Calibration of the Magnitude Determination Reveals the Smallest Earthquakes Localized in Switzerland

For the earthquakes encountered in the scope of the geothermal energy project in St. Gallen, it has only been possible up to now to determine the earthquake magnitudes for a small number of slightly stronger events. The majority of the events were so small that they were recorded only at a few nearby stations. Thus, the signals are strongly influenced by local conditions and do not allow a reliable calculation of the magnitude.

For the earthquakes near St. Gallen, the SED previously used the logarithm of the signal strength (log10(amplitude)) at the borehole station SGT00 as a measure of the size of the events (see left panel of the figure). The values were then normalized to the magnitude of the strongest earthquake (ML = 3.5) that was reliably determined at the stations of the national monitoring network.

Using the data acquired in the context of the geothermal energy project in St. Gallen, it was possible to calibrate the magnitude calculation specifically for the stations of the St. Gallen network, thereby allowing the local magnitudes to now be reliably determined for small quakes at the geothermal borehole (see right panel of the figure). In the future, these magnitude values will be used for the figures.

Thanks to the new calibration of the magnitudes, it is clear that the SED not only recorded the largest induced quake in Switzerland during the geothermal energy project in St. Gallen, but also localized the smallest earthquakes ever recorded in Switzerland. These quakes have magnitudes down to ML = –1 and occurred on rupture surfaces with diameters of just a few decimeters. (Events with magnitudes less than –1 are, in fact, measured, but cannot be localized with any degree of accuracy.)

Earthquakes of this magnitude close the gap between laboratory experiments and natural seismicity and may provide a better understanding of the physics of earthquakes and help improve earthquake models.

The calibrated local magnitudes are somewhat smaller than the magnitude values for the larger St. Gallen events calculated in the standard way. Thus, there are small differences at present between the values in the earthquake lists and those in the figures.