Source parameters of southern California earthquakes
Abstract
The source characteristics of southern California earthquakes with local magnitudes M_{L} between 2 and 7 have been estimated from the gross spectral properties of the locally recorded S waves written on broad band torsion seismograph systems that have operated in California since 1932. The seismic moments M_{0} are consistent with the magnitudemoment relation obtained by M. Wyss and J. N. Brune from surface wave amplitudes, although a single straight line given by log M_{0} = 1.5 M_{L} + 16.0 fits the data equally well. Source dimensions 2r vary from about 0.6 to 25 km, and stress drops Δσ lie between 0.3 and 200 bars. Neither parameter is a welldefined function of magnitude, although source size roughly increases with increasing M_{L}. A theoretical relation between M_{L} and source parameters is developed by using Brune's source model and the displacement response of the WoodAnderson seismograph. The result is M_{L} = log M_{0}  32 log r  17.8, where 3 < M_{L} < 7, M_{0} is in dynes centimeters, and r is in kilometers. The M_{L} may be accurately obtained by this equation, and the result is used to construct the following relations: log M_{0} = 2.0 M_{L} + 14.2  log Δσ, where Δσ is in bars, and log 2r = ⅔ M_{L} + 2.9  ⅔ log Δσ. Singlevalued relations are not expected unless Δσ is constant. However, all available data are bracketed by these equations if stress drops vary over the range observed in this study. Radiated energies computed by spectrum integration agree with a theoretical result, log E_{s} = 2.0 M_{L} + 8.1, where E_{s} is in ergs. Both are consistent with B. Gutenberg and C. F. Richter's energymagnitude relation above M_{L} = 4.5 but depart considerably from their result at lower magnitudes. The apparent stress parameter can provide source information that is independent of stress drop only if the highfrequency spectral falloff is a welldetermined source property, which is unlikely for most of the data examined in this and other recent spectrum studies. Regional differences in source dimensions and stress drops within the southern California area are suggested by the spectral observations. However, the pattern is complex, and there are significant variations in these source parameters within any one region. Closein seismic measurements are needed to substantiate the suggested regional differences, and determination of fault zone properties at depth is required to more fully understand the regional and local variations in seismic source parameters suggested by this study.
 Publication:

Journal of Geophysical Research
 Pub Date:
 December 1973
 DOI:
 10.1029/JB078i035p08547
 Bibcode:
 1973JGR....78.8547T
 Keywords:

 Seismology: General or miscellaneous;
 Information Related to Geographic Region: General or miscellaneous