Teleseismic observations of aftershocks immediately following an underground explosion

Abstract

Underground explosions often trigger a series of earthquakes (aftershocks) at least one magnitude unit less than the magnitude of the explosion. In the first one or two hours following an explosion the rate of occurrence of the aftershocks can be so high that the signals from separate shocks overlap. Few if any of the P-waves from these aftershocks have been observed at teleseismic distances. Here, however, evidence is presented that the aftershock sequence triggered by at least one explosion (the Nevada Test Site explosion, Greeley) commenced a second or two after the firing of the explosion and that the radiation from some of the first of these aftershocks was detected at teleseismic distances; the radiation being present in the coda of the P signal from the explosion. The evidence for the presence of the aftershocks is that for the Greeley explosion the coda of the P signal contains a much higher proportion of high-frequency energy than the initial P pulse and yet the absence of high-frequency energy in the P pulse seems to be a source effect and not the result of losses on the transmission path. So, as the explosion source radiated little high-frequency energy it is proposed that the high-frequency energy in the coda is from aftershocks which have P-wave spectra with corners well above the corner frequency of that of the explosion. The body-wave magnitude of the largest of the aftershocks is estimated to be about one unit less than that of the explosion. The low magnitude of the aftershocks does not, however, appear to rule out the possibility that the radiation from the aftershocks can account for all the radiation from the explosion source that is usually attributed to a double-couple type of source occurring simultaneously with or immediately following the explosion.