Late Quaternary paleovalley systems detections through mHVSR technique: two case studies from the Adriatic coastal plain of Italy

Late Quaternary paleovalley systems are sedimentary bodies, tens of m thick and a few km wide, that are typically buried beneath modern deltas and coastal plains and that have no obvious geomorphological expression. Paleovalley systems are increasingly studied worldwide as they are considered possible amplifiers of earthquake damage due to the sharp contrast between their soft and unconsolidated sediment fill and the adjacent substrate. In this study, using previous high-resolution stratigraphic reconstructions of the Pescara and Manfredonia paleovalleys in the Adriatic coastal plain (Italy), we investigate the potential of the microtremor-based horizontal-to-vertical spectral ratio technique (mHVSR) to identify these sediment bodies in the subsurface. We acquired 23 microtremor measurements in the Pescara area and 54 at Manfredonia along two transects transversal to the paleovalley axes. At both sites, we were able to detect resonance peaks that we correlated with stratigraphic data. In the Pescara paleovalley system, we identified low-amplitude resonance peaks at frequencies varying between 0.9 and 4 Hz, clearly denoting a U-shaped feature with lower frequencies in the central part. At Manfredonia, the resonance peaks are more prominent (with mHVSR amplitude up to 7), and the paleovalley system is denoted by resonance frequencies between 0.9 and 2.5 Hz, with a more complex geometry shaped by the interactions of the Candelaro, Cervaro, and Carapelle rivers. Using the well-known facies architecture as a guide, we constrained the mHVSR resonance peaks to create a Frequency-Depth model and transformed the mHVSR curves from the frequency to the spatial domain to reconstruct paleovalley geometries and infer Vs models. We thus obtained the 2D models of the paleovalleys profiles. The Pescara mHVSR model shows a sedimentary cover thickness varying from 10 m (on the interfluves) to 40 m (in the depocentre). At Manfredonia, the sedimentary cover has similar thickness, in the range of 10-45 m, with variations that reflect its complex internal geometry. At both sites, paleovalley fills are characterized by low Vs velocities: comparable Vs values were obtained from the two depocentres (about 180 m/s at Pescara and 140 m/s at Manfredonia), which places paleovalley fills into the ground type D of the Standard Eurocode 8. The fundamental resonance frequencies show considerable variability along the investigated transects on very short distances (few hundred meters), in a range of frequencies that can interact with the most common building types. We mapped this variability and observed excellent correlation with the geologic cross-sections, proving the mHVSR to be an effective tool for mapping these particular sediment bodies. The 2D models obtained will serve as a basis for future seismic response simulations.