Rhinegraben Area Research Articles

Estimation of small surface displacements in the Upper Rhine Graben area from a combined analysis of PS-InSAR, levelling and GNSS data

The intra-plate deformation of the Upper Rhine Graben (URG) located in Central Europe is investigated using geodetic measurement techniques. We present a new approach to calculate a combined velocity field from InSAR, levelling and GNSS measurements. As the expected tectonic movements in the URG area are small (less than 1mma−1), the best possible solutions for linear velocity rates from single-technique analyses are estimated in a first step. Second, we combine the velocity rates obtained from InSAR (line of sight velocity rates in ascending and descending image geometries), levelling (vertical velocity rates) and GNSS (horizontal velocity rates) using least-squares adjustment (LSA). Focusing on the Northern URG area, we analyse SAR data on four different image stacks (ERS ascending, ERS descending, Envisat ascending, Envisat descending) using the Persistent Scatterer (PS) approach. The linear velocity rates in ascending and descending image geometries, respectively, are estimated in an LSA from joint time-series analysis of ERS and Envisat data. Vertical velocity rates from levelling are obtained from a consistent adjustment of more than 40 000 measured height differences using a kinematic displacement model. Horizontal velocity rates in east and north direction are calculated from a time-series analysis of daily coordinate estimates at 76 permanently operating GNSS sites in the URG region. As the locations, at which the measurement data of PS-InSAR, levelling and GNSS reside, do not coincide, spatial interpolation is needed during several steps of the rigorous processing. We use Ordinary Kriging to interpolate from a given set of data points to the locations of interest with a special focus on the modeling and propagation of errors. The final 3-D velocity field is calculated at a 200 m grid, which carries values only close to the location of PS points, resulting in a mean horizontal and vertical precision of 0.30 and 0.13 mm a−1, respectively. The vertical component of the combined velocity field shows a significant subsidence of about 0.5 mm a−1 in the northern part of the graben coinciding with a well-known quaternary basin structure. Horizontal displacement rates of up to 0.8 mm a−1 in southeast direction are observed outside the graben, in reasonable alignment with the average direction of maximum horizontal stress. Within the graben, the velocity directions rotate toward east in the non-subsiding part, while an opposite trend is observed in the subsiding part of the graben. The complexities of the observed velocity field are compatible to the geomechanical situation in our investigation area which is characterized by a transition from a restraining to a releasing bend setting. Glacial isostatic adjustment is another potential source influencing the observed velocity field, as well as anthropogenic signals due to mining, oil exploration and groundwater usage that have been identified in some places.

Relocation of the 5 December 2004 Waldkirch, Germany, Earthquake with Regional 1D and 3D Seismic Velocity Models in the Presence of Upper-Mantle Anisotropy

Abstract The exact hypocenter location, that is, the depth of the 5 December 2004 Waldkirch earthquake in the southern Rhinegraben area, has been at the center of debate for some time. After various relocation efforts by several geophysical institutions, a depth range of 9–12 km was eventually reported for this event. Here we examine whether the Waldkirch event can be located with improved accuracy by a regional network with newly derived regional 1D and 3D seismic velocity models for the crust and upper mantle that also take into account the existing Pn upper‐mantle anisotropy beneath Germany. The preliminary arrival‐time analysis of the phase data hints at possible misidentifications of several phases ( Pg versus Pn ), in addition to that of errors due to anisotropy that lead to unrealistic shallow depth locations in the range of 0–5 km. After removal of these adverse effects on the relocation, the more realistic depth range of 10–15 km is obtained. The effects of an anisotropic upper mantle, the influence of the V P / V S ratio, and the influence of a varying Moho depth on the event relocation are then investigated. With the 1D model, the Waldkirch depth is located between 14 and 15 km, when P waves only or both P and S waves are used, whereas with the 3D P ‐wave velocity model, the depth again becomes 14 km. The effect of the Pn anisotropy is stronger for the epicenter than for the depth. The statistical location precision is determined by calculation of covariances and confidence ellipses as well as by Monte Carlo simulations with both randomly perturbed arrival times and initial hypocenters. In addition, the effects of uncertainties in the velocity model on the relocation are investigated. Based on this comprehensive analysis, the optimal Waldkirch event depth is determined to be 14 km, with a 95% confidence interval of ±2.5 km. Online Material: Tables of original phase data and statistical results with various relocations, and figures showing effects of phase corrections, 3D tomographic and anisotropic models, and results of Monte Carlo relocations.

Seismicity wedge beneath the Upper Rhine Graben due to backwards Alpine push?

The distribution of hypocentres in the Upper Rhine Graben area is re-examined, and discussed with respect to the present day tectonic framework. Most earthquakes occur within a N60° striking wedge, located on top of a Moho dome. This wedge is limited by the surface and at depth, by a plane which, in the south of the dome, coincides with the SE dipping Conrad discontinuity. In depth, the seismicity shows a normal distribution the maximums of which are located on a surface dipping 6° towards SE, parallel to the south-eastward dipping Conrad and Moho. This surface outcrops along the north-western edge of the uplifted crystalline Vosges and Black-Forest. The main shocks in earthquake swarms in the region often occur in the vicinity of this surface and along pre-existing N–S to NE–SW Variscan or Tertiary faults and show focal mechanisms of strike-slip. In contrast, part of the aftershocks show focal mechanisms of reverse faulting associated with SE–NW striking compression. The seismic wedge and the north-westward rising seismic surface suggest initiation of crustal ramp, which starts at the south-eastern rim of the Conrad dome and which may become a thrust plane if SE–NW compression continues. In the south-eastern edge of the graben and above the south-eastern ridge of the Moho dome, where evidences for extension have been found, we identify clustering of hypocentres along a surface that strikes N150°, parallel to the main compression and dipping towards NE. Dominant normal faulting mechanisms along this surface suggests initiation of a normal, probably listric fault. At depth, the onset of the future fault plane is located on top of the NW–SE striking ridge of the lower crust and Moho, which act as a an indenter. In addition to thrusting of the whole wedge towards NW, increasing of NW–SE compression would lead to the formation of a half graben at the place of the present Sierentz depression.

Determination of displacements in the upper Rhine graben Area from GPS and leveling data

The Upper Rhine graben is a north–northeast trending rift system, which belongs to the European Cenozoic rift system. Today, the southern part of the graben is seismically still active. Earthquakes of magnitude 5 have a recurrence time of approximately 30 years. In order to monitor and to determine the displacements in the study area, GPS measurements have been carried out in two campaigns (1999 and 2000), and observations of the available permanent stations have been processed in 2002. Owing to the small size of deformations expected, high accuracy requirements must be met by the GPS processing. In order to achieve these requirements, precise antenna modeling has been introduced into the processing. As expected the short time span has not enabled to detect significant displacements from the GPS measurements. The deformation analysis shows that the horizontal displacement rates do not exceed 1 mm/year, which is compatible with the geological information. Owing to the fact that the accuracy of positioning with GPS for the vertical coordinates is lower than for the horizontal coordinates, the determination of vertical displacements has been carried out using the leveling technique. In the area of Freiburg, first-order and second-order leveling lines have been chosen for the detection of local displacements on the Weinstetten, on the Lehen-Schonberg and on the Main Border Fault (MBF). Some sections of these faults are still active today. Significant vertical displacements have been observed at the Weinstetten fault in the area of Bad Krozingen and on the MBF in Freiburg. The displacement rates (1925–1984) are 0.17±0.01 mm/year and 0.25±0.02 mm/year respectively. The results agree very well with the results of seismotectonic investigations, and show that ongoing displacements can be found on the northern part of the Lehen-Schonberg fault in the vicinity of Eichstetten, and on the MBF in the vicinity of Freiburg.

Simulation of earthquakes in the Upper Rhinegraben using empirical Green functions

SUMMARY In low seismicity areas such as southwestern Germany, quantitative statements on strong ground, motion suffer from the lack of instrumental strong motion data although the historic catalogue indicates that events of moment magnitudes up to MW = 6 are conceivable. This is the situation in the Upper Rhinegraben area were moderate earthquakes with low probability but high impact caused by the significant aggregation of population, infrastructure and industrial facilities are expected to happen. We have developed scenarios of crustal earthquakes with moment magnitudes MW = 6.0 for a source region southeast of Basel (Switzerland), itself being struck by a strong earthquake in 1356 with an intensity of up to IX (EMS98 intensity). In order to synthesize strong ground motion seismograms, we are using an empirical Green’s function (EGF) approach. We utilize records in the frequency range of 0.5‐25 Hz of permanent stations in the Rhinegraben and the adjacent Black Forest in southwestern Germany as EGFs. To be consistent with the local seismicity, we model earthquakes with a left-lateral strike slip focal mechanism. The applied EGF approach allows one to define independently a variety of rupture scenarios on the fault plane by varying the hypocentre location, rupture velocity, fault plane geometry, slip distribution and rise time. Therefore, we can create worst-case scenarios for the chosen sites. We compare peak horizontal acceleration (PHA) and response spectra in terms of spectral accelerations and PHAs with attenuation laws proposed for Europe. Our results encourage the application of the EGF approach as a supplementary tool for site-specific strong ground motion prediction in low-seismicity regions such as the Upper Rhinegraben area.

Active faulting and paleoseismology along the Bree fault, lower Rhine graben, Belgium

Paleoseismic analysis of the 10‐km‐long Bree fault scarp in the lower Rhine graben yields numerous lines of evidence of earthquake activity in the Holocene and late Pleistocene. This active normal fault, a part of the Feldbiss fault system, dips 70°NE and is expressed at the surface by a prominent NW‐SE trending 7 to 20 m high scarp, formed since the deposition of the Maas River main terrace <700 kyr. B.P. Trenches and geophysical prospecting show that the fault, which is known to have ∼100 m of vertical offset since the late Pliocene, breaks late Pleistocene and Holocene deposits. Ground‐penetrating radar, seismic refraction, and electric tomography suggest that at shallow depth the amount of displacement is larger than the youngest vertical offset visible in the trenches and corresponds to cumulative fault displacements. The analysis of 36 leveling profiles across the scarp indicates that its height can be classified into three groups, likely corresponding to different events. A morphologic dating gives approximate ages of 2±1.5 kyr B.P., 14±5 kyr B.P., and 41±6 kyr B.P. for the past three surface‐faulting earthquakes. Analysis of faulted stratigraphy and earthquake‐induced deformation structures exposed in trenches suggests the occurrence of three large earthquakes during the past 45×l03 years and yields 0.07 mm/yr of relative vertical deformation rate. The most recent seismic event occurred between A.D. 610 and 890. The first identification of an active fault with surface ruptures in the lower Rhine graben area emphasizes that large earthquake sources exist within intraplate Europe and that at least some of these events are preserved in the geologic record.

Anisotropic Measurements in the Rhinegraben Area and the French Massif Central: Geodynamic Implications

Anisotropic Measurements in the Rhinegraben Area and the French Massif Central: Geodynamic Implications

Lower crustal thinning in the Rhinegraben: Implications for recent rifting

Seismic near‐vertical and undershooting experiments carried out in the southern upper Rhinegraben area between 1984 and 1991 show pronounced lateral variations of deep crustal properties. Significant differences in the apparent thickness of the reflective lower crust and the transparent upper crust appear to be related to different structural settings. A 12‐ to 14‐km‐thick reflective lower crust beneath a 15‐km‐thick transparent upper crust of the eastern graben shoulder (Black Forest) probably originated during Permo‐Carboniferous reequilibration of thickened Variscan orogenic crust. Thinning of this lower crust by about 5 km beneath the graben and rifted domains transitional between graben and shoulder (the Dinkelberg block) is interpreted to be related to Cenozoic extensional faulting. A discrepancy between moderate extension of upper crust and lower crustal geometry indicates mechanical decoupling at depth during extension. Congruent modification of other physical properties is suggested by strong single reflective elements in the topmost parts of the thinned lower crust. In the transitional Dinkelberg block, such an anomalously strong reflector occurs at a depth of 20 km below a pronounced local maximum of earthquake activity; it is interpreted to be the presently active zone of decoupling which in time shifted from the rift axis to the eastern transition area.

Quantification of surface faulting potential in a low to moderate active region: an example from the southern Rhinegraben area : Loew, S; Jenni, J P; Blanc, B Int Assoc Engng Geol BullN40, Oct 1989, P111–117

Quantification of surface faulting potential in a low to moderate active region: an example from the southern Rhinegraben area : Loew, S; Jenni, J P; Blanc, B Int Assoc Engng Geol BullN40, Oct 1989, P111–117

Evidence for the permutation of stresses σ1 and σ2 in the Alpine foreland: the example of the Rhine graben

The orientation of the principal stresses ( σ 1, σ 2 and σ 3) and the ratio R = (σ 2 − σ 3) (σ 1 − σ 3) which characterizes the shape of the stress tensor can be calculated using earthquake focal mechanisms in the Rhine graben area. Two types of stress tensor can be determined. One of them is an extensional stress state (subvertical σ 1 horizontal σ 2 N130 °) in the northern part of the Rhine graben; the other is a strike-slip stress state (horizontal σ 1 N150 °) in the southern part. These results show that the maximum horizontal stress is constant in an approximately N130 ° –150 ° direction ( σ 1 in the south, σ 2 in the north). This permutation of the stresses σ 1 and σ 2 may be imputed to the interaction between the decrease in Alpine pressure and the asthenospheric uprise under the Rhenish Massif. The superficial in situ stresses measured from the Alps to the lower Rhine embayment showing a decrease in maximum horizontal stress ( σ hmax ) support this hypothesis.

Crustal evolution of the Rhinegraben area. 1. Exploring the lower crust in the Rhinegraben rift by unified geophysical experiments

Abstract Unified geophysical investigations of the lithosphere in the Rhinegraben rift system have revealed new details of the lower crust and its role in the rifting process. The new findings allow an assessment of the compatibility of four different geophysical notions and properties of the lower crust: 1. (1) the lower crust as the layer beneath the Conrad discontinuity with a P-wave velocity of about 6.5 km/s or greater (refraction seismics); 2. (2) the laminated band of reflections, as seen in the near-vertical reflection seismic experiments in many parts of the continents; 3. (3) the ductile part of the crust below the brittle-ductile transition, devoid of earthquakes in seismically active regions; 4. (4) the electrical conductivity of the lower crust indicative of dry or wet conditions, or still unknown conduction phenomena. In the Rhinegraben area the lamination of the lower crust serves as an outstanding marker of deep tectonic activity during the rifting process, in which the crust of the Rhinegraben rift system has been subjected to three different natural dynamic processes: (1) Uplift by 2 to 3 km with subsequent erosion of the Rhinegraben shoulders (Black Forest and Vosges Mountains) caused decompression possibly leading to the formation of a low-velocity/high-electrical-conductivity zone right on top of the laminated lower crust beneath the elevated shoulders of the Black Forest. (2) The brittle crystalline wedge of the graben proper subsided nearly undeformed into the lower crust, which became about 5 to 7 km thinner below the graben than below the shoulders. (3) The deepest hypocentres in the Black Forest (Dinkelberg area), if projected onto the neighbouring reflection profile, would be located 7 to 8 km within the laminated lower crust beneath the southern Black Forest, indicating a discrepancy between the top of the lower crust as defined by the brittle-ductile transition as seen by the deepest earthquakes and by the top of the laminated reflection band. The Rhinegraben rift system reveals the properties and behaviour of the lower crust under a wide variety of tectonic situations.

Mathematical foundation of non-tectonic effects in geodetic recent crustal movement models

A general mathematical formulation for the integrated determination of recent crustal movements as well as non-tectonic effects from geodetic and other data is presented. As an example, regional and local features of vertical motions in West Germany are computed using the proposed collocation model. Further investigations on the map of recent crustal movements considering surface deformation analysis are shown. As a second example, computations in the Rhinegraben area (F.R.G.) were performed where besides crustal movements a considerable subsidence due to groundwater table variations takes place. The separation shows that the subsidence in the last decade can be described by this non-tectonic influence.

A geophysical study of the Rhinegraben ? I. Kinematics and geothermics

Summary The temperature distribution beneath the Rhinegraben area is constrained by independent geophysical findings: (a) the gravity field and its relation to the crust/mantle boundary, (b) the thinning of the lithosphere. The interrelationships between these geophysical properties are discussed. An attempt is made to construct the temperature field as a result of a kinematic model. It is postulated that hot mantle material has been rising during the rifting process. The mass displacement is of the order of 1 mm yr−1. In order to determine the transient temperature field the differential equation of heat transport in a moving medium is solved by a two-dimensional numerical method. The resulting lateral temperature differences reach values of about 350°. This deep-reaching thermal anomaly is compared with other geophysical findings relevant to temperature. It is shown that it matches well with the pronounced elevated asthenosphere. Taking into account the temperature dependence of density it can be demonstrated that the thermally induced gravity effect serves as gravitational compensation for the upwarped upper mantle (see also part II of this paper, Kahle & Werner 1980).

The quaternary uplift of the rhenish shield in germany

Abstract The Rhenish shield in Central Europe is made up by a series of slates, quartzites and limestones of dominantly Devonian age which were strongly folded by the Hercynian orogeny. The area has been uplifted to about 300 m since the Pliocene. Rates of uplift increased during the last 600,000 years as shown in the uplifted terraces of the Rhine river. The drainage pattern was forced to erode deep antecedent river valleys in the uplifted plateau. Two late Quaternary volcanic fields have developed on the Rhenish shield: The West Eifel, covering an area of some 500–600 km2 with c. 220 eruptive centers; and, about 25 km to the east, the smaller East Eifel (Laacher See area) with some 70 volcanic centers distributed over c. 400 km2. Magmas of both fields are moderately potassic. Most West Eifel magmas are nephelinites and leucitites and have high CaO/Al2O3-ratios (up to about 1.45) and 87 86 Sr-ratios (0.7039–0.7044); in contrast, intermediate (tephritic) and highly differentiated (phonolitic) magmas are common in the East Eifel, where magma chambers of different compositions have developed at varying depths in the crust, as evidenced by a variety of crustal xenoliths. Primitive East Eifel magmas are dominantly basanites with Mg numbers 87 86 Sr-ratios (0.7041–0.7047). The Quaternary Eifel magmas must have come from a different mantle area than the Tertiary magmas, whose 87 86 Sr-ratio ranges from 0.7035–0.7040. Most of the existing seismic-refraction profiles in the Rhenish shield have been reinterpreted using travel time and amplitude information. The general pattern of observed phases can be divided into three types: (1)Throughout the central shield a strong P-phase reflection from the crust—mantle boundary is recorded. The average crustal thickness is 28–29 km and the average crustal velocity is 6.2–6.3 km/sec. (2)Beneath the southern part of the shield strongly reflected phases show clear intracrustal and Moho discontinuities. Along profiles crossing major volcanic features the Mdiscontinuity is disrupted and an intermediate intracrustal boundary at about 20 km depth forms the main reflector. Beneath this boundary the velocity increases gradually from about 7 km/sec to upper-mantle velocities. (3)For profiles crossing the northern Rhinegraben area as well as for a line east of the Lower Rhine embayment, the observed phases indicate only one major seismic boundary at a depth of about 23 km where the velocity increases to 7.3 km/sec. Below this boundary the velocity increases gradually with depth reaching 8 km/sec at 27–28 km. The Rhenish shield is part of an incoherent block mosaic of the Alpine foreland. The Alps are under rapid Holocene uplift and sideward extension. Between the Alps and the Rhenish shield there is a major foreland block which is shifted northwestward to be sheared along the Rhinegraben rift with a sinistral polarity. The northern apex of this block wedges in the Rhenish shield and provokes an axial deformation and, conditioned by that, a split-up of the northwestward frame of the shield. It is shown that the shield uplift evolved as a consequence of horizontal deformations of the unit. Subcrustal rift propagation under the tectonically incompetent crustal unit of the Rhenish shield had favored magmatic activity, thermal rise and subsequent plateau uplift.

Structure of the uppermost mantle from long-range seismic observations in southern Germany and the Rhinegraben area

The crustal structure has been determined in the area between the Lorraine, the Bohemian massif and the northern Alps with considerable detail in recent years. But up to now little has been known about the velocity—depth structure of the uppermost mantle in this area. The situation changed recently when two recent seismic events near the northern and southern end of the Rhinegraben rift system were recorded to distances of 400 km. The explosions at the westernmost shotpoint of the international alpine refraction profile in 1975 were also observed in the Rhinegraben area up to the same distance. Earlier refraction seismic experiments between Steinbrunn near Basle and Boehmischbruck at the western border of the Bohemian massif also reach distances of 400 km. All these data lead to the rather high P-wave velocities of 8.5–8.6 km/s at depths between 40 and 50 km. These velocities are considerably higher than the average velocities of 8.2 km/s under other areas of western and central Europe, as for example the Bretagne in northwestern France and the North German Plain. There are indications of a minor velocity inversion in the uppermost mantle between Steinbrunn and Boehmischbruck. From the dispersion of surface waves there is good evidence that the regional high P-wave velocities are limited to a certain depth range only. This indicates rather pronounced lateral variations of the velocity—depth structure in the uppermost mantle of central Europe.

Geodetic high-precision measurements in active tectonic areas; example: The Rhinegraben

Abstract Geodetic measurements in the Rhinegraben area are discussed. Repeated levellings, together with horizontal control data, torsion-balance measurements, very dense gravity coverage, tiltmeter observations and a high-precision gravity test net were available in order to test present concepts of the Rhinegraben structure and sources of recent crustal movements. The ambiguities of levelling data are stressed, but investigations of high-precision levelling data clearly indicate subsidence of several centimetres in the graben area, whereas multivariate studies of the data available on the graben shoulders do not indicate any significant movement on the shoulders themselves.

Erdw�rme und Lithosph�re im Raum S�dwestdeutschland/Schweiz

A geothermal survey of the Central Alps, the Swabian volcano, and the Rhinegraben area is presented. New results from seismic surface-wave investigations are used to estimate temperatures at the base of the lithosphere. Cross sections show the high position of the 1000 °C isotherm under the Rhinegraben in relation to its surroundings.

Main features of crustal structure in western and southern Europe based on data of explosion seismology

During the past two decades deep seismic sounding measurements have been carried out in western and southern Europe, mainly using the refraction method. These investigations were performed partly on a national basis but as well within international cooperative programs under the sponsorship of the European Seismological Commission. In France, a systematic study has been executed to determine the main feature of deep structures under the Central Massif and the Paris Basin. In the Forez and Margeride regions, the sub-crustal velocity is lower (7.2 km/sec) than the normal value (8.0 km/sec) observed in the adjacent areas. The central and southern part of Western Germany is covered by an extensive network of refraction profiles. The crustal thickness varies, similarly to France, from 25 to 35 km. A great amount of deep reflection data was obtained by commercial and special reflection work. The crust beneath the Rhinegraben area shows the typical “rift system” structure with a low subcrustal velocity (7.4–7.7 km/sec). Very intensive refraction work has been carried out in the Alpine area. The maximum crustal thickness found near the axis of the negative gravity anomaly is about 55–60 km. Furthermore, a clear lowvelocity layer at a depth between 10 and 30 km has been detected. A key position with regard to the geotectonic structure of the Alps is held by the zone of Ivrea characterized by a pronounced gravity high. From the refraction work it may be concluded that there material of the lower crust and the upper mantle (7.2–7.5 km/sec) is overlying a layer of extremely low velocity (5.0 km/sec) which is interpreted as sialic crust. Three years ago, a systematic study of crustal structure of the Italian peninsula has been started. Reversed profiles were observed on Sicily, in Calabria, and in Puglia. On Sicily, the structure is very complicated; the crust of the western part looks like a transition between a continental and oceanic structure whereas the eastern side shows a continental-type crust. In Calabria and Puglia, the crustal thickness has been determined to be about 25–35 km.