Possible origin of the spatiotemporal evolution of the 2008 earthquake swarm in Northwest-Bohemia, Czech Republic

Abstract

The phenomenon `earthquake swarm' is well known from numerous locations on Earth but its mechanism and trigger process are still an enigma. The influence of fluids on fluid-driven seismicity is generally well accepted but the temporal behaviour and the migration of activated phases is still an open question. We attempt to answer these open questions by combining the results of research on induced-seismicity with new reliable analyses of full moment tensors (FMTs) of the swarm earthquakes of 2008 to 2018 in Northwest-Bohemia, Czech Republic by Vavryˇcuk et al. (2017, 2021). The FMTs reveal evidence of compressive fracturing as the prevailing focal mechanism during the swarms. Our proposed hypotheses considered these earthquakes as the origin of subsequent pore pressure pulses in the respective active swarm phase. The diffusion of these pore pressure pulses, which could trigger the nearby earthquakes as well as the successive swarm phase in the vicinity after a respective diffusion process in space and time, are the target of this studies. Missing information regarding the real diffusion distances are compiled by a strict statistical approach of a one-to-one analysis of the swarm earthquakes. The analyses suggest that parts of the swarm earthquakes could be triggered by pore pressure diffusion with an estimated diffusivity range of approximately D = 0.01 to 3.0 m2/s. In exceptional cases, which make about 5% of all cases, we have to consider also diffusivities up to 15 m2/s. Examples of hydraulic diffusivities observed in our region of interest confirm this range. Such a process can also be assumed during further swarms in the years 2011 to 2018 because of the continuous and predominant occurrence of compressive fracturing as a focal mechanism beside pure shear and tensile fracturing. Our results could explain for the firsttime the missing link regarding the temporal and spatial migration of the subsequent swarm phases in ourstudied region at Nový Kostel (Northwest-Bohemia, Czech Republic).