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Scenario-dependent site effects for the determination of unbiased local magnitude

James Holt, Benjamin Edwards and Valerio Poggi
Scenario-dependent site effects for the determination of unbiased local magnitude
Bulletin of the Seismological Society of America (October 2019) 109 (6): 2658-2673

Abstract

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


ISSN: 0037-1106
EISSN: 1943-3573
Coden: BSSAAP
Serial Title: Bulletin of the Seismological Society of America
Serial Volume: 109
Serial Issue: 6
Title: Scenario-dependent site effects for the determination of unbiased local magnitude
Affiliation: University of Liverpool, Department of Earth, Ocean and Ecological Sciences, Liverpool, United Kingdom
Pages: 2658-2673
Published: 20191022
Text Language: English
Publisher: Seismological Society of America, Berkeley, CA, United States
References: 52
Accession Number: 2019-093029
Categories: Seismology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. sketch maps
N30°00'00" - N45°00'00", E129°00'00" - E147°00'00"
Secondary Affiliation: National Institute of Oceanography and Applied Geophysics, ITA, Italy
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute. Abstract, Copyright, Seismological Society of America. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201949
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