No abstract

This study determines site-response factors that can be applied as corrections to a rock-attenuation relationship for use in probabilistic seismic-hazard analysis. The site-response factors are amplitude and site-class dependent. These amplification factors are determined by averaging ratios between observed and predicted ground motions for peak ground acceleration (PGA) and for 5% damped response spectral acceleration at 0.3, 1.0, and 3.0 sec oscillator periods. The observations come from the strong-motion database of the Southern California Earthquake Center (SCEC), and the predictions are based on the Sadigh (1993) rock-attenuation relation. When separated and averaged according to surface geology, significantly different site-response factors are found for Quaternary and Mesozoic units, but a subclassification of Quaternary is generally not justified by the data. The low inputmotion amplification factors are consistent with those obtained from independent aftershock studies at the PGA and 0.3-second period. An observed trend of decreasing Quaternary site amplification with higher input motion is consistent with nonlinear soil behavior; however, the trend exists for Mesozoic sites as well, implying that this may be an artifact of the Sadigh relationship. There is a correlation between larger site-response factors and lower average shear-wave velocity in the upper 30 m for low predicted PGA input motions, with an increase in the correlation with increasing period. The 0.3-sec site response factors for Quaternary data in southern California determined in this study are consistent with 0.3-sec NEHRP site-response correction factors; however, at 1.0-sec period some inconsistencies remain. A trend is also seen with respect to sediment basin depth, where deeper sites have higher average siteresponse factors. These results constitute a customized attenuation relationship for southern California. The implication of these customized attenuation models with respect to probabilistic hazard analysis is examined in Field and Petersen (2000).

Weak-motion geotechnical array recordings at 38 stations of the Japanese strong-motion network KiK-Net from the 2003 M w 7:0 Miyagi-Oki aftershock sequence are used here to quantify the amplification and attenuation effects of nearsurface formations to incident seismic motion. Initially, a seismic waveform optimization algorithm is implemented for the evaluation of high-resolution, low-strain velocity (V s ), attenuation (Q s ), and density (ρ) profiles at the sites of interest. Based on the inversion results, V s versus Q s correlations are developed, and scattering versus intrinsic attenuation effects are accounted for in their physical interpretation. Surfaceto-downhole traditional spectral ratios (SSR), cross-spectral ratios (c-SSR), and horizontal-to-vertical (H/V) site-response estimates are next evaluated and compared, while their effectiveness is assessed as a function of the site conditions classified on the basis of the weighted average V s of the upper 30 m (V s30 ) of the formations. Single and reference-station site-response estimates are successively compared to surface-to-rock outcrop amplification spectra and are evaluated by deconvolution of the downhole records based on the inversion results; comparison of the observed SSR and estimated surface-to-rock outcrop amplification spectra illustrates the effects of destructive interference of downgoing waves at the downhole instrument level as a function of the site class. Site amplification factors are successively computed in reference to the National Earthquake Hazards Reduction Program (NEHRP) B-C boundary site conditions (V s30 760 m=sec), and results are compared to published values developed on the basis of strong-motion data and site-response analyses. Finally, weak-motion SSR estimates are compared to the mainshock spectra, and conclusions are drawn for the implications of soil nonlinearity in the near surface. Results presented in this article suggest that currently employed site classification criteria need to be reevaluated to ensure intraclass consistency in the assessment of amplification potentials and nonlinearity susceptibility of near-surficial soil formations.

Online Material: Tabulation of parameters of the finite source model for the M w 5.4 earthquake; tabulation of stress drops, including the event ID and other earthquake parameters.

in the southern part of the Walker Lane shear zone (Figure 1) were felt throughout southern California and produced a vigorous aftershock sequence. These events led to rapid deployments of seismic arrays across and around the Ridgecrest earthquake sequence (Catchings et al., 2020). Kinematic rupture processes of the Mw 6.4 and Mw 7.1 events, surface deformation, and properties of the aftershocks show complex patterns, with strong variations both along strike of the rupture zones and in depth (e.g.,