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Evidence for -cube scaling from amplitudes and periods of the Rat Island sequence (1965)

APPLIED SEISMOLOGY GROUP LINCOLN LABORATORY MASSACHUSETTS INSTITUTE OF TECHNOLOGY, CAMBRIDGE, MASSACHUSETTS 02142

Abstract

An attempt has been made to deduce seismic scaling information from the amplitudes A and periods T of P waves from 868 earthquakes of the Rat Island sequence of 1965, with m_b values ranging from 4 to 5. Consistently reliable data were reported in the Earthquake Data Reports from five VELA array stations in the U.S., falling in an azimuthal sector of 60° to 80° from the source region with distances ranging from 44° to 68°. The most striking feature of the data is the strong linear trend of log (A/T) with T at each station over a wide period range of 0.5 to >1.5 sec. Using a subset of 245 events reported jointly at all five VELA stations we calculated a least squares magnitude versus period relation given by m_b = (3.24 ± 0.08) + (1.32 ± 0.09) T. In order to interpret this trend in terms of source models, synthetic records, including earth attenuation were calculated for -square models of Sharpe and Brune, and an -cube model suggested by Savage. Under the assumption of geometric similarity, whereby the zero-frequency source level is proportional to source volume, the measured trend of log (A/T) on the synthetic records indicates that an -cube rather than either -square model is more appropriate for the Rat Island data.

Extrapolation of the synthetic source spectra to D.C. for m_b 4.7 yields a moment of 6.65 x 10²² dyne cm and for the sequence a stress drop of 25 bars, in reasonable agreement with previous results obtained from long-period P-wave spectra.

These results indicate that crude seismic scaling information for moderate-sized earthquakes can be extracted from good bulletin data without using spectral analysis. Large amounts of data are required, however, to establish a reliable trend for interpretation.

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