The seismic magnitude of the 1989 Loma Prieta mainshock determined from strong motion records

Abstract

The work investigates the usefulness of regional broadband recordings to estimate rapidly the size of large earthquakes. Strong motion records of the Loma Prieta earthquake from 20 free-field stations at distances 15 < Δ < 105 km from the source are used to compute the local magnitude of the mainshock. Frequency-domain convolution and integration are used to obtain appropriate synthesized signals. An azimuthal average of ML = (6.7 ± 0.09) was estimated from the maximum trace amplitudes measured on synthesized Wood-Anderson torsion records. The low-gain Wood-Anderson torsion seismograph (100 magnification) at the University of California Seismographic Station gave ML = 7.0. Comparison between published local and teleseismic estimates indicates that the usual strong motion accelerometers in the near field of large earthquakes yield acceptable values of magnitude. Efforts to obtain estimates of the overall average radiated seismic energy from the integrated ground velocity-squared records demonstrated the need for special site calibration.