ALBERT

Description: Three-component wide-angle seismic data acquired in southern Tibet during Project INDEPTH show strong P-to-S converted reflections from reflectors that are aligned at a depth of ∼15 kilometers beneath the northern Yadong-Gulu rift. These converted reflections are locally higher in amplitude than the corresponding P-wave reflections. Modeling of reflection mode conversion as a function of incidence angle indicates that this condition obtains for a reflector that is a solid over fluid interface; it is not typical of a solid-solid interface. The likely candidates for a fluid trapped within the crystalline crust of southern Tibet are granitic magma and water (brine).

Description: The PISCO’94 (Proyecto de Investigatión Sismólogica de la Cordillera Occidental, 1994) seismological network of 31 digital broad band and short-period three-component seismometers was deployed in northern Chile between the Coastal Cordillera and the Western Cordillera. More than 5300 local seismic events were observed in a 100 day period. A subset of high-quality P and S arrival time data was used to invert simultaneously for hypocenters and velocity structure. Additional data from two other networks in the region could be included. The velocity models show a number of prominent anomalies, outlining an extremely thickened crust (about 70 km) beneath the forearc region, an anomalous crustal structure beneath the recent magmatic arc (Western Cordillera) characterized by very low velocities, and a high-velocity slab. A region of an increased Vp /Vs ratio has been found directly above the Wadati-Benioff zone, which might be caused by hydration processes. A zone of lower than average velocities and a high Vp /Vs ratio might correspond to the asthenospheric wedge. The upper edge of the Wadati-Benioff zone is sharply defined by intermediate depth hypocenters, while evidence for a double seismic zone can hardly be seen. Crustal events between the Precordillera and the Western Cordillera have been observed for the first time and are mainly located in the vicinity of the Salar de Atacama down to depths of about 40 km.

Description: A local, broadband, seismic network of four observatory-quality stations (KTB-NET) was operated during the drilling of the KTB hole, within the framework of the interdisciplinary German Continental Deep Drilling Program (KTB). The aim was to investigate the seismic activity with regard to the tectonic stress field and to compare it with data from in situ measurements in the 9.1-km deep borehole sections (bottom temperature of 260°C and heat flow of 82–85 mW/m2). From October 1990 to November 1995, over 80 local microearthquakes with magnitudes from 0.2 to 2.8 ML were recorded: eight small events by the KTB-NET only and four earthquake swarms with 73 events by the KTB-NET and stations of the Vogtland/Western Bohemia networks. Six of the small events are located within or close to the KTB-NET. The swarm events occurred at the southwestern extension of the Ohre rift, in an area 20 km north of the drill site, which is revealed to be part of the Vogtland/Western Bohemia seismotectonic unit, characterized by swarm activity. The hypocenters are limited to the upper 13 km of the crust, with a distinct concentration between 10 and 12 km. All types of fault plane solutions are found, but at depths greater than 8 km, reversed faulting mechanisms predominate. P axes are very uniformly oriented in a NNW-SSE direction, corresponding to the well-known regional stress orientation in central Europe and in agreement with the special in situ stress measurements of the KTB program. The focal mechanism of a ML=1.2 event induced by a fluid injection experiment fits into the results obtained from the natural events. Possible indications for the brittle-ductile transition are discussed in view of the observed earthquake depth and focal mechanism distributions.

Description: 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.

Description: For almost 10 years the KTB superdeep drilling project has offered an excellent field laboratory for adapting seismic techniques to crystalline environments and for testing new ideas for interpreting seismic reflections in terms of lithological or textural properties of metamorphic rock units. The seismic investigations culminated in a three-dimensional (3-D) reflection survey on a 19 × 19 km area with the drill site at its center. Interpretation of these data resulted in a detailed, structural model of the German Continental Deep Drilling Program (KTB) location with dominant, steep faults in the upper crust. The 3-D reflection survey was part of a suite of seismic experiments, ranging from wide-angle reflection and refraction profiles to standard vertical seismic profiles (VSP) and more sophisticated surface-to-borehole observations. It was predicted that the drill bit would meet the most prominent, steeply dipping, crustal reflector at a depth of about 6500–7000 m, and indeed, the borehole penetrated a major fault zone in the depth interval between 6850 and 7300 m. This reflector offered the rare opportunity to relate logging results, reflective properties, and geology to observed and modeled data. Post-Variscan thrusting caused cataclastic deformation, with partial, strong alterations within a steeply dipping reverse fault zone. This process generated impedance contrasts within the fault zone on a lateral scale large enough to cause seismic reflections. This was confirmed by borehole measurements along the whole 9.1 km deep KTB profile. The strongest, reflected signals originated from fluid-filled fractures and cataclastic fracture zones rather than from lithological boundaries (i.e., first-order discontinuities between different rock types) or from texture- and/or foliation-induced anisotropy. During the interpretation of seismic data at KTB several lessons were learned: Conventional processing of two-dimensional (2-D) reflection data from a presite survey showed predominantly subhorizontal layering in the upper crust with reflectivity striking in the Variscan direction. Drilling, however, revealed that all rock units are steeply dipping. This confirms that surface common depth point (CDP) seismics strongly enhances subhorizontal reflectivity and may thus produce a very misleading crustal image. Although this was shown for synthetic examples earlier, the KTB provides the experimental proof of how crucial this insight can be.

Description: The PUNA (Plateau Untersuchung Nw Argentinien) seismograph network was deployed across the Andes at ∼23.5°S. The array was centered in the backarc, atop the Puna high plateau in NW Argentina and was in operation for approximately 100 days between late August and late November 1997. Most stations were equipped with short-period1-Hz 3-component seismometers and PDAS data loggers recording continuously 100 sps. The deployment was part of the collaborative research center „Deformation processes in the Andes - SFB267”. Waveform data are available from the GEOFON data centre, under network code ZB under CC-BY 4.0 license.