Seismic amplification of Late Quaternary paleovalley systems: 2D seismic response analysis of the Pescara paleovalley (Central Italy)

Abstract

Robust site characterization and ground response analysis require a thorough understanding of subsurface features, including geophysical properties and geometries of sediment bodies. Late Quaternary paleovalley systems, often overlooked in seismic hazard assessments, represent a potential threat due to their unconsolidated infill (with shear wave velocities <200 m/s) and sharp contrast with the adjacent substrate. Through an integrated approach that combined geophysical and stratigraphic data, we characterized the subsurface of the Pescara paleovalley system. Geostatistical interpolation of microtremor measurements enabled mapping resonance frequencies, highlighting abrupt changes and delineating the paleovalley boundaries. High-resolution core descriptions were then correlated with resonance frequencies, enabling the reconstruction of a 3D geophysical depth model of the buried paleovalley morphology. Furthermore, analyzing velocity profiles from down-hole tests led to the identification of five main seismic/stratigraphic layers within the valley fill. The geometry and facies architecture were reconstructed through a cross-section transversal to the paleovalley axis and then implemented into a 2D finite element model. Seismic response was computed, revealing significant amplification factors at frequencies closely matching the direct observations. Amplification factors peaked at frequencies between 0.9 and 1.1 Hz in the paleovalley center and up to 5.5 Hz towards the flanks, reaching a factor of 4.6. These findings suggest a notable increase in amplification amplitude compared to simpler geological contexts and emphasize the potential impact on common building types. Response spectra show strong amplifications in the paleovalley system, potentially leading to an underestimation of spectral accelerations compared to NTC18 guidelines. The comparisons of 1D and 2D modeling approaches revealed minimal differences, indicating that the generally flat geometry of the valley may not exhibit clear 2D effects. However, local subsurface stratigraphy strongly influences lateral changes in seismic response, emphasizing the importance of detailed subsurface knowledge for realistic seismic response estimates.