Characterizing Subsoil Seismic Behavior: A Case Study of Tbilisi

Subsoil conditions are known to play a crucial role in modifying earthquake motions at the surface, affecting not only the amplitude but also the duration and frequency characteristics of seismic waves. Seismic microzonation studies, therefore, focus on identifying and mapping areas with homogeneous seismic responses to improve hazard assessment and mitigation efforts. 

In this study, we present the preliminary results of the first comprehensive site effects assessment conducted in different parts of Tbilisi, the Capital of Georgia. The analysis includes the characterization of site conditions based on the average shear wave velocity in the upper 30 meters (Vs30), dominant frequencies, and amplification factors. Shallow shear wave velocity (V_S30) is a critical parameter in seismic hazard assessment, as it significantly influences the amplification and frequency content of seismic ground motion. While Vs30 in Georgia has traditionally been estimated using refraction techniques, this work incorporates a suite of methodologies, including single-station analysis with borehole data, refraction, Multichannel Analysis of Surface Waves (MASW), and 2D (passive) array measurements. We compare the advantages and limitations of each method, highlighting their effectiveness in different geological and geophysical settings.

Additionally, we generated a resonance frequency distribution map and estimated amplification factors using standard acceleration response spectra as per the EC8 classification guidelines. These findings provide critical insights for improving the seismic resilience of urban infrastructure and serve as a baseline for future seismic hazard studies in the region.

Acknowledgment

This study has been funded by project FR-23-10514 of the SRNSF