Which is a better proxy, site period or depth to bedrock, in modelling linear site response in addition to the average shear-wave velocity?

This study aims to identify the best-performing site characterization proxy alternative and complementary to the conventional 30 m average shear-wave velocity V S 30 , as well as the optimal combination of proxies in characterizing linear site response. Investigated proxies include T 0 (site fundamental period obtained from earthquake horizontal-to-vertical spectral ratios), V Sz (measured average shear-wave velocities to depth z , z = 5, 10, 20 and 30 m), Z 0.8 and Z 1.0 (measured site depths to layers having shear-wave velocity 0.8 and 1.0 km/s, respectively), as well as Z x - infer (inferred site depths from a regional velocity model, x = 0.8 and 1.0, 1.5 and 2.5 km/s). To evaluate the performance of a site proxy or a combination, a total of 1840 surface-borehole recordings is selected from KiK-net database. Site amplifications are derived using surface-to-borehole response-, Fourier- and cross-spectral ratio techniques and then are compared across approaches. Next, the efficacies of 7 single-proxies and 11 proxy-pairs are quantified based on the site-to-site standard deviation of amplification residuals of observation about prediction using the proxy or the pair. Our results show that T 0 is the best-performing single-proxy among T 0 , Z 0.8 , Z 1.0 and V Sz . Meanwhile, T 0 is also the best-performing proxy among T 0 , Z 0.8 , Z 1.0 and Z x - infer complementary to V S 30 in accounting for the residual amplification after V S 30 -correction. Besides, T 0 alone can capture most of the site effects and should be utilized as the primary site indicator. Though ( T 0 , V S 30 ) is the best-performing proxy pair among ( V S 30 , T 0 ), ( V S 30 , Z 0.8 ), ( V S 30 , Z 1.0 ), ( V S 30 , Z x - infer ) and ( T 0 , V Sz ), it is only slightly better than ( T 0 , V S 20 ). Considering both efficacy and engineering utility, the combination of T 0 (primary) and V S 20 (secondary) is recommended. Further study is needed to test the performances of various proxies on sites in deep sedimentary basins.