Thomas Jahr

Crustal structure of the Main Ethiopian Rift from gravity data: 3-dimensional modeling

Abstract A three-dimensional interpretation of the newly compiled Bouguer anomaly map of the Main Ethiopian Rift is presented. A high-resolution 3-D model constrained with the seismic results reveals a possible crustal thickness and density distribution beneath the graben. The Bouguer anomalies along the axial portion of the rift floor, as deduced from the results of the regional and residual separation, are mainly caused by the deep-seated structures. The inferred zone of intrusion, which is the main subject of the present study, coincides with the maximum gravity anomaly of the rift floor. The intrusion is displaced at several sectors along the east–west direction, and the two major displacements coincide with the locations of the major rift offsets on the surface. Because of the asthenospheric uplift, the crust under the Main Ethiopian Rift is slightly thinned. The zone of crustal thinning (≤31 km) coincides with the location of the intrusion beneath the rift floor, and the maximum of which is attained in the northern and central sectors of the graben. The trend of the crustal thinning zone, which is from south to north, is the same as the one obtained in the Afar depression. The southeastern and western plateaus, on the other hand, show by far the largest crustal thickness in the region (38–51 km). In contrast to the Afar depression, where the crust is partly oceanized, the thickness and density of the crust suggest that the Main Ethiopian Rift is underlain by a purely continental crust. The deep and relatively large nature of the intrusion leads to the conclusion that a large-scale asthenospheric upwelling might be responsible for the thinning of the crust and subsequent rifting of the graben.

Three-dimensional gravimetric modelling to detect the deep structure of the region Vogtland/NW-Bohemia

Abstract The occurrence of swarm earthquakes in the Vogtland/NW-Bohemia area results probably from the physical interactions of fluids, the stress field and the geometry of the geological units. Therefore the present study aims at the development of a 3-D density model of the region with a vertical range of 35 km. A new Bouguer anomaly map is presented containing about 17 000 gravity data points. Prominent Bouguer anomalies are produced by the granites of Eibenstock and Karlovy Vary (low with −75 mGal), the metabasites near Marianzke Lazně (high with 5 mGal) and the Munchberg Gneiss Massif (gravity high of Hof with 10 mGal). The geometry of the internal model structures correspond to geological units and, thus, the modelled gravity fits well the observed Bouguer anomaly. The 3-D gravimetric modellings indicate detailed geometries of the geological settings. With regard to the periodic occurrence of swarm earthquakes in the Vogtland region the existence of an upwelling mantle or a magmatic body is investigated. Precise information only can be given, if the vertical extension of the near surface bodies is known.

Deep structure and evolution of the Harz Mountains: results of three-dimensional gravity and finite-element modeling

A new Bouguer anomaly map is presented for the region of the entire Harz Mountains based on more than 60,000 gravity values. The various gravity anomalies are discussed and interpretation is carried out by high-resolution 3-D gravity modeling. One of the main subjects of interest in the investigation is the northern boundary fault zone of the Harz Mountains, separating the Mesozoic sediments in the north from the Palaeozoic rocks of the Harz in the south. Dip and vertical displacement are determined for this fault zone; mean values are 3400 m and 70°, respectively. Gravity modeling shows that the Brocken and the Ramberg Granites are distinctly different. The Brocken Granite is shallow, whereas the Ramberg Granite has a maximum depth of 8.5 km and a NS dimension of 37 km. The prominent Benneckenstein Gravity High is explained by two different models, one based on a granodioritic intrusion (2900 kg/m3) with a center-depth of 14 km and the other based on phyllites (2740 kg/m3) on a depth of 3–4 km. Studies on the geodynamic evolution of the Harz Mountains are carried out using the finite-element method. On the basis of a 3-D model, vertical displacements that can be related to horizontal forces are computed. For the period of the Variscan Orogeny an uplift of 600 m in the Harz area is calculated, for Late Cretaceous and Tertiary 400 m are determined. The total amount of 1000 m is about 12 of the vertical displacement of the northern boundary fault zone of the Harz Mountains shown by the gravity modeling. These results do not contradict geological ideas.

Geodynamic modelling of the recent stress and strain field in the Vogtland swarm earthquake area using the finite-element-method

Abstract The study of several swarm earthquake regions led us to the assumption that swarm quakes mostly occur in combination with fluid migration and often in the neighbourhood of volcanoes. Observations of the strong carbon dioxide flux and investigations of Weinlich et al. (1999) about recent volcanism in the Vogtland/ Western Bohemia are the linking part to global observations of swarm earthquakes. This idea and the theoretical modelling of Yamashita (1999) were the starting point to develop a model using the finite-element-method with the software package ABAQUS concerning stress field, pore pressure, temperature and deformation in the Vogtland swarm quake area. The model is intended to reveal the interaction between fluid migration, temperature changes and deformation in this area. Its horizontal dimensions are 55 km×60 km and 30 km in depth. The main geological units include the Marianske Lazne Fault Zone and the Eger Rift. We focus on that part of the Vogtland/Western Bohemia region where these two fault zones cross. The model takes into consideration the stress field, Mohr–Coulomb failure, thermal stresses as well as creep and poro-elastoplastic rheologies. The results show that the rate of deformation and the stress accumulation caused only by the regional stress field is not high enough to generate the swarm earthquakes. But periodic change of pore pressure (which includes fluid migration) in combination with temperature changes has a severe influence on the rate of deformation and is in the dimension to cause swarm quakes.

Recording and interpretation/analysis of tilt signals with five ASKANIA borehole tiltmeters at the KTB

In June 2003, a large scale injection experiment started at the Continental Deep Drilling site (KTB) in Germany. A tiltmeter array was installed which consisted of five high resolution borehole tiltmeters of the ASKANIA type, also equipped with three dimensional seismometers. For the next 11 months, 86 000 m(3) were injected into the KTB pilot borehole 4000 m deep. The average injection rate was approximately 200 l/min. The research objective was to observe and to analyze deformation caused by the injection into the upper crust at the kilometer range. A new data acquisition system was developed by Geo-Research Center Potsdam (GFZ) to master the expected huge amount of seismic and tilt data. Furthermore, it was necessary to develop a new preprocessing software called PREANALYSE for long-period time series. This software includes different useful functions, such as step and spike correction, interpolation, filtering, and spectral analysis. This worldwide unique installation offers the excellent opportunity of the separation of signals due to injection and due to environment by correlation of the data of the five stations with the ground water table and meteorological data.

Hydrological signals in gravity — foe or friend?

Although hydrological effects on gravity are known nearly as long as the influence of barometric pressure, they are not as well understood as the latter. The improvement of gravity data quality during the last years adds weight to the importance of understanding the hydrological influence. Moxa observatory is one station at which studies regarding hydrological effects are carried out. From soil moisture, water level and meteorological observations the effects of different hydrological contributors including snow can be modelled and compared to the gravity residuals of the superconducting gravimeter (SG). The total peak-to-peak amplitude amounts to 35 nm/s2. Contributions from the various areas around the observatory partly compensate due to the hilly morphology. The comparison between residuals and computed total hydrological effect yields a good agreement, but also shows that not all hydrological influences have been taken into account. A significant additional hydrological influence is due to the hill flank near the SG.

Three dimensional inversion of gravity data from the main ethiopian Rift

Abstract A three-dimensional interpretation of the newly compiled Bouguer anomaly map of the Main Ethiopian Rift is discussed. Then, the crustal thickness distribution beneath the Main Ethiopian Rift is confirmed using a — dimensional inverse approach to gravity data interpretation. The depths to the crust-upper mantle interface form the inversion parameters. Both approaches are constrained with the results of the seismic refraction experiments of the region. The degree of ambiguity of the final model parameters is then quantified. The Bouguer anomalies along the axial portion of the rift floor, as deduced from the results of the regional and residual separation, are mainly caused by deep-seated structures. The high resolution 3-D forward modeling reveals a possible crustal thickness and density distribution beneath the graben. The results of the inversion confirm a strong crustal attenuation zone (≤ 31 km) closely associated with the rifting of the graben and an abrupt fall of the Moho interface on either side of the rift (up to 51 km) related to the formation of the western and southeastern plateaus. However, no indication of crustal separation is observed. The ambiguity analysis reveals that greater ambiguity of the model parameters exists in the southeastern plateau. There, these model parameters represent the depths to the Moho interface where the seismic control is relatively less.

High Precision Deformation Monitoring at the Geodynamic Observatory Moxa/Thuringia, Germany

With strainmeters the observation of crustal deformation is possible with a resolution better than 10−9 m. At the Geodynamic Observatory Moxa in Thuringia/Germany an assembly of strainmeters of different types is recording deformation. Deformation mainly results from the tidal forces of sun and moon acting on the Earth, but also comes from seismic wave propagation or regional and local sources. Here we describe the results of an analysis of five time-series, each spanning 482 days, obtained from the different instruments and areal strain. We focus on the Earth tides but also look on the resonance of the Earth’s core to tidal forcing, the Nearly Diurnal Free Wobble. Even if not all five time-series show the resonance, its finding, especially in strain data, confirms the high data quality and sensitivity of the instruments. The analysis of the strainmeter data shows the comparability of the data from the different instruments as well as the good data quality connected to the very low noise level at the Geodynamic Observatory Moxa. Comparison with ocean loading shows that strong effects from local conditions like topography or rock inhomogeneities exist.

Influence of temperature variations on the noise level of the data of the LaCoste and Romberg Earth tide gravity meter ET18

Gravity records include a wide spectrum of signals based on mass changes in the Earths crust. Today, the detectable amplitudes of the signals of interest become smaller and smaller (e.g., hydrological effects). From this it follows that the accuracy and resolution of measurements must be very high and the noise level as low as possible. In this study, the influence of the temperature variations as noise signal in the gravity records was investigated. Therefore, two 215 day time series of the LaCoste and Romberg gravity meter ET18 in the broadband Geodynamical Observatory Moxa were analyzed and compared. Each time series was recorded in another room. The first room is characterized by a temperature variation of ±4°C per year. The second room has a constant temperature of 19°C. As reference data, the parallel recorded time series of the superconducting gravity meter (SG) CD-034, installed in the temperature-stabilized room were used. The result of this study shows a clear influence of temperature variation on the noise level in the spectra of the ET18 compared to the SG spectra. Depending on the frequency ranges, the ratios of the noise levels of the ET18 and SG spectra are between 2 and 5. In the frequency range up to 0.01 cph the ET18/SG factor is 2, between 0.01 cph and 0.09 cph the factor is 3, and above 0.09 cph the factor is 5.

Three-dimensional inversion of gravity data from the Main Ethiopian Rift

Abstract The crustal thickness distribution beneath the Main Ethiopian Rift is determined using a 3-D inverse approach to gravity data interpretation. The depths to the crust-upper mantle interface form the inversion parameters. These initial model parameters are constrained with the results of the seismic refraction experiment of the region. The degree of ambiguity of the final model parameters is then quantified using a Q-mode factor analysis approach. The results of the modelling indicate a strong crustal attenuation zone (≤31 km) closely associated with the rifting of the graben and an abrupt fall of the Moho interface on either side of the rift (51 km) related to the formation of the western and southeastern plateaus. However, no indication of crustal separation is observed. The semi-quantitative ambiguity analysis reveals the association of the model parameters from the southeastern plateau with the strong ambiguity region. These model parameters represent the depths to the Moho interface where the seismic control is relatively less.