PERCENTAGES ASSOCIATED WITH THE DETECTION OF LONG-PERIOD SURFACE WAVES FROM LOW-MAGNITUDE EVENTS

Abstract

Abstract : A statistical survey of Long-Range Seismic Measurements (LRSM) bulletin data was made to determine the percentage of earthquakes from which long-period surface waves are recorded, as a function of event magnitude. Bhe bases for the percentages were the events located by USC+GS, regardless of depth or epicentral distance. Similar detection-versus-magnitude functions were computed for short-period signals and for long-period surface waves from earthquakes at depths less than 75 km. The long-period seismographs at two LRSM sites, Las Cruces, New Mexico (LC-NM), and La Paz, Bolivia (LZ-BV), are capable of operating at magnifications of 100K. To estimate the capabilities of these sites, a joint distribution of detection percentage versus magnitude and distance was determined for long-period surface waves recorded at LC-NM. For comparison a similar distribution was computed for a typical moderate-gain site, Marysville, California (MV-CL). A survey of the seismological bulletin based on data from the five VELA-UNIFORM observatories (WMSO, CPSO, UBSO, TFSO, and BMSO) determined detection percentage for Rayleigh waves as a function of epicentral distance, magnitude, and magnification of the recording instrument. From this information, functions of detection percentage versus magnitude were computed for instruments operating at 10K, 20K, and 30K. The 50 percent detection levels as functions of distance and magnitude were also calculated for these three magnification levels.