VS30 and Dominant Site Frequency (⁠fd⁠) as Provisional Station ML Corrections (⁠dML⁠) in California

Bulletin of the Seismological Society of America
By: , and 

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Abstract

New seismic stations added to a regional seismic network cannot be used to calculate local magnitude (MLML) until a revised regionwide amplitude decay function is developed. Each station must record a minimum number of local and regional earthquakes that meet specific amplitude requirements prior to recalibration of the amplitude decay function. Station component adjustments (dMLdMLUhrhammer et al., 2011) are then calculated after inverting for a new regional amplitude decay function, constrained by the sum of dMLdML for long‐running stations. Therefore, there can be significant delay between when a new station starts contributing real‐time waveform packets and when data can be included in magnitude determinations. We propose the use of known estimates of seismic site conditions such as the time‐averaged shear‐wave velocity (VSVS) of the upper 30 m (VS30VS30) and the site dominant frequency (fdfd) to calculate dMLdML. Previously established dMLdML, measured VS30VS30, and fdfd data are available for between 126 and 458 horizontal components (east–west and north–south) at 81 seismic stations in the California Integrated Seismic Network; dMLdML data range from −1.10 to 0.39, VS30VS30 values range from 202 to 1464  m/s1464  m/s, and 440 fdfd values are compiled from earthquake and microseismic records that range from 0.13 to 21 Hz. We find VS30VS30 and dMLdML exhibit a positive coefficient of determination (R=0.59R=0.59), indicating that as VS30VS30 increases, dMLdML increases. This implies that greater site amplification (lower VS30VS30) results in smaller dMLdMLfdfd and dMLdML also generally exhibit a positive correlation (R2<0.56R2<0.56), which implies lower dMLdML values are related to site resonance at depth‐dependent frequencies. Using the developed relationships, VS30VS30 or fdfd measurements can be used to establish a provisional dMLdML for newly established stations. This procedure allows new stations to contribute to regional network MLML determinations immediately without the need to wait until a minimum set of earthquake data has been recorded.

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Additional publication details

Publication type Article
Publication Subtype Journal Article
Title VS30 and Dominant Site Frequency (⁠fd⁠) as Provisional Station ML Corrections (⁠dML⁠) in California
Series title Bulletin of the Seismological Society of America
DOI 10.1785/0120200130
Edition Online First
Year Published 2020
Language English
Publisher Seismological Society of America
Contributing office(s) Earthquake Science Center
Country United States
State California
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