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.

Description: Fluctuations in the length of day (Δlod) can be caused by temporal variations of the inertia tensor described by the excitation function and by disturbances of the torque balance between core and mantle. Recent models of the atmospheric excitation of the variations of the lod certainly failed with respect to longer periods (e.g., at about 70 years), but seem to be responsible for the annual period, the 22 years period and for a part of the nearly 30 years period. So, another geophysical phenomenon is needed which is responsible for the remaining part of unexplained lod variations. Previous studies of the geomagnetic core-mantle coupling were re-examined using lod values from which atmospherically excited parts were removed. The remaining part of the 30 years period could be explained by core-mantle coupling. Additionally, the torque blance was realized by assuming lower values of the electrical conductivity at the bottom of the mantle. It was concluded that the consideration of geophysical processes within atmosphere and hydrosphere will be important for future refinements of the core-mantle coupling models of the decade fluctuations in the lod.

Description: A 2-D profile for the shear wave velocity and anisotropy between Egypt and Spitsbergen is presented. The profile is constructed using fundamental- and higher-mode Love and Rayleigh waves recorded by stations of the NARS-DEEP, IRIS and GEOFON networks. The surface wave data have been inverted for shear velocity and anisotropy using a waveform inversion. In the eastern Mediterranean lithosphere we observe a large difference (7 per cent) between SH and SV velocities. We interpret this anomaly as an anisotropic oceanic lithosphere beneath the eastern Mediterranean, an interpretation which is consistent with tectonic reconstructions of the region. The east European continent is imaged as a high-velocity body whose thickness increases with the estimated age of the lithosphere. The continental root of the Ukrainian and Baltic shields and east European platform extends to a depth of 200 km. This is in contrast to the surrounding younger continental regions which appear to be less than 100 km thick. We further studied the structure of the continental lithosphere by investigating a possible relation between seismic velocities and tectonic age. Both a logarithmic and a square root relation have been fitted to the average seismic velocities in each tectonic region. The data slightly favour a logarithmic relation but a square root relation cannot be excluded.

Description: In sequence of the GFZ93 high resolution gravity models (Gruber Th., et al, 1993) a new model, named GFZ95A, which is complete to degree and order 360 of a spherical harmonic series was computed. The model is based on new data sets, which were collected during the last months. This new data promises a major step towards a more precise high resolution gravity model. Especially from new available data over CIS (Community of Independent States) major progress can be expected with respect to the former models, which were based on predicted data in this area.

Description: Within the BASIN'96 project (Basin Analysis and Seismic Investigation in North Germany 1996) a marine survey gathered seismic data in the southwest BalticSea to study the Caledonian collisional zone between Baltica to the north and Avalonia to the south. Additionally, the airgun pulses have been observed by several land stations. Special attention was paid to record good quality data on Rügen island aiming at linking offshore and land-seismic wide-angle observations. Therefore, a complex geophone array (the ‘Star of Rügen’) was designed to enhance the signal quality. The Star of Rügen data and other onshore observations in NE Germany and on Bornholm, an island in the BalticSea, have been used for P-wave velocity modelling. Important structures of the transition zone from Palaeozoic western Europe to the Precambrian Baltic Shield are imaged in the resulting three-dimensional velocity model. A pronounced basement high north of Rügen island indicates the location of the Caledonian Deformation Front (CDF). Velocities also reflect other prominent features, such as the Rønne Graben, a pull-apart structure of the Tornquist Zone, and the basement outcrops around Bornholm island. In the investigated area the Moho depth varies between 27 and 33 km. A high-velocity layer in the lower crust below the NE German mainland may indicate genetic relations to Baltica. The structural continuity, however, is disturbed by a velocity decrease in an area north of the CDF. Sporadic, but strong reflections from the upper mantle below the CDF seem to be consistent with the reflection seismic data [DEKORP-BASIN Research Group, 1999. Geology 27(1), 55–58].

Description: Fast polarization directions α of split SKS waves in Central Europe change from NE/ENE in the western part to dominatingly E/ESE orientation towards north and east. This coincides strikingly well with the dominating trend of Hercynian deformational crustal features. It hints to frozen anisotropy related to paleo-crustal fabric. But when considering plausible anisotropy values of about 2–3% then only a small fraction (δt 〈 0.3 s) of the rather large observed average delay-times (δt = 0.83 ± 0.31 s) between the two split waves could be attributed to structural anisotropy in the relatively thin Central European crust. Therefore, the main “anisotropy signal” has to be associated with lattice-preferred orientation (LPO) of olivine below the crust. It may be either frozen in the subcrustal lithosphere since Hercynian times or have developed more recently in the asthenosphere. The thickness of the lithosphere varies significantly beneath Europe and the depth contours show systematic changes in trend. The latter varies from dominatingly NE in the southwest to SE in the north and east. The polarization directions α of the fast split SKS waves observed at seismic stations in proximity to the southern and northeastern boundaries of Central Europe are subparallel to the trends of these strong anomalies in lithosphere topography. A causal relationship is assumed and a new model proposed to explain the observations in α and δt. It takes into account the possible effects of paleo-deformational events. They may have produced both anisotropic crustal fabric and probably still preserved and similarly trending frozen LPO in the subcrustal lithosphere. The model also considers the influence of recent absolute motion of the West European lithospheric plate towards NE and the effect of its pronounced lower boundary topography on the formation and trend of LPO in the asthenosphere. Accordingly, the effects of anisotropy of different nature and age at different depth levels but with similar trend may superimpose constructively. This could explain the rather large delay-times observed at Central European stations which are too large to be attributed to frozen anisotropy in the lithosphere alone. The model would even permit the total effect observed to be attributed to asthenosphere flow controlled by absolute plate motion direction and lithosphere-asthenosphere boundary topography.

Description: During the TOR-1 passive seismic experiment in 1996/97, a maximum of 139 temporary seismograph stations were operating over the Sorgenfrei-Tornquist Zone (STZ) in an area extending from northern Germany through Denmark to central Sweden. One of the objectives was to study horizontal anisotropy directions in the subcrustal lithosphere and asthenosphere across the Trans-European Suture Zone. To achieve this goal,broad-band and intermediate-period (5 s) data of the TOR-1 stations and additional stations of permanent networks (GRSN, GEOFON) were analysed for splitting of SKS and SKKS phases. As a result of the relatively dense station spacing, the method offers good lateral resolution of anisotropy.Preliminary results suggest that the directions of the fast horizontal S wave velocity are affected by the STZ. In central Europe and southern Sweden, far away from the STZ, fast S wave directions are approximately E-W while they turn more northerly closer to the STZ where they are approximately parallel to the trend of the STZ. No significant shear wave splitting was observed north of 57 degr. N and east of 14 degr. E. Small delay times between 0.2 and 0.5 s observed at the northernmost TOR-1 station T40S and T60S may be controlled by anisotropy in a thickened crust. The mantle contribution of horizontal anisotropy within the STZ is probably constrained to an approximately 60-km-thick zone in the depth range between 70 and 300 km. The observations are consistent with a model where azimuthally anisotropy is not governed by present-day mantle flow in the asthenosphere, but rather is frozen into the subcrustal lithosphere during the last episode of tectonic activity.

Description: Data from the Projecto de Investigacion Sismologica de la Cordillera Occidental (PISCO) seismic network and from six broadband seismographs that were operating in northern Chile were used to investigate the mantle in the convergent boundary zone between Nazca plate and the South American continent for the presence of anisotropy. Broadband data as well as long-period filtered data of teleseismic SKS and PKS phases were analyzed for the presence of shear wave splitting as a possible indicator for seismic anisotropy in the mantle beneath the PISCO network. Measurable shear wave splitting was observed with maximum delay times between the slow and fast split wave of the order of 1 s. Splitting of S waves from intermediate-depth events located directly beneath the PISCO network in the descending Nazca plate is generally associated with small delay times of the order of 0.1 s, a value typical for the continental crust. Near-vertical ScS reflections from two deep earthquakes in Argentina and one nearby intermediate-depth earthquake have similar splitting parameters as the SKS phases. This means that the anisotropic zone causing the splitting of the core phases can be constrained to the Pacific mantle underlying the subducting Nazca plate. It probably does not extend deeper than about 260 km. The majority of the anisotropy directions inferred from the core phases are parallel to the absolute plate motion (APM) direction of the Nazca plate, which is about N80°E. At some stations, however, the fast polarization direction is pointing N160°E, nearly parallel to the strike of the trench and the Andes which would be compatible with the trench-parallel flow model for South America proposed by Russo and Silver [1994]. This direction is observed over an approximately 100-km-wide band to the west of the active volcanic zone. It may represent either a second anisotropy regime in the mantle, a small-scale diversion of slab-entrained mantle flow, or a relatively small area where slab entrainment of mantle flow is reduced or ceases to exist. The large number of observed APM-parallel fast directions suggests, however, that the mantle beneath the descending Nazca plate in northern Chile deforms mainly as the result of slab-entrained mantle flow. The large variations of anisotropy directions in the Andean subduction zone indicate that asthenospheric flow in the Pacific mantle has a complex pattern which may vary over scale lengths of a few hundred kilometers and which may be governed by slab morphology.

Description: A stack of the wide‐angle reflection/refraction component of the URSEIS‐95 experiment provides the first well‐resolved imaged of the Moho beneath the southern Urals. The processing consisted of low pass filter (0–6 Hz), CMP sorting, and a NMO correction without stretch. The PmP phase, a very narrow band and low frequency (up to 6 Hz) wavelet, changes character from west to east along the transect. In the depth converted section, the Moho reaches a maximum depth of 53±2 km beneath the Magnitogorsk arc. Thickness estimates determined from high amplitudes at near critical distances also support a 53 km thick crust. A selective offset stack consisting of traces at 150–250 km offset indicate an undulating, irregular Moho, suggesting either strong lateral velocity variations or high topographic relief beneath the Magnitogorsk arc.

Description: [...] We develop a regularizing solution procedure accounting for recent theoretical stability estimates. The capabilities of the procedure are shown for a single magnetic field component of the spherical harmonic field expansion beginning from the year 1900 by varying the mantle conductivity model and the degree of smoothness in the regularization. As an example, the radial component of a global (5,5) core-mantle boundary field is calculated for two epochs.

Description: The coupled plate interface of subduction zones—commonly called the seismogenic zone—has been recognized as the origin of fatal earthquakes. A subset of the after-shock series of the great Antofagasta thrust-type event (1995 July 30; Mw = 8.0) has been used to study the extent of the seismogenic zone in northern Chile. To achieve reliable and precise hypocentre locations we applied the concept of the minimum 1-D model, which incorporates iterative simultaneous inversion of velocity and hypocentre parameters. The minimum 1-D model is complemented by station corrections which are influenced by near-surface velocity heterogeneity and by the individual station elevations. By relocating mine blasts, which were not included in the inversion, we obtain absolute location errors of 1 km in epicentre and 2 km in focal depth. A study of the resolution parameters ALE and DSPR documents the importance of offshore stations on location accuracy for offshore events. Based on precisely determined hypo-centres we calculate a depth of 46 km for the lower limit of the seismogenic zone, which is in good agreement with previous studies for this area. For the upper limit we found a depth of 20 km. Our results of an aseismic zone between the upper limit of the seismogenic zone and the surface correlates with a detachment zone proposed by other studies; the results are also in agreement with thermal studies for the Antofagasta forearc region.

Description: A major problem in geodynamics with seismic data is discussed: How deeply do the continents penetrate into the mantle? Differential travel times of underside reflections from mantle discontinuities that appear as precursors to SS, in large parts of the globe, show a clear correlation with oceans and continents. They are significantly larger beneath the Asian and North American continents than underneath the neighbouring Pacific. From this observation we conclude that the Asian and North American continents affect the mantle well below 410 km. Changes in the thickness of the transition zone can explain our observations, which are in agreement with the hypothesis of petrological phase changes causing the 410 km and 660 km seismic discontinuities. Average thickness of the transition zone underneath continents is about 14 kim thicker than beneath oceans. Moreover, our findings imply temperature variations about 100–200 K in the mantle transition zone. Weak reflections from 520 km depth corresponding to an impedance contrast of about 2% can be observed only in some areas of the Earth, while observations from other locations definitively show no signal from this depth. Therefore, we propose that the 520 km reflector is only a regional feature.

Description: In December of 1994 a fluid injection experiment which triggered several hundreds of microearthquakes was conducted at the KTB main borehole (Oberpfalz, Germany). These events were recorded with a temporal seismic network at the surface. Out of the complete data set, a cluster of five events recorded at four mini-arrays consisting of eight or nine stations was used to investigate the crustal scattering properties in the vicinity of the KTB. For this purpose, the ‘Double Beam Method’ (DBM; Krüger et al. 1993, 1995, 1996) and the ‘Double Beam Imaging Method’ (Scherbaum, Krüger & Weber 1997) were extended to curved wave fronts to drop the restriction of plane-wave propagation. This technique is used for imaging the crustal scattering strength using earthquake clusters recorded at close-by mini-arrays. The results of the array analysis show that the composition of the P coda is mainly affected by the site location of the arrays. Near-surface and deeper crustal scattering contribute in a very complicated pattern. Furthermore, with the present data set it was possible to identify reflections from the top of the Erbendorf Body. This is a very pronounced arrival in most of the recorded traces. In one of the arrays its amplitudes are even greater than the direct P phases. Five to eight coherent phases could be identified by the mini-arrays. Using only these phases, synthetic P-coda traces were constructed, which only contain the coherent part of the observed wavefield. By subtracting the synthetic coherent wavefield from the original traces we achieve a variance reduction in the P coda of up to 37 per cent. This leads to the conclusion that a large amount of the P coda at the KTB can be modelled by a simple deterministic single-scattering model using a small number of individual scatterers.

Description: A fluid injection-induced seismicity experiment was undertaken in the KTB (German Continental Deep Drilling Program) main borehole at 9 km depth. Several hundred microearthquakes were recorded by a three-component geophone at 4 km depth in the pilot hole of the KTB about 200 m west of the main hole. More than 100 of these events were also recorded with good signal-to-noise ratio by a 73-element temporary network at the surface. Several different clusters of microearthquakes with distinct waveforms were defined. Compound fault-plane solutions for the two most prominent clusters of seismic events were determined: a strike-slip mechanism for cluster 1 at an average depth of about 8.9 km and a strike-slip/reverse mechanism for cluster 4 (with the “main” ML = 1.2 event) at an average depth of 8.6 km. For both fault-plane solutions, the P axis is subhorizontal and oriented NNW-SSE, similar to the N160°E direction of maximum horizontal stress observed in the well bore. Both clusters were analyzed using an empirical Green's function method to derive the relative source time function (RSTF). Azimuthal variations of the RSTF were used to determine rupture directions and velocities. By combining the information about rupture directions with fault-plane solutions, it was possible to identify the active fault planes (NE striking nodal planes) for both clusters. Although injection-induced events are supposed to exhibit a dilatational component due to the tensile character of the source, the moment tensor inversion for both microearthquake clusters resulted in a double-couple contribution of about 90% and P axes similar to the direction of maximum horizontal stress observed in the borehole. The isotropic components of the moment tensors are insignificant due to the size of the location uncertainties. From records of the sensor at 4 km depth, we found seismic moments of the microearthquakes ranging from 107 to 1011 N-m. The spectra were corrected for Q [Q(f) = 420 f0.5 for P, and Q(f) = 230 f0.5 for S-waves, which were determined assuming an ω2 model]. Following Brune (1970, 1971), we found source radii between 12 and 28 m and stress drops between 0.01 and 6 MPa. The average ratio of S- to P-wave energy was determined as 14.2. Our relation between seismic moment and ML is log M0 = 1.01 ML + 9.68, and between energy and seismic moment, log E = 2.0 log M0

Description: The amplitude spectra of global geophysical phenomena were investigated to motivate research of physical connections between them. The suggested causality was derived from comparison of the spectra, and from cross correlation functions. The following global parameters were discussed: for the earth rotation by the variations of the length of day, for the gemagnetic variation by the global field intensity, changes of the dipole axis and the westward drift, and for climate change by the atmospheric excitation function derived from air pressure variations, and temperature variations. The model of atmospheric excitation, which can be proved most exactly for the annual variation of length of day, is responsible for the 11 and 22 years periods, too. It failed for longer periods e. g., partially for the 30 years periods and completely for the 60 to 80 years periods, which were also discovered in the mean temperature and geomagnetic field variations. Therefore, it was suggested that longer periods in climate change and in the variations of the earth's rotation are caused independently by the same process in the earth core, provided that a physical influence of the geomagnetic field on climate will be accepted in future. The investigation was completed by comparison with the spectra of some local temperature variations in Europe.

Description: High noise levels hamper teleseismic shear wave splitting measurements, which bandpass filtering does not always help. To investigate how robust splitting measurements are to noise, we analysed a set of synthetic records with known splitting parameters and added fixed levels of noise. In the presence of weak anisotropy, single-waveform splitting measurements are unreliable when operating with noisy data sets. A practical rule in terms of S/N ratio and splitting delay time parameters is that splitting is confidently detectable at S/N 〉 8, regardless of the wave's original polarization orientation. However, for the evidence of weak anisotropy to be detectable and measurable at an S/N value of 4, the backazimuth separation of the phases from the fast polarization direction needs to be higher than 20°. Stacks of individual measurements consistently yield reliable results down to S/N values of 4. Applying stacking to data from DSB (Dublin, Ireland), the fast polarization direction φ and lag time δt are 58° and 0.95 s. This orientation reflects surface trends of deformation in the area, as found elsewhere in the UK. Our result thus reinforces the proposed model that the detected anisotropy in the British Isles originates from lithospheric coherent deformation preserved from the last main tectonic episode.

Description: Local seismic network in Northern Chile, Southern Bolivia. (Grant-number: GIPP199604) Waveform data is available from the GEOFON data centre. License: “Creative Commons Attribution-ShareAlike 4.0 International License” (CC BY-SA).

Description: The vibroseis reflection profiling component of the URSEIS '95 experiment provides a high-resolution crustal-scale image of the unextended southern Uralide orogen. A marked lateral and vertical variation of reflectivity throughout the entire crust differentiates the former margin of the East European craton to the west from accreted terranes to the east. Between these regions is a less reflective zone corresponding to the Magnitogorsk volcanic arc and the crustal root zone. Continuous reflections and reflective domains that correlate at the surface with major tectonic features define a bivergent orogen in which the Paleozoic collisional structure has been largely preserved.

Description: The tremor wave‐field at Mt.Etna was recorded with an array of three 3‐component broadband seismometers. The analysis of its polarisation and dispersion properties leads to the conclusions that (1) the wave‐field consists mainly of surface waves and that (2) two of the observed spectral peaks are caused by a wave guide effect along the propagation path.

Description: This paper presents the application of the electrical resistivity tomography (ERT) method to the investigation of the Tertiary maar structure of Baruth (Germany) known from previous gravimetric surveys. ERT was applied to support the optimum location for a palaeoclimatological drill hole. Special modifications of data acquisition, signal processing and inversion are introduced to adapt the method of ERT to the special requirements for the 3-D investigation of structures with horizontal extensions of 1 km or more. More than 5000 dipole–dipole combinations were recorded at three concentric circular electrode arrangements using stand-alone transient data acquisition systems (RefTek). We present a fast approximate imaging technique based on the simultaneous iterative reconstruction technique (SIRT). As the complete calculation of the inverse Frechét matrix is avoided, the algorithm is especially suitable for large data and model spaces, where complete inversion is beyond the limits of available computing hardware. The single-step method is applicable to arbitrary irregular electrode layouts. Synthetic tests show that the imaging procedure reconstructs the main features of the subsurface. A low-resistivity body could be interpreted as limnic sediments filling the interior of the Tertiary maar crater. Considering the horizontal resistivity gradient, estimates for the lateral and depth extents of the structure were made. An optimum position for a palaeoclimatological borehole was found, and was in good agreement with the gravimetric minimum.

Description: A marine seismic reflection survey was carried out in 1996 by DEKORP, Potsdam, and BGR, Hannover in the SW Baltic Sea. Several tectonic lineaments were crossed nearly perpendicularly, for example, the Caledonian Deformation Front which is assumed to mark the northern boundary of the terrane Avalonia which accreted to Baltica ca. 400 Ma. Here, a bivergent collision is clearly observed, confirming early ideas from the BABEL survey. The NE-dipping reflections in the uppermost mantle are interpreted as signs of the subducted Tornquist Ocean. A similar tectonic style of compression and indentation is observed in the Proterozoic crust northeast of Bornholm, where in addition a remarkable crustal thickening and a strong increase of reflection power is observed. The three DEKORP lines in this area provide a certain three-dimensional control and allows extension of similar observations from the BABEL line A southward. This Proterozoic terrane accretion seems to be connected to major tectonic lineaments in southern Sweden, either to the Gothian Thrust or the Sveconorwegian Front.

Description: Explosive-source deep seismic reflection data from the southern Ural Mountains of central Russia provided a lithosphere-scale image of the central Eurasian plate that reveals deep reflections (35 to 45 seconds in travel time; ∼130 to 170 kilometers deep) from the mantle. The data display laterally variable reflectivity at the base of the crust that deepens beneath the central part of the profile, documenting a crustal thickness of ∼55 to 60 kilometers beneath the axis of the orogen. These data provide an image of the structure of the crust and underlying mantle lithosphere in a preserved collisional orogen, perhaps to the base of the lithosphere.

Description: Oxygen-isotope ratios of precipitation (δ18OP) inferred from deep-lake ostracods from the Ammersee (southern Germany) provide a climate record with decadal resolution. The record in detail shows many of the rapid climate shifts seen in central Greenland ice cores between 15,000 and 5000 years before the present (B.P.). Negative excursions in the estimated δ18OP from both of these records likely reflect short weakenings of the thermohaline circulation caused by episodic discharges of continental freshwater into the North Atlantic. Deviating millennial-scale trends, however, indicate that climate gradients between Europe and Greenland changed systematically, reflecting a gradual rearrangement of North Atlantic circulation during deglaciation.

Description: Analyses of magnetotelluric (Mn and magnetovariational (MV) data from 20 sites spanning southe~ Kenya has revealed significant three-dimensional effects. Consideration of both regional geology and induction arrow transfer functions implies the importance of two principal strike directions : One delineated by the N-S trending rift, the other controlled by a continental scale, Protorozoic , NW-SE striking , fault fabric. The hypothesis that the induction arrows might be explicable in terms of the near-surface expression of the conductive infill of the rift, together with a deep-seated , NW-SE striking conductor extending into the upper mantle is borne out by three-dimensional modelling . Spatially and directionally , the mantle conductor may correspond with a low velocity zone modelled from teleseismic data and both geophysical signatures may relate to a major, NW-SE striking shear zone system which has been mapped at the surface. A low velocity zone modelled from seismic refraction data recorded along a profile which crosses the rift at the same latitude as the MT measurements has been speculatively attributed to a few percent partial melt in the mantle directly below the rift. No support for this interpretation is provided by the MV data.

Description: The history of the study of Polar Motion began with the derivation of the equations for the rotation of a rigid body by Euler published in 1758. This was followed by further theoretical contributions made by Lagrange (1788) and Poinsot (1834). To confirm the existence of the polar motion of the Earth in terms of a variation of latitude, intensive efforts were undertaken at several observatories toward the end of the 19th century. During the 7th General Conference of the European Arc Measurement in Rome in October 1883, Fergola had already suggested to investigate the problem. The latitude variation was detected by K¨ustner at the Berlin Observatory in 1888. Following this, during the Annual Conference of the Permanent Commission of the ”Internationale Erdmessung” in Salzburg in September 1888, Foerster proposed to consider the variation of latitude more systematically by a Special Commission. Activities began quickly, and after considerable effort the International Latitude Service (ILS) started in September 1899. The paper reviews, in which way the Geodetic Institute Potsdam contributed to the foundation of the ILS and took part in its activities through the work of Helmert, Albrecht,Wanach and Mahnkopf. Based on international scientific cooperation, the results were the rectangular coordinates of the Polar Motion from 1890.0 to 1922.7 at 0.1 year intervals, in particular those derived from the latitude observations at independent stations from 1890.0 to 1899.8, and those derived from the latitude observations at the ILS stations from 1899.9 to 1922.7

Description: Long period magnetotelluric and magnetovariational (MT /MV) measurements in West Bohemia carried out along the profile Cheb- Tachov- Nyrsko, close to the Czech-German border, display a striking similarity with analogous data from the immediate vicinity of the KTB borehole. On the other side, broad band AMT /MT /MV data along the seismic reflection traverse 9HR, measured by the Free Univ. of Berlin and Geofyzika Inc. Brno, show a substan tially more complex geoelectrical structme in the region to the east of the West Bohemian fault zone, indicating a discontinuous geoelectrical character when approaching the western margin of the Bohemian Massif. To assess the large scale geoelectrical changes in the E-W direction, additional long period MT measurements have been carried out at 8 localities in the area between the two basic NW-SE profiles. The new stat ions were arranged along two short profiles, one traversing the West Bohemian faults into the TeplaBarrandien zone and the other running just along the Central Bohemian deep fault. Preliminary analysis of those measurements is presented in this contribution.

Description: Magnetotelluric fieldwork was carried out in the southwest of Ireland as part of an integrated study of the Variscan Front. Data from a network of stations in southwest Ireland were collected, processed and modeled using a variety of equipment and computational methods. Extensive 2D modeling and inversion yielded models which gave insights into Variscan processes. 3D modeling yielded a strong yet surprising result showing the interaction of Variscan and Caledonian processes. Profile and regional models are developed and described.

Description: Well-known Dey & Morrison's [l] finite difference (FD ) algorithm for the 2-D modelling of direct currents is modified for generally anisotropic 2-D structures. By Fourier transforming the general current conservation equation with respect to the strike coordinate, the original 2½-D problem for the potential of a single feeding electrode is decomposed into an infinite number of 2-D problems in the wave number domain. Applying the area discretization (volume integration) scheme to the transformed 2-D PDEs, a 9-point FD stencil is obtained at each mesh node within the anisotropic structure, with generally complex elements for the direct neighbours of the central node. The resulting FD matrix is banded, 9-diagonal, complex and non-symmetric, but Hermitian. Gaussian elimination for real, symmetric and banded matrices is slightly modified to apply to the Hermitian matrices, and used to solve for the wave number potential components . Numerical tests and modelling examples of 2-D anisotropic structures are presented. Extension of the technique to 3-D models with anisotropy is discussed.

Description: Variations of annual and semiannual oscillations in rotation parameters have been investigated on the basis of length-of-day (LOD) as well as atmospheric-angular-momemtum (AAM) time series. These oscillations were deter- mined using band-pass filters. In order to show the character of variations of seasonal oscillations, amplitudes, phases and periods were computed by a least-squares adjustment with the method of modified harmonic analysis at quarterly intervals. In addition, the seasonal imbalances in LOD and AAM budgets were determined and analysed in a similar way. These discrepancies were corrected for tidally excited effects. The non-atmospheric oscillations without the annual tide effect Sa and the semiannual tide effect Ssa have changeable amplitudes between 0.02 and 0.10 ms.

Description: The large-scale seismic experiment POLONAISE'97 investigated the seismic structure of the crust and the uppermost mantle in the Polish region of the Trans-European Suture Zone. This paper covers the interpretation of seismic data along the 300-km-long P3 profile, which is located in the Precambrian East European Craton (EEC) parallel to the Teisseyre–Tornquist Zone. The recordings were of high quality, with seismic energy detectable out to 300 km offsets. The crustal model developed by two-dimensional raytracing forward modelling and waveform analysis is characterized by a subhorizontal uniform structure, although the boundaries of several large basement features are crossed. The crystalline crust consists of three parts: the upper, middle and lower crust with P-wave velocities of 6.1–6.4 km/s, 6.55–6.7 km/s and 7.05–7.15 km/s respectively. The PmP wave can usually be correlated at distances beginning at about 100 km, but it has variable reflection character. Moho depth increases from 38 km at the NW end of the profile to 44 km at the SE end. The Pn velocity of 8.05–8.1 km/s is less than that previously found in neighbouring areas of the EEC. The VP/VS ratio was determined separately for the upper/middle and lower crust to be 1.67 and 1.77 respectively. Following the Pn wave, another phase with an apparent velocity of about 8.3 km/s is interpreted as a weak reflection from a low-contrast discontinuity in the uppermost mantle.

Description: Was löste den erst jüngst erloschenen Vulkanismus in der Eifel aus? Zur Klärung dieser Frage wurde der Untergrund der Region mit knapp 250 Erdbeben-Meßstationen – dem bisher umfangreichsten europäischen Geophon-Netz – abgehorcht. Die Daten werden in ein hochaufgelöstes dreidimensionales Bild des Erdinneren bis in mindestens 400 Kilometer Tiefe liefern.

Description: The wave-field of volcano-induced seismic signals at Stromboli volcano (Italy) was observed using a network of four broadband seismometers. In addition to the standard spectral analysis of the recorded signals, new spectral techniques such as wavelet transform and wavelet coherence analysis were also used in order to discriminate between signals related to volcanic tremor and volcanic explosions. The common features of these two types of signals are characterised by the maximum of the power spectral density values located in the frequency band > 2.0 Hz, and the main orientation of the principal axes of the particle motion ellipsoids in the frequency bands analysed. The seismic signals of the volcanic explosions at Stromboli cover the total frequency band, whereas the spectral content of the volcanic tremor is restricted to narrow frequency bands ranging from 0.3–5.0 Hz. In order to localize both types of signals, a modified inversion technique was used. The result of this beam-forming on diffraction hyperboloids of both types of signals reveal a surprisingly shallow source location ranging from 0.2 km–0.7 km above sea level, suggesting that the sources of the volcanic explosions, as well as of the volcanic tremor, are restricted to the uppermost parts of the volcano. This is in agreement with several nonseismological observations of this volcano. The combination of spectral analysis of volcanic tremor and volcanic explosions with the analysis of ground displacement implies a mechanism which can be described by an ascending bubble of volcanic gas in a nearly closed volcanic system, and finally the forced ejection of the material through a narrow channel at the top of the magma column.

Description: A fluid injection-induced seismicity experiment was conducted in the German Continental Deep Drilling Program (KTB) main borehole at 9.1 km depth (in situ temperature of 260°C) to extend knowledge about stress magnitudes and brittle faulting to depths and temperatures approaching the brittle-ductile transition. Almost 400 microearthquakes were induced at an average depth of 8.8 km by injection of KBr/KCl brine into a ∼70 m open hole section near the bottom of the borehole. Although most focal plane mechanisms were poorly constrained due to the very small size of the induced earthquakes, several different clusters of microearthquakes with distinct mechanisms were defined. Most of the microearthquakes for which focal plane mechanisms were determined were strike-slip events with a NNW trending P axis, essentially parallel to the direction of maximum horizontal compression observed in the borehole. The largest induced earthquake, M 1.2, occurred 18 hours after injection was started. This event was a strike-slip/reverse faulting event which also had a NNW trending P axis. Utilization of a precise relative location technique indicates that many of microearthquakes occurred relatively far (50–100 m) from the well bore. Modeling of the pore pressure disturbance caused by injection suggests that many of the earthquakes were induced by extremely small pore pressure perturbations (〈1 MPa) less than 1% greater than the ambient, approximately hydrostatic pore pressure at depth. Thus it is apparent that there are critically stressed, permeable fault zones in the crust, even at great depth and temperature. A frictional analysis of the focal plane mechanisms of the induced microearthquakes indicates that fault slip is consistent with the stress magnitudes and orientations determined in situ at depths to 7.7 km in the borehole and relatively high coefficients of friction (∼0.6–0.7) reported by Brudy et al. [this issue]. This and the observation that very small pore pressure perturbations were able to trigger seismicity appear to confirm the hypothesis that “Byerlee's law” (i.e., that differential stresses in situ are limited by the frictional strength of well-oriented, preexisting faults) is valid to great crustal depth and that the crust is in brittle failure equilibrium at depths and temperatures approaching the brittle-ductile transition, even in this relatively stable intraplate area.

Description: Global Seismic Network data were used to image upper-mantle seismic discontinuities. Stacks of phases that precede the PP phase, thought to be underside reflections from the upper-mantle discontinuities at depths of 410 and 660 kilometers, show that the reflection from 410 kilometers is present, but the reflection from 660 kilometers is not observed. A continuous Lamé's constant λ and seismic parameter at the 660-kilometer discontinuity explain the missing underside P reflections and lead to a P-wave velocity jump of only 2 percent, whereas the S-wave velocity and density remain unchanged with respect to previous global models. The model deemphasizes the role of Lamé's constant λ with regard to the shear modulus and constrains the mineralogical composition across the discontinuity.

Description: The small satellite mission CHAMP was initiated and is primarily funded by the German Space Agency (DARA) as a lead project for the East German space industry. It is defined in its main mission goals by researchers of the GeoForschungsZentrum Potsdam (GFZ), and is conducted under lead of GFZ in cooperation with the German Aerospace Establishment (DLR) and the industry . After completion of an initial feasibility study (Phase A) and of the project's definition/specification phase (Phase B), followed by a two months redesign phase (Phase ~B), CHAMP is supposed to enter into Phase C/D in late 1996. CHAMP as a geoscientific mission with a multi-purpose and complementary payload shall substantially contribute to one of the basic research objectives of studies of planet Earth, that is, to the determination of the composition, structure, and dynamics of the solid planet, its oceans and atmosphere, and its surrounding envelope of charged particles and fields. CHAMP being one element in a timely sequence of Earth observations and platforms, satellites, and mini-satellites could be a contributor to the acquisition of global, synoptic and long-term measurements of global processes through space and ground instrumentation . CHAMP shall fulfil the criteria of a small satellite mission, i.e., only a few years of development time through the usage of existing sensors and commercial spacecraft subsystem components, and reduced costs through protoflight approach, reduced quality standards and test efforts. The most challenging parts of the CHAMP mission are the variety of payload components especially the accelerometer and the magnetometers, each one with demanding environmental requirements. It is designed to observe both the gravitational as well as the magnetic potential from one platform in order to get a complementary scientific payback. The GPS-receiver on-board CHAMP being employed for gravity field recovery, simultaneously will perform atmosphere and ionosphere profiling by Earth limb sounding. It is also for the first time a three-axes accelerometer will be flown to measure with a required accuracy of 10^8m/s2 the non-gravitational forces, e.g. air drag, perturbing the satellite's motion.

Description: Rischmüller, H.: Technical Limits for Scientific Drilling and Downhole Measurements. p. 5-15. Storms, M. A.: Science/Industry Cooperation in the Research & Development of ODP Drilling/Coring Technology. p. 19-48. Khakhaev, B. N.: Technological Problems while Drilling Superdeep Wells. p. 51-66. Basovitch, V. S.: Requirements for Borehole Verticality in Superdeep Drilling. p. 69-73. Saito, S.: High Temperature Experiences and Developments in Geothermal Drilling in Kakkonda, Japan. p. 77-95. Guangzhi, Liu: The Strategy and Concept of the Proposed Pilot hole Drilling in China. p. 99-118. Beswick, A. J.: High Quality Coring for Scientific Purpose on Hard Rock to 2000 m. p. 121-123. Beswick, A. J.: Summary of Deep Slimhole Wireline Drilling in South Africa. p. 127-130. Holtrop, J. F.: Some Remarks to Mud Technology and Wellbore Stability. p. 133-134. Rowley, J. C.: Drilling and Coring Technologies for CSD Products. p. 137-148. Gloth, H.: Scientific Drilling, Sampling and Testing up to 5000 m Depth. p. 151-173. Jürgens, R.: Slim Hole and Horizontal Drilling. p. 177-184. Young, F., Hohoff, H., Kerk, T.: UTB: Rig Layout and Rig Operation for Ultradeep Scientific Drilling. p. 187-203. Boyeldieu, C.: Logging while Drilling and other New Logging Technologies. - Their Potential for Lithosphere Research. p. 209-237. Kehrer, P., Bram, K., Draxler, J. K., Kück, J., Kessels, W., Zoth, G.: Subsurface Data Acquisition for Science by Downhole Measurements. p. 241-250. Lysne, P.: Evolution of Downhole Measurement Systems. p. 253-260. Draxler, J. K., Bram, K.: Measurement Systems for in situ Data Acquisition in Normal, Superdeep, Slim and Tough Boreholes. p. 263-277.

Description: Umsonst, Th., Lauterjung, J., Tran Viet, T., Wöhrl, Th.: Technical aspects and sampling procedures at the KTB-Hauptbohrung. p. A1-A11. Duyster, J., Grawinkel, A., Kontny, A.: Petrographic and Structural Characterization. p. B1-B80. Machon, L.: Geochemistry/Fluid Analysis. p. C1-C28. Rauen, A., Winter, H.: Petrophysical Properties. p. D1-D45. Röckel, Th., Natau, O.: Rock Mechanics. p. E1-E9. Conze, R.: Electronic Data Processing at KTB. p. F1-F13.

Description: This technical report has been initiated, compiled and edited until March 1994. Its objective was to collect the existing geoscientific database and state of the art on the Uralides. It serves as a platform for financial and operational decisions by national and international funding agencies on the URSEIS95-project (Urals Reflection Seismic Experiment and Integrated Studies) under the EUROPROBE umbrella. The German DEKORP2000 (GFZ Potsdam), funded by BMBF, decided to operate this project in June 1994. Until November 1994, the COCORP project (Cornell University, U.S.A), funded by NSF, and CICYT (Barcelona, Spain) got funding to join a western consortium on the operation ofURSEIS in 1995. Until the end of 1994, a Russian consortium, funded by ROSGEOLCOM, was built under the leadership of SPETSGEOFISIKA (Moscow), integrating the BAZHENOV Geophysical Expedition (Sheelite, Ekaterinburg), BASHNEFTEGEOFISIKA (Ufa) and the GEON institute (Moscow). All partners in the project join forces in financing, acquisition, processing, interpretation and publishing the results of the seismic experiment. The field parameters of the experiment (chapter VI) have been slightly modified in the meantime and include the Vibroseis source technique. The field campaign is scheduled for May-September 1995. The interdisciplinary studies (chapter Vll) have been approved by the E.C. in the frame of the INTAS-programme (International Association for the Promotion of Cooperation with Scientists from the Independent States of the Former Soviet Union). in October 1994 and started their operational phase.

Description: For many years, in situ stress in the brittle crust has been measured at relatively shallow depth and related to the mechanical behavior of the crust as inferred from laboratory studies and faulting theory. A continuous profile of the magnitudes and orientations of the three principal stresses has been estimated to depths of 7.7 km and 8.6 km in the German Continental Deep Drilling Program (KTB). This was achieved by hydraulic fracturing tests at relatively shallow depth (1–3 km), estimates of the magnitude of the least horizontal principal stress provided by modified hydraulic fracturing experiments at 6 km and 9 km depths, and analysis of compressional (breakouts) and tensile (drilling-induced tensile wall fractures) failures of the borehole wall over nearly the entire depth of the KTB borehole. The orientation of the maximum horizontal principal stress was found to be uniform with depth with an orientation of N160°±10°E, which is consistent with the average orientation found throughout western Europe. The only significant change in stress orientation was observed directly below a major fault zone crosscutting the borehole. The profile of stress magnitudes we have obtained demonstrates that to a depth of 8 km, the state of stress in the brittle crust in southern Germany is in frictional equilibrium. That is, the ratio of shear to normal stress as resolved on preexisting faults which are well-oriented to the in situ stress field is comparable to their frictional strength based on predictions of Coulomb faulting theory for a coefficient of friction of about 0.7 and near-hydrostatic pore pressure.

Description: The goal of the BEECD project is to prepare a basic parametric earthquake catalogue of Europe and a database of primary data, with special reference to long-term seismicity. This paper discusses the background of the project, with special reference to the reasons and the procedures according to which the catalogue and the database will be compiled. In the following the first results of the project are presented: first, procedures and problems in the compilation of the working file, including the analysis of the input catalogues; next, the definition of a tool for classifying the supporting data sets and its application to the entries of the input catalogue for the time-window before 1990, which shows that the average quality of the supporting data is rather poor; finally, some results of earthquake investigation, some potential use and the expected results.