Integrated geophysical and geotechnical characterization of accelerometer stations sites in Crete, Greece

Estimating strong motion parameters in shallow geological formations is crucial in seismic hazard assessment and earthquake engineering, which can be achieved through the development of Ground Motion Models (GMMs). These parameters provide essential information for designing earthquake-resistant structures and infrastructure. Accelerometer networks may cover diverse geological settings and therefore require comprehensive site characterization for reliable ground motion recordings. In this study, Crete Island in Greece was selected as a region prone to seismic hazards for the subsurface characterization of 13 accelerometer sites located along the island by integrating surface and borehole geological and geophysical data from Spectral and Multichannel Surface Waves Analysis (SASW & MASW), Refraction Microtremor (ReMi or MAM-Microtremor Array Measurement), Seismic Refraction Tomography and Electrical Resistivity Tomography (SRT & ERT). This research resulted in the estimation of several geophysical and geotechnical parameters, such as the P and S-wave velocity, the Young's modulus, the Poisson's ratio and the maximum shear modulus. Based on these parameters, engineers can estimate strong motion intensity measures on the surface, using corresponding values on engineering bedrock. The resulting velocity and impedance models provide physically based constraints on site amplification, high-frequency attenuation, and basin effects, supporting future development of site-specific and non-ergodic GMMs. Thus, the dynamic properties of shallow geological formations may rationally serve in understanding effects of subsurface complexity structure. In addition, strong ground motion recordings in the characterized stations may be effectively utilized for hosting to target sites (Host To Target Adjustment: HTTA). The area investigated comprises a variety of geological and tectonic settings, making it an ideal location for investigating the effects of different geological conditions on ground motion intensity measures. This study provides a methodological example for the refinement of the geological model of a site by integrating surface high-resolution geological mapping, geophysical imaging, and borehole validation. The findings of this work indicate that the methods used in this study contributed to the mapping of different physical properties in different depth ranges. MASW and SRT provided the shallow S- and P-wave velocity structure, respectively, which affects the high-frequency ground motion, while Ambient Vibration Arrays (AVA) mapped deeper seismic impedance contrasts that control long-period seismic response. ERT is a valuable tool for estimating a formation's lithology and heterogeneity, but it cannot directly quantify its mechanical properties. Finally, borehole and in-situ geotechnical tests provided high vertical resolution information and guided geophysical data interpretation. By the integration of these complementary methods, we achieved the evaluation of a scale-consistent and realistic site subsurface model.