Role of crustal fluids in triggering the West Bohemia/Vogtland earthquake swarms: Just what we know (a review)

Abstract

We summarise the results of seismological studies related to triggering mechanisms, driving forces and source processes of the West Bohemia/Vogtland earthquake swarms with the aim to disclose the role of crustal fluids in the preparation, triggering and governing of the swarms. We present basic characteristics distinguishing earthquake swarms from tectonic mainshock-aftershock sequences and introduce existing earthquakes swarm models. From the statistical characteristics and time-space distribution of the foci we infer that self-organization is a peculiarity of West Bohemia/Vogtland swarms. We discuss possible causes of the foci migration in these swarms from the viewpoint of coseismic and/or post-seismic stress changes and diffusion of the pressurized fluids, and we summarize hitherto published models of triggering the 2000-swarm. Attention is paid to the source mechanisms, particularly to their non-shear components. We consider possible causes of different source mechanisms of the 1997-and 2000-swarms and infer that pure shear processes controlled solely by the regional tectonic stress prevail in them, and that additional tensile forces may appear only at unfavourably oriented faults. On data from the fluid injection experiment at the HDR site Soultz (Alsace), we also show that earthquakes triggered by fluids can represent purely shear processes. Thus we conclude that increased pore pressure of crustal fluids in the region plays a key role in bringing the faults from the subcritical to critical state. The swarm activities are mainly driven by stress changes due to co-seismic and post-seismic slips, which considerably depend on the frictional conditions at the fault; crustal fluids keep the fault in a critical state. An open question still remains the cause of the repeatedly observed almost simultaneous occurrence of seismic activity in different focal zones in a wider area of West Bohemia/Vogtland. The analysis of the space-time relations of seismicity in the area between 1991 and 2007 revealed that during a significant part of this time span the seismicity was switching among distant focal zones. This indicates a common triggering force which might be the effect of an increase of crustal-fluid pore-pressure affecting a wider epicentral region. K e y w o r d s : West Bohemia/Vogtland, earthquake swarms, crustal fluids, triggering of earthquakes, models of earthquake swarms J. Horálek and T. Fischer 456 Stud. Geophys. Geod., 52 (2008)

The 2008 West Bohemia earthquake swarm in the light of the WEBNET network

“…Such solution confirms our previous idea that the faulting during individual swarms in the region is controlled mainly by the regional tectonic stress (Horálek and Fischer, 2008) and that the N-S leftlateral (sinistral) movements with southward subsidence prevail in the NK. However, deeper understanding of the faulting and forces acting during the swarm requires a detailed study of source mechanisms in the full moment tensor description.…”

“…According to Horálek and Fischer (2008), the faulting during individual swarms is mostly controlled by the regional tectonic stress. However, the faults are brought to a critical state by pore pressure increase.…”

“…Triggering mechanisms acting during such rupturing were investigated on the basis of the 2000 swarm data by Hainzl and Fischer (2002), Parotidis et al (2003), Hainzl and Ogata (2005), and Fischer and Horálek (2005). Horálek and Fischer (2008) summarized outcomes of the 1997 and 2000 swarm studies. They inferred that faults in this region are in a subcritical steady state as a result of the regional tectonic stress and their portions can be brought to a critical state due to the increase of pore pressure of crustal fluids After reaching instability, a self-organization of the swarm activity prevails and fluids mainly act in the background, i.e., they keep the fault segments with the swarm activity in a critical state.…”

Mantle lithosphere control of crustal tectonics and magmatism of the western Ohře (Eger) Rift

“…The periodic occurrence of earthquake swarms (Horálek and Fischer 2008) and neotectonic crustal movements (Bankwitz et al 2003;Schunk et al 2005) indicate that the crust deformation continues even nowadays. It is interesting to note that almost all earthquakes in the western BM occur within the ST crust (Figs 6 and 8) and are located within a small focal area near Nový Kostel ).…”

“…Active tectonics is primarily manifested by Cenozoic volcanism (Ulrych et al 2000), gas emanations containing high proportions of mantle-derived CO 2 and He (Weinlich et al 1999), as well as by the periodic occurrence of shallow earthquake swarms (Horálek and Fischer 2008) and neotectonic crustal movements (Bankwitz et al 2003;Schunk et al 2005). The question of a possible source of mantle fluids is important namely in relation to the recent increase of He ratios observed by Bräuer et al (2005).…”