Abstract
On 22 March 2020, Zagreb was struck by an M5.5 earthquake that had been expected for more than 100 years and revealed all the failures in the construction of residential buildings in the Croatian capital, especially those built in the first half of the 20th century. Because of that, extensive seismological, geological, geodetic and structural engineering surveys were conducted immediately after the main shock. This study provides descriptions of damage, specifying the building performances and their correlation with the local soil characteristics, i.e., seismic motion amplification. Co-seismic vertical ground displacement was estimated, and the most affected area is identified according to Sentinel-1 interferometric wide-swath data. Finally, preliminary 3D structural modeling of the earthquake sequence was performed, and two major faults were modeled using inverse distance weight (IDW) interpolation of the grouped hypocenters. The first-order assessment of seismic amplification (due to site conditions) in the Zagreb area for the M5.5 earthquake shows that ground motions of approximately 0.16–0.19 g were amplified at least twice. The observed co-seismic deformation (based on Sentinel-1A IW SLC images) implies an approximately 3 cm uplift of the epicentral area that covers approximately 20 km2. Based on the preliminary spatial and temporal analyses of the Zagreb 2020 earthquake sequence, the main shock and the first aftershocks evidently occurred in the subsurface of the Medvednica Mountains along a deep-seated southeast-dipping thrust fault, recognized as the primary (master) fault. The co-seismic rupture propagated along the thrust towards northwest during the first half-hour of the earthquake sequence, which can be clearly seen from the time-lapse visualization. The preliminary results strongly support one of the debated models of the active tectonic setting of the Medvednica Mountains and will contribute to a better assessment of the seismic hazard for the wider Zagreb area.
Highlights
The city of Zagreb is the Croatian capital and is situated in the contact area of three major regional tectonic units: the Alps in the northwest, the Pannonian Basin in the east and the Dinarides in the south
There were quite a number of people who spread alarming news, such as the idea that Zagreb was on the verge of collapse because it lies above an underground volcano; they fabricated mud volcanoes, and some observed bluish flames at the top of the Medvednica Mountains and new hot springs in Stubica
Based on the analysis of data recorded in Croatia and around the world, the focal mechanisms of the main earthquake and the strongest aftershock were calculated (Figure 4)
Summary
The city of Zagreb is the Croatian capital and is situated in the contact area of three major regional tectonic units: the Alps in the northwest, the Pannonian Basin in the east and the Dinarides in the south. The earthquakes in the area are the result of the interface between crustal fragments bordered by active faults [6,7]. The earthquake mechanisms reveal predominantly N–S directed P-axes in the study area that indicate the prevalence of compressional tectonics with reverse faulting [7]. These data are in agreement with stress calculations and kinematics of Quaternary structures obtained from geological studies [6,9]. While Prelogović et al [6] suggested an active longitudinal transpressive positive flower structure of the Medvednica Mountains bordered by steep divergent reverse faults, Matoš et al [8]. Indicated unidirectional top-to-the-north steep reverse faults along the Medvednica Mountains as the main earthquake sources
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