From a time-domain study, the amplitude of 10-Hz P waves in the New Madrid seismic zone was found to fall off as the inverse 1.4 power of the epicentral distance. The amplitude of 10-Hz Lg waves was found to decay as for an Airy phase with a coefficient of anelastic attenuation of 0.006 km-1. In almost all cases the Lg motion was found to be larger than that of P, even at epicentral distances of 5 km and less. Thus design earthquake motions need to be concerned with Lg waves, whose coda also have a longer duration than those of the P waves.

The apparent Q for the 10-Hz Lg waves is 1500, identical to that found previously by Nuttli (1973) for higher mode 1-Hz Lg waves. The attenuation of 10-Hz Lg waves is sufficiently small that one must pay attention to their damage potential at distances as large as a few hundred kilometers.

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