Friedemann Wenzel

Partially molten middle crust beneath southern Tibet : Synthesis of project INDEPTH results

INDEPTH geophysical and geological observations imply that a partially molten midcrustal layer exists beneath southern Tibet. This partially molten layer has been produced by crustal thickening and behaves as a fluid on the time scale of Himalayan deformation. It is confined on the south by the structurally imbricated Indian crust underlying the Tethyan and High Himalaya and is underlain, apparently, by a stiff Indian mantle lid. The results suggest that during Neogene time the underthrusting Indian crust has acted as a plunger, displacing the molten middle crust to the north while at the same time contributing to this layer by melting and ductile flow. Viewed broadly, the Neogene evolution of the Himalaya is essentially a record of the southward extrusion of the partially molten middle crust underlying southern Tibet.

Direct mapping of seismic data to the domain of intercept time and ray parameter -A plane-wave decomposition

Marine seismic data recorded as a function of source‐receiver offset and traveltime are mapped directly to the domain of intercept or vertical delay time and horizontal ray parameter. This is a plane‐wave decomposition based on beam forming of wide‐aperture seismic array data to determine automatically the loci of coherent seismic reflection and refraction events. In this computation, semblance, in addition to the required slowness or horizontal ray parameter stack, is found for linear X — T trajectories across subarrays. Subsequently, semblance is used to derive a windowing filter that is applied to the slowness stack to determine the points of stationary phase and eliminate aliasing. The resulting filtered slowness stacks for multiple subarrays can then be linearly transformed and combined according to ray parameter, range, and time. The resulting function of intercept time and horizontal ray parameter offers significant computational and interpretational advantages for the case of horizontal homogeneou...

Seismogenic index and magnitude probability of earthquakes induced during reservoir fluid stimulations

An important characteristic of seismicity is the distribution of magnitudes of earthquakes. Fluid injection in rocks, aimed to create enhanced geothermal systems (EGS), can sometimes produce significant seismic events (e.g., Majer et al., 2007). This is rarely the case in hydraulic fracturing of hydrocarbon reservoirs. However, in any case the behavior of the seismicity triggering in space and in time is controlled by the process of stress relaxation and pore-pressure perturbation that was initially created at the injection source. This relaxation process can be approximated by pressure diffusion (possibly a nonlinear one) in the pore fluid of rocks (e.g., Shapiro and Dinske, 2009). At some locations the tectonic stress in the Earths crust is close to a critical stress, causing brittle failure of rocks. Increasing fluid pressure in such a reservoir causes pressure in the connected pore and fracture space of rocks to increase. Such an increase in the pore pressure consequently causes a decrease of the eff...

P-wave mantle velocity structure beneath northern Eurasia from long-range recordings along the profile Quartz

High-resolution seismic sounding of the mantle using two Peaceful Nuclear Explosions (PNEs) along the 3950 km long Quartz profile in northern Eurasia reveal major unexpected vertical and lateral mantle inhomogeneities. In contrast to standard seismological earth models such as PREM and IASP91 three, rather than two, mantle discontinuities are found between 400 and 700 km depth. They are located at about 420, 550 and 670 km depth. Lateral variations in depths amount to 10–20 km, and associated P-wave velocities vary by 0.1 km s−1. In the upper mantle, the uppermost layer through which Pn propagates extends down to about 100 km depth. The mantle below this depth, but above the 410 km discontinuity, is characterized by alternating high and low P-wave velocities in the depth range between 100 and 200 km. The velocity fluctuates between 8.3 and 8.7 km s−1 and shows variations from shotpoint to shotpoint. Average velocities are clearly higher than standard seismological reference models. This part of the mantle overlies a low-velocity zone between about 200 and 300 km depth. This channel also displays substantial lateral variation in depth which could be interpreted as a laterally variable asthenosphere. Beneath the West Siberian basin the asthenosphere seems to be well established whereas it is only poorly defined under the Uralian belt and the tectonic units to the northwest. With the addition of more data from this and other profiles, future aims include two-dimensional and three-dimensional tomographic modelling, as well as modelling of petrological composition, of the mantle down to 850 km depth beneath northern Eurasia.

Crustal evolution of the Rhinegraben area. 1. Exploring the lower crust in the Rhinegraben rift by unified geophysical experiments

Abstract Unified geophysical investigations of the lithosphere in the Rhinegraben rift system have revealed new details of the lower crust and its role in the rifting process. The new findings allow an assessment of the compatibility of four different geophysical notions and properties of the lower crust: 1. (1) the lower crust as the layer beneath the Conrad discontinuity with a P-wave velocity of about 6.5 km/s or greater (refraction seismics); 2. (2) the laminated band of reflections, as seen in the near-vertical reflection seismic experiments in many parts of the continents; 3. (3) the ductile part of the crust below the brittle-ductile transition, devoid of earthquakes in seismically active regions; 4. (4) the electrical conductivity of the lower crust indicative of dry or wet conditions, or still unknown conduction phenomena. In the Rhinegraben area the lamination of the lower crust serves as an outstanding marker of deep tectonic activity during the rifting process, in which the crust of the Rhinegraben rift system has been subjected to three different natural dynamic processes: (1) Uplift by 2 to 3 km with subsequent erosion of the Rhinegraben shoulders (Black Forest and Vosges Mountains) caused decompression possibly leading to the formation of a low-velocity/high-electrical-conductivity zone right on top of the laminated lower crust beneath the elevated shoulders of the Black Forest. (2) The brittle crystalline wedge of the graben proper subsided nearly undeformed into the lower crust, which became about 5 to 7 km thinner below the graben than below the shoulders. (3) The deepest hypocentres in the Black Forest (Dinkelberg area), if projected onto the neighbouring reflection profile, would be located 7 to 8 km within the laminated lower crust beneath the southern Black Forest, indicating a discrepancy between the top of the lower crust as defined by the brittle-ductile transition as seen by the deepest earthquakes and by the top of the laminated reflection band. The Rhinegraben rift system reveals the properties and behaviour of the lower crust under a wide variety of tectonic situations.

Detailed look at final stage of plate break‐off is target of study in Romania

Geophysical experiments next year in Romania may provide insight into a common but short-lived seismic process that can be observed and understood at only one spot on Earth at present. About 150 stations will be set up in the Vrancea area in the southeast Carpathian Mountains to, in effect, record the terminal phase of the detachment of a subducting slab of oceanic lithosphere. This is a major regional tomographic study using a large number of broadband seismometers, which will operate for 6 months. Images will be used for hazard assessment as well as for a delineation of detachment history. Active subduction of oceanic lithosphere at convergent plate boundaries involves earthquakes, magmatism, metamorphism, and deformation—some of the most vivid manifestations of any plate tectonic process. The initiation and termination of subduction, however, remains relatively poorly understood. When convergence of lithospheric plates ceases and the suction force of the subducting plate becomes negligible, the subducting slab moves into an almost vertical position. If subduction occurs in an arcuate geometry, the slab is likely to be segmented.

A deep reflection seismic line across the Northern Rhine Graben

Abstract Two reflection seismic lines across the Tertiary Rhine Graben in Central Europe were recorded in 1988 as a joint venture of the French ECORS and the German DEKORP deep seismic reflection programs. In this paper the line across the northern graben is presented. The main results are: The asymmetry of the graben as documented by the sedimentary fill is accompanied by asymmetric features throughout the entire deep crystalline crust: a thin (3.7 s TWT) reflective lower crust in the east—a thick (5.5 s TWT), relatively transparent lower crust in the west, total crustal thicknesses of 8.7 s TWT in the east vs. 10.5 s TWT in the west. Provided a laterally homogeneous crust existed prior to rifting significant differences in upper and lower crustal thinning must be postulated. Extension occurred along localized shear zones that are located in the upper crust and at the crust/mantle boundary. The entire lower crustal layer acts as a decoupling zone.

Seismic wide-angle constraints on the crust of the southern Urals

A wide-angle seismic reflection/refraction data set was acquired during spring 1995 across the southern Urals to characterize the lithosphere beneath this Paleozoic orogen. The wide-angle reflectivity features a strong frequency dependence. While the lower crustal reflectivity is in the range of 6–15 Hz, the PmP is characterized by frequencies below 6 Hz. After detailed frequency filtering, the seismic phases constrain a new average P wave velocity crustal model that consists of an upper layer of 5.0–6.0 km/s, which correlates with the surface geology; 5–7 km depths at which the velocities increase to 6.2–6.3 km/s; 10–30 km depths at which, on average, the crust is characterized by velocities of 6.6 km/s; and finally, the lower crust, from 30–35 km down to the Moho, which has velocities ranging from 6.8 to 7.4 km/s. Two different S wave velocity models, one for the N-S and one for the E-W, were derived from the analysis of the horizontal component recordings. Crustal sections of Poissons ratio and anisotropy were calculated from the velocity models. The Poissons ratio increases in the lower crust at both sides of the root zone. A localized 2–3% anisotropy zone is imaged within the lower crust beneath the terranes east of the root. This feature is supported by time differences in the SmS phase and by the particle motion diagrams, which reveal two polarized directions of motion. Velocities are higher in the central part of the orogen than for the Siberian and eastern plates. These seismic recordings support a 50–56 km crustal thickness beneath the central part of the orogen in contrast to Moho depths of ≈ 45 km documented at the edges of the transect. The lateral variation of the PmP phase in frequency content and in waveform can be taken as evidence of different genetic origins of the Moho in the southern Urals.

Seismotectonics of the Romanian Vrancea Area

The seismicity of the Romanian Vrancea area has peculiar features: (1) strong earthquakes occur at intermediate depths in a very narrow source volume; (2) the seismogenic zone is situated beneath continental crust, at the SE corner of the highly arcuate Carpathian arc; (3) no evidence for active ongoing subduction is found today. Several geophysical models were developed that tried to provide an explanation for the localization of seismicity at depth (Fuchs et al., 1979; Oncescu, 1984; Tavera, 1991). They contain ideas on interaction of a paleo-subduction zone with more recent subduction and include initial concepts of slab break-off. In recent years new facts and concepts came up that merit a re-evaluation of the tectonic scenarios related to Vrancea seismicity.

Understanding tectonic stress in the oil patch The World Stress Map Project

Knowledge of the present-day tectonic stress is essential for numerous applications in petroleum exploration and production and in civil and mining engineering, such as improving the stability of boreholes and tunnels and enhancing petroleum production through natural or induced fractures. The World Stress Map (WSM) Project is a collaborative project between academia, industry, and government that is building a comprehensive global database of present-day stress information to better understand the state and sources of contemporary tectonic stress in the lithosphere (Figure 1).