Evidence for triggering of the Vogtland swarms 2000 by pore pressure diffusion

Abstract

Seismicity triggered by fluid injection in boreholes usually has certain typical spatiotemporal characteristics. These characteristics can be partly recognized as signatures, when pore pressure diffusion is the dominant triggering mechanism. Starting from these diffusion‐typical signatures of man‐made earthquakes, we seek analogous patterns for the earthquakes in Vogtland/NW Bohemia (VB) at the German/Czech border region in central Europe. VB is characterized by recurring intraplate earthquake swarms with magnitudes up to ML 4.5. There is strong geophysical evidence that the seismic events are correlated to fluid‐related processes in the crust. This study aims to investigate the possible role of fluids in VB. We test the hypothesis that ascending magmatic fluids trigger earthquakes by the mechanism of pore pressure diffusion (i.e., relaxation). This triggering process is mainly controlled by two physical fields, the hydraulic diffusivity and the seismic criticality (i.e., critical pore pressure value leading to failure; stable locations are characterized by higher critical pressures), both heterogeneously distributed in rocks. The results of the analysis of the year 2000 VB earthquake swarm data support this concept. We were able to recognize diffusive signatures and to obtain scalar estimates of hydraulic diffusivities with values between 0.3 and 10 m2/s for the seismically active region. Using a numerical model, where spatially correlated diffusivity and criticality patches (where patches with higher diffusivity are assumed to be less stable) are considered, we successfully simulate a general seismicity pattern of the swarms 2000, including the spatiotemporal clustering of events and the migration of seismic activity.