extends from about 30 ± ; 5 km depth to the Moho. The LVZ is characterised by an increased vP/vS-ratio ranging from 1.81 to 1.93 depending on the possible extents of this body. This is in contrast to the surrounding crust where a ratio of only about 1.76 is observed. In the same area, the results of a teleseismic tomography study show a P-wave low-velocity anomaly of − 3%. The seismic data can be explained by either an anorthositic body directly above the Moho in the region of the Chyulu Hills or by the presence of partial melt. Directly beneath the Chyulu Hills, a P-wave velocity of 7.9 km/s is determined for the uppermost mantle ; this velocity is 0.2– 0.3 km/s lower than that of the surrounding mantle region. The teleseismic tomography model suggests a P-wave low-velocity anomaly of − 2.5 to − 3.5% in the uppermost mantle (

This paper presents an updated interpretation of seismic anisotropy within the uppermost mantle of southern Germany. The dense network of reversed and crossing refraction profiles in this area made it possible to observe almost 900 traveltimes of the P(tief)n phase that could be effectively used in a time-term analysis to determine horizontal velocity distribution immediately below the Moho. For 12 crossing profiles, amplitude ratios of the P(tief)n phase compared to the dominant crustal phase were utilized to resolve azimuthally dependent velocity gradients with depth. A P-wave anisotropy of 3–4 per cent in a horizontal plane immediately below the Moho at a depth of 30 km, increasing to 11 per cent at a depth of 40 km, was determined. For the axis of the highest velocity of about 8.03 km s(hoch)-1 at a depth of 30 km a direction of N31°F was obtained. The azimuthal dependence of the observed P(tief)n amplitude is explained by an azimuth-dependent sub-Moho velocity gradient decreasing from 0.06 s(hoch)−1 in the fast direction to 0 s(hoch)−1 in the slow direction of horizontal P-wave velocity. From the seismic results in this study a petrological model suggesting a change of modal composition and percentage of oriented olivine with depth was derived.

The catastrophic consequences of the 2004 Indian Ocean and the recent (17 July) Java tsunamis demand the development of modern and robust tsunami early warning systems.The greatest challenge of the German Indonesian Tsunami Early Warning System (GITEWS), led by the National Center of Geosciences (GeoForschungsZentrum) in Potsdam, Germany, is to provide early tsunami warnings for the Indian Ocean coast of Indonesia where tsunamis can arrive 20–40 minutes after an earthquake. This article shows that reliable prediction of tsunami wave heights at the Sumatra coast cannot be provided using traditional, earthquake-magnitude-based methods. Instead, a novel, real-time warning technique based on special types of near-field global positioning system (GPS) arrays—a ‘GPS shield’—may be more appropriate for Sumatra and elsewhere around the globe.