Measures to combat COVID-19 also reducing seismic noise in Switzerland

Seismic measuring stations do not only record seismic waves. They also pick up tremors caused by other factors: these are known as seismic background noise. Humans are partly responsible for these constant movements of the Earth. For instance, road traffic or industrial activities may cause vibrations in the Earth’s crust. Their impact is typically greater during the day and lower at night and at the weekend. Wind, waves and the weather also cause the Earth to vibrate constantly. International studies have shown that levels of human-induced seismic background noise have decreased in many locations since the outbreak of the coronavirus pandemic. Measuring stations are thus indirectly detecting the effects of the lockdown and the associated drop in human activity. The Swiss Seismological Service at ETH Zurich has observed this phenomenon in Switzerland too.

Some of the stations belonging to the Swiss Strong Motion Network (SSMNet) in particular, many of which are located in urban areas, have recorded significant drops in seismic background noise. This has been the case for the stations in Martigny, Zurich, Basel and Geneva, among others. Since the state of emergency was declared, levels of background noise in these towns and cities have been almost as low on working days as they were at weekends before the lockdown began. Saturday and Sunday evenings have also been far quieter than usual since the start of the lockdown. The seismic noise levels recorded on weekend evenings have fallen to the level typically observed on a normal weekday evening in the relevant towns and cities – as a rule, more seismic noise is generated on weekend evenings than on weekday evenings.

Conversely, rural or alpine stations belonging to the national broadband network (SDSNet) have only recorded slight decreases in background noise because these areas are considerably less affected by vibrations from road traffic, trains and other human activities. Strong winds and other weather factors may nevertheless lead to increased levels of background noise in some areas, as may have been the case in northeastern Switzerland during the period under study (23 to 29 March 2020).

Due to the exceptional situation in Switzerland, measuring stations are now able to detect somewhat smaller earthquakes whose signals would otherwise be lost amid the background noise. The COVID-19 lockdown has therefore increased the sensitivity of earthquake monitoring in some parts of Switzerland, although the overall effect, at just 0.1 to 0.2 magnitude units, is not very great. By way of comparison, the sensitivity of monitoring is on average 0.5 magnitude units greater during the night than during daytime working hours. Latest data show at some stations that the seismic background noise has slightly increased over the last days. However, statements about compliance with lockdown measures cannot be derived from these data.