Discrimination of Explosions and Earthquakes: An Example Based on Spectra and Source Parameters of the 11th May 1998 Pokhran Explosion and the 9th April 2009 Earthquake

摘要

We compare the P-, S-and Lg-spectra of the 11th May, 1998 Pokhran underground nuclear explosion (NE) with those of an earthquake (EQ) of comparable magnitude that occurred in its vicinity (similar to 100 km west) on 9th April, 2009, utilizing the waveforms recorded by a Global Seismograph Network station at Nilore (NIL), Pakistan. The contiguous occurrence of these events and the similarity of the travel paths provided a good opportunity to discriminate the nature of the sources. Our results suggest that the Pn/Lg and Pn/Sn amplitude ratios of the explosion and earthquake waveforms exhibit distinct differences in the higher frequency window. Further, since the P-phases have high signal to noise ratio compared to their S counterparts, we utilize their spectra to derive the source parameters of the NE and EQ sources. Our results show that the seismic moment, corner frequency and source dimension of the explosion are similar to 1.58X10(17) Nm, 1.18 Hz and similar to 0.793 km respectively. The moment magnitude (MW) and surface wave magnitude (M (S)) for the nuclear explosion are estimated to be similar to 5.4 and similar to 3.57 respectively. The values of M (W) (5.3) and M (S) (4.3) obtained by us for the earthquake are consistent with the estimates in the Harvard catalog and earlier published results. The estimate of M (W) for the nuclear explosion was hitherto not available. Lastly, we estimate the yield of the NE to be similar to 50 kt from the surface wave magnitude and discuss the various limitations related to its estimation.