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Local Magnitude Determinations for Intermountain Seismic Belt Earthquakes from Broadband Digital Data

James C. Pechmann¹, Susan J. Nava^*,1, Fabia M. Terra^,1 and Julie C. Bernier^,1

¹ University of Utah Seismograph Stations
University of Utah
Department of Geology and Geophysics
135 South 1460 East Room 705 WBB
Salt Lake City, Utah 84112-0111

Present address: U.S. Geological Survey, St. Petersburg, Florida 33701.

Present address: U.S. Geological Survey, St. Petersburg, Florida 33701.

Present address: U.S. Geological Survey, St. Petersburg, Florida 33701.

The University of Utah Seismograph Stations (UUSS) earthquake catalogs for the Utah and Yellowstone National Park regions contain two types of size measurements: local magnitude (M_L) and coda magnitude (M_C), which is calibrated against M_L. From 1962 through 1993, UUSS calculated M_L values for southern and central Intermountain Seismic Belt earthquakes using maximum peak-to-peak (p-p) amplitudes on paper records from one to five Wood–Anderson (W-A) seismographs in Utah. For M_L determinations of earthquakes since 1994, UUSS has utilized synthetic W-A seismograms from U.S. National Seismic Network and UUSS broadband digital telemetry stations in the region, which numbered 23 by the end of our study period on 30 June 2002. This change has greatly increased the percentage of earthquakes for which M_L can be determined. It is now possible to determine M_L for all M 3 earthquakes in the Utah and Yellowstone regions and earthquakes as small as M <1 in some areas.

To maintain continuity in the magnitudes in the UUSS earthquake catalogs, we determined empirical M_L station corrections that minimize differences between M_Ls calculated from paper and synthetic W-A records. Application of these station corrections, in combination with distance corrections from Richter (1958) which have been in use at UUSS since 1962, produces M_L values that do not show any significant distance dependence. M_L determinations for the Utah and Yellowstone regions for 1981–2002 using our station corrections and Richter’s distance corrections have provided a reliable data set for recalibrating the M_C scales for these regions. Our revised M_L values are consistent with available moment magnitude determinations for Intermountain Seismic Belt earthquakes.

To facilitate automatic M_L measurements, we analyzed the distribution of the times of maximum p-p amplitudes in synthetic W-A records. A 30-sec time window for maximum amplitudes, beginning 5 sec before the predicted Sg time, encompasses 95% of the maximum p-p amplitudes. In our judgment, this time window represents a good compromise between maximizing the chances of capturing the maximum amplitude and minimizing the risk of including other seismic events.