Subsurface basin structure, seismic site conditions and earthquake damage: case study of the Gölbaşi basin, Türkiye

Abstract

Gölbaşı (Adıyaman) Town, located within a basin along a left-stepping segment of the East Anatolian Fault Zone in Türkiye, experienced severe damage during the 6 February 2023 Mw 7.8 and Mw 7.6 Kahramanmaraş earthquakes. Widespread liquefaction, ground subsidence, lateral spreading, bearing capacity failures and building collapses occurred, with their spatial distribution suggesting a strong association between basin geometry and ground motion amplification. This study investigates the subsurface geometry and sediment thickness of the Gölbaşı Basin using an integrated approach combining microgravity measurements and Spatial Autocorrelation (SPAC) analyses. A total of 120 microgravity measurements were conducted to delineate the lateral and vertical extent of low-density alluvial deposits, and two-dimensional gravity models were constructed along four profiles. Additionally, SPAC measurements at 20 locations yielded deep shear-wave velocity (Vs) profiles, from which Vs30 values were derived. Basin-depth proxies (Z 1.0 and Z 2.5 ) were subsequently estimated from Vs30 using empirical relationships. The basin contains up to approximately 700 m of unconsolidated sediments. Variations in sediment thickness and basin geometry produce significant lateral heterogeneity in seismic velocity and impedance contrasts across the basin. Areas underlain by the thickest sediments experienced the most severe structural damage. The results demonstrate a strong spatial association between sediment thickness, basin geometry, seismic site conditions, and the observed distribution of structural damage. A neighborhood-based Pearson correlation analysis revealed a very strong positive relationship between the severe-damage ratio and sediment thickness ( r  = 0.905, p  < 0.001), a strong positive correlation with Z 2.5 ( r  = 0.763, p  = 0.017) and a strong negative correlation with Vs30 ( r  = − 0.698, p  = 0.037). Although the correlations with Z 1.0 and the gravity anomaly did not reach the 5% significance level, they exhibited similarly strong spatial trends. Given the limited number of neighborhoods analyzed ( n  = 9), these results are interpreted as exploratory spatial associations rather than evidence of causal relationships. Nevertheless, the agreement among the gravity-derived basin geometry, SPAC-derived site parameters, and the damage analysis indicates that sediment thickness and subsurface basin structure were significant contributors to the spatial distribution of earthquake damage.