Can hydroseismicity explain recurring earthquake swarms in NW-Bohemia?

Fluid driven seismicity has been observed worldwide. The occurrence of intraplate seismicity triggered by pore pressure perturbations is a widely accepted process. Past analysis of earthquake swarms in the NW-Bohemia/Vogtland region provided evidence for the diffusion of pore pressure fronts during the migration of earthquakes within each swarm. Here, we test the hypothesis whether the diffusion of hydraulically induced pore pressure perturbations from the surface to the hypocentral depth could be a valid trigger mechanism.We test this hypothesis for the earthquake swarms in the Nový Kostel focal zone based on the analysis of 121 earthquake swarms and microswarms which occurred between 1992 and 2016. During the past 30 yr, 90 per cent of the earthquake swarms clustered beneath the village Nový Kostel within a depth range between 6 and 12 km. The spatial distribution of the swarms suggests a main diffusion pathway to the hypocentres, namely conduits along a NW-SE trending fracture zone which can be correlated at the surface with a part of the Libocký creek. A secondary branch of this fault zone exits in the southern part below the Horka reservoir. The filling of this reservoir began in 1969 and might have triggered the first significant earthquake swarm during the last century within the Nový Kostel area in the year 1985/1986. Since then, both the number of potentially triggered swarms as well as the number of microswarms increased with time. According to a robust statistical evaluation, hydraulically induced pore pressure perturbations could have triggered at least 19 per cent of the analysed earthquake swarms assuming effective diffusivities of 0.5 m2 s-1. Thus, hydroseismicity seems to be a valid option to explain at least a part of the earthquake swarms. The alternate hypothesis that ascending/intruding magmatic fluids trigger the remaining part of the earthquake swarms is another potential but weak option.