GEOPHYSICAL RECONSTRUCTION OF BEDROCK DEPTH AT THE LARGE SINKHOLE IN THE 2020 PETRINJA EARTHQUAKE

Abstract

This paper shows the results of bedrock depth analysis around the largest cover-collapse sinkhole that occurred during the 2020-2021 Petrinja earthquake sequence. Horizontal to Vertical Spectral Ratio (HVSR) data was collected by the Geotechnical Extreme Events Reconnaissance (GEER) team after the Mw 6.4 December 2020 Petrinja earthquake in Croatia. In addition, the GEER team collected other data to assess the damage and geologic conditions, including two geotechnical boreholes with field and laboratory data and Multichannel Analysis of Surface Waves (MASW) profiles. Out of 61 HVSR readings performed during reconnaissance, 15 are around the largest sinkhole, S001, about 25 m wide and 12 m deep, with vertical walls and groundwater. The soil in the area consists of a clayey cover that is 4.0 m to 10.0 m thick, with sporadic gravel lenses. Clays are mostly over-consolidated, with varying degrees of saturation with intensely karstified carbonate rocks underneath. The HVSR data was analyzed using the HVSRweb platform and associated Python-based modules incorporating various statistical assessment models include single azimuth, multiple-azimuth, and geometric mean. The geometric mean results based on resampling frequencies between 3 Hz and 10 Hz indicate karst depths between 12.0 m and 18.0 m, which is generally consistent with the bedrock assumed from the sinkhole depth. Furthermore, an evaluation of the spatial variability of the resonance frequencies and the corresponding depth estimates assesses the presence and orientation of karstic features around S001. Based on the assessed data, HVSR measurements appear to be a helpful tool for evaluating variations in subsurface impedance contrasts and can be used to augment geotechnical data and other geophysical measurement techniques due to the relative ease of deployment and rapid data acquisition.