Surface displacement and source parameters of the 2020 Zagreb, Croatia earthquakes

On 22 March 2020 at 5:24 UTC, a M_W 5.4 earthquake occurred on the outskirts of Zagreb and was followed by a M_W 5.0 aftershock at 6:01 UTC on the same day. These moderate-magnitude events resulted in ground motion at the edge of detectability with satellite data but caused considerable damage in Zagreb’s historic centre. To illuminate the seismic sources, we compute the ground motion and invert seismic data using a state-of-the-art Bayesian inversion method that accounts for uncertainties in the data and in the Earth structure model. We compare the results to a well-established technique that uses a large number of hand-picked first-motion polarities.

Sentinel-1 Interferometric Wide data both from the ascending and descending orbit is obtained and processed using the European Space Agency SNAP toolbox. The resulting images show a phase difference of about 2π corresponding to ground displacement of 3 cm. The hypocentre and centroid locations of the events suggest that both were initiated at a depth of around 10 km, the rupture propagated upward, and most of the energy was released at a depth of 4 to 5 km. The shallow depth of the earthquakes possibly led to measurable surface displacement and resulted in considerable damage in the epicentral area and the centre of Zagreb.