## 2 Study Area: Geological Features and Seismicity
### Seismicity of the Zagreb Area  
Several seismic zones (Zagreb, Novo Mesto-Krsko, Karlovac-Metlika and Pokupsko-Petrinja) are situated on the wider Zagreb area as can be seen on Figure 2 (marked with coloured shadowing). The most important one is the Zagreb seismic zone which encompasses the Medvedrica Mountains and the city of Zagreb with its surrounding areas. Epicentre locations of the strongest earthquakes are mostly situated on the north-western slopes of the Medvedrica Mountains. The strongest earthquake was on 9 November 1880, close to Kasina, in the village Planina, and is known as the \"Great Zagreb earthquake\". This was the first Croatian earthquake for which the value of the focal depth (16 km) was determined based on macroseismic investigations. The analysis of the earthquake effects enabled the determination of the epicentral intensity (VIII \({}^{\circ}\)MCS). The earthquake itself is extremely well documented [26] due to extensive material damage. Of the 3670 buildings (Zagreb had around 30,000 inhabitants at the time) all were damaged and about 13% were destroyed. After 1900 the strongest event that happened in Zagreb area occurred on 22 March, 2020, 140 years after the \"Great Zagreb earthquake\" [15]. Its epicentre was 7 km north of the centre of Zagreb, in the vicinity of Markusevac and Cucerje, and the hypocentre was at a depth of 10 km. This M5.5 earthquake was felt with a maximum intensity of VII-VIII \({}^{\circ}\)MSK. Regarding other recent seismic events, prior to the Zagreb event of 2020 there was only a single strong earthquake in this area. The M5 event occurred in 1990, 3.5 km south of Kraljev Vrh, where the worst of the damage was observed. The strongest earthquakes in the Novo Mesto-Krsko seismic zone occurred mainly in the period up to 1900. The maximum intensity of these events did not exceed VIII \({}^{\circ}\)MCS. The strongest earthquakes (with the maximum intensity VIII \({}^{\circ}\)MCS) occurred in 1634 (Novo Mesto area) and 1917 (around Brezice). The strongest earthquake after 1900 was in 1974. It occurred approximately 15 kilometres north of Krsko with the magnitude of 5.1. Seismic zone Karlovac-Metlika encompasses the areas of Zumberacka gora and Samoborska gora together with Ozalj and Jastrebarsko. It is characterized by the occurrence of a large number of small earthquakes and smaller number of stronger ones. The majority of earthquakes (95%) that have occurred in this area have a magnitude of less than 3.0. Some authors have mentioned two historical earthquakes southeast of Metlika, within this seismic zone, with the intensity of IX \({}^{\circ}\)MCS, in 567 and 1097. However, the information that is available on these earthquakes is very unreliable. We will emphasize the earthquakes whose locations and intensities are known with a higher degree of reliability. These are two earthquakes of intensity VIII \({}^{\circ}\)MCS, in 1645, 6 km west of Ozalj and in 1699,with the epicentre in Metlika. In addition to these two events, four more occurred with intensities greater or equal to VII \"MCS (in 1645, 1881, 1887 and 1974). The locations of epicentres of strong earthquakes in the Pokupsko-Petrinja seismic zone stretch in the N-S direction. The strongest event here is one of the best-known earthquakes, not only in Croatia but also globally. It is the earthquake that occurred on 8 October, 1909, with the epicentre 9 km north of Pokupsko. Therefore, it is known as the Pokupsko earthquake. The magnitude of the earthquake was 5.8, with the focal depth of 14 km, and the effects that it caused were rated with a maximum intensity of VIII \"MCS. The Pokupsko earthquake is one of the few historical earthquakes that are regularly referred to in seismological textbooks and other books on the history of science due to it's study leading to important discoveries that have marked a turning point in the understanding of earthquakes and their effects.