Analysis of amplitude spectra ofPwaves from earthquakes and underground explosions

Abstract

A step-by-step display of the theoretical P-wave amplitude spectrum of shallow (focal depth ≤ 40 km) earthquakes with mb values ranging from 5.9 to 6.3 and underground explosions of comparable magnitudes as the signal propagates from source to receiver indicates that the factors most influential in shaping the respective spectra are the source crust transfer function, which includes the focal depth effect, and the source function. The results, although not unequivocal, favor a peak in the amplitude spectrum of the teleseismic P coda of earthquakes rather than a flat spectrum at low frequencies. The effect of focal depth appears to have more influence than the effect of source parameters on the peak frequency f0 in the spectra of earthquakes, whereas the source function is the important parameter for shallow underground nuclear explosions. For shallow seismic events of comparable mb the f0 of earthquake spectra is smaller than the f0 of explosion spectra by approximately an order of magnitude. The pronounced minimum (hole) at about 0.1 Hz in the long-period spectrum of earthquakes is associated with the focal depth effect and can be identified for several events. However, theoretically predicted holes starting at about 0.5 Hz in the short-period spectrum of assumed unilateral faulting earthquakes are due to the extended source effect and are not identifiable on any of the experimental spectra. For short-period records of Longshot and Milrow, Haskell's analytic formulation of the Werth and Herbst displacement potential, in conjunction with the outlined theory, predicts fairly accurately the increase (approximately 0.15 sec) in the duration of the first half-cycle with increasing yield. For the corresponding long-period records of Milrow his formulation predicts a value smaller than a mean of the measured values by about 15% (0.32 sec). Haskell's equations, together with the rest of the theory, appear to predict fairly accurately the variation in mb with yield for these two events when amplitude measurements are restricted to the first half-cycle.