Towards a Better Understanding of κ0 at Bedrock Level
Session: How Well Can We Assess Site Effects So Far? I
Type: Oral
Date: 4/20/2021
Presentation Time: 10:30 AM Pacific
Description:
Site-specific hazard estimates adjust ground motion models from surface (and soft) outcrop rock to deep (and hard) bedrock conditions below the soil column where the soil column is used to compute the site-specific amplification. Such adjustments imply the knowledge of the high-frequency attenuation parameter κ0,bedrock at depth. Bedrock refers in this context to the rock below the soil column where shear-wave velocities are 1500-2500 m/s (defined differently by different authors) or higher.
In this study, we will try to answer two questions:
1. Can κ0,bedrock be obtained from surface recordings?
2. Is κ0,bedrock more related to the regional attenuation or is there still some influence of the site?
To address these issues we use data from Kik-net in Japan where each site is equipped with a surface and a borehole station. We select only deep sites where the shear-wave velocity at depth is 800 m/s or larger (soft rock to hard rock conditions).
Mayor et al. (2018) and Pilz et al. (2019) have shown that scattered coda waves might be independent of the local site response. We therefore compute κ on the coda wave and compare it to the S-wave κ at borehole depth. Furthermore, we directly infer the quality factor of coda waves in time domain (Qc) for comparison with κ0 of S-waves. Finally, we use the program Qopen from Eulenfeld and Wegler (2016) to measure the intrinsic and scattering quality factor at the surface and at depth.
Our results show that it is not possible to derive κ0,bedrock from surface recordings. The discrepancy between Q measurements and κ0 values, and a strong correlation between κ0 measured at the surface and at depth suggests that κ0,bedrock is more influenced by site-specific properties than by the regional attenuation structure. We finally discuss the implication of the site-dependency of κ0,bedrock for site-specific hazard assessment.
Presenting Author: Annabel Haendel
Student Presenter: No
Authors
Annabel Haendel Presenting Author Corresponding Author annabel.haendel@gmail.com GFZ German Research Centre for Geosciences |
Marco Pilz pilz@gfz-potsdam.de GFZ German Research Centre for Geosciences |
Fabrice Cotton fcotton@gfz-potsdam.de GFZ German Research Centre for Geosciences |
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Towards a Better Understanding of κ0 at Bedrock Level
Category
How Well Can We Assess Site Effects So Far?