A team of scientists from Potsdam, Leipzig, Halle and Hanover has investigated the cause of unusually deep earthquakes in 2015 and 2017 in the Leipzig-Halle metropolitan region in a current study now published in the Journal of Seismology. For their research geophysicists from the University of Leipzig, the State Office for Geology and Mining in Halle and scientists from the Federal Institute for Geosciences and Natural Resources Hannover (BGR) under the leadership of the GFZ German Research Centre for Geosciences in Potsdam have applied new methods. For the first time ever, this makes it possible to unravel details of the fracture mechanics of these weak quakes and to place them in a tectonic context. This indicates that large-scale geological fault systems between Halle and Leipzig, which run through the entire Earth's crust and which were previously classified as inactive, could be reactivated by earthquakes.

Should this theory be confirmed, then earthquakes that could lead to damage in the metropolitan region would also be possible. The scientists are investigating scenarios for earthquakes and are calling for more research so as to be better prepared for such cases.

In the years 2015 and 2017 there were two earthquakes between Halle and Leipzig - each with a magnitude of about 3. These earthquakes were felt by the population up to 50 kilometres away from the epicentre, but did not cause any damage. The two earthquakes were the strongest instrumentally recorded earthquakes so far north of the earthquake zone between Vogtland and Gera. The Seismology Association of Central Germany, the group of institutions in Saxony, Saxony-Anhalt and Thuringia working in the field of seismology, has been monitoring this region since 1996 and has since then repeatedly localised weaker earthquakes around Leipzig. What is special about the earthquakes in 2015 and 2017 is that they were noticeable from afar and originated unusually deep in the lower crust between 22 and 29 kilometres below the Earth's surface. "Seismologists are worried by earthquakes at this depth because they give us indications of major distortions and make the possibility of strong earthquakes more likely," says Sigward Funke, head of the earthquake monitoring at the University of Leipzig.

For such reasons, BGR seismologists have been looking specifically for small deep-sea earthquakes in central and northern Germany for some time. What has remained completely open in the region around Halle and Leipzig so far, however, was the question on which faults these could take place and how they could be oriented in space. "In the investigation of the fracture mechanics of the earthquakes in Halle and Leipzig, we used, for the first time, new methods that we developed especially for the analysis of weak quakes at the GFZ German Research Centre for Geosciences in Potsdam," says Torsten Dahm, head of the Earthquake and Volcanic Physics section at the GFZ. "This has shown that both quakes most likely occurred on the same fracture surface only a few kilometres apart." The researchers, therefore, see the possibility that the segments of the rupture zone between the events to date could break in the future. "To see what this could mean, we have developed scenarios of such possible earthquakes and simulated the wave propagation and expected ground movements for such events."

The results alert the team: Earthquakes such as that of Roermond in the Dutch-German border area in 1992 (magnitude 5.3) would cause similarly strong ground movements and damage in the Leipzig Bay. Sigward Funke: "We are not well prepared for such events, and in addition to the continuation of earthquake observation to date, more geophysical research would be important to minimize possible consequences for the region.“

This is a joint press release by the University of Leipzig and the GFZ German Research Centre for Geosciences.

Original study: Dahm, T., Heimann, S., Funke, S., Wendt, S., Rappsilber, I., Bindi, D., Plenefisch, T., Cotton, F., 2018. Seismicity in the block mountains between Halle and Leipzig, Central Germany: centroid moment tensors, ground motion simulation, and felt intensities of two M ≈ 3 earthquakes in 2015 and 2017. Journal of Seismology, DOI: 10.1007/s10950-018-9746-9

Graphics and further information may be found on the GFZ Earthquake poster