Natural earthquakes occur throughout Switzerland, including in and around St. Gallen, though the region's seismicity is rather low compared to the rest of the country (see also Seismic Hazard in Switzerland). Only the St. Gallen Rhine Valley, bordering the area to the east, has a high level of seismic activity.
Between 1975 and 2008, the SED recorded 93 earthquakes with magnitudes of between Mw = 1.2 and Mw = 3.7 within a circle of 20 km around the site of the geothermal projects. The quakes in question occurred at depths of up to 30 km, though the seismic focus of most of them was at depths of between 0 and 10 km.
The strongest earthquake had a magnitude of Mw = 3.7 and occurred on 24 August 1996 under Kirchberg, at a depth of roughly 30 km. Another typical feature of seismic activity in Switzerland is the occurrence of earthquake swarms. In 1990, a spatiotemporal cluster of earthquakes of similar strength occurred south of Oberuzwil.
The strongest ever earthquake recorded in or around St. Gallen occurred on 11 August 1771 near Niedersommeri, to the north of Amriswil. Since at the time of course there were no instruments to record that quake, its magnitude of Mw = 5.1 can only be estimated from historical accounts of the event. The following entries stem from the diary of Prince-Abbot Beda Angehrn (St. Gallen Abbey Archive, vol. 282, 1767-1773):
11 August 1771: Circa mediam nonam matutinam factus est terrae motus com valde magna concussione sine tamen ullo damno. Deo sint laudes. (At around eight-thirty in the morning a violent earthquake occurred, but did not cause any damage. Thanks be to God.)
12 August 1771: Received reports from various places about yesterday's earthquake, which was also very strongly felt in Wyl where it toppled a number of chimneys and caused serious damage to the front end of the church. One or two cracks are also said to have appeared in the cupola of the church in Kirchberg (May God protect [us] from any further evil).