ALBERT

Description: The combined passive and active seismic TRANSALP experiment produced an unprecedented high-resolution crustal image of the Eastern Alps between Munich and Venice. The European and Adriatic Mohos (EM and AM, respectively) are clearly imaged with different seismic techniques: near-vertical incidence reflections and receiver functions (RFs). The European Moho dips gently southward from 35 km beneath the northern foreland to a maximum depth of 55 km beneath the central part of the Eastern Alps, whereas the Adriatic Moho is imaged primarily by receiver functions at a constant depth of about 40 km. In both data sets, we have also detected first-order Alpine shear zones, such as the Helvetic detachment, Inntal fault and Sub-Tauern ramp in the north. apart from the Valsugana thrust, receiver functions in the southern part of the Eastern Alps have also observed a north dipping interface, which may penetrate the entire Adriatic crust [Adriatic Crust Interface (ACI)]. Deep crustal seismicity may be related to the ACI. We interpret the ACI as the currently active retroshear zone in the doubly vergent Alpine collisional belt.

Description: In this study, we present an interpretation of seismic refraction profiles from the PISCO 94 experiment in northern Chile. As the PISCO experiment was a combined active and passive seismological study, we also discuss results of the passive part in the context of the seismic refraction model. Previous seismic refraction and gravimetric studies indicate a maximum crustal thickness of about 70 km beneath the Pre- and Western Cordillera. The new seismic refraction data lead to a differentiated image of the Andean crust which shows strong varying characteristics. The crustal discontinuities (up to five are detected) dip from W to E. The upper crust has a thickness of 18 km (Precordillera) to 23 km (magmatic arc) underlain by the recent middle crust down to 35-45 km where the velocity increases to about 7 km/s at its base. This crustal level is interpreted as old continental lower crust and its base as blurred continental (paleo) Moho. Beneath the Precordillera, a strong discontinuity at 70 km depth with a velocity increase to about 8 km/s was detected, interpreted as the recent geophysical Moho. For the magmatic arc, this deep discontinuity could not be found by active seismic measurements. The tomographic models of the seismological studies, in general, confirm the seismic refraction results. Anomalously high vp/vs rations in the deeper part of the forearc indicate a hydrated mantle wedge consisting of serpentine and amphibole-bearing peridotite and the 70 km discontinuity is interpreted as the boundary between these two different stages of the hydrated mantle wedge. A zone of high attenuation (Qp) and high vp/vs ratios beneath the magmatic arc coincides with the low velocity zones and indicates partially molten rocks from a depth of 20 km down to the asthenospheric wedge.

Description: The more than 1000 km long trans-continental Dead Sea Transform (DST) forms the boundary between the African and Arabian plates in the Middle East. Magnetotelluric (MT) data were recorded at more than 200 sites, focusing on the DST in the Arava valley in Jordan. 2D inversion results of the MT data indicate very clearly that the DST is associated with a strong lateral conductivity contrast. The most prominent feature on the MT image is a conductive half-layer beginning at a depth of approximately 1.5 km, which may be caused by brines in porous sediments. The DST can be identified as a sharp vertical conductivity boundary on the east side of the feature and directly beneath the surface trace. On a coincident high-resolution seismic tomography image of the upper crust, a strong increase of the P wave velocities to values exceeding 5 km/s is observed west of the DST, where the MT model indicates lower conductivities. The seismic velocities are consistent with metamorphic basement rocks; however the observed resistivities (50-250 $Omega m$) are unusually low for unaltered metamorphic rocks. Fractured metamorphic rocks with interconnected fluid bearing veins could explain both the seismic and MT observations. However, the conductivity model suggests furthermore that the DST acts as an impermeable barrier to cross-fault fluid flow. In stark contrast, a prominent flower structure is observed along the central segment of the SAF and interpreted as evidence of pervasive along-fault fluid flow. Here, the high conductivity is attributed to the circulation of saline fluids within the damage zone of the fault system. The width of the conductive zone (0.5 km) is in the same order of magnitude as the width of a seismic low-velocity zone inferred from fault-zone-guided wave observations, while its depth extent (3 km) coincides with the occurrence of a cluster of small earthquakes. It is possible, that the damage zone of the DST is so narrow that it cannot be resolved even with the dense site spacing of the MT experiment. This observation is supported by preliminary results from geological mapping and a seismic study using fault-guided waves that suggest a very narrow low-velocity wave-guide of 3 to 10 m width. The reason for this difference between the DST (very narrow fault zone) and the SAF (wide gouge zone) is not yet clear, but seems to coincide with generally slower slip rates and the relatively low recent seismicity associated with this segment of the DST.

Description: A striking feature of Russian long-range seismic refraction data from Peaceful Nuclear Explosions is the observation of a high-frequency teleseismic P(tief)n, phase, which travels with a group velocity of 8.0 km s(hoch)-1 out to distances of several thousands of kilometres. Modelling using the reflectivity method shows that this phase can be understood as the response of an upper mantle that contains random RMS velocity fluctuations of about ±4 per cent superimposed on a positive velocity gradient. This class of model explains the existence of the teleseismic P(tief) n, its high-frequency content and its coda length. A teleseismic P(tief)n can only be generated if velocity flucluations are strong enough to cause multiple scattering and occur on a subwavelength scale. Cross-correlation properties of P- and S-wave velocity fluctuations exert a substantial influence on the wavefield. A completely unexpected phase can be observed if the fluctuations are imposed on a negative gradient

Description: Short-period, three-component recordings of the seismic wave field of peaceful nuclear explosions recorded on deep seismic sounding profiles (Quartz and Ruby-I, collected in 1984 and 1988, respectively) in northern Russia are used to constrain the nature of the high-frequency teleseismic Pn phase, which can be observed for receiver distance of 〉3000 km. We suggest that this phase is caused by velocity fluctuations in the upper mantle acting as scatterers. To test this hypothesis and to determine the properties of the upper mantles scatterers, the elastic reflectivity method (one-dimensional isotropic models) was used to model the coda of the high-frequency teleseismic Pn phase. Both the observed data and the synthetic record sections were analyzed by examining the coda decay rates of the teleseismic Pn phase at 5 Hz. Synthetic seismograms are computed for different models based on the global model IASP91 with an added zone of randomly distributed velocity fluctuations (lamellae) just below the crust-mantle boundary. These models may be characterized by the thickness of the scattering layer L, the vertical heterogeneity correlation length a, and the heterogeneity standard deviation sigma. The numerical simulation of the wave propagation in these models generated a high-frequency teleseismic Pn. Comparing the coda decay rates of synthetic seismograms with the observation, we tried to constrain the properties of the velocity model containing fluctuations in the upper mantle, L, a and sigma. In the distance range of the uninterfered high-frequency teleseismic Pn phase beyond 1300 km, the coda decay rates for our best fitting model are similar to the observed ones. This model has a 75 km thick zone of scatterers below the Moho, containing lamellae with an average thickness of 2 km and a RMS velocity perturbation of 5%. The modeling results show also that two other possible models, the whispering gallery phase along the crust-mantle boundary and scattering in the lower crust, taken alone are not able to explain the coda properties of the high-frequency teleseismic Pn phase.

Description: It is well known that the Seychelles microcontinent resulted from episodic rifting between Africa and India. However, the mechanics of such rifting are poorly understood. The aim of the SEISM project (Seismic Experiment to Investigate the Seychelles Microcontinent) is a better understanding of microcontinent formation through a seismic study of the upper-mantle and crustal structure beneath the Seychelles. From Feb. 2003 and Jan. 2004, 8 broadband and 18 intermediate-period three-component seismometers were deployed on 18 islands in the Seychelles. During this time period 240 events of Mb $〉$ 5.8 were recorded. Microseismic noise was a problem on most islands and resulted in low signal to noise ratios in seismic recordings. A polarisation filter developed by Du et al. (GJI, 2000) has been used to enhance teleseismic signals. The first stage of data analysis has been a study of upper-mantle anisotropy using shear-wave splitting in core phases such as SKS. In general the degree of splitting is similar to global averages (i.e., $sim$1 second). Variability in the orientation of the anisotropy suggests a mechanism related to deformation associated with microcontinent formation during the breakup of the Madagascar/Seychelles/India land mass, rather than a mechanism associated with current plate motions. The granitic inner-islands of the Seychelles plateau show a coherent NNE-SSW trend in anisotropy, whilst stations near the Mascarene plateau show more E-W trends. Such trends may support ideas of an anticlockwise rotation of the Seychelles during rifting events, or influence from local plume upwelling. The boundaries between oceanic and continental parts of the plateau are poorly know. Crustal thicknesses can be used to determine oceanic and contenental parts. Receiver functions will be used to estimate crustal depths and deeper mantle discontinuities.