## 1 Introduction  
Civil engineering regulations often incorporate vertical PGA by simply multiplying the horizontal PGA by a constant. In Eurocode 8 [1], the vertical design PGA values are simply calculated by multiplying the horizontal PGA by a constant that depends only on the spectrum type [1]. Furthermore, the effect of local soil on vertical ground motion is not taken into account at all in many design regulations worldwide. In Eurocode 8 [1], vertical design spectra remain the same for all ground types. However, a recent study on the vertical ground motion at deep soils in Osijek, Croatia, showed that the vertical PGA values can be approximated as 0.61 of the horizontal PGA values, which is a 37% higher ratio than the one given by Eurocode 8 for Type 2 spectra [2]. Another study on the vertical ground motion recorded during the two 6 February 2023 Kahramanmaras earthquakes showed that it was higher than expected and that the vertical earthquake effects were not assessed correctly in the Turkish code as well [3]. The reason why most codes do not take into account local soil effects on the vertical PGA can be found in to the so-called H/V methodology, first presented by [PERSON] and [PERSON] [4; 5] and later popularized by [PERSON] [6; 7; 8]. This methodology is widely used to determine the natural period of soil vibrations using microtrenorms. The H/V approach assumes that local soil amplification happens only horizontally, not vertically. Although recent research has shown that this method will not always provide realistic amplification factors [9], most codes continue to be based on the assumption that local soil effects are not existent in the vertical ground motion direction. In this study, we will define site effects in terms of \"local soil\" and \"deep geology\". The term \"deep geology\" will be used to describe the geological site conditions on a scale of at least a few hundred meters, while the deep geology categories are defined as rock, intermediate sites, and sediments, according to [PERSON] and [PERSON]'s classifications [10]. The term \"local soil\" will be used to describe soil with a thickness of up to 100 or so meters, and the local soil types are defined in this study according to classifications by [PERSON] et al. [11; 12]. We use the same classifications as in our previous studies [2; 13; 14; 15; 16; 17] for the same key reasons. First, we acquire the associated deep geology data from the EQINFOS database [18], which contains the majority of our input strong motion data and is the only strong motion database besides the Californian database [19] for which such data exist. Following the classification suggested by [PERSON] and [PERSON] [10], an international team of 13 geologists and earthquake engineers interpreted the description of the corresponding site geology and defined the deep geology site condition parameters for the recording sites [20]. As for local soil classification, [PERSON] et al. [20] initially presented the local soil classification of the accelerograph locations in the former Yugoslavia, following the classifications of [PERSON] et al. [11; 12]. [PERSON] and [PERSON] [21] and [PERSON] (1994) [22] subsequently revised and improved these classifications. Rock soil sites are defined by an average shear wave velocity, V\({}_{\text{S}}\), of more than 800 m/s and a soil layer thickness of less than 10 m. Stiff soil sites feature a soil layer that is 15-75 m deep on top of a layer with a V\({}_{\text{S}}\) > 800 m/s. Deep soil locations have an overlaying soil layer that is more than 100 m deep. The second key reason why we use these classifications is that many recent regional microzonation research studies [23; 24; 25; 26; 27], which used GMPEs that take into account deep geology and are based only on the data recorded in former Yugoslavia, demonstrated that the resulting empirical predictions were in excellent agreement with newer regional ground motion records and observed macro-seismic intensities.