Site Amplification during Strong Earthquakes Investigated by Vertical Array Records

A number of vertical array records during eight destructive earthquakes in Japan are utilized, after discussing criteria for desirable requirements of vertical arrays, to formulate seismic amplification between ground surface and outcrop base for seismic zonation. A correlation between peak spectrum amplification and Vs (S-wave velocity) ratio (base Vs/surface Vs) was found to clearly improve by using Vs in an equivalent surface layer wherein predominant frequency or first peak is exerted, though the currently used average Vs in top 30 m is also meaningful, correlating positively with the amplification. We also found that soil nonlinearity during strong earthquakes has only a marginal effect even in soft soil sites on the amplification between surface and outcrop base except for ultimate soil liquefaction failure, while strong nonlinearity clearly appears in the vertical array amplification between surface and downhole base. Its theoretical basis has been explained by a simple study on a two-layered system in terms of radiation damping and strain-dependent equivalent nonlinearity.

An Empirical Method for Estimating Source Vicinity Ground-Motion Levels on Hard Bedrock and Annual Exceedance Probabilities for Inland Crustal Earthquakes with Sources Difficult to Identify in Advance

ABSTRACT The locations and scales of the seismic sources of inland crustal earthquakes without surface fault traces (Mw≲6.5 in Japan) are difficult to identify in advance, even by conducting detailed surveys, and in such a case, it seems rational to uniformly evaluate ground-motion levels in the regions with similar seismogenic conditions. For such earthquakes, we first developed a technique to estimate ground-motion levels in a specific area by calculating the response spectra corresponding to nonexceedance probabilities (NEPs) based on probability density functions derived using strong-motion records. These records were used in the analysis after adjustments to the condition of being and on hard bedrock (VS≈2000–3000 m/s) in the source vicinity. Next, we developed an empirical method to estimate the correspondence between the NEP spectrum levels and their annual exceedance probabilities (AEPs) by considering annual occurrence frequencies for the target event group. Moreover, we showed an example that applied our approach to all over Japan, where a large number of downhole records on stiff baserock (VS≈700–3000 m/s) have been obtained by the KiK-net, a dense nationwide network of vertical array stations (pairs of surface and downhole recordings). In the example, we demonstrated that the empirical AEP spectral levels using our method are consistent with AEP response spectra, that is, uniform hazard spectra, derived from the probabilistic seismic hazard analysis using the kinematic fault rupture modeling method in a previous study.