V S 30 and Dominant Site Frequency (fd) as Provisional Station ML Corrections (dML) in California

Abstract

ABSTRACT New seismic stations added to a regional seismic network cannot be used to calculate local magnitude (ML) 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 (dML; Uhrhammer et al., 2011) are then calculated after inverting for a new regional amplitude decay function, constrained by the sum of dML 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 (VS) of the upper 30 m (VS30) and the site dominant frequency (fd) to calculate dML. Previously established dML, measured VS30, and fd 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; dML data range from −1.10 to 0.39, VS30 values range from 202 to 1464  m/s, and 440 fd values are compiled from earthquake and microseismic records that range from 0.13 to 21 Hz. We find VS30 and dML exhibit a positive coefficient of determination (R=0.59), indicating that as VS30 increases, dML increases. This implies that greater site amplification (lower VS30) results in smaller dML. fd and dML also generally exhibit a positive correlation (R2<0.56), which implies lower dML values are related to site resonance at depth-dependent frequencies. Using the developed relationships, VS30 or fd measurements can be used to establish a provisional dML for newly established stations. This procedure allows new stations to contribute to regional network ML determinations immediately without the need to wait until a minimum set of earthquake data has been recorded.