## 2 Study Area: Geological Features and Seismicity  
### Geological Features of the Zagreb Area  
The city of Zagreb (the Croatian capital) is situated in the contact area of three major regional tectonic units [16; 17]: the SE Alps, NW Dinarides, and Tisza Unit in the southwestern margin of the Pannonian Basin (Figure 1a). Interaction of the units causes earthquakes in the upper crust [18] that are distributed predominantly along the regional active faults ([19], Figure 1b). The NW verging reverse fault system striking along the Medvednica Mountains is probably related to a major seismogenic thrust fault [15] in the wider city of Zagreb region, and is dissected by active transversal strike-slip faults (Figure 1b). A system of south-dipping longitudinal reverse faults has been mapped on the surface along the Medvednica Mountains ([20; 21; 22], Figure 1c), and interpreted in one of the latest tectonic models as the youngest (Pliocene to Quaternary) inverse structure ([19], Figure 1c). A master thrust could represent a basal detachment (decollement) of the system of steeper reverse faults striking along the mountain range. Although the frontal part of the thrust could be steeply inclined towards the surface along the northern margin of the Medvednica Mountains, or it could be buried below the Neogene Hrvatsko Zagorje basin ([15], Figure 1c).  
Due to the Pliocene-Quaternary N-S directed shortening, Medvedrica Mountains experienced approx. up to 2000 m of differential uplift and now exposes pre-Neogene and Neogene tectonic and stratigraphic units that are surrounded by Pliocene-Quaternary sediments [23]. According to  
Figure 1: (**a**) Simplified tectonic map of the Alpine, Carpathian and Dinaric orogenic belts and the Pannonian basin. (**b**) Tectonostriagraphic map of the Medvedrica Mountains and surrounding areas near the Alpine-Dinaridic junction. (**c**) Longitudinal (**A**) and transversal (**B**) cross-sections of the Medvedrica Mountains (location on the Figure 1b). (from [19]). Location of seismological stations Puntijarka (PT)-on the top of Medvedrica Mountains) and Zagreb (ZAG-on the unconsolidated alluvial cover) are marked with yellow triangles.  
the DEM-based morphometric analysis the ongoing tectonic uplift in Medvedrica Mountains area is concentrated along the SW corner of the NW mountain front and along a narrow boundary zone that divides the Medvedrica Mountains into the NE and SW regions. According to [24] this area is considered to be tectonically active, with low uplift rates of approx. 0.023 mm/y which is in good agreement with the 0.17-0.4 mm/y Quaternary uplift rate reported by [25] for the central part of Medvedrica Mountains. During the recent rupture (March 2020) along a segment of an active NW verging thrust fault in the subsurface, the central part of the mountain was uplifted by up to 3 cm [15].  
The mountain is considered as uplifted bedrock, composed of Paleozoic and Triassic metamorphic and volcanic rocks in the core, obducted ofiolitic melange and overlying Cretaceous carbonate and flysch rocks. Miocene transgressive successions of the Pannonian basin unconformably cover the eroded Phanerozoic basement all around the mountain ([20; 21; 22], Figure 1c). Predominantly unconsolidated superficial Pliocene-Quaternary sediments (gravel, sand and clay) are up to 2000 m thick in the subsurface of Sava basin on the south and the Hrvatsko Zagorje basin on the north (Figure 1c), and pinch-out towards Medvedrica Mt. ([20; 21; 22]. Thus, the seismic station Puntijarka (PTJ)) is situated on the bedrock that can be considered as a firmly consolidated basement, while the seismic station Zagreb (ZAG) is situated on the Quaternary alluvial sediments of the Sava river that are mostly unconsolidated materials (Figure 1a).