Robust Estimates of Moment Magnitude for Small- to Moderate-Size Aftershocks of the Mw 5.7 Magna, Utah Earthquake
Session: Intermountain West Earthquakes in the Spring of 2020 I
Type: Oral
Date: 4/20/2021
Presentation Time: 03:00 PM Pacific
Description:
To improve seismic hazard characterizations, particularly for induced seismicity, it is desirable to directly estimate moment magnitude (Mw) for small earthquakes. Mw is preferred over other magnitude types in most applications. However, Mw is challenging to obtain for earthquakes with local magnitude (ML) < 3–3.5 using conventional moment tensor (MT) inversion methods. The 2020 Mww 5.7 Magna, Utah, seismic sequence presents an ideal opportunity to illustrate and evaluate the use of spectral methods to determine Mw for small earthquakes. Using a catalog of 2103 earthquakes (ML<5.6), we estimate Mw with two independent spectral methods—one based on direct waves (Mw,direct), and the other based on coda waves (Mw,coda). For the direct-wave method,we present a non-parametric (NP) inversion scheme that solves for apparent geometrical spreading and site corrections. The NP inversion is constrained using Mws derived from MT inversion for nine Magna sequence earthquakes. We recover statistically robust and physically acceptable geometrical spreading and site corrections and compute Mw,direct for 635 earthquakes down to ML 0.7. For the coda-wave method, we considered two separate calibrations using different sets of Mws (from MT inversion) and computed Mw,coda for 311 earthquakes down to ML 1.0. There were 280 events that were assigned both an Mw,direct and Mw,coda, which show a high correlation (r=0.98), and their differences have a mean of 0.05 and a std. dev. of 0.13. For ML < 3.6, both Mw,direct and Mw,coda agree with the theoretically predicted relationship of Mw=(2/3)ML+C, where C is a regional constant. Our results imply that seismic network operators can use spectral-based Mw estimates to replace a significant number of ML estimates for events with ML ≥1.0, and possibly smaller. The main requirement is a small dataset (3 or more) of pre-computed MT solutions for calibration.
Presenting Author: James Holt
Student Presenter: No
Authors
James Holt Presenting Author Corresponding Author jholt@seis.utah.edu University of Utah |
Katherine Whidden katherine@whidden.net University of Utah Seismograph Stations |
Keith Koper koper@seis.utah.edu University of Utah Seismograph Stations |
Kristine Pankow pankowseis2@gmail.com University of Utah Seismograph Stations |
Kevin Mayeda kevin.mayeda@us.af.mil Air Force Technical Applications Center |
James Pechmann pechmann@seis.utah.edu University of Utah Seismograph Stations |
Benjamin Edwards edwardsb@liverpool.ac.uk University of Liverpool |
Rengin Gök gok1@llnl.gov Lawrence Livermore National Laboratory |
William Walter walter5@llnl.gov Lawrence Livermore National Laboratory |
Robust Estimates of Moment Magnitude for Small- to Moderate-Size Aftershocks of the Mw 5.7 Magna, Utah Earthquake
Category
Intermountain West Earthquakes in the Spring of 2020