Jökull - 01.01.2010, Side 75

Intraplate earthquake swarms in Central Europe
Figure 6. Magnitude-time course of the 1997, 2000 and 2008 swarms being depicted on the same time scale. –
Stærð skjálfta í hrinunum 1997, 2000 og 2008.
non-DC part of the source mechanism consisting of
both volumetric ISO (volumetric) and CLVD (com-
pensated linear-vector dipole) components. The per-
centage of the non-DC part in the moment tensor is
fairly sensitive to the deviation of the slip vector, for
example, a deviation of a mere 5◦ off the fault plane
generates roughly up to 20% of the non-DC compo-
nent (S̆ílený, 2009). Tensile earthquakes have been
reported particularly from volcanic and geothermal
regions (e.g. Julian et al., 1997; Shimizu et al., 1988).
Note that non-DC part of the source mechanism is not
only attributed to the tensile earthquake, it can be also
due to anisotropy of the focal area. But a tensile earth-
quake can be distinguished from an anisotropy of the
medium thanks to different non-DC patterns result-
ing from the decomposition of the respective moment
tensors.
Our knowledge of source mechanisms in the West
Bohemia/Vogtland earthquake swarms is based on the
moment tensor studies of the 1997 and 2000 swarms
presented by Horálek et al. (2002, 2010). The swarms
were located about 1 km from each other: the 2000
swarm on the NK fault plane in its southern sub-
cluster, while the 1997 one in the northern sub-cluster
in two segments across the NK fault plane (Figure 5).
We retrieved moment tensors of 70 and 100 events
which covered the whole swarms of 1997 and 2000,
respectively. For this purpose we inverted the ground-
displacement peak amplitudes of direct P and SH
waves. In that case the source-time function is as-
sumed to be the step function; the inversion problem is
reduced to estimate six components of MT, thus linear
and robust. The number of stations used for the inver-
sion varied from 6 to 9 stations for the 1997 swarm
and from 7 to 18 for the 2000 swarm. Response of
the medium to elementary dipole excitation - Green’s
function was constructed for a 1D vertically inhomo-
geneous isotropic P- and S-wave velocity model of
the upper crust of the West Bohemia swarm region
by Málek et al. (2005). To solve the linear system of
equations, we used the singular value decomposition
method (SVD) and applied the library routine from
Numerical Recipes by Press et al. (1992), the resultant
MTs were decomposed into percentages of the ISO,
DC, and CLVD components according to Vavryc̆uk
(2001).
In the January 1997 swarm, two types of source
mechanisms (A and B in our notation) were found
to be predominant. The A-mechanisms were oblique
normal, nearly pure shears; earthquakes possessing
these mechanisms occurred during the first phase of
the swarm. The B-mechanisms implied oblique-thrust
faulting with a combined source (DC with significant
ISO and CLVD components) indicating tensile earth-
quakes of the rupture-opening type. The B-events
dominated in the second swarm phase when the other
fault segment was active. Percentage of the non-DC
part varies between 1% and 25% for the A-events
whereas between 25% and 53% for the B-events. Be-
side the A and B events there were identified further
six source mechanism types with various percentages
of the non-DC parts in the 1997 swarm (see Horálek
et al., 2000) but none of them (including the A and B
types) matched the geometry of the NK fault plane.
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