V_(S30) and Dominant Site Frequency (⁠f_d⁠) as Provisional Station M_L Corrections (⁠dM_L⁠) in California

Abstract

New seismic stations added to a regional seismic network cannot be used to calculate local magnitude (⁠M_L⁠) 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 (⁠dM_L⁠; Uhrhammer et al., 2011) are then calculated after inverting for a new regional amplitude decay function, constrained by the sum of dM_L 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 (⁠V_S⁠) of the upper 30 m (⁠V_(S30)⁠) and the site dominant frequency (⁠f_d⁠) to calculate dM_L⁠. Previously established dM_L⁠, measured V_(S30)⁠, and f_d 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; dM_L data range from −1.10 to 0.39, V_(S30) values range from 202 to 1464  m/s⁠, and 440 f_d values are compiled from earthquake and microseismic records that range from 0.13 to 21 Hz. We find V_(S30) and dM_L exhibit a positive coefficient of determination (⁠R=0.59⁠), indicating that as V_(S30) increases, dM_L increases. This implies that greater site amplification (lower V_(S30)⁠) results in smaller dM_L⁠. f_d and dM_L also generally exhibit a positive correlation (⁠R²<0.56⁠), which implies lower dM_L values are related to site resonance at depth‐dependent frequencies. Using the developed relationships, V_(S30) or f_d measurements can be used to establish a provisional dM_L for newly established stations. This procedure allows new stations to contribute to regional network M_L determinations immediately without the need to wait until a minimum set of earthquake data has been recorded.

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