2nd Croatian Conference on Earthquake Engineering 2CroCEE

Almost 14000 aftershocks have been located in the first six month after the Mw 6.4 Petrinja earthquake of 29 December 2020. Most epicentres lie close to the NW–SE striking right-lateral causative fault, but considerable activity has been recorded in the surrounding area up to about 50 km away. The hypocentres reach depths of over 20 km, with most of activity recorded between the depths of 5 and 16 km. Their spatial pattern reveals activation of a number of smaller faults. The 75 focal mechanism solutions computed using the first motion polarities read from the seismograms of the local and regional seismic networks, indicate that about half of the aftershocks exceeding magnitude M = 3.0 occurred on strike-slip faults, while the large majority of the remaining ones were due to almost pure reverse dip-slip faulting. Analyses of the Coulomb stress change on optimally oriented strike-slip and dip-slip faults following the mainshock rupture, reveals that a large majority of aftershocks occurred in the volumes characterised by the Coulomb static stress increase, whereas the areas where the effective stress decreased remained mostly quiet. The distribution and preferred strike directions of strike-slip and reverse faulting inferred by the Coulomb stress transfer analyses are also in good agreement with individual focal mechanisms.