Comparison of P-wave spectra of underground explosions and earthquakes

The P-wave displacement spectra of the underground explosions Milrow and Longshot are compared with those of four shallow earthquakes of similar body-wave magnitude (mb) in the Aleutian Islands. The spectral data (0.5≤T≤33 sec) have been obtained from three vertical instruments at Pasadena, approximately 50° from the epicentral region common to the six events. A Novaya Zemlya explosion of comparable mb was also analyzed. The peak spectral amplitude for the earthquakes occurs at periods several times larger than for comparable mb explosions, indicating that the earthquakes have source dimensions several times larger than the explosions. For T<1.5 sec, the spectra for both types of events are comparable; the absolute level and the rates of decay (ω−2) with increasing frequency are the same. The Q-corrected explosion spectra peak sharply in the interval 2 sec>T≥1 sec and decrease at least as far as ω for longer periods. The earthquake spectra, however, continue to increase to a long-period level. The greatest differences in spectral amplitudes for the events considered occur at periods longer than 3 sec. The observed differences in the P-wave spectra provide a promising method for discriminating between earthquakes and underground explosions.