False alarm: Czech earthquake initially reported as magnitude 5.5 turns out to be minor tremor of 1.5–1.8
A magnitude 5.5 earthquake reported in western Czech Republic on Thursday evening was later corrected to a minor tremor of 1.5–1.8, likely caused by quarry blasting, after an automatic detection system error at Germany's GFZ.
Initial report of a strong quake
On Thursday around 18:00 local time, the German Research Centre for Geosciences (GFZ) reported an earthquake of magnitude 5.5 in western Czech Republic, with the epicentre between Plzeň and Příbram. The news, carried by Reuters, said the quake was also felt in Slovakia, Austria and southern Germany. The hypocentre was initially placed at a depth of 10 kilometres.
Correction and explanation
Within hours, GFZ withdrew the alert. Spokesperson Josef Zens told the Czech News Agency that the magnitude could not be confirmed. "At this moment we cannot confirm an earthquake of that magnitude. It was an automatically assessed signal. Perhaps there was an error in one of the measurement stations or something else," he said. A later GFZ statement read: "Due to an error in our automatic earthquake detection system, we published incorrect information." The manual analysis revised the magnitude to 1.5, while the GFZ ultimately settled on 1.8. The depth was also corrected to about 1 kilometre.
At this moment we cannot confirm an earthquake of that magnitude. It was an automatically assessed signal. Perhaps there was an error in one of the measurement stations or something else.
What a real 5.5 would have meant
Aleš Špičák, director of the Czech Institute of Geophysics, put the false alarm into perspective. The strongest earthquake ever recorded in the country was a magnitude 4.6 event in December 1985 near Nový Kostel in the Cheb region. A genuine 5.5 tremor would have been 30 times stronger. "Such an earthquake is absolutely unprecedented in Europe, our territory has not experienced anything like it since the beginning of the Quaternary," Špičák explained. It would have been felt in Prague and across the entire country.
If the Thursday tremor had really been magnitude 5.5, it would have been 30 times stronger than the strongest earthquake ever recorded in the Czech Republic. Such an earthquake is absolutely unprecedented in Europe, our territory has not experienced anything like it since the beginning of the Quaternary.
No damage, likely quarry work
No damage or casualties were reported. GFZ later said the weak tremor was "most likely caused by routine quarry work" near Plzeň. The agency apologised for raising the alarm. The European-Mediterranean Seismological Centre also confirmed the lower magnitude.
