Reconstructing Late Quaternary Paleovalley Systems of Italy Through mHVSR: A Tool for Seismic Hazard Assessment in Modern Coastal Lowlands

Effective site characterization in highly urbanized coastal lowlands requires accurate stratigraphic
and geophysical investigations. In these regions, which typically host shallowly buried paleovalley systems
formed in response to Quaternary glacio-eustatic fluctuations, the marked lithologic contrast between soft
sediment paleovalley fills and the adjacent, stiff substrate has the potential to modify earthquake motions,
and assessment of critical parameters, such as shear wave velocities (VS) and resonance frequencies (f),
should be coupled with detailed stratigraphic architecture. To evaluate the potential of the microtremor
horizontal-to-vertical spectral ratio (mHVSR) for paleovalley recognition and mapping, we performed
mHVSR measurements along the Adriatic coastal plain of Italy, where two paleovalley systems (Pescara and
Manfredonia) have been recently identified. In both areas, we detected rapid lateral variations in resonance
frequencies and highlighted laterally continuous impedance contrasts. Relying on a robust stratigraphic
framework, we carefully evaluated the relation between geological and geophysical data and identified the
stratigraphic surfaces responsible for the observed resonances. We derived VS models for the sediment fill,
reconstructing the geometry of the two buried paleovalleys. We address the importance of evaluating the
geological context when designing microzonation studies, for a reliable interpretation of changes in resonance
frequencies.