Technical Committee Geophysics
Active Tectonics in Northwest Germany: Glacial Isostatic Adjustment and/or a Consequence of Hydrocarbon Production?
The aim of DGMK project 773 was to characterize and distinguish recent seismic events in northern Germany on the basis of their triggering mechanisms. The project consists of three main parts: (I) seismological analyses, which include relocalization as well as the calculation of focal mechanisms of the events, (II) a geological 3-D subsurface modeling of the regions in northern Germany with increased seismicity, and (III) the numerical simulation of the glacial-isostatic compensation movements at the identified fault locations.
The characteristic features of anthropogenically induced seismicity, especially the spatio-temporal definition, can be applied to almost all seismic events in northern Germany. The seismic events at depths between 5 and 9 km with magnitudes ML ≤ 3.9, which are located in close proximity to the natural gas fields, are concentrated along faults with thrust kinematics of the Permian Rotliegend rift system. These NNW-SSO or N-S striking faults are not optimally oriented to receive increased reactivation potential from glacial isostatic offset movements. For these reasons, natural gas production is assumed to be the most likely triggering mechanism for the weak events in northern Germany.
Slightly stronger events (ML ≥ 4.0) from greater depths, such as the 2004 Rotenburg earthquake, probably have a more complex triggering mechanism. The faults on which these events are located are not optimally oriented for the induced stresses of glacial isostatic compensation motions. Stress changes that bring the fault close to the rupture criterion are possible. In addition to the glacial isostatic compensation movements, a pressure drop in a very nearby reservoir could also be sufficient to trigger a movement on the suspected seismogenically active fault.
The causes of the five deep earthquakes cannot yet be clearly determined, as very little data is available on the subsurface conditions. However, the stress changes triggered by the glacial-isostatic compensation movements form a possible mechanism to trigger these deep earthquakes. However, further investigations are needed