Two active earthquake swarms in Valais

Since June of this year, two earthquake sequences have been active in Valais – largely unnoticed by the general population – in locations around 12 km apart. One is occurring approximately 5 km north-east of Sion, while the other is located about 6 km north of Sierre.

The earthquake swarm near Sion was preceded by a magnitude 3.0 earthquake on 20 June 2015. Although seismicity directly after the event was very limited, the SED has recorded an increase in seismic activity since the beginning of October. The majority of the recorded earthquakes occurred at a depth of around 8 km, most probably in the crystalline bedrock. A similar series of earthquakes was observed in 2011 approximately 2 km north-west of Sierre. The density of the SED’s seismic network, coupled with new methods of analysis, means that it is possible to observe even very small earthquakes with a magnitude of less than zero.

To the north of Sierre, the earthquake swarm increased in intensity toward the middle of October. During this period, the strongest of the swarm earthquakes measured thus far occurred, with a magnitude of 3.1 (10/14/2015). The second-strongest earthquake, with a magnitude of 2.7, was recorded on 11/18/2015. The earthquakes’ hypocenters were at a depth of approximately 4 km. A look at the history books shows that this area has experienced similar rates of activity in the past. In the summer of 1996, three earthquakes with a magnitude of 2.5 or greater took place.

In spite of the spatial and temporal proximity of the two swarms, there is currently no indication that they are directly linked. Furthermore, historic analyses show that such sequences are not unusual for Valais. Swarm activities generally end after a few weeks or months; in rare cases, the strength and number of the earthquakes increase over time.