ALBERT

Beschreibung: The analysis of crustal stresses is a prerequisite for a detailed investigation of intraplate tectonic processes. This study restricts to the interpretation of the directions of maximum horizontal stresses, both from earthquake fault plane solutions (direction of pressure axes) and sigma-1 axes of in situ stress-measurements. In the W part of Central Europe the well-known NNW-SSE-directed compressive stresses are dominant. But other directions characterize the region joining eastward in the forefield of the East European platform. Numerical model calculations were carried out to interpret the collected stress features. Therefore, the plane stress state of the investigated region was simulated by an elastic plate under prescribed boundary loading. The governing equilibrium equations have been solved by a finite element procedure.-Authors

Beschreibung: Additional keywords: lithospheric bending, obduction, Saxothuringicum, Moldanubicum, Auflasten, Durchbiegung der Lithosphaere

Beschreibung: Abstract: Along the left-lateral strike-slip fault on the eastern border of the Tibetan plateau, 22 M〉=6 earthquakes have occurred since 1725. The fault is divided into three segments with two aseismic gaps between the segments. Using the elasticity theory of dislocations, the spatial and temporal variations in stress are calculated using source parameters from the earthquakes such as epicentre, time, length of surface rupture, mean coseismic displacement and magnitude. Further, a total slip rate is assumed in order to calculate the stress increase due to plate motion. The model is optimised towards the smallest variations in the preseismic stresses. Under this constraint, the present stress field is calculated by varying the spatial distribution of the ratio of seismic slip to total slip and the unknown initial stress field. The greatest seismic hazard is found in the central segment, if a spatially constant slip rate is assumed. However, different seismic slip rates for different fault segments lead to the southern segment as the region where the next large earthquake may occur. Additional keywords: source parameters, creep, Dislokationen, numerische Modelle, historische Seismizitaet, Kriechen, tektonische Spannungen, Erdbeben-Gefaehrdung, Erdbeben-Serie

Beschreibung: An earthquake near Ottawa (45.20"SUP o"N, 75.75"SUP o"W, focal depth 12 km) of unusualy large size for the region's seismicity (m"SUB b"(Lg) = 4.1) provided good P-wave first-motion data for a focal-mechanism solution. The mechanism is thrust faulting with a predominantly horizontal pressure axis trending 154"SUP o". The nodal planes have strike 71"SUP o", dip 75"SUP o", and strike 221"SUP o", dip 17"SUP o". The spatial distribution of aftershocks suggests the gently-dipping nodal plane could be the fault plane. There is uncertainty about the seismotectonics of the region, and the orientation of neither nodal plane correlates with known geological features. More earthquake fault-plane solutions are required to interpret the seismotectonics and the stress regime. -from Authors

Beschreibung: Sets of seismological, geological and geophysical maps have been developed or compiled for the E part of the W European platform. The area under study includes the GDR, SW Poland, and N Czechoslovakia. The maps mentioned are presented and briefly discussed with regard to their suitability for a seismogenetic analysis. A first seismotectonic interpretation is made as a rough lineation of seismoactive zones based on the seismological as well as non-seismological data.-from Authors

Beschreibung: The coincidence of a low velocity zone in a depth of 8-17 km, an estimated temperature of 250°C in the upper crust, relatively high recent lateral compression and relatively high shear strain in an area of the S part of the GDR (Vogtland region) may be favourable for the occurrence of small earthquakes. Further possible prerequisites for the causes of earthquakes have been taken into consideration.-Authors

Beschreibung: Abstract: Since 1984 in a German-Turkish project on earthquake prediction research has been operating the North Anatolian Fault Zone in Turkey. Among many other parameters changes in radon emission are investigated. Plastic nuclear track detectors (Kodak cellulose nitrate LR 115) are used to record alpha-particles emitted from radon and thorn atoms and their daughter isotopes. At regular intervals of 3 weeks the detectors are replaced and analysed. Thus a quasi-continuous time sequence of the radon soil gas emission is recorded. The authors present a comparison between measurements made with electronic countermand with plastic track detectors

Beschreibung: 22 hornblende K-Ar ages and 10 39Ar-40Ar spectra were obtained for hornblende garbenschists from the Western Tauern Window. The post-kinematic amphiboles were produced during the late Alpine prograde metamorphism (6-10 kb and 500-570° C). Two nearly potassiumfree cummingtonites rimming hornblende yield K-Ar ages of 120 Ma, while the 20 tschermakitic hornblendes scatter between 17 and 37 Ma. The reason for this scatter is excess Ar, possibly incorporated into amphiboles during healing of fractures, now traceable by trails of fluid inclusions. Excess Ar is semiquantitatively corrected for by combining cogenetic hornblende and cummingtonite with K-Ar isochrons. It can be quantified in 4 out of 10 hornblendes by 39Ar-40Ar stepwise heating experiments. Ages of 18-20 Ma result for corrected hornblendes. The retentivity of 40Ar, after correction for excess, shows no correlation with chemistry within the narrow compositional range observed; rather, it shows intriguing correlations with irregularities in Ca/K spectra, pointing to a microstructurally controlled mechanism for Ar loss. This observation leads to a critical evaluation of the closure temperature "constant", which apparently depends on an incompletely known number of mineralogical and environmental parameters. In particular those 39Ar-40Ar release spectra which yield low temperature steps with younger ages than the plateaus are not interpretable in terms of a synchronous closure. This gives evidence that loss of radiogenic isotopes proceeds by a more complex mechanism than simple volume diffusion through isotropic media.

Beschreibung: The frequency distribution of earthquakes has been compared with different geological and geophysical data. Coincidence exists between the frequency of epicentres and the transition zone of Bouguer anomalies (positive/negative) in the border zone of the Bohemian massif. The distribution of earthquakes is dominated by E- W and N-S directions which may be related to right-handed transcurrent faults. In a superordinate scale they could be connected with a clockwise tendency of rotation of the Bohemian massif. -Authors

Beschreibung: In recent years more and more data has become available about focal depth of intra-crustal earthquakes as well as structure and distribution of physical and geologic-petrological parameters as a function of depth. This allows us to study the causes for increased earthquake occurrence at certain depth intervals. Besides structural and material inhomogeneities this is mainly controlled by depth and temperature-dependent variations of rheological properties of crustal materials. Intra-crustal earthquakes occur in an intraplate environment almost exclusively in the depth-range 2km 〈 h 〈 20km with a clear maximum of activity between some 5 to 15km. The petrophysical as well as possible geologic-petrological reasons are discussed. The brittle fracture behaviour of crustal material is strongly controlled by the v"SUB p"/v"SUB s"-ratio. In crustal regions with v"SUB p"/v"SUB s" = or 〉 1.8 there is obviously no seismic activity. -from Authors

Beschreibung: The course and the character of the anomalies of geophysical fields indicate significant differences between the E European platform (EEP) and Central and S Europe. There can be derived a lense-shaped low viscosity zone within the upper mantle in a depth range between about 100 and 300 km between the African plate in the S and the E European platform in the NE. This zone defines a broad transition zone between the colliding African and Eurasiatic plates. The SW border of the EEP represents the real plate boundary of the Eurasiatic plate.-Authors Akademie der Wissenschaften der DDR, Forschungsbereich Geo- und Kosmoforschung, DDR-1500 Potsdam, Telegrafenberg, GDR.

Beschreibung: Abstract: The stress field before and after strong (M 〉= 6) earthquakes in the western part of the North Anatolian Fault Zone is modelled. Assuming an initially homogeneous state of stress, the elastic co-seismic stress-drops, the inelastic post-seismic stress variations and the increase of stress due to plate motion are taken into account, using the theory of dislocations. The model consist of an elastic layer (upper crust) above a half-space which is described by elastic properties and alternatively by a standard linear solid. The results of the latter inelastic model differ only from that of the pure elastic one. One of the main results is that most of the earthquakes fall into regions for which the model calculations predict high stress levels prior to the events. At present, the highest stress values are obtained for an area around 34 degrees E to 35 degrees E. They are higher than the average pre-seismic stress value for the series of eight earthquakes investigated in this study. All results are critically dependent on the assumption of the initial stress field being homogeneous along the western part of the North Anatolian Fault Zone. Additional keywords: earthquake migration, plate tectonics, seismic hazard, rupture planes, source parameters, tektonische Spannungen, Seismizitaet, Platten-Tektonik, Erdbeben-Gefaehrdung, Erdbeben-Serie

Beschreibung: New Hornblende K-Ar and 39Ar-40Ar and mica Rb-Sr and K-Ar ages are used to place specific timemarks on a well-constrained pressure-temperature path for the late Alpine metamorphism in the Western Tauern Window. After identification of excess 40Ar the closure behavior of Ar in hornblende is compared with that of Sr and Ar in phengite and biotite. Samples were collected in three locations, whose maximum temperatures were 570° C (Zemmgrund), 550° C (Pfitscher Joch), and 500-540° C (Landshuter Hütte). The average undisturbed age sequence found is: Phengite Rb-Sr (20 Ma)〉 hornblende K-Ar (18 Ma)〉 phengite K-Ar (15 Ma)〉biotite Rb-Sr, K-Ar (13.3 Ma)〉apatite FT (7 Ma). Except for the phengite Rb-Sr age, the significance of which is debatable, all ages are cooling ages. No compositional effects are seen for closure in biotite. Additionally, Rb-Sr phengite ages from shearzones possibly indicate continuous shearing from 20 to 15 Ma, with reservations regarding the validity of the initial Sr correction and possible variations of the closure temperatures. The obviously lower closure temperature (Tc) for Ar in these hornblendes than for Sr in the unsheared phengites indicates that the T(c sequence in the Western Tauern Window is different from those observed in other terrains. In spite of this discrepancy, valuable geological conclusions can be drawn if the application of closure temperatures is limited to this restricted area with similar T, P and PH2O: (1) All ages of samples located on equal metamorphic isotherms decrease from east to west by about 1 Ma which is the result of a westward tilting of the Tauern Window during uplift. (2) In a PT-path, the undisturbed cooling ages yield constantly decreasing uplift rates from 3.6mm/a to 0.1 mm/a. (3) Use of recently published diffusion data for Ar in hornblende (Tc=520° C) and biotite (Tc=320° C) suggests an extrapolated phengite closure temperature for Sr at 550° C. This suggests that the prograde thermal metamorphism at this tectonic level of the Tauern Window lasted until some 20 Ma ago.