Scenario‐Dependent Site Effects for the Determination of Unbiased Local Magnitude

Abstract

AbstractWe explore the role of scenario‐dependent site amplification on local magnitude (ML) and possible bias it may introduce. ML is strongly influenced by local site response, which is conditioned by unique local geological factors. To isolate the effect of the near‐surface amplification on ML, relative differences between station‐specific ML at the surface and borehole (ΔML,STN) are studied for 34 sites from the KiK‐net network, Japan. We find strong moment magnitude (M) dependent scenario‐specific ΔML,STN trends over the range 3.0<M<6.5. To model these trends, we employ the stochastic method, initially using empirical surface‐to‐borehole (S/B) Fourier spectral ratios for the site term. Simulated data, ΔML,STN(M), based on the available site‐response information are shown to closely match the empirical ΔML,STN trends. Subsequently, the site term is replaced with (a) linear 1D shear‐wave (horizontal) transfer function (1D‐SHTF) amplification, (b) horizontal‐to‐vertical ratios, and (c) quarter wavelength amplification to calculate ΔML,STN(M) in the absence of S/B. We find that ΔML,STN(M) trends are best estimated with S/B as the site term, but in many cases using a linear 1D‐SHTF model is adequate. Furthermore, we discuss how this phenomenon may be related to the observed inequality between M and ML at low magnitudes and how ΔML,STN(M) may be used in the future to compute unbiased ML with greater confidence.