# Horizontal PGA Estimates for Varying Deep Geological Conditions--A Case Study of Banja Luka
## 5 Discussion and Conclusions  
In our previous studies, we either exclusively examined the geological rock sites that exist in Kopaonik, Serbia [6], or the deep soil sites that are found in Osijek, Croatia [57; 58; 59; 60]. This is the first time we have examined a region that has varying deep geological and local soil conditions. Our study's primary weakness is the size of our database, which only includes 436 horizontal components of regional strong motion records. Unfortunately, no other sets of data in the world include deep geology data in parallel with local soil data, with the exception of California, which has different seismo-tectonic properties than the Balkans [8]. We are, therefore, constrained to rely on our current database. In this study, we examined horizontal PGAs for Banja Luka, which served as a case study area for regions with varying deep geological conditions and with a history of catastrophic earthquakes. This city is situated between the Pannonian basin and the Dinaric Alps. In addition to deep geological sediments, Banja Luka has scattered outcrops of geological rock and also areas with complex deep geology. This city was also devastated by the 27 October 1969 Banja Luka earthquake with \(M_{\text{W}}=6.1\), and the epicenter around 10 km  
Figure 7: Various probabilistic PGA estimates for the location in Banja Luka with coordinates \(44^{\circ}46.5^{\prime}\) N, \(17^{\circ}15^{\prime}\) E, computed in this study for different return periods, to the PGAs from current official seismic hazard maps of Bosnia and Herzegovina [14] and PGAs estimated based on the macroseismic intensity degrees from ex-Yugoslavian maps [56]. from the city center. The estimated predominant intensity in Banja Luka during the 1969 earthquake was VIII \({}^{\circ}\)MCS. Regional empirical GMPEs for horizontal PGAs are first presented. These GMPEs consider the effects of local soil and deep geology simultaneously. Despite the modest number of the strong-motion accelerograms from former Yugoslavia, which were used to derive the GMPEs, the empirical PGA estimates are shown to be in excellent agreement with the PGA values that were recorded in the past in Banja Luka. Our conclusions can be summarized as follows:  
* Regardless of the type of deep geological formation, the effects of deep geology are comparable in scale to those of local soil and must not be overlooked. * Only the combined effects of stiff soil sites and (deep) geological rock can result in median empirical PGA estimates comparable to the ones associated with the intensity of the 1969 Banja Luka earthquake (VIII \({}^{\circ}\)MCS). * Real re-occurrence (return) periods of the earthquakes that are most contributing to seismic hazard vary with the so-called \"return period\", \(T\!r\), which represents just a reciprocal value of \(N(a)\) (see Equations (6) and (10)); moreover, real return periods are significantly shorter than the corresponding return periods, \(T\!r\). * PGA probabilistic hazard estimates are dominated by local seismicity and are less influenced by earthquakes at distances larger than around 100 km. * The latest official (2018) hazard maps [14] provide PGA estimates that are comparable to our estimates of horizontal PGAs for Banja Luka. Table 5 shows that depending on the GMPE used (Equation (3) or Equation (4)), the effects of deep geology may even outweigh the effects of local soil. For example, if Equation (4) is used, the largest site amplification of 1.67 is obtained for deep geology when comparing the PGA at geological rock to that at intermediate sites. Compared to the local soil amplification of 1.34, which is obtained at stiff soil sites (compared to the \"rock\" soil), the deep geology amplification of 1.67 is 25% higher. We believe that our PSHA estimates, which were computed using regional GMPEs, can be perceived as a first step toward defining more realistic PGA probabilistic estimates in Banja Luka.