Correcting Biases from Velocity Structures in Moment Magnitude Estimates

Moment magnitude Mw is derived from seismic moment and was defined with the intent to be a consistent measure of earthquake size. Here, we demonstrate that reported Mw values depend systematically on the rigidity structure assumed in the moment tensor inversion, which varies between regions and users. We perform regional moment tensor inversions for 70 target earthquakes in the Walker Lane and southern California regions of the western United States, comparing magnitude results across a suite of commonly used 1D velocity models. Inversions performed with slower, lower rigidity velocity models produce lower Mw estimates than inversions performed with faster, higher rigidity velocity models: the epistemic uncertainty from the choice of velocity model averages ±0.08 for the analyzed earthquakes, with larger uncertainties for shallower events. Using magnitudes based on seismic potency (Mp) instead of Mw can reduce this uncertainty by a factor of 2–3. We introduce a potency-based magnitude Mpv that, by correcting for both the rigidity at source depth and characteristic rigidity of the velocity model, produces a robust magnitude scale that is independent of the velocity model. This formulation can be used to generate uniform and consistent regional and global earthquake catalogs necessary for a wide range of applications in earthquake science and engineering.