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The effect of the earthquake radiation pattern on m_b—A study using aftershocks in the 1976 Gazli sequence

AWE Blacknest, Reading, RG7 4RS. United Kingdom

Abstract

We use published focal mechanisms to estimate radiation coefficients to four short-period arrays recording teleseismic P from 38 aftershocks in the 1976 Gazli, Uzbekistan, earthquake sequence. We divide the observed P-wave amplitude by its radiation coefficient to estimate the P-wave amplitude that would be observed if it was from the maximum of the double-couple radiation pattern. We use this new P-wave amplitude to calculate a P-wave magnitude, m^C_b, that is independent of the P-radiation pattern if the focal mechanisms are without error. Analysis of variance shows that the random error in m^C_b is reduced relative to that in the original P-wave magnitudes m^C_b and that this reduction is statistically significant at the 11% level. Further, analysis of variance demonstrates that the radiation coefficients calculated from the focal mechanisms contain error but that this error is probably not large enough to mask the detection of the radiation effect in m^O_b. Published averages of the logarithm of P-radiation coefficients allow an assessment of the differences in network-averaged m_b due to the radiation pattern of point earthquake and explosion sources. Network-averaged m_b from a vertical strike-slip earthquake can differ from an explosion of similar scalar moment by as much as 1.0 m.u. (magnitude units). However, this difference can be as little as 0.2 m.u. if the earthquake mechanism is 30° dip slip. We argue that, if m_b is required to be independent of the earthquake mechanism, the most appropriate network average is ^C_b – 0.48.

This article has been cited by other articles:

				J. Schweitzer and T. Kvaerna Influence of source radiation patterns on globally observed short-period magnitude estimates (mb) Bulletin of the Seismological Society of America, April 1, 1999; 89(2): 342 - 347. [Abstract] [PDF]