Toward Improved Seismic Hazard Analysis Using Spectral Modeling to Estimate the Moment Magnitudes of Small Earthquakes

Computing magnitudes is a key part of characterizing earthquakes. They are especially important for developing the magnitude–frequency distributions that are used to forecast shaking from future large earthquakes. Although dozens of magnitude scales have been proposed, moment magnitude (Mw) is preferred because it does not saturate for large events and it is related to physical source properties such as fault area. However, it is challenging to resolve Mw for small earthquakes (Mw<3.5), which dominate regional earthquake catalogs, and seismic networks typically report regional magnitudes (e.g., mbLg, ML, Mc, and Md). Therefore, it is common to develop simple relations that convert regional magnitudes into Mw before calculating magnitude–frequency distributions. Conversion relations can add bias and variance to seismic hazard analysis, and ideally Mw would be calculated directly for all earthquakes. To address this issue, we applied a spectral modeling framework to estimate Mw from Sg waves recorded at distances of 10–300 km for 25,712 natural tectonic earthquakes that occurred in or near Utah between 1 January 2001 and 30 June 2025. The procedure is simple, fast, and robust, and can be run automatically in near-real time, thus potentially contributing to earthquake response products. Refinements to time windows or waveform selection can be implemented during analyst review, similar to the existing workflow for ML or Mc. We determined high-quality moment magnitudes for 63% of the target earthquakes, increasing the number of Utah events with an Mw by a factor of ∼170. The resulting Mw catalog contains 16,173 values ranging from 0.54 to 5.54 with a median of 2.06. A declustered version has a magnitude of completeness of 2.2 and a b value of 1.14 ± 0.02. Its large size will enable improved resolution of spatial and temporal variations in b value, which may allow inference of crustal stress variations in Utah.