ABSTRACT:

This paper presents few examples of the calibration and benchmarking of 3D Finite Element models using a range of geoscientific data. The models simulate the in-situ stress state of the Upper Rhine Graben (URG), which is a continental graben in SW-Germany at the borders to France and Switzerland. The aim of this study is to predict the present-day stress state and kinematic behavior of the graben and to identify the active structures. This requires the model to represent realistic stress magnitudes and orientations as well as realistic strain. A regional, crustal model has been constructed. Its state of stress was calibrated to borehole data. For further analysis of the kinematics of a sub-region of the URG, a fault block model has been constructed. Both models provide data on the present-day 3D stress state, the kinematic behavior of the graben and faults implemented as well as the surface displacement. In addition, the sub-model predicts active fault segments, which have not been identified as being active previously. In order to evaluate these results, information from geology, geomorphology, geodesy, seismicity and paleoseismicity on relative fault block motions and on fault kinematics has been taken into account.

1. INTRODUCTION

The Upper Rhine Graben (URG) is a 300 km long and 30 - 40 km wide graben in the central part of the European Cenozoic rift system (figure 1). At present, the URG is characterized by relatively slow tectonic deformation and low to moderate intra-plate seismic activity. The URG area is highly faulted. Despite the lack of large earthquakes presently observed, the occurrence of damaging earthquakes in the future cannot be fully excluded and thus the numerous pre-existing faults have to be considered potential sources for future earthquakes. Since the area is intensely populated and industrialized a detailed understanding of the fault reactivation is of great economical and societal importance. The URG is among the best studied grabens worldwide. It has been the target of many seismic and geological investigations, most of which have been carried out for hydrocarbon exploration during the 1960s and 1970s. This work resulted in detailed knowledge on the Tertiary stratigraphy of the graben fill and on intra- graben faulting and enabled reconstruction of the Tertiary evolution of the rift [e.g. 1, 2].

Fig. 1. Shaded relief map of the Upper Rhine Graben and surroundings showing the surface topography and the outline of the 300 km long graben structure. The stress orientations in this area are relatively homogenous. Data source: World Stress Map [3].(available in full paper)

The URG is no longer of economical interest for oil and gas, but has been targeted for geothermal energy since the 1990s. A scientific geothermal drilling site is located in the central part of the URG near the town of Soultz- sous-Forêt (figure 1). Several wells have been drilled at this site to depths of 3 to 5 km [4]. For these depths, a large data set of stress orientations, stress magnitudes and rheological parameters is available, which is a very valuable dataset to be used for calibration of the Finite Element (FE) model of this study.

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