Toward Robust and Routine Determination of Mw for Small Earthquakes: Application to the 2020 Mw 5.7 Magna, Utah, Seismic Sequence

To better characterize seismic hazard, particularly, for induced seismicity, there is an increasing interest in methods to estimate moment magnitude (⁠$Mw$⁠) for small earthquakes. $Mw$ is generally preferred over other magnitude types, but, it is difficult to estimate $Mw$ for earthquakes with local magnitude (⁠$ML$⁠) $<3–3.5$⁠, using conventional moment tensor (MT) inversion. The 2020 $Mww$ 5.7 Magna, Utah, seismic sequence provides an opportunity to illustrate and evaluate the value of spectral methods for this purpose. Starting with a high‐quality seismic catalog of 2103 earthquakes (⁠$ML<5.6$⁠), we estimate $Mw$ using two independent spectral methods—one based on direct waves, yielding $Mw,direct$⁠, and the other based on coda waves, yielding $Mw,coda$⁠. For the direct‐wave method, we present a non‐parametric (NP) inversion scheme that solves for apparent geometrical spreading, G(R), and site effects (S), similar to other NP procedures that have been used to calibrate regional $ML$ scales. The NP inversion is constrained using $Mw$s derived from MTs for nine events in the Magna sequence. We recover statistically robust and physically reasonable G(R) and S and compute $Mw,direct$ for 635 Magna earthquakes down to $ML$ 0.7. For the coda‐wave method, we consider two separate calibration schemes involving previous MT solutions and compute $Mw,coda$ for 311 earthquakes down to $ML$ 1.0. For 280 of the events that were processed with both methods—$Mw,direct$ and $Mw,coda$—are strongly correlated (r = 0.98), with a mean difference of only 0.05. We compare $Mw,direct$ and $Mw,coda$ with $ML$ and find reasonably good agreement for $ML<3.6$ with the theoretically predicted relationship of $Mw=(2/3)ML+C$⁠, in which $C$ is a regional constant. Our results imply that seismic network operators can use spectral‐based $Mw$ estimates to replace $ML$ estimates for events with $ML≥1.0$⁠, and possibly smaller. The main requirement is the existence of a small number of MT solutions for calibration purposes.