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Beyond Site Response: On the Importance of Installation Depth on the Quality of Seismic Recordings - Example of Measurements Carried Out at Epos-France Seismological Network Stations

Description: 

Recording ground motions is essential to estimate the seismic hazard. Signals from the databases are often considered to be in free-field, i.e. at the natural surface of the ground, without disturbances due to buildings or neighboring structures. Nevertheless, analysis of the seismological networks has highlighted the coexistence of numerous installation methods, and in particular the depth of installation (station installed in post-hole, in tunnel, in the basement of buildings…). We present results obtained at several sites, mainly Epos-France seismological stations, belonging to the French accelerometric (RAP) and broad-band (RLBP) networks.

At these sites, we set up temporary surface stations placed vertically above the deep permanent stations. We then calculated SSR ratios on signal segments corresponding to earthquakes. We observed strong signal de-amplifications at certain frequencies due to destructive interference between upgoing and downgoing waves. These effects are visible at high frequencies from very shallow depths (sometimes even less than one meter in the case of soft soils). We discuss the importance of these effects on the use of depth-recorded signals for ground motion parameters and magnitude estimation. We also draw attention to the disadvantages of using these stations as reference stations if no corrections are made (e.g. as part of the application of the generalized inversion method - GIT).

Session: From Earthquake Recordings to Empirical Ground-Motion Modelling - I

Type: Oral

Date: 5/2/2024

Presentation Time: 08:30 AM (local time)

Presenting Author: Fabrice

Student Presenter: No

Invited Presentation: 

Authors