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| Titel |
Space and time distribution of foci and source mechanisms of West-Bohemia/Vogtland earthquake swarms - a tool for understanding of their origin |
| VerfasserIn |
Josef Horalek, Hana Čermáková, Tomáš Fischer |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250091599
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| Publikation (Nr.) |
 EGU/EGU2014-5901.pdf |
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| Zusammenfassung |
| The origin of earthquake swarms remains still an enigma. The swarms typically accompany
volcanic activity at the plate margins but also occur in intracontinental areas. West
Bohemia-Vogtland (border area between Czech Republic and Germany) represents one of the
most active intraplate earthquake-swarm regions in Europe. Above, this area is
characteristic by high activity of crustal fluids. Swarm earthquakes occur persistently in
the area of about 3 000 km2. However, the Nový Kostel focal zone (NK), which
shows a few tens of thousands events within the last twenty years, dominates the
recent seismicity of the whole region. There were swarms in 1997, 2000, 2008 and
20011 followed by reactivation in 2013, and a few tens of microswarms which
forming a focal belt of about 15 x 6 km. We analyse geometry of the NK focal zone
applying the double-difference method to seismicity in the period 1997 - 2013. The
swarms are located close to each other in at depths from 6 to 13 km. The 2000
(MLmax = 3.3) and 2008 (MLmax = 3.8) swarms are “twins” i.e. their hypocenters
fall precisely on the same portion of the NK fault; similarly the 1997 (MLmax =
2.9), 2011 (MLmax = 3.6) and 2013 (MLmax = 2.4) swarms also occurred on the
same fault segment. However, the individual swarms differ considerably in their
evolution, mainly in the rate of the seismic-moment release and foci migration.
Source mechanisms (in the full moment-tensor description) and their time and space
variations also show different patterns. All the 2000- and 2008-swarm events are
pure shears, signifying both oblique-normal and oblique-thrust faulting but the
former prevails. We found a several families of source mechanisms, which fit well
geometry of respective fault segments being determined on the basis of the event
location: The 2000 and 2008 swarms activated the same portion of the NK fault, hence
the source mechanisms are similar. The 1997 and 2011 swarms took place on two
differently oriented fault segments, thus two different source mechanisms occurred:
the oblique-normal on the one segment and the oblique-thrust type on the other
one.
Furthermore, we disclose that all the ML ≥ 2.7 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 existing results do not allow us to explain properly an origin of earthquake swarms.
Nevertheless, some results point to a connection between pressurized fluids in the crust and
the earthquake swarm occurrence. Taking this into account, we may infer that earthquake
swarms occur on short fault segments with heterogeneous stress and strength, which are
affected by crustal fluids. Pressurized fluids reduced 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. |
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