# Horizontal PGA Estimates for Varying Deep Geological Conditions--A Case Study of Banja Luka
## 1 Introduction  
Peak ground acceleration (PGA) is still a very important parameter in earthquake engineering [1, 2, 3], and it is imperative to estimate it as reliably as possible. Most empirical formulae for scaling PGA values only take into account the effects of local soil, ignoring the effects of deep geology [4]. There are, however, strong motion studies [5, 6, 7, 8] that show that evaluating the severity of surface ground motion requires taking into account both deep geology and local soil conditions to minimize bias. The 2004 version of Eurocode 8 [9] also acknowledges the significance of deeper geological conditions, stating in Clause 3.1.2(1) that the National Annex may specify the classification scheme that will account for deep geology. The term \"deep geology\" refers to geological site surroundings on a scale of a few hundreds of meters or even kilometers [10], whereas \"local soil\" refers more to a geotechnical description on a scale of a few tens of meters, usually down to the first layer, with \(\rm V_{S}\geq 800\) m/s. Furthermore, most GMPEs (Ground Motion Prediction Equations) classify local soil conditions solely based on the top 30 m of the stratigraphic profile, regardless of research showing that the average shear-wave velocity of the top 30 m does not correlate with a site resonance [11, 12], particularly for deep soil sites [13]. This method is still extensively employed, because data on deep geology is limited, and investigating a soil profile todepths of at least 100 m would be prohibitively costly for the design of common civil engineering structures. In Bosnia and Herzegovina, seismic hazard maps for use in the application area of Eurocode 8 [14; 15] are defined for ground type A, which is defined as a rock or \"other rock-like geological formation\" with up to 5 m of weaker surface material and V\({}_{\text{S}}\) > 800 m/s in the top 30 m of the soil profile. The PGA value from the official hazard map is then multiplied by the soil factor, \(S\), to obtain the PGA at the top of local soil. Table 1 shows how \(S\) varies by ground type. \(S\) is additionally dependent on the type (Type 1 or Type 2) of spectrum, which is determined by the magnitude of the \"most contributing earthquakes\" [9]. It should be noted that Eurocode 8 defines the magnitudes of the most significant earthquakes using the \(M_{\text{S}}\) scale. However, as the majority of modern GMPEs and seismic source models utilize the \(M_{W}\) magnitude, the majority of seismic hazard maps in Europe were created using this magnitude. Furthermore, recent studies have demonstrated the \(M_{W}\) scale's significant shortcomings and support the implementation of the \(M_{\text{wg}}\) scale in its place [16; 17; 18]. Since our hazard analyses utilize a European source zone model defined by other authors (see Section 4), converting magnitude scales will be outside the purview of this research. We do, however, intend to include magnitude conversion of the seismicity data into our upcoming studies. The goal of this study was to estimate PGA values at different deep geological conditions for regions with moderate to strong earthquakes. The case-study region encompasses the city of Banja Luka, Bosnia and Herzegovina, for which our associated studies [19; 20; 21] examined vertical PGA values as well as uniform hazard spectral estimates. We calculated the seismic hazard for a location in Banja Luka using regional GMPEs for PGA and compared the results to all previous and current estimates of PGA and macroseismic intensity. In this study, local soil will be classified in compliance with [PERSON] et al. [22; 23]. The same local soil classification was used in former Yugoslavia to define site conditions for recorded strong motion data and to develop regional GMPEs [24; 25; 26]. According to this classification, \"rock\" soil sites are those with V\({}_{\text{S}}\) > 800 m/s, stiff soil has a 15 to 75 m thick soil layer atop that with V\({}_{\text{S}}\) > 800 m/s, while deep soil has a soil layer with