Jökull - 01.01.2010, Side 81

Intraplate earthquake swarms in Central Europe
The plot of the areal density of the IA (Figure 10a)
clearly evidenced that vast majority of immediate af-
tershocks was located close to prior events. Hence,
we estimated the areal density of the immediate af-
tershocks as a function of normalized distance from
the centre of the PEs to find out whether the after-
shocks were generated preferably near the edge of the
PE rupture. As it is apparent from Figure 10b, the IA
density increases from a small value within the rup-
ture area to a narrow maximum at distances between 1
and 2 PE radii; at greater distances the density rapidly
decreases. The areal density for 5th and 50th after-
shocks shows similar trend; however, it shows much
smaller maximum in both cases. These results ap-
parently point out triggering effect of the prior events
upon the subsequent ones and signify prevailing ori-
gin of aftershocks close to the edge of the PE rupture.
Inter-event-times in the 2000 swarm, which range
from less than 1s up to more than 104 s, indicate vari-
ous speed of interaction between prior and subsequent
events: about, 40% and 8% of the ML>0.5 IAs were
linked with the PEs by velocities above 10 m/s and
100 m/s, respectively. We examined the PE – IA in-
teractions in term of their rapidity and preferential oc-
currence directions. To this purpose we calculated the
number of the IAs in the angular sectors of 10◦ on the
whole fault plane for different intervals of inter-event
velocities. The resulting IA-rate polar plots for all the
ML>0.5 IAs, and for inter-event velocities higher than
10 m/s and 100 m/s are depicted in Figure 11. All
three plots show a distinct prevalence of the IA occur-
rence along the direction of 30◦ to 40◦ from horizontal
plane which corresponds to the slip-parallel direction
revealed by the retrieved source mechanisms. The po-
0 1 2 3 4 5 6 7 8 9 10
0
5
10
15
20
25
30
35
40
45
Normalized aftershock distance
5th
IA
Areal density of
aftershocks per
unit rupture area
50th
a) b)
Figure 10. Areal density of positions of immediate aftershocks (IAs) relative to the position of previous events
(PEs); the coordinates are projected on a common fault plane. Density plot in a) shows the number of events
per 100 m2; in b) the coordinates of IAs are normalized by nominal radius of PEs defined as R=30 x 100.35ML ,
and the areal density of 1st, 5th and 50th aftershock is shown. – Þéttleiki staðsetninga eftirskjálfta miðað
við staðsetningu skjálftans á undan. Hnitunum er varpað á sameiginlegan brotflöt. Myndin til vinstri sýnir
fjölda skjálfta fyrir hverja 100 m2. Myndin til hægri sýnir fjarlægð eftirskjálfta eftir stöðlun með mati á stærð
skjálftans, R=30 x 100.35ML . Myndin sýnir þéttleikann fyrir fyrsta, fimmta og fimmtugasta eftirskjálftann.
JÖKULL No. 60 81