11:15 〜 11:30
[S13-2-04] earthquake statistics, spatiotemporal distribution of foci and source mechanisms as a key to understanding of causes leading to the West Bohemia/Vogtland earthquake swarms
The origin of earthquake swarms is still unclear. West Bohemia-Vogtland represents one of the most active intraplate earthquake-swarm areas in Europe. There were swarms in 1997, 2000, 2008 and 20011 followed by reactivation in 2013 which forming a focal belt of about 15 x 6 km, focal depths vary from 6 to 15 km. An exceptional non-swarm activity up to magnitudes ML = 4.5, stroke the region in 2014, the events were also located in the NK focal belt.
We analysed geometry of the NK focal zone applying the double-difference method to seismicity in the period 1997 – 2014. The swarms are located close to each other at depths between 6 and 13 km. The most events obey the b-value = 1.0 distribution, however, a group of the largest events (~ML > 2.8) depart significantly from it. Furthermore, we disclose that all the ML > 2.8 swarm events, which occurred in the given time span, are located in a few dense clusters. It implies that the most of seismic energy in the individual swarms has been released in step by step rupturing of one or a few asperities. The mechanism patters of the individual swarms indicate their complexity. MTs of the most analysed events signify pure shears except for events the second phase of the 1997 swarm the MTs of which indicate significant amount of non-DC components.
We infer that the individual earthquake swarms in West Bohemia-Vogtland are mixture of the mainshock-aftershock sequences which correspond to step by step rupturing of one or a few asperities. The swarms occur on short fault segments with heterogeneous stress and strength, which may be affected by crustal fluids. Pressurized fluids may reduce normal component of the tectonic stress and lower friction. Thus, critically loaded and favourably oriented faults are brought to failure and the swarm activity is driven by the differential local stress.
We analysed geometry of the NK focal zone applying the double-difference method to seismicity in the period 1997 – 2014. The swarms are located close to each other at depths between 6 and 13 km. The most events obey the b-value = 1.0 distribution, however, a group of the largest events (~ML > 2.8) depart significantly from it. Furthermore, we disclose that all the ML > 2.8 swarm events, which occurred in the given time span, are located in a few dense clusters. It implies that the most of seismic energy in the individual swarms has been released in step by step rupturing of one or a few asperities. The mechanism patters of the individual swarms indicate their complexity. MTs of the most analysed events signify pure shears except for events the second phase of the 1997 swarm the MTs of which indicate significant amount of non-DC components.
We infer that the individual earthquake swarms in West Bohemia-Vogtland are mixture of the mainshock-aftershock sequences which correspond to step by step rupturing of one or a few asperities. The swarms occur on short fault segments with heterogeneous stress and strength, which may be affected by crustal fluids. Pressurized fluids may reduce normal component of the tectonic stress and lower friction. Thus, critically loaded and favourably oriented faults are brought to failure and the swarm activity is driven by the differential local stress.