Earthquake swarms represent an intensive, long lasting, low magnitude seismicity that contrasts with more typical mainshock-aftershock sequences.
Such seismicity can be felt by the population, sometimes with damages of buildings. They are nowadays recognized in many regions worldwide under
different tectonic settings, mainly in volcanic and geothermal fields or at margins of tectonic plates. They can also represent intraplate activity
as is the case of the West Bohemian region. However, their mechanism is still not fully disclosed. They can occur as precursors of larger earthquakes
as, e.g., during recent L’Aquila 2009 earthquake in Italy.
In West Bohemia, at present, the highest concentration of earthquake activity occurs in the area of the Cheb Basin, near three Quaternary volcanoes
and at the intersection of major tectonic lines. It seems that the earthquake swarms are related to the re-activation of a complex system of faults.
The most intensive earthquake swarms were in the years 1896/1897, 1903, 1908/1909, 1985/1986, 2000, 2008, 2011, and 2014, but a weaker activity is
recognizable almost continuously.
Seismic activity in the Nový Kostel swarm area, which dominated the activity during 1997 – 2014. Left: map of earthquake epicentres
demonstrating that the foci of events align along steeply dipping fault. Right bottom: earthquake hypocentres along vertical cross-section
parallel to the fault. Right top: occurrence of the earthquakes with time, vertical axis shows Richter magnitude of individual events.
Seismic activity is colour coded according to time of occurrence.