Source Studies in the Near and Far Field

Abstract

Abstract : Several continuing investigations of the seismic source using near- and far-field data are discussed. We completed our analysis of near-field acceleration data recorded at Bear Valley, California, by examining two small events which occurred on 7 February 1974 and 6 July 1974. The February event, using only spectral analysis, was found to have a moment of 3 x 10 to the 21st power dyne-cm with an equivalent circular dislocation radius of 0.18 sq km. Both spectral and time domain waveform fitting techniques were applied to the July event. The solutions using each method were in reasonable agreement, with a seismic moment in the range of 2 to 4 x 10 to the 21st power dyne-cm over an equivalent dislocation radius of about 0.1 to 0.3 sq cm. Using far-field surface wave data, two central California area earthquakes (22 June 1973 and 28 November 1974) were re-examined using two new earth models which were determined by McEvilly (1975) for the northeast and southwest side of the San Andreas fault. Several far-field solutions for the source mechanism were obtained by using various combinations of stations and earth models, including use of both structures at once for the first time. However, for the central California event of 28 November 1974, the mechanism which agreed closely with that obtained by other means used only those stations on the west side of the fault, with the southwest structure used in the spectral fit.