Historical seismicity Sample Clauses

Historical seismicity. Over the past 500 years, the region around St. Gallen repeatedly experienced seismicity, although not along the fault in the target region. The closest seismic event was an earthquake in 1987 with a magnitude of ML 3.2 located near Abtwil SG, WNW of St. Gallen (Xxxxxx et al., 2017). The distance between the epicentre of the 1987 earthquake and the 2013 St. Gallen earthquake is about 2 km. A normal faulting regime was observed at the 1987 earthquake (Moeck et al., 2015). Due to the lack of recent seismic activity, the public utility company of St. Gallen (project operator St.Galler Stadtwerke) concluded that the fault zone is hardly seismically active (Xxxxxx et al., 2017). However, during the step-rate injection test, 12 micro-earthquakes were detected with magnitudes ML <= 0.9, followed by seismic events in vicinity of the injection with magnitudes ML <= 1.2 during the acidization, and further increasing seismicity with the ML 3.5 event during the well control operation after the gas kick. The focal mechanism of the main event indicates a strike-slip faulting regime with the maximum principal stress oriented NNW-SSE. Xxxxx et al. (2017) relocated the seismicity around the St. Gallen site and concluded that the majority of earthquakes (more than 800 events in 2013) is „[...] located in the pre-Mesozoic basement, likely within a PCT below the targeted hydrothermal aquifer.“ This supports the possibility that the gas migrated upward from the PCT (Xxxxxxx et al., 2018)