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Session: Constructing and Testing Regional and Global Earthquake Forecasts [Poster]

Type: Poster

Date: 4/20/2021

Time: 11:30 AM

Time: 11:30 a.m.-12:30 p.m. Pacific

Description: Constructing and Testing Regional and Global Earthquake Forecasts [Poster]

Earthquake magnitudes are fundamental measures of earthquake size and represent critical inputs in earthquake forecasts. Although moment magnitude (M_w) has become the established standard for moderate and large earthquakes, difficulty in reliably measuring seismic moment for small (generally M_w<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 M_w. 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 M_L, m_bLg and M_w for M ~2-4 earthquakes examined here. More troublingly, we find that relations designed to convert other magnitudes to M_w, 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

Additional Authors

David R Shelly

Presenting Author Corresponding Author

dshelly@usgs.gov

U.S. Geological Survey, Denver, Colorado, United States

Kevin Mayeda

kevin.mayeda@us.af.mil

AFTAC, Livermore, California, United States

William R Walter

walter5@llnl.gov

Lawrence Livermore National Laboratory, Livermore, California, United States

Rengin Gök

gok1@lanl.gov

Lawrence Livermore National Laboratory, Livermore, California, United States

Justin Barno

barno1@llnl.gov

Lawrence Livermore National Laboratory, Livermore, California, United States

Katherine M Whidden

katherine@whidden.net

University of Utah, Salt Lake City, Utah, United States

Justin L Rubinstein

jrubinstein@usgs.gov

U.S. Geological Survey, Moffett Field, California, United States

Paul S Earle

pearle@usgs.gov

U.S. Geological Survey, Golden, Colorado, United States

William L Yeck

wyeck@usgs.gov

U.S. Geological Survey, Golden, Colorado, United States

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

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