Surface-to-Bedrock Imaging of the Sakarya Basin Based on Integrated Analysis of Ambient Noise and Seismic Data

Recent earthquakes in Türkiye, including the 2020 Samos and 2023 Kahramanmaraş events, have once again underscored the significant impact of local ground conditions and the three-dimensional structure of alluvial basins on earthquake ground motion. In such settings, seismic wave propagation is strongly controlled by basin geometry, sediment thickness, and shear-wave velocity (Vs) contrasts, which can significantly affect ground motion characteristics and increase uncertainties in seismic hazard assessments. For this reason, generating reliable and spatially detailed Vs models at the basin scale has become increasingly important.

The Sakarya Basin, located within the active tectonic framework of the North Anatolian Fault Zone, represents a suitable case study due to its young alluvial deposits and high seismic potential. In this study, the shear-wave velocity structure of the basin is investigated from the surface down to the engineering bedrock through an integrated analysis of ambient noise and seismic data, combining both active and passive seismic methods. Field investigations comprise 533 MASW and ReMi measurements, including 316 newly acquired sites, providing dense coverage of the shallow subsurface. To constrain deeper velocity structures, ambient noise array recordings collected at 61 locations were analyzed using the Spatial Autocorrelation (SPAC) method. The resulting one-dimensional Vs profiles were interpreted together with existing geological and geophysical information and integrated within a GIS-based framework to construct a coherent surface-to-bedrock shear-wave velocity model of the Sakarya Basin.

The resulting model reveals extensive low-velocity sedimentary zones that can delay seismic wave propagation and lead to ground motion amplification within specific frequency ranges. These observations improve the understanding of basin-related site effects and support the identification of areas that may be more vulnerable to seismic amplification. The main contribution of this study lies in the basin-scale integration of high-density active seismic measurements with SPAC-derived ambient noise data, enabling surface-to-bedrock imaging with a level of spatial resolution not previously available for the Sakarya Basin. The resulting Vs model provides an improved representation of both shallow and deep subsurface conditions, offering valuable insights for site classification and basin-related ground motion studies. This research was conducted within the scope of the TÜBİTAK-funded project no. 124Y188.