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The 26 December 2004 Sumatra-Andaman earthquake of Mw 9.3 triggered a massive tsunami in the Indian Ocean. We here report on observations of the Indian Ocean tsunami at broadband seismic stations located on islands in the area. The tsunami induces long-period (> 1000 s) signals on the horizontal components of the sensor. Frequency-time analysis shows that the long-period signals cannot be due to seismic surface waves, but that it arrives at the expected time of the tsunami. The waveforms are well correlated to tide gauge observations at a location where both observations are available. To explain the signals we favour tilt due to coastal loading but we cannot at the present stage exclude gravitational effects. The density of broadband stations is expected to increase rapidly in the effort of building an earthquake monitoring system. They may unexpectedly become useful tsunami detectors as well
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
Ambient vibration techniques are promising methods for assessing the subsurface structure, in particular the shear-wave velocity profile (V-s). They are based on the dispersion property of surface waves in layered media. Therefore, the penetration depth is intrinsically linked to the energy content of the sources. For ambient vibrations, the spectral content extends in general to lower frequency when compared to classical artificial sources. Among available methods for processing recorded signals, we focus here on the spatial autocorrelation method. For stationary wavefields, the spatial autocorrelation is mathematically related to the frequency-dependent wave velocity c(omega). This allows the determination of the dispersion curve of traveling surface waves, which, in turn, is linked to the V-s profile. Here, we propose a direct inversion scheme for the observed autocorrelation curves to retrieve, in a single step, the V-s profile. The powerful neighborhood algorithm is used to efficiently search for all solutions in an n- dimensional parameter space. This approach has the advantage of taking into account the existing uncertainty over the measured curves, thus generating all V-s profiles that fit the data within their experimental errors. A preprocessing tool is also developed to estimate the validity of the autocorrelation curves and to reject parts of them if necessary before starting the inversion itself. We present two synthetic cases to test the potential of the method: one with ideal autocorrelation curves and another with autocorrelation curves computed from simulated ambient vibrations. The latter case is more realistic and makes it possible to figure out the problems that may be encountered in real experiments. The V-s profiles are correctly retrieved up to the depth of the first major velocity contrast unless low-velocity zones are accepted. We demonstrate that accepting low-velocity zones in the parameterization has a dramatic influence on the result of the inversion, with a considerable increase in the nonuniqueness of the problem. Finally, a real data set is processed with the same method
The Cretaceous eo-Alpine collisional event in the European Eastern Alps is generally accepted to induce W-NW- directed thrusting both in basement and in sedimentary cover units. This study presents the first evidence of eo-Alpine W-NW directed normal kinematics along the Schneeberg Normal Fault Zone, which separates eo-Alpine high-pressure rocks in a footwall position from pre-Alpine basement rocks in a hanging wall position. New Garnet Sm-Nd data indicate that exhumation of the high-pressure rocks along the normal fault zone started around 95 Ma ago and continued up to low greenschistibrittle conditions at 76 Ma, as indicated by a Rb-Sr age from a low temperature mylonite. The occurrence of pre-Alpine basement rocks both in the hanging wall and the footwall of eo-Alpine high-pressure rocks suggests exhumation by extrusion processes. Despite the displacement or removal of parts of the lower portion of the high-pressure unit by Tertiary strike-slip faults, eo-Alpine top-to-ESE thrusting, as expected for the structurally lower part of an extruding wedge, was found at and below the base of the eo-Alpine high-pressure rocks. A Rb-Sr age of 77 Ma from a greenschist facies mylonite in this thrust shear zone shows the contemporaneity of deformation at the base and the top of the wedge. The tectonic transport direction within the extruding wedge was E-SE, opposite to the W-NW direction so far reported for the eo-Alpine event in the Eastern Alps. The contemporaneity of opposite tectonic transport directions during continental subduction may be explained by a double-vergent wedge model with a narrow zone of ductile flow, where the high-pressure rocks were exhumed. (c) 2005 Elsevier B.V. All rights reserved
Thermobarometrical and mineral-chemical investigations by electron microprobe and LA-ICP-MS on a sillimanite- bearing pegmatoid from the Reinbolt Hills provide important constraints on the P-T-X-age relations of part of East Antarctica during Pan-African tectonism. U-Th-total Pb ages of monazite imply that the pegmatoid of originally Grenvillan age (zircon U-Pb age of ca. 900 Ma) underwent a major, late Pan-African (Cambrian) regional, granulite-facies metamorphism between 500 and 550 Ma. Most of the monazite formed during this event, as result of apatite metasomatism owing to infiltration of high-grade metamorphic fluids. Apatite-biotite and other mineral thermobarometers define the peak metamorphic temperatures and pressures with 850-950 degrees C and 0.8-1.0 GPa. The F-Cl-OH relations in apatite, and biotite, the chemistry of fluid inclusions and the presence of K-feldspar microveins suggest that the metasomatising fluid was a CO2-bearing, diluted KCl brine. The pegmatoid is the first record of monazite-(Ce) formed from fluorapatite that is rich in U (up to 2.6 Wt% UO2) and possesses Th/U ratios <1 (0.09 on average). These chemical signatures are direct reflection of the U and Th concentration patterns in the parental fluorapatite
Facies analysis and basin architecture of the Neogene Subandean synorogenic wedge, southern Bolivia
(2005)
Foreland sedimentation in the Subandean Zone of south-central Bolivia spans from the Upper Oligocene to present. It records sediment dispersal patterns in an initially distal and later proximal retroarc foreland basin, and thereby contains stratigraphic information on the tectonic evolution of the adjacent Andean fold-thrust belt. Within the Neogene orogenic wedge individual siliciclastic-dominated depositional systems formed ahead of an eastward-propagating deformation regime. We defined, described, and interpreted eight architectural elements and 24 lithofacies from 15 outcrop locations representing the Neogene foreland basin in the Subandean Zone and the Chaco Plain. These are combined to interpret depositional settings. The up to 7.5 km-thick Neogene wedge is subdivided in five stratigraphic units on the basis of facies associations and overall architecture: (1) The basal, Oligocene-Miocene, up to 250 m-thick Petaca Formation consists dominantly of calcrete, reworked conglomeratic pedogenic clasts, and fluvial sandstone and mudstone. This unit is interpreted to represent extensive pedogenesis under an and to semiarid climate with subordinate braided fluvial processes. (2) The overlying, Upper Miocene, up to 350 m thick Yecua Formation records numerous small-scale transgressive-regressive cycles of marginal marine, tidal, and shoreline facies of sandstone, ooid limestones, and varicoloured mudstone. (3) The Upper Miocene, up to 4500 m-thick Tariquia Formation principally consists of sandstone with interbedded sandstone-mudstone couplets representing frequent crevassing in anastomosing streams with an upsection- increasing degree of connectedness. (4) The up to 1500 m-thick Lower Pliocene Guandacay Formation represents braided streams and consists principally of granule to cobble conglomerate interbedded with sandstone and sandy mudstone. (5) The Upper Pliocene, up to 2000 m-thick Emborozu Formation consists predominantly of alluvial-fan-deposited cobble to boulder conglomerate interbedded with sandstone and sandy mudstone. The coarsening- and thickening-upward pattern and eastward progradation, coupled with the variable proportions of overbank facies, channel size, and degree of channel abandonment, in the Tariquia, Guandacay, and Emborozu Formations reflect a distal through proximal fluvial megafan environment. This long-lived megafan grew by high sedimentation rates and a north east-through-southeast radial paleoflow pattern on large, coarse-grained sediment lobes. The marked overall upsection change in pattern and depositional styles indicate fluctuations in accommodation space and sediment supply, regulated by basin subsidence, and are attributable to Andean tectonics and climatic controls. (c) 2005 Elsevier B.V. All rights reserved
Fe K-edge X-ray absorption near edge structure (XANES) and Mossbauer spectra were collected on synthetic glasses of basaltic composition and of glasses on the sodium oxide-silica binary to establish a relation between the pre- edge of the XANES at the K-edge and the Fe oxidation state of depolymerised glasses. Charges of sample material were equilibrated at ambient pressure, superliquidus temperatures and oxygen fugacities that were varied over a range of about 15 orders of magnitude. Most experiments were carried out in gas-flow furnaces, either with pure oxygen, air, or different CO/CO2 mixtures. For the most reduced conditions, the samples charges were enclosed together with a pellet of the IQF oxygen buffer in an evacuated silica glass ampoule. Fe3+/Sigma Fe x 100 of the samples determined by Mossbauer spectroscopy range between 0% and 100%. Position and intensity of the pre-edge centroid position vary strongly depending on the Fe oxidation state. The pre-edge centroid position and the Fe oxidation state determined by Mossbauer spectroscopy are nonlinearly related and have been fitted by a quadratic polynomial. Alternatively, the ratio of intensities measured at positions sensitive to Fe2+ and Fe3+, respectively, provides an even more sensitive method. Pre- edge intensities of the sample suite indicate average Fe co-ordination between 4 and 6 for all samples regardless of oxidation state. A potential application of the calibration given here opens the possibility of determining Fe oxidation state in glasses of similar compositions with high spatial resolution by use of a Micro-XANES setup (e.g., glass inclusions in natural minerals). (c) 2004 Elsevier B.V. All rights reserved