SCEC Award Number 21167 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Seismic Moment and Corner Frequency of Ridgecrest Earthquakes Determined with Two Novel Methods
Name Organization
William Ellsworth Stanford University
Other Participants Postdoctoral research associate to be named later
SCEC Priorities 1d, 1e, 2d SCEC Groups FARM, Seismology, EFP
Report Due Date 03/15/2022 Date Report Submitted 03/16/2022
Project Abstract
This project is part of the Community Stress Drop Validation Study. We are developing and testing a new method for retrieval of source information from time domain amplitude measurements. The work is being done in coordination with analysis made using spectral methods for calibration, testing and validation. The method is inspired by the simplicity and durability of local magnitude (Ml), which despite its well-understood limitations continues to be the primary measure of earthquake size for small to moderate earthquakes in many regions today. We derive a measure of seismic moment and source duration from narrow band measurements of the peak displacement amplitude in the time domain. This requires correction for propagation, just as Richter's Ml scale uses a calibrated "attenuation" relationship. To date, we have calibrated the narrow band attenuation curves for the broader Ridgecrest region and successfully recovered moment values for earthquakes, including the theoretically predicted departure of linear scaling between Ml and Mw. Work is now shifting to determination of the appropriate narrow band filters for accurate recovery of the source corner frequency.
Intellectual Merit This work, while still in progress, already indicates that simple and easy-to-implement procedures for measuring seismic moment can significantly enhance the quantitative value of earthquake catalogs. The nonlinearlity of the relationship between Ml and Mw complicates almost every use of magnitude-frequency statistics. Rather than ignore it, as is commonly done, a path for eliminating it appears feasible.
Broader Impacts Rapid and accurate assessment of the strength of an earthquake appears to be within reach for almost any earthquake using the simple and straightforward methodology we are developing. The method has clear application to earthquake early warning and immediate post-event response, for one, where accurate knowledge of the moment magnitude is needed to map the extent of strong ground motion for both warning and response purposes.
Exemplary Figure Figure 5. Left: Comparison between Trugman’s seismic moment measurements and those determined in this study. Right: Comparison between catalog ML and determined in this study from the calibrated seismic moments.

The significant result is shown in the right panel, comparing our rapid, time domain measurement of moment magnitude (Mw) to local magnitude (Ml). The break in scaling around Ml = 3 is expected and a consequence of the corner frequency of the earthquake moving through the instrumental corner of the Wood-Anderson seismograph response.