Investigating the source processes of underground-mining induced earthquakes based on geodetic and seismic observations

Underground mining induces seismicity and surface displacement. In Poland, in the Legnica Glogow Copper District near Wroclaw, induced earthquakes are particularly frequent with earthquakes of Mw3 and larger occurring many times a year. These earthquakes have shallow hypocentres of often less than 1 km and mostly above the mined copper layer.

The area around the mines also experiences a fast continuous surface subsidence of several millimeters per year caused by rock as well as groundwater extraction. This surface motion is observed through geodetic measurements on the ground and from space. The rate of surface motion is spatially very heterogeneous. Across wide areas above the active mining it even exceeds 10 mm/yr. Also sudden coseismic acceleration of surface motion is observed at the time of the larger earthquakes through space-borne InSAR. In these cases we often observe motion of several centimeters within a few days and with spatial extensions reaching a few kilometers.

Despite safety measures, the occurrence of some, also larger earthquakes is unexpected in space and time, which poses a particular threat to workers in the mines and but also to the subsurface mine structures as well as generally to the people, settlements and infrastructure above ground.

 

Our study investigates a number of larger events of the recent years by analyzing the locally recorded seismic waveforms jointly with measurements of the surface displacements based on InSAR and partly GNSS measurements. We aim to precisely locate the source processes of larger induced earthquakes and to characterize them as an interplay between shear-failure and collapse using full moment tensor models in a fully Bayesian inference framework. Potentially we can relate collapse and failure to the mining activities or other influences and improve our understanding of these unwanted events for mitigation measures.

The observations are best explained by large negative isotropic components accompanied by apparently significant shear failure mechanisms. Another finding is that our moment estimates systematically exceed the local catalog values. Challenges to be discussed are the impact of our single short-duration source model for possibly an accumulation of multiple events, possibly involving a larger volume and a longer duration, and the potential bias introduced by a simplified velocity model.