Crustal deformations in the epicentral area of the West Bohemia 2008 earthquake swarm in central Europe

Abstract

[1] In West Bohemia, central Europe, during October 2008 an earthquake swarm of 25,000 shocks with a maximum event of M L $ 3.7-3.8 occurred at depths of 7-11 km. In 2007, annual GPS campaigns were launched. During the co-seismic phase, displacements of a few centimeters were detected at GPS sites. Maximum displacement was revealed at the KOPA site, which subsided by 167 mm. The epicentral area is covered by eluvium of 4-10 m thick, and is located in undulating pastures and well-forested valleys where visible surface soil effects could not be observed. To test possible fault manifestations, rough geomorphologic, geoelectric, and geochemical surveys were performed. GPS and seismic data, with geologic materials, were used to build a forward model for surface displacements, crustal deformations, and shear and normal stress fields. The fields enabled us to better determine crustal deformations and stresses that appeared within the seismic cycle, during the pre-, co-, and post-seismic phases. During the co-seismic phase, modeled fault motions along N-S faults located within the epicentral zone reached 0.6-1 mm/day. Possible structural block rotations were comprised of these motions. A dominant role for stress accumulation, release, and relaxation was assigned to the Mariánské Lázně fault zone and the Nový Kostel zone. Strain loads slowly, and when local PT conditions with an action of deep magmatic fluids reach instability, the strain is released and stress balancing occurs. The process leads to the reversible motions known for silent earthquakes. A forward crustal deformation model for West Bohemia is also presented within.

Seismotectonic significance of the 2008–2010 Walloon Brabant seismic swarm in the Brabant Massif, Belgium

“…This association relies both on the direct observation of hot springs and fluid-or gas outflow at the surface accompanying the swarm and on the observation of well-defined depth-related migration of seismicity (e.g. Daniel et al, 2011;Fischer et al, 2014;Lindenfeld et al, 2012;Schenk et al, 2012). According to the study of seismic swarms, triggering of seismicity by fluid overpressure at depth is often explained to be a likely cause (Fischer et al, 2014).…”

“…In an intraplate continental tectonic setting, seismic swarms are commonly associated with stress perturbations caused by magmatic intrusions, volcanic activity and with gradual fluid transport in the seismogenic part of the crust (Hainzl, 2004;Hiemer et al, 2012;Schenk et al, 2012;Špičák, 2000). In volcanic areas, continental rift and subduction zones, large fluid-and gas movements such as CO 2 release along prominent faults or fault intersections can generate earthquake swarms (e.g.…”

The October 2008 Nový Kostel earthquake swarm and its gas geochemical precursor

“…Simultaneous fault movements along the Mariánské Lázně fault system (Schenk et al . ) may have contributed to the stress build‐up and can have led to an additional compression of the Plesná valley‐Počátky fault system in the vicinity of the Mariánské Lázně fault. This compression affecting the fault permeability explains the observed gas concentration recordings.…”

“…During the coseismic phase, the basement blocks shifted back to the south, combined with vertical subsidence of 167 mm 3 km SE of Nový Kostel (Schenk et al . ). Faber et al .…”

“…For a detailed description of the swarm area, see Schenk et al . (). Quaternary volcanoes; north – Komorní Hůrka, south – Železná Hůrka.…”