The epicenter of the earthquake was about six kilometers southwest of Wägitalersee, an artificial lake located in the canton of Schwyz.Image: KEYSTONE
A magnitude 4.4 earthquake shook central Switzerland last week. According to a seismologist from the Swiss Seismological Service, long periods of heavy rain could be the cause.
Bruno Knellwolf / ch media
An earthquake measuring 4.4 shook Switzerland this week. Such seismic intensity could cause damage, and an earthquake is perceptible from a magnitude of 2.5.
“The earthquake near Einsiedeln was nothing ordinary”
Philippe Roth from the Swiss Seismological Service at ETH Zurich
However, this is not exceptional either. Statistically, such an earthquake occurs in Switzerland every one to two years.
What could be exceptional is the cause of this strong earthquake in central Switzerland.
“It is indeed possible that the large amounts of precipitation in recent weeks, combined with seasonal snowmelt, could have played a role”
Philip Roth
Excess water may have entered the fault zone near the surface. This phenomenon was favored by the local karst geology, which gives great permeability to the rocks. The resulting change in pressure in the soil pores could have triggered the earthquake, which occurred at very shallow depths. According to the seismologist, similar cases where water played a triggering role have also been observed or suspected in the Alps on other occasions.
Same as the tremors in the projects geothermal
“Physically, these rain-induced earthquakes are the same phenomenon as those caused by deep geothermal projects,” says Roth. In geothermal energy, water is intentionally injected into rock, which can cause earthquakes. This phenomenon can also occur when filling a reservoir, when water enters the basement, which can also cause shaking.
For last Tuesday’s earthquake, however, this remains a hypothesis that still needs to be confirmed.specifies the seismologist.
In general, earthquakes are caused by tectonic stresses exerted on rock. However, rain can have an influence, because in addition to these tectonic forces, there is a stabilizing pressure exerted by the rock above the seismic focus. Rainwater seeping into the rough surface of the fault can reduce this stabilizing pressure, which could impact earthquake triggering.
Thus, an earthquake is not triggered by exceeding a tectonic threshold, but because water lowers this tectonic threshold. The earthquake can therefore occur more quickly because of the rain.
“This was first observed in Switzerland after heavy rains in August 2005, which triggered small earthquakes in central Switzerland”
Philip Roth
The 4.4 magnitude earthquake was followed by aftershocks on Tuesday, reaching a magnitude of 2.4, six hours after the main tremor. “These aftershocks have been few in number until now, for an earthquake of this intensity”explains the seismologist.
Since then, the Swiss Seismological Service of the ETH Zurich has installed two stations in the epicentral region in order to locate the aftershocks more precisely and to determine more precisely the apparently very shallow depth of these earthquakes. Apart from a few isolated and difficult to verify reports from further away, the earthquake was only felt within a radius of 30 to 35 kilometers.
“This is explained by the shallow depth of the seismic focus”
Philip Roth
Translated and adapted by Tanja Maeder
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