On the Importance of the in Situ Characterization of Rock Stations: Differences Between Inferred and Measured Information From the French Accelerometric Network (RAP)

Date: 4/26/2019

Time: 06:00 PM

Room: Grand Ballroom

Most of the previous estimations of site conditions (soil class, V_S30) of the French accelerometric network (RAP) stations have been inferred on the basis of indirect information, as geological map interpretation.

Over the last years, geophysical surveys allowed performing in situ measurements of ~50 stations in very various geological contexts. These surveys consisted in surface wave-based methods involving both active (Multichannel Analysis of Surface Waves: MASW) and passive methods (Ambient Vibration Array: AVA) using array from 10 m up to 1 km of aperture. This acquisition layout allowed extracting shear wave velocity profiles from surface up to few hundreds of meters, providing at the same time a rather good vertical resolution within the shallow layers.

Unsurprisingly, the new measured V_S30 values are significantly lower in comparison with the previous inferred values. We discuss the difference between inferred and measured values and we try to understand the origin of this difference, introducing the nature of lithology (limestone, granite,…) but also using weathering and fracturing estimation from a priori information. We propose a ‘rescaling’ of methods used to infer value from indirect information. This allows proposing new inferred V_S30 value for stations that did not benefit so far from in situ characterization.

We also discuss about the high frequency amplification induced at most rock site due to the weathering of shallow layers and its impact on seismologic data analysis.

Presenting Author: Edward M Cushing

Authors