Jökull - 01.01.2010, Page 84

Horálek and Fischer
portion of the NK focal zone, which implies reacti-
vation of the same fault segment. A total seismic mo-
ment of the 2000 and 2008 swarms corresponds to a
single event with magnitude ML∼5.3 which is very
likely the maximum expected earthquake in the region
of West Bohemia and Vogtland.
(iv) The seismicity pattern of the last nineteen
years indicates a slow N to S migration along the NK
fault plane. The 2000 and 2008 swarms show simi-
larity in general trends in the hypocenter migration:
activity of both of them started at the bottom of the
fault segment while the latest events occurred in the
uppermost tail.
(v) Source mechanisms (in the moment tensor de-
scription) of the 1997 and 2000 swarms differ. The
source type diagrams (Hudson et al., 1989) indicate a
dipole character of all the 1997 sources: slightly com-
pressive dipoles dominating in the first swarm phase
were replaced by pronounced tensile dipoles in the
second phase, whereas all of the 2000-events were
pure shears parallel to the NK fault plane.
(vi) The magnitude-frequency distribution of all
investigated swarms shows b-value≈1.0. Scalar seis-
mic moments M0 and magnitudes ML are related
according to power-law M0∝10MML which con-
siderably differ from moment magnitude Mw given
by Kanamori (1977) who shows power-law relation
of M0∝101.5Mw . Different scaling of the ML and
Mw magnitudes result in different b-values of the
magnitude-frequency distribution and we have to con-
sider bL=bW /1.5.
(vii) Analysis of the immediate aftershocks in the
2000 swarm revealed that both static and dynamic
Coulomb stress changes upon the fault plane due to
co-seismic slip contribute significantly to triggering
swarm events. The fact that majority of immediate af-
0 1 2 3 4 5 6 7
10!8
10!6
10!4
10!2
100
log
10
of interevent time [s]
pr
ob
ab
ilit
y
de
ns
ity
[1
/s
ec
]

all events
distant events
random distant
Figure 12. Probability density function of inter-event times of the West-Bohemia/Vogtland seismicity in the
period of 1991–2009 measured among all events (full line), distant events separated by more than 5 km (broken
line) and distant events with random origin times (thin line with symbols). – Þéttleikafall fyrir tímabil á milli
skjálfta í Vestur Bæheimi og Vogtlandi á árunum 1991–2009. Feit lína er fyrir alla skjálfta, brotin lína er fyrir
skjálftapör þar sem fjarlægðin á milli skjálftanna er meiri en 5 km, grönn lína með hringjum er fyrir fjarlæga
skjálfta.
84 JÖKULL No. 60