A Big Problem for Small Earthquakes: Benchmarking Routine Magnitudes and Conversion Relationships With Coda-Envelope-Derived Mw in Southern Kansas and Northern Oklahoma
Session: Constructing and Testing Regional and Global Earthquake Forecasts [Poster]
Type: Poster
Date: 4/20/2021
Presentation Time: 11:30 AM Pacific
Description:
Earthquake magnitudes are fundamental measures of earthquake size and represent critical inputs in earthquake forecasts. Although moment magnitude (Mw) has become the established standard for moderate and large earthquakes, difficulty in reliably measuring seismic moment for small (generally Mw<4) earthquakes has meant magnitudes for these events remain plagued by a patchwork of inconsistent measurement scales. Because of this, magnitudes of small earthquakes and statistics derived from them can be biased. Furthermore, because small earthquakes are much more numerous than large ones, many applications, such as seismic hazard analysis, depend critically on analysis of events characterized by magnitudes other than Mw. To assess this problem, we apply coda-envelope analysis to reliably determine moment magnitudes for a case study of small earthquakes from northern Oklahoma and southern Kansas. Not surprisingly, we find significant differences among ML, mbLg and Mw for M ~2-4 earthquakes examined here. More troublingly, we find that relations designed to convert other magnitudes to Mw, which are relied upon for important applications such as seismic hazard analysis, often increase rather than decrease this bias for our dataset. Converted magnitudes can result in a systematic bias sometimes exceeding 0.5 magnitude units, a difference that typically corresponds to a factor of ~3 in seismicity rate. Moreover, we find a correspondingly large bias in Gutenberg-Richter b-values, caused primarily by inaccurate magnitude scaling in the conversion relationships. Although this study focuses on a relatively small geographic area, we can expect similar issues exist with varying severity in other regions. Therefore, magnitudes of small earthquakes and their associated statistics (including seismicity rates and b-values), should be treated with extreme caution.
Presenting Author: David R. Shelly
Student Presenter: No
Authors
David Shelly Presenting Author Corresponding Author dshelly@usgs.gov U.S. Geological Survey |
Kevin Mayeda kevin.mayeda@us.af.mil AFTAC |
William Walter walter5@llnl.gov Lawrence Livermore National Laboratory |
Rengin Gök gok1@lanl.gov Lawrence Livermore National Laboratory |
Justin Barno barno1@llnl.gov Lawrence Livermore National Laboratory |
Katherine Whidden katherine@whidden.net University of Utah |
Justin Rubinstein jrubinstein@usgs.gov U.S. Geological Survey |
Paul Earle pearle@usgs.gov U.S. Geological Survey |
William Yeck wyeck@usgs.gov U.S. Geological Survey |
Andrea L Llenos allenos@usgs.gov U.S. Geological Survey, Golden, Colorado, United States |
Morgan P Moschetti mmoschetti@usgs.gov U.S. Geological Survey, Golden, Colorado, United States |
A Big Problem for Small Earthquakes: Benchmarking Routine Magnitudes and Conversion Relationships With Coda-Envelope-Derived Mw in Southern Kansas and Northern Oklahoma
Category
Constructing and Testing Regional and Global Earthquake Forecasts