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Real-Time Seismograms

The following figure shows seismograms produced at monitoring stations in the area around St. Gallen. Seismograms record ground movements over time, i.e. the velocity of seismic waves registered within the last five minutes by the earthquake stations SGT00 to SGT05.

The horizontal axes represent the flow of Greenwich Mean Time (GMT), which is one hour ahead of Central European Time (CET). During Central European Summer Time (CEST), GMT is two hours ahead. The vertical axes show wave velocity in micrometres per second (μm/s). These seismograms are filtered before being displayed (using a band-pass filter of 2 to 10Hz) so that small-scale local earthquakes show up more clearly.

Generally speaking, seismograms can be said to depict seismic noise naturally occurring in the vicinity of the monitoring stations. Particularly quiet locations are characterised by thin wavy signals with minor deviations (e.g. station SGT04). This regular wavy signal is caused by storm systems in the Atlantic and described as marine microseismics. In the summer its amplitude is greater than in winter and it gives several days' advance warning of the arrival of low-pressure fronts.

What Do Seismograms Show?

At locations with more seismic noise, the marine microseismics are increasingly masked by brief interference (e.g. station SGT01, which is very close to St. Gallen), which are usually due to human activity and follow a typical daily pattern. However, strong winds and storms can cause similar interference, too. Man-made interference can be very substantial over very brief periods, e.g. when an aircraft flies past the station, but in most cases it is limited to the local area and only shows up in the recording made by the single, closest station.

With a little luck, seismograms can pick up signals of earthquakes or explosions. Recordings of these events are characterised by a brief increase in amplitude that shows up on seismograms recorded by several stations within a few seconds. Earthquakes and quarry explosions in Switzerland its neighbours can be picked up by these stations and be seen on seismograms, as can more distant earthquakes (so-called 'teleseismic events'). However, filtering tends to prevent most teleseismic events from showing up on seismograms.

Do you think you've spotted an earthquake or explosion on the seismogram? Find out if you were right by checking your suspicion against the list of local earthquakes and global events.