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

We explore the role of scenario-dependent site amplification on local magnitude (M-L) and possible bias it may introduce. M-L 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 M-L, relative differences between station-specific M-L at the surface and borehole (Delta M-L,M-STN) are studied for 34 sites from the KiK-net network, Japan. We find strong moment magnitude (M) dependent scenario-specific Delta M-L,M-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, Delta M-L,M-STN(M), based on the available site-response information are shown to closely match the empirical Delta M-L,M-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 Delta M-L,M-STN (M) in the absence of S/B. We find that Delta M-L,M-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 M-L at low magnitudes and how Delta M-L,M-STN(M) may be used in the future to compute unbiased M-L with greater confidence.