Geophysical Institute of the ASCR

consecutive event pairs occurring systematically at greater distances (called fast IAs as opposed to
remaining ones denoted slow IAs). These characteristics point out an evident causal link between
consecutive swarm earthquakes and a triggering effect of the coseismic stress perturbation, which
is induced by a fault slip during the prior earthquake.
To examine conditions of triggering the aftershocks upon the main fault plane on account of the
prior events we calculated the time-space distribution of both dynamic and static components of
the Coulomb stress change on the fault plane surrounding a synthetic rupture (see Fig. 21). We
found a close relation between the spatial and temporal distribution of the relative position vectors
and of the Coulomb stress changes. The static Coulomb stress field decays rapidly with distance
likewise the average areal density of the IAs; oval-like character of the isobars and their elongation
in the slip direction matches well the spatial distribution of the slow IAs. The dynamic stress
changes generated by transient deformation due to passage of the near-field seismic waves decay
much slower with distance and show a two-lobe shape resembling the pattern of the fast IAs. This
implies that both static and dynamic coseismic stress changes significantly affect the stress field in
the surroundings of the rupture and so they have a substantial impact on self-organization of the
swarm activity. Majority of subsequent events at smaller distances are presumably triggered by the
static stress field, while the fast IAs occurring at greater distances appear to be generated by the
dynamic stress transition. The relatively small magnitude of Coulomb stress changes upon the fault
plane (a few hundredths of MPa at distances of a few radii of the prior rupture) might still be
capable of governing the aftershock distribution as reported e.g. by Harris (1998). Moreover, highpressured
crustal fluids, generally presumed in the region concerned, could locally elevate the pore
pressure and thus substantially reduce the shear strength of the fault.
Fig. 21. a) Space distribution of the complete stress field on the fault plane surrounding the rupture induced
by an instantaneous stress drop of 10 MPa due to strike slip on a circular area with radius of 100 m in
a homogeneous half-space. The shear stress component at the time of 0.35 s after the rupture is depicted
(normal stress is zero). The stress field breaks with time into the permanent (static) part as a result of the nearfield
deformation and the transient (dynamic) part carried by the passage of seismic waves. b) Angular
dependence of the rate of IAs. The left panel shows all aftershocks, the right panel shows the fast aftershocks
linked by speed higher than 100 m/s.
c) The interactions between subsequent earthquakes of the 2000-swarm at the shortest interevent
times were studied by Fischer (2005) by the analysis of the multiple-events whose complicated
waveforms indicated multiple rupturing episodes. The source-time functions of such earthquakes
consist of several pulses whose relative positions provide information on the mutual position of the
sub-events. In order to determine the P- and S-wave onsets of the sub-events the waveform
modelling method was used (Fig. 22). The P- and S-multiple waveforms were modelled as the sum
of waveforms of single sub-events with different hypocentre coordinates and scalar moments, the
38