# Destructive M6.2 Petrinja Earthquake (Croatia) in 2020--Preliminary Multidisciplinary Research
## 8 Discussion  
The fault system is a textbook example of a major failure in the upper crust along the pre-existing fault net [5; 14] at the critical moment of likely centennial release of generally north-south oriented compressional strain that has accumulated in the crust due to continuous northward movement of the Adriatic microplate or Adria [2]. Up to a few mm/yr, the Adria GPS generally northern velocities that are measured in the Adriatic foreland are generally accommodating along major External Dinarides active faults, since the Internal Dinarides GPS velocities are only 1-2 mm/yr, while the velocities in the Pannonian basin are near zero [2]. The dextral Pokupsko-Kostajnica-Banja Luka Fault could be one of the main inherited active faults between the crustal segments of Adria, while the sinistral Petrinja Fault could represent a reactivated Mesozoic transform fault that bordered crustal fragments and probably played a significant role in the formation of the Dinarides fold-and-thrust belt [72]. The Dinarides could be considered as the detached and deformed upper crust in the northern part of once greater Adria [73], while the transform faults were probably inherited from the Jurassic closure of the ocean that existed north of the Adria. Such transform faults could dissect also the micro-continent margins during the main phases of the orogenesis, and could be still active. The construction of the PGA and response spectra prediction assessment for the Petrinja M6.2 earthquake depends on judgments of the applicability of local and global ground motion models to the target region [65]. The HMI01, MHH102 and Sk20 models reflect the local GMPEs that were developed from regional datasets: hence, they are expected to reflect the regional tectonic characteristics better than other similar models. However, the uncertainties that are introduced by local GMPEs exceed those of global GMPEs because they are based on statistically less stable and more limited datasets. For this reason, we have used candidate NGA14 models BSSA14 and CY14, which were chosen by the BSHAP working group [65] based on behavioural analyses of the BSHAP strong motion dataset with these approaches. We used the ASB14 most recent empirical GMPE model for point source and extended source crustal earthquake scenarios in Europe to evaluate the convergence with NGA14 models. The Bora14 model was used to evaluate the convergence with the Sk20 preliminary model from the analogue BSHAP strong motion data, which will be significantly updated in the following years with new empirical data [71]. In the close Petrinja epicentral area, significant ground failures were reported within local site effects: karst sinkholes, visible ruptures at the surface, landslides and liquefaction. Due to these effects, we believe that soil nonlinearity is potentially masked in GMPE models and that the amount of local site amplification was significant and likely more linear within RVT estimations in the close epicentral areas at various micro locations. However, this depends heavily on the knowledge of the upper 10 or 20 m of the soil lithology, which is masked in a single \(V_{S30}\) value. Based on field observations, geological data and borehole data [72], clay is dominant in the upper layers. Medium to high plasticity clays exhibit linearly elastic behaviour for large \(PGA_{rock}\) values and can be amplified at least twice, whereas soil nonlinearity is certainly most prominent in soft sites with deep deposits of sand and gravel as described by [PERSON] et al. [73]. Based on reported ground failures and building damage, the resulting \(PGA_{site}\) values were likely in the approximate range of 0.4-0.6 g depending on the local site characteristics and distance from the epicentre and are reasonably comparable with USGS ShakeMap soil site predictions at the epicentre and at close distances (Figure 12).