Jökull - 01.01.2010, Page 80

Horálek and Fischer
a)
Figure 9. (a) Cumulative magnitude-frequency distribution for the swarms of 1997 (light blue), 2000 (green),
and 2008 (red). It is apparent that all these swarms are characterized by a b-value∼1.0. (b) Relation between
seismic moment M0 and local magnitude ML for the 1997 and 2000 swarms. Seismic moments M0 are taken
from moment tensors of 70 and 100 events of the 1997 and 2000 swarms, respectively. – Uppsafnaður fjöldi
skjálfta sem fall af stærð (a). Hrinan 1997 er ljósblá, hrinan 2000 græn og 2008 rauð. b) Samband mismunandi
stærðarákvarðana á skjálftum í hrinunum 1997 og 2000.
Applying the relation (1) to the WEBNET cata-
logues we estimated total seismic moments released
during the individual swarms. These moments are as
follows: 4.3×1016 Nm for the 2008 swam, 1.5×1016
Nm for the 2000 swarm and only about 6×1014
Nm for the 1997 swarm; these values correspond to
single events of magnitudes ML∼5.1, ML∼4.6 and
ML∼3.5, respectively. Assuming hypothetically that
the 2000 and 2008 swarms, which occupied over-
lapping fault areas, would have ruptured simultane-
ously, the corresponding single event would amount
to ML∼5.3, which is inferred as the seismic poten-
tial of the NK area. This inference is supported by
the fact that two largest earthquakes, which occurred
in the West Bohemia/Vogtland region during the last
150 years (March 1872 and November 1908), are es-
timated to be of magnitude ML=5.0.
Internal triggering forces
To complete the review of the West Bohemia/-
Vogtland swarms we refer to our study of internal trig-
gering mechanisms (Fischer and Horálek, 2005). We
investigated the space-time relations between consec-
utive swarm earthquakes in 2000 and their associa-
tion with the Coulomb stress changes. To this pur-
pose we analyzed the relative position vectors of about
2300 events with ML>0.5 which were constructed in
the following way: the events were projected onto the
fault plane (in reality the foci formed a cluster sur-
rounding the fault plane) and the relative position vec-
tors between the events of every consecutive-event
pairs were measured. The first and second events in
each event pair are considered as prior events (PE) and
immediate aftershocks (IA), respectively, and subse-
quent evens after PE are considered as aftershocks.
We are aware of oversimplification due to disregard-
ing of the IA and PE magnitudes. On the other
hand, the magnitude differences within majority of the
swarm-event pairs are rather small, so that the event
pairs with much larger IA than PE seldom occur. This
is why this simplification does not influence signifi-
cantly the following results that are of statistical char-
acter.
80 JÖKULL No. 60