Surface-wave attenuation and crustal anelasticity in Central North America

Abstract

The southeastern Missouri earthquake of October 21, 1965 generated fundamental- and higher-mode Love and Rayleigh waves which were recorded at numerous North American stations. Love-wave amplitude radiation patterns were determined and found to be consistent with theoretical patterns predicted by a fault-plane solution previously inferred from Rayleigh-wave data. The radiation patterns were used to estimate the source spectrum and values for Love-wave attenuation coefficients for the mid-continent of North America by a least-squares iterative process. The source spectrum derived from Love-wave amplitudes exhibits a peak at periods between 5 and 9 sec and decreases to a lower DC level at longer periods, in agreement with the source spectrum determined previously for Rayleigh waves. The Love-wave attenuation coefficients decrease rapidly from about 0.0018 km⁻¹ at a period of 4 sec to about 0.0001 km⁻¹ at a period of 20 sec. At periods between 20 and 40 sec the values seem to remain nearly constant. The crust in the mid-continent of North America is characterized by relatively low Q_β values, 75 to 300, in its upper portion. At depths between 15 and 20 km, Q_β increases sharply and decreases again at greater depths. The decrease can be explained as being due to increasing temperature in a homogeneous material, but the sharp increase requires a change in the chemical constitution of the material at mid-crustal depths.