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 1/2 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.     

Interpretation of 1992–1994 Gravity Changes around Mayon Volcano,Philippines, Using Point Sources

Significant gravity changes observed around the Mayon Volcano (Philippines) between 1992 and 1994 at 26 stations are interpreted in terms of an increase of mass and pressure changes at several point sources modelled using a fast inversion process. This inversion approach attempts to fit gravity and elevation changes by combining a random search for the positions of the sources and a linear least-squares fit for the incremental mass, pressure and possible common regional values for gravity or elevation changes. Some stabilizer terms are included in the misfit function. Models with one and two sources were tested against the observed changes at Mayon. Models with only one-source give a best fit for a shallow source with a positive mass increment, horizontally displaced far from the summit. The study using two sources gives a best fit that is similar to the one-source model, but in addition indicates anomalous behavior at stations in the SW. Neglecting the stations located southward from a local fracture, the best-fitting model suggests one central positive mass change source, which is likely to be an intrusion of about 0.5 MU with a depth of about 5 km beneath the volcano. Standard deviation for the residuals ranges from 7–8 μGal for one-source models to 6–7 μGal for models with two sources. Both of the cases are below the error value of 9.4 μGal estimated for the gravity data, so that it is not possible to discriminate between both possible interpretations without additional information.     

Seismic Hazard Assessment of the East Thuringian Region/Germany – Case Study

East Thuringia/Germany, especially the region Gera-Ronneburg, is part of the large Kyffhäuser-Jachymov-Fault-Zone and displays moderate seismicity. However, its seismic hazard is significantly higher than that of the surrounding area including the Vogtland/Northern Bohemian region. The earthquake catalogue of Germany contains for this region besides the well-investigated Central German Earthquake (March 1872, I ₀ =VII-VIII) entries of up to I ₀ =VIII (14th century). Epicentral intensities and coordinates of these historical earthquakes are considered as uncertain. In seismic hazard analysis historical events which are uncertain are often neglected. But, especially in regions of moderate seismicity and infrequent larger earthquakes, the time window considered should be extended as far as possible. Apart from the necessity to study the historical sources of the strongest 14th century earthquakes, we investigate the influence of these events on the seismic hazard, taking into account the uncertainties of their size and location. Generally, the investigations clearly reveal the importance of defining source regions on the one hand and the significance of the local relevant attenuation function on the other hand. A further important point in seismic hazard assessment is the strong influence of the geological site conditions on seismic hazard (amplification or damping phenomena). For both points the well-known Central German Earthquake (1872) supplies important information.     

The East Thuringian Seismic Network

The East Thuringian Seismic Network (OTSN) was installed in 1997. It started its operation with five and now consists of six seismic stations, the GRSN (German Regional Seismic Network) station MOX and a control and analysis centre. All stations are equipped with 3-component GÜRALP and short-period seismometers, RefTek 24-bit data acquisition systems (dynamic range 23.5 bit), hard disks, GPS-receivers, modems and communication computers for dial-up purposes. The seismic signals are sampled at 100 Hz and stored on the hard disk. Simultaneously, the signals are processed by a STA/LTA detector which generates an extended event list. The central station calls these event lists once per day, analyses them, produces a list of real seismic events and calls the waveform data for these events only from the single stations. All stations operate completely autonomously and the whole system works automatically, but all operations can also be carried out interactively. The event analysis is performed manually using common seismic analysis programs. The main purpose of installing the seismic network is to investigate the local seismicity, its relation to recent tectonics, the stress field and structure of the upper crust in order to render more precisely the seismic hazard of East Thuringia. A further aim of the network is to improve the seismic monitoring situation for the neighbouring regions, especially the Vogtland/Northern Bohemia and the Western Saxony area.     

Evolution of the Variscan foreland-basin: modelling the interactions between tectonics and surface processes

The geology of Western and Central Europe is significantly influenced by the Variscan orogen that developed during Devonian and Carboniferous time. Numerical models are essential in understanding and quantifying the involved endogenous and exogenous processes and their interactions. These are mainly based on the large-scale mass redistribution caused by erosion and fluvial sedimentary transport. The sedimentary mass flux leads to changing loads on the lithosphere and affects therefore the evolution of the orogen and the foreland-basin. The complex feedback-mechanism of the surface and tectonic processes is studied by three-dimensional elastic–plastic numerical models. The calculated uplift rates are used to model the interaction between tectonic and surface processes such as erosion and sedimentation. An iterative application of the numerical models for the tectonic and surface processes yields a detailed view of the evolution of the foreland-basin. The tectonic model itself (excluding surface processes) already shows some of the palinspastically reconstructed important features of the lower Carboniferous like the London-Brabant Massif, and the northward propagation of the Variscan deformation front. The results obtained from the coupled analysis can be compared to studies of the sedimentary record (i.e. time, thickness, and sedimentation rates) and other geological concepts (i.e. stability of geological provinces). The results demonstrate that both processes are essential in understanding the complex structural evolution of the Variscides and their foreland. The numerical approach on the tectonic–surface process interaction can also be applied easily to other geological settings.     

Accuracy assessment of ocean tide loading computations for precise geodetic observations

The increasing resolution of ground based gravity measurements (e.g. by superconducting gravimeters) as well as satellite based gravity field studies allows to study very small signals, globally as well as local. On the other hand, this requires the correction of such signals to uncover others. To study the Earth’s deep interior and the on-going dynamic processes requires the correction of disturbing signals, and one of these signals is related to ocean tidal loading. Although new ocean tide models are being derived from current satellite missions, there are still uncertainties.In this paper we present an intercomparison ocean tide models to test their fit to world-wide observations. Therefore, three TOPEX/POSEIDON (T/P) satellite derived models (CSR3.0, FES95.2 and TPXO.2) beside the classical SCHW80 model were selected for an accuracy assessment study. The selected models have been subjected to an intercomparison test, tide gauge validation test and comparison to 59 tidal gravity stations.The intercomparison test shows a good agreement between the T/P-based models for the open ocean and remarkable disagreement between the selected models in the coastal regions indicating that such models are still problematic in these regions. The tide gauge validation shows that the T/P derived models fit tide gauges better than SCHW80, with a better fit for the semidiurnal constituents than for the diurnal constituents. Comparing the gravimetric ocean-tide loading computed from the selected models with the residuals from a set of 59 tidal gravity stations shows that there is an improvement of the T/P derived models with respect to the Schwiderski model, especially in M2. However, this improvement is not as significant as the result of the comparison with the pelagic data. The procedure developed for the comparison of T/P derived models with SCHW80 is presented. The results provide not only information and improvement with regard to SCHW80, but also information about the properties of the new models. It is intended to continue this work applying the very recent models to see how they perform compared to this study.With this study we provide boundary conditions for the improvement of new ocean-tide models in order to benefit from the gravity measurements now possible regarding the evaluation of Earth structures and dynamic processes.     

Earth's free core nutation determined using C032 superconducting gravimeter at station Wuhan/China

The long series tidal gravity observations from 1997 to 2002 recorded with C032 superconducting gravimeter (SG) at station Wuhan/China are used in order to study the Earth's geodynamics. The tidal gravity parameters are determined precisely using Eterna software package after careful data pre-processing. The Earth's free core nutation (FCN) resonant parameters (eigenperiods, quality factors and resonant strengths) are determined accurately. The results show the determined eigenperiod to be 431.0 sidereal days with an accuracy of ±1.81%, the quality factor is a negative one as of −7002, and the resonance strength can be explained by the elastic property of the Earth's mantle. The discrepancy of the eigenperiods when using various ocean models can amount to ±1.8%. The 30 sidereal days difference between the determined eigenperiod in this paper and the one in theoretical computation given by Wahr and Bergen can be explained by the real dynamic ellipticity of the Earth's liquid core, i.e., it is about 5% larger than the one under the hydrostatic equilibrium assumption.     

Results from 44 months of observations with a superconducting gravimeter at Moxa/Germany

Results for more than 42 months of observations with the superconducting gravimeter CD-034 at the Geodynamic Observatory Moxa are discussed. Moxa observatory is one of the newer stations within the ‘Global Geodynamics Project’ (GGP). A special feature of the gravimeter at Moxa is its dual sensor system; differences in the results obtained from the two sensor recordings are generally well within the standard deviations of the tidal analysis. One significant difference concerns the slightly different drift rates of 31 and 49.5 nm/s² per year for upper and lower sensor; both sensor drifts can be fitted by a linear function. We find that the noise levels are close to the ‘New Low Noise Model’ for the seismic-modes and are also low in the tidal bands. Due to this low noise, Moxa is a station well suited to search for small geodynamic signals. The long-period variation in the gravity residuals correlates well with the polar motion.The difference signal between the two sensor recordings has a peak-to-peak amplitude of about 6 nm/s² and shows systematic variations. Its spectrum is characterised by instrumental noise between 0.2 and 0.4 cph. The noise level of the difference and of the sum of the two residual datasets are clearly lower, respectively, higher than the noise contents of the gravity residuals themselves. This is a strong indication for the existence of broadband signals common to the two residual datasets, leading to the conjecture that the reduction of environmental effects is still not sufficient.Our results once more emphasize the necessity to correct the data for barometric pressure effects when analyzing the data for seismic modes. The reduction visibly increases the signal-to-noise ratio in the low frequencies of the mode band and helps to avoid misinterpreations of peaks. Besides the well known barometric pressure influence we can establish hydrological effects in the data which are probably caused by soil moisture and groundwater table variations as well as by batch-wise water movement within the weathering layer. As the major part of the observatory surroundings is above gravimeter level, an anticorrelation between hydrological and gravity changes is observed. In addition, it can be shown that global hydrological effects reach an order of magnitude that makes it necessary to consider these effects when analyzing long-period signals like polar motion. Vice versa these effects are large enough to be detectable in the gravity data. A first joint analysis of five datasets from the GGP network shows no indications for signals related to the Slichter triplet or core modes.     

Structure and State of Stress of the Chilean Subduction Zone from Terrestrial and Satellite-Derived Gravity and Gravity Gradient Data

It is well known that the quality of gravity modelling of the Earth’s lithosphere is heavily dependent on the limited number of available terrestrial gravity data. More recently, however, interest has grown within the geoscientific community to utilise the homogeneously measured satellite gravity and gravity gradient data for lithospheric scale modelling. Here, we present an interdisciplinary approach to determine the state of stress and rate of deformation in the Central Andean subduction system. We employed gravity data from terrestrial, satellite-based and combined sources using multiple methods to constrain stress, strain and gravitational potential energy (GPE). Well-constrained 3D density models, which were partly optimised using the combined regional gravity model IMOSAGA01C (Hosse et al. in Surv Geophys, 2014, this issue), were used as bases for the computation of stress anomalies on the top of the subducting oceanic Nazca plate and GPE relative to the base of the lithosphere. The geometries and physical parameters of the 3D density models were used for the computation of stresses and uplift rates in the dynamic modelling. The stress distributions, as derived from the static and dynamic modelling, reveal distinct positive anomalies of up to 80 MPa along the coastal Jurassic batholith belt. The anomalies correlate well with major seismicity in the shallow parts of the subduction system. Moreover, the pattern of stress distributions in the Andean convergent zone varies both along the north–south and west–east directions, suggesting that the continental fore-arc is highly segmented. Estimates of GPE show that the high Central Andes might be in a state of horizontal deviatoric tension. Models of gravity gradients from the Gravity field and steady-state Ocean Circulation Explorer (GOCE) satellite mission were used to compute Bouguer-like gradient anomalies at 8 km above sea level. The analysis suggests that data from GOCE add significant value to the interpretation of lithospheric structures, given that the appropriate topographic correction is applied.