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IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS006 Poster presentation 1941
Applying Different Techniques in Studying an Active Strike-slip Fault;
Example of a Seismotectonic Study in the Southern Alps, NW Slovenia
Mrs. Vanja Kastelic
University of Ljubljana, Department of Geology M.Sc IASPEI
Jure Bajc, Mladen IvčIć
We combine several techniques to obtain better insight into earthquake processes on the active fault
zone in NW Slovenia. This particular fault system is of major concern with respect to seismic hazard in
Slovenia one of the strongest events in the last century occurred there, the Mw = 5.6 in 1998 and Mw =
5.2 in 2004. In areas with absence of recent sedimentation or with active erosion processes, no young
sediments or sedimentary rocks are present. Therefore, studying the seismic history of a fault in such
an area cannot be done by standard paleoseismological investigation technique like trenching.
Quantitative assessment of deformation that occurred on a particular fault must be approached by
alternative methods due to lack of typical geological markers, such as displacements between
stratigraphy or lithological boundaries. The Julian Alps (Eastern part of Southern Alps) in NW Slovenia
are a region where both, absence of recent deposits and lack of geological markers is typical. The 1998
and 2004 earthquakes occurred along a NW-SE striking fault that runs through this region. We applied
detailed structural mapping of the area along the fault trace, computed the stress orientation from
measured kinematic data, and enriched both results with joined hypocenter (re)locations and focal
mechanism calculation. The combination of the results of these different techniques provides better
understanding of the seismicity of the area. We show that the fault exhibits a segmented geometry of
individual fault segments and step-over zones of local transpressional basins that bound these
segments. The geometrical relation between the length of individual segments and their overlap and
separation distances in the step-over zones play an important role in spatial distribution of earthquake
sequence. The step-over zone with overlap to separation ratio around 1:1 was easily breached by Mw =
5.6 event, while the one exhibiting the ratio around 5:1 was not breached by the same event and the
aftershock cluster stopped at this step-over zone. Comparison of the fault area marked by the
aftershocks and the fault area, obtained from the scaling relation between the magnitude and the area
that ruptured, shows that the aftershocks span a significantly larger area than the segment that was
activated during the main event. On the other hand, the aftershocks that occur within the first few
hours after the main shock do mark the activated fault area of the main event reasonably well.
Comparison between both methods shows, that in case of good local coverage of seismic stations in the
epicentral area distributed evenly in all direction from the epicenter, polarity of first arrivals technique
gives very good and reliable focal mechanisms. These in turn allow detailed seismotectonic
interpretation of the earthquake processes in the area. The solution of focal mechanisms computation
for both main earthquakes, that yield dextral strike-slip deformation, are also in a very good agreement
with results obtained from field measurements of kinematic indicators and the solution of inversion of
the data used for calculating stress orientation. Results give a N-S oriented direction for the maximum
compressive stress and a strike-slip solution for the kinematic behaviour of the fault within this tectonic
phase.
Keywords: active tectonics, fault segmentation, fault plane solution