A Comparative Study of the Elastic Wave Radiation from Earthquakes and Underground Explosions*

A detailed analysis of the surface wave radiation from two underground explosions (BILBY and SHOAL) and an earthquake near Fallon, Nevada, whose epicentre is only 60 km from SHOAL, indicates that: (1) at long periods the surface wave radiation from the earthquake can be explained by a pure quadrupole (double couple) source, but at higher frequencies the radiation pattern contains asymmetries which suggest effects due to rupture propagation; these would require higher-order multipole terms in the source equivalent representation; (2) the surface waves from the explosions can be explained by superimposed monopole and quadrupole sources, with no indication of higher-order multipole terms, at least in the period range comparable to that in which the earthquake signal was recorded; (3) the principal conclusion of this study is that the anomalous radiation from the explosions is probably due to stress relaxation around the shock-generated shatter zone and not due to earthquake triggering. Comparative analysis of SHOAL and FALLON shows that: (1) the ratio of the Love wave amplitude generated by the earthquake to the Love wave amplitude from the explosion increases with period, which implies a larger source dimension for FALLON; (2) the normalized spectral ratio of Love wave amplitude to Rayleigh wave amplitude, considered as a function of period, is nearly constant and close to unity for the explosions, but larger for the earthquake by a factor of two or three, and increasing with period. These differences might be useful in distinguishing earthquakes from explosions (at least in the magnitude range of the events used in this study, mb 4.4 and above), as well as for estimating source parameters, such as stress, which are of fundamental geophysical interest.