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Impact of Shallow Subsurface Stratigraphic Architecture on Shear-Wave Velocity Prediction: Examples From the Po Plain and Other Coastal Lowlands of Italy

Description: 

Detailed knowledge of shallow subsurface geology is essential to evaluate ground response analysis and seismic site characterization. However, for the investigation of the shallow velocity structure, stratigraphic architecture is often largely oversimplified. In this study, by adopting refined characterization of sediment cores, we portray a new modeling approach that can be applied to a wide range of coastal plains with no topographic relief, and used for reliable prediction of S-wave velocities at the scale of the whole sedimentary basin.

Within the uppermost 30 m, modern alluvial plains and coastal lowlands exhibit predictable facies architectures that reflect sedimentary evolution under global climate and sea-level changes. Following 25 years of research into the late Quaternary stratigraphy of the Po River plain, in Italy, we carried out the geophysical characterization of seven sedimentary units (lithofacies associations) that accumulated in distinct depositional environments (floodplain, swamp, fluvial channel, lagoon/bay, beach, shelf) or under particular conditions (paleosols). Individual lithofacies associations are typified by: (i) unique mechanical properties that cannot be captured by traditional engineering geology databases, (ii) distinctive (sheet-like, lenticular, or wedge-shaped) geometries, (iii) specific Vs values that can be estimated through Down-Hole measurements and calibration with high-resolution analysis of sediment cores. The thickness of unconsolidated Holocene deposits (and thus the position of the engineering bedrock) was observed to vary remarkably (10-55 m) from site to site, especially across the buried Adriatic paleovalleys, in a predictable manner. Using accurate three-dimensional models of facies architecture, we can predict the proportion of different lithologies along a stratigraphic column, and thus estimate shallow shear-wave velocity profiles at any point. The resulting Vs30 map enables precise soil classification that primarily reflects subsurface geology and that can be used effectively for seismic site effect evaluation and earthquake engineering design purposes.

Session: How Well Can We Predict Broadband Site-Specific Ground Motion and Its Spatial Variability So Far? -III

Type: Oral

Date: 5/1/2024

Presentation Time: 02:30 PM (local time)

Presenting Author: Alessandro

Student Presenter: No

Invited Presentation: 

Authors