Research in Seismology
Semi-annual technical rept.
SAINT LOUIS UNIV MO DEPT OF EARTH AND ATMOSPHERIC SCIENCES
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The report concerns research related to the discrimination between small-magnitude earthquakes and explosions. The principal findings are The source spectra of small-magnitude earthquakes in the New Madrid Lower Mississippi Valley seismic region can be divided into two classes, one of which is deficient in low-frequency energy. In most cases the source spectrum of P waves from deep-focus South American earthquakes is flat at the low frequencies and falls off as the inverse square of the frequency at the high frequencies. The absorption of surface waves is much smaller east of the Rocky Mountains than west of them. This fact must be taken into account when computing surface-wave magnitudes of small events.