| Description |
Magnitudes scales are the principal measure of earthquake size and the moment magnitude (Mw) is the gold standard as it is directly tied to the physics of earthquake rupture. However, it is difficult to obtain reliable Mw estimates for small earthquakes (M < 4), which constitute the vast majority of global seismicity. Thus, less physically based magnitude scales that are biased relative to Mw are employed for estimating the size of small earthquakes, leading inconsistencies in earthquake catalogs, seismic hazard mapping, and induced seismicity monitoring. An established method for estimating Mw for small events is through coda envelope analysis. The technique uses an empirical calibration that models earthquake coda energy producing moment-rate spectra which can then be used to measure Mw. I apply this technique using the Coda Calibration Tool (CCT) to all earthquakes occurring in Utah from the last seven months of 2021. The CCT involves a time-intensive manual revision of automatically-picked data envelopes which is a barrier to routine coda Mw calculation at University of Utah Seismograph Stations (UUSS). Using this 2021 Utah dataset, I compare Mws computed from the CCT automated picks to Mws calculated from manually revised picks and I find that the Mws are consistent on most, but not all, of the dataset. I also compare the Mws from manual picks in CCT with the UUSS cataloged local magnitudes (ML). I find that in the ML comparison, there are two trends in the data: one for deep events and another for shallow events. The deeper events have a linear trend between Mw and ML that I model using an orthogonal regression while the shallow events appear to overestimate Mw. I propose a static offset from the deeper-event linear model that appears to fit the shallow event data. I then reexamine the comparison between MWs calculated from automated and manually revised picks using only the deeper events from the ML comparison and find that the automatic picks from CCT calculate Mws consistent with the Mws calculated from the manual picks. This result has promising implications for routine, rapid estimation of unbiased and accurate coda Mws in Utah. |
