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

Abstract

New seismic stations added to a regional seismic network cannot be used to calculate local magnitude ( ⁠ M L "> 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 ( ⁠ d M L "> d M L ⁠ ; Uhrhammer et al. , 2011 ) are then calculated after inverting for a new regional amplitude decay function, constrained by the sum of d M L "> d M 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 "> V S ⁠ ) of the upper 30 m ( ⁠ V S 30 "> V S 30 ⁠ ) and the site dominant frequency ( ⁠ f d "> f d ⁠ ) to calculate d M L "> d M L ⁠ ⁠ . Previously established d M L "> d M L ⁠ ⁠ , measured V S 30 "> V S 30 ⁠ ⁠ , and f d "> 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; d M L "> d M L ⁠ d M L "> data range from −1.10 to 0.39, V S 30 "> V S 30 ⁠ values range from 202 to 1464 &#x2009;&#x2009; m / s "> 1464 m / s ⁠ , and 440 f d "> f d values are compiled from earthquake and microseismic records that range from 0.13 to 21 Hz. We find V S 30 "> V S 30 ⁠ and d M L "> d M L ⁠ exhibit a positive coefficient of determination ( ⁠ R = 0.59 "> R = 0.59 ⁠ ), indicating that as V S 30 "> V S 30 ⁠ increases, d M L "> d M L ⁠ increases. This implies that greater site amplification (lower V S 30 "> V S 30 ⁠ ⁠ ) results in smaller d M L "> d M L ⁠ ⁠ . f d "> f d and d M L "> d M L ⁠ also generally exhibit a positive correlation ( ⁠ R 2 &lt; 0.56 "> R 2 < 0.56 ⁠ ), which implies lower d M L "> d M L values are related to site resonance at depth‐dependent frequencies. Using the developed relationships, V S 30 "> V S 30 ⁠ or f d "> f d measurements can be used to establish a provisional d M L "> d M L ⁠ for newly established stations. This procedure allows new stations to contribute to regional network M L "> M L determinations immediately without the need to wait until a minimum set of earthquake data has been recorded.