# Destructive M6.2 Petrinja Earthquake (Croatia) in 2020--Preliminary Multidisciplinary Research
## 7 Estimation of _PGA_ and Response Spectra of the M6.2 Petrinja 2020 Earthquake  
Following the destructive 29 December 2020 M6.2 earthquake near Petrinja, ShakeMap, which was developed by the U.S. Geological Survey (USGS), rapidly mapped earthquake ground motions [59] and portrayed the distribution and severity of shaking in the area (Figure 12). The peak ground acceleration was estimated to be approximately 0.5 g in an area approximately 5 km from the epicentre, 0.2 g at distances of 20-30 km and 0.1 g at a distance of 50 km (Figure 12a). Comparison of predictions and observations of a _PGA_ scaling with rupture distance were estimated by GMPE Peak Ground Acceleration prediction (Figure 12b) considering local site effects [59]. We tested USGS ShakeMap using the attenuation of the peak horizontal acceleration in the Dinarides area relations that were developed by [PERSON] et al. ([60], HMI01) and [PERSON] et al. ([61], MHH102) to the level of the average rock or very stiff soil sites (Figure 13a). We also selected three global, empirical ground-motion equations (GMPEs) that might be applicable to this region: (1) the hypocentral distance model of [PERSON] et al. ([62], ASB14), which was derived using pan-European datasets and two NGA-WEST 2 models (referred to as NGA14) with weights of 0.5: (2) the model of [PERSON] et al. ([63], BSSA14) and (3) the model of [PERSON] ([64], CY14), which was tested by [PERSON] et al. [65] for the Western Balkan BSHAP strong motion dataset (Figure 13b). Additionally, we tested the [PERSON] et al. ([66], RVT-Bora14) model, which uses random vibration theory (RVT) to predict Fourier amplitude spectra and the duration of ground motion to estimate the response spectra from the RESORCE-2012 strong motion database of the SIGMA project. The same RVT method was used by [PERSON] ([67], RVT-Sk20) on the analogue recordings in the BSHAP strong motion database [65; 68] and was used in this preliminary analysis, together with the Bora14 model, for comparison (Figure 13c). Since this is a preliminary analysis and we do not have a full seismotectonic analysis of the fault mechanism, GMPE estimations were made for a strike-slip fault with a 90-degree dip using a point source distance model (\(R_{\rm e}=R_{\rm IB}\) and \(R_{\rm hyp}=R_{\rm rup}\)), similar to [65]. For consistency and comparison, all _PGA_-distance models in Figure 13 are scaled to the epicentral distance \(R_{\rm e}\). Figure 12: _Cont._The preliminary analysis showed that for the M6.2 (or M\({}_{\rm w}\)6.4) earthquake in the epicentral area (1 km) for the rock site (\(V_{S30}=800\) m/s), the estimated _PGA_ values from the Croatian attenuation relations are 0.327 g (HMI01) and 0.290 g (MHH102), those from the GMPE models are 0.317 g (ASB14) and 0.298 g (NGA14), and those from the RVT models are 0.436 g (Bora14) and 0.321 g (Sko20). These values are reasonably comparable with USGS ShakeMap rock predictions in the epicentre [59], namely 0.3 g. However, in comparison to Figure 12, site effects significantly influence the amplification of seismic ground motion. According to the Croatian Seismic Hazard Map [32] for the 95- and 475-year return periods (accepted as a part of the National Annex to Eurocode 8), the Petrinja area is located in a relatively moderate seismic hazard zone: 0.08 g (95 yrp) and 0.16 g (475 yrp) peak ground accelerations. The estimated _PGA_ values in Figure 13 for M\({}_{\rm w}\)6.4 and for the close epicentral area (up to 10 km) substantially exceed the seismic hazard 95- and 475-yrp values. However, they are comparable with 0.33 g for 1000 yrp. This proves the significance of this earthquake within the known historical seismicity in this area. More important is the question of what type of impact this earthquake will have on future seismic hazard calculations not only for the epicentral area but also for the Zagreb city area, within which over 1 million people live.