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

摘要

To better characterize seismic hazard, particularly, for induced seismicity, there is an increasing interest in methods to estimate moment magnitude (M-w) for small earthquakes. M-w is generally preferred over other magnitude types, but, it is difficult to estimateMw for earthquakes with localmagnitude (M-L) < 3-3:5, using conventionalmoment tensor (MT) inversion. The 2020 M-ww 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 (M-L < 5.6), we estimate M-w using two independent spectral methods-one based on direct waves, yielding M-w;direct, and the other based on coda waves, yielding M-w,M-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 M-L scales. The NP inversion is constrained using M(w)s derived from MTs for nine events in the Magna sequence. We recover statistically robust and physically reasonable G(R) and S and compute M-w,M-direct for 635 Magna earthquakes down to M-L 0.7. For the coda-wave method, we consider two separate calibration schemes involving previous MTsolutions and computeMw; coda for 311 earthquakes down to M-L 1.0. For 280 of the events that were processed with both methods-Mw; direct and M-w,M-coda-are strongly correlated (r = 0.98), with a mean difference of only 0.05. We compareMw; direct and M-w,M-coda withML and find reasonably good agreement for M-L < 3.6with the theoretically predicted relationship of M-w = (2/3)M-L + C, in which C is a regional constant. Our results imply that seismic network operators can use spectral-based Mw estimates to replace M-L estimates for events with M-L >= 1.0, and possibly smaller. The main requirement is the existence of a small number of MT solutions for calibration purposes.