Seismic and geological zonation of the part of the City of Zagreb area

Abstract

The area of the City of Zagreb is relatively large (~ 640 km 2 ) and urbanized (> 800,000 residents). The general geomorphological setting of the City is on the alluvial plane of the Sava River, on the southern slopes of Medvenica Mountain and on the northern slopes of Vukomeričke Gorice, i.e. hilly area. Within this area geohazard events occur, for example: numerous landslides during last decades, great flood in 1964 and on 22 nd March 2020 Zagreb was struck by an M5.5 earthquake. These events cause great damages and can endanger or even take lives. Seismic and geological zonation of the part of the City of Zagreb area (~ 175 km 2 on the southern slopes of Medvednica Mountain) was carried out as one of the geohazard mitigation measure. The zonation was financed by the City of Zagreb government and performed according to Eurocode 8 in the period of 2017-2019. The results of zonation were presented in Study where the geological, geotechnical, geophysical and seismic characteristics of the research area were compiled and addressed. The data sets were organized and presented in GIS project, i.e. in form which is easily usable by officials or public users. At the same time the Study contributes to better understanding of soil and rock properties of the research area and increases the available data and knowledge fund. The research results were also presented on developed Seismic zonation map in accordance with Eurocode 8 in scale of 1:25,000 where areas of equal soil amplification relative to the bedrock are depicted. The developed map can provide (thematic) basic seismic background info necessary for urban planning. Different thematic (geohazard) zonation maps are necessary in modern and quality urban development and they are prerequisite in development of quality hazard and risk management.

Semi-empirical estimation of the Zagreb ML 5.5 earthquake (2020) ground motion amplification by 1D equivalent linear site response analysis

“…Com parison between the predicted ground motion amplification factor of 1.8-2.2 with DAF from HVSR (for chosen earthquake scenario M = 6.0, D = 15 km, h = 10 km) along with the superimposed profile (Fig. 1; details in Miklin et al, 2019; sum marized in Padovan et al, 2021) shows similar pattern in site amplification. But here needs to be mentioned clear distinction between the quantitative discrep ancy of these two approaches, AF amplification (Herak et al, 2008) that is ap propriate for small scale perturbations like microtremors.…”

“…Dynamic Amplification Factor (DAF) map of the part of the Zagreb area (first described in Herak, 2008) was presented based on Horizontal-to-Vertical Spectral Ratio (HVSR) approximation (Sović et al, 2019). Padovan et al (2021) summarized all these reports. Main limitation of the Zagreb microzonation study is that estimation of the ground motion amplification based on the site response modelling was not done for different earthquake scenarios (deterministic and probabilistic seismic hazard analysis) within various steps involved in seismic microzonation (e.g., James et al, 2014).…”