Technical Committee Geophysics
High Resolution Seismic Survey of the Seismicity near Rotenburg and Comparison of Survey Methods
The Rotenburg region was monitored by a small-scale, high-resolution seismic station network. The station network consisted of 3-component, mini array and classic array stations. Due to the high local ground unsteadiness the installation of the station network was a challenge. Nevertheless, a total of 3 years of continuous data recording could be evaluated. The event identification was performed with automatic data processing like pattern recognition, cross correlation, Fisher-ratio and source scanning. In addition, a visual inspection of the super sonograms was applied manually, so that a detection threshold of ML 0.5 could be achieved for the whole region.
The detected and localized seismicity showed significant differences between the regional clusters and their proximity to the producing gas fields. Only in the vicinity of the three gas fields Walsrode, Rotenburg/Taaken and Söhlingen seismic events could be observed. The Walsrode cluster shows a typical magnitude-frequency distribution with many small and few large events. In contrast, only two earthquakes with local magnitudes of 1 to 3 were detected in the Rotenburg/Taaken and Söhlingen regions, despite the lower detection threshold. To account for the observed deviations of the SPhase arrival times at constant vP/vS ratio, the depth of the hypocenters was determined with an improved 1-D velocity model. Since the focal depths are in the area of the gas reservoirs, a connection with gas production is possible and thus it could be induced seismicity.
During the observation period, four deep-crustal earthquakes could be identified, which could be attributed to tectonic intraplate processes due to (post-) glacial compensation movements.
The highly accurate localization of the ML 1.5 Soehlingen event in February 2017 allowed a re-examination of the aftershock sequence of the October 2004 Rotenburg earthquake (ML 4.5) using waveform comparisons. The determination of the S and converted phases indicate a division of the aftershock series into two focal depth ranges: one at 12 km, corresponding to the focal depth of the main earthquake; the other in the reservoir area at 5 km, which could indicate a stress redistribution to an adjacent fault.