Seismicity patterns before strong earthquakes in Greece |
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| Authors: | Vassilis Karakostas |
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| Institution: | (1) Geophysics Department, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece;(2) Geology Department, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece;(3) Geophysical Institute, Bulgarian Academy of Sciences, Sofia, Bulgaria |
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| Abstract: | The spatial distribution of the stress field around a fault, before its failure, depends on the focal mechanism of the ensuing
rupture. In the preseismic stage, the main fault is locked and background seismicity is distributed in the surrounding area
along small faults due to the raise of the stress level. This distribution is well correlated with changes of the stress field,
when these are calculated considering the fault slipping on the opposite sense (back-slip model), before the incoming strong
event. Studies of the seismicity around faults with known geometry, dimensions and slip properties could contribute in the
seismic hazard assessment.
In the present work, the spatio-temporal distribution of the smaller magnitude seismic activity before the occurrence of the
four most recent (1995–2008) strong (M ≥ 6.4) shallow earthquakes in Greece is compared with the stress pattern necessary for the generation of the strong earthquakes.
Studying the annual rate of occurrence it is found that in areas of positive pre-stress changes a sharp increase of the number
of earthquakes is observed several years before the occurrence of the main shock. This increase lasts for a few years and
then for several years before the main shock the occurrence rate declines but usually remains higher than it was before the
sharp increase. Seismicity distribution in the respective areas of negative pre-stress changes shows a rather stable occurrence
rate. |
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