# Destructive M6.2 Petrinja Earthquake (Croatia) in 2020--Preliminary Multidisciplinary Research
###### Abstract  
On 28 December 2020, seismic activity in the wider Petrinja area strongly intensified after a period of relative seismological quiescence that had lasted more than 100 years (since the well-known M5.8 Kupa Valley earthquake of 1999, which is known based on the discovery of the Monrovicic discontinuity). The day after the M5 foreshock, a destructive M6.2 mainshock occurred. Outcomes of preliminary seismological, geological and SAR image analyses indicate that the foreshocks, mainshock and aftershocks were generated due to the (re)activation of a complex fault system--the intersection of longitudinal NW-SE right-lateral and transverse NE-SW left-lateral faults along the transitional contact zone of the Dinarides and the Pannonian Basin. According to a survey of damage to buildings, approximately 15% of buildings were very heavily damaged or collapsed. Buildings of special or outstanding historical or cultural heritage significance mostly collapsed or became unsurceivable. A preliminary analysis of the earthquake ground motion showed that in the epicentral area, the estimated peak ground acceleration _PGA_ values for the bedrock ranged from 0.29 to 0.44 g. In the close Petrinja epicentral area that is characterized by the superficial deposits, significant ground failures were reported within local site effects. Based on that finding and building damage, we assume that the resulting peak ground acceleration (_PGA_sito) values were likely between 0.4 and 0.6 g depending on the local site characteristics and the distance from the epicentre. P ARTIC  
2021  
11 March 2021  
Multidisciplinary Research. R.  
In the Croatian Earthquake Catalog (CEC, with the updated version first described in [4]), the first data about a strong earthquake in this area correspond to 17 August 1903. This earthquake had an epicentral intensity of VI MCS in Petrinja and an estimated magnitude of 4.6. The earthquake caused moderate damage in Petrinja near the epicentre. According [PERSON] [5], \"... in Petrinja, the windows rattled, the rocks cracked, plaster fell from the vault and the vaults cracked.\"  
The strongest earthquake that occurred in this area (until 2020) after 1900 is one of the most famous earthquakes, not only in Croatia but across the world. It was an M5.8 event that occurred on 8 October 1909 with an epicentre north of Pokupsko (Kupa Valley) and is known as the Kupa Valley earthquake. The Kupa Valley earthquake is included in the several most important historical earthquakes that are regularly discussed in seismological textbooks and other books on the history of science. Based on this earthquake's recordings, the great Croatian scientist, geophysicist [PERSON], improved knowledge of the mechanism of spreading near-earthquake waves through the earth and was the first in the world to discover the velocity discontinuity that separates the crust from the earth's mantle. This discontinuity, in his honour, was called the Mohorovic discontinuity. The main Kupa Valley earthquake was followed by a series of more than 50 aftershocks by the end of 1910. The strongest event occurred on 29 January 1910, with an epicentre about 15 kilometres east of the main earthquake epicentre and a magnitude of 5.3. In the last 30 years, until Dec 2020, the strongest earthquake within this area occurred in 1996, with a magnitude of 4.5. In Petrinja, Komarevo, Glina, Maja, Sisak and Veliki Svinjicka, it caused damage: Objects fell from shelves in houses and plaster, roofing tiles and parts of chimneys cracked. However, at the end of 2020, in the wider Pokupsko-Petrinja area, an earthquake of higher magnitude than expected occurred that we will remember for a long time. After the seismic sequence that followed, it was clear that this is an area for which the seismic potential exceeds that suggested in previous studies (e.g., analyses of seismicity, seismic hazard map of Croatia), and the geodynamic model is highly complicated (much more complicated than expected and with a larger part hitherto unknown).