Local Magnitude Practices in the United States of America
Description:
Twelve U.S. Advanced National Seismic System (ANSS) participant organizations produce the authoritative parameters such as magnitude, latitude, longitude, and depth for earthquakes in the U.S.A. Due to regional and historical differences, these various contributors have practices that differ in how the parameters are determined. The ANSS has initiated an endeavor to document these practices in a central location, starting with the local magnitude. The local magnitude is based on the original Richter magnitude from Southern California, which used the peak amplitudes measured from Wood-Anderson seismic instruments, a distance correction, and where needed, a station correction. It defined that a magnitude 3 earthquake recorded at 100 km distance would have an amplitude of 1 mm on a Wood-Anderson instrument. Nowadays, data from digital instruments are instrument-corrected and convolved with a Wood-Anderson response to mimic the old practice. For much of the U.S., the local magnitude is the preferred magnitude for small to moderate-sized earthquakes (M<6). However, because the local magnitude saturates near magnitude 7, the National Earthquake Information Center’s moment magnitude derived from the W-phase is preferred for earthquakes of magnitude 6 and greater.
We document and report how each ANSS catalog contributor performs the instrument response corrections, how they measure the peak amplitude, which distance correction they use, whether they use station corrections, and, finally, how they aggregate the station magnitude contributions to determine the final magnitude and uncertainty estimate. We discuss to what extent the different approaches may affect the consistency of local magnitudes across the nation.
Session: Network Seismology: Recent Developments, Challenges and Lessons Learned - III
Type: Oral
Date: 4/15/2025
Presentation Time: 02:30 PM (local time)
Presenting Author: J Renate
Student Presenter: No
Invited Presentation:
Poster Number:
Authors
J Renate Hartog Presenting Author Corresponding Author jrhartog@uw.edu University of Washington |
Emily Morton emily.morton@utah.edu University of Utah |
Paul Earle pearle@usgs.gov U.S. Geological Survey |
Gabrielle Tepp gtepp@caltech.edu California Institute of Technology |
Michael West mewest@alaska.edu University of Alaska Fairbanks |
William Savran wsavran@unr.edu University of Nevada, Reno |
Julien Marty jmarty@berkeley.edu University of California, Berkeley |
Jacob Walter jwalter@ou.edu University of Oklahoma |
Jefferson Chang jchang@usgs.gov U.S. Geological Survey |
Elizabeth Vanacore elizabeth.vanacore@upr.edu University of Puerto Rico, Mayagüez, Puerto Rico, United States |
Alexandros Savvaidis alexandros.savvaidis@beg.utexas.edu University of Texas at Austin, Austin, Texas, United States |
Mitchell Withers mwithers@memphis.edu University of Memphis, Memphis, Tennessee, United States |
Local Magnitude Practices in the United States of America
Session
Network Seismology: Recent Developments, Challenges and Lessons Learned