Spectral characteristics of the 11 May 1998 Pokhran and 28 May 1998 Chaghai nuclear explosions

The concept of spectral magnitudes and spectral seismograms, recently devel oped utilizing broadband seismic data permits finer insight into the seismi c-source processes. It has been observed that on a spectral seismogram disp lay, nuclear explosions have peak energy content in higher frequencies comp ared to earthquakes of the same magnitude at similar distances. The broad-b and GEOSCOPE seismic station (HYB) at NGRI, Hyderabad, recorded the Pokhran explosion of 11 May 1998 as well as the Chaghai explosion of 28 May 1998. These two explosive sources exhibit a distinct difference in the energy con tents in various frequency ranges. The Chaghai source had more energy in lo wer frequencies compared to the Pokhran sources. To ensure that this observ ation is not specific to the recording site, we analysed records of the new ly established broad-band seismic stations of the India Meteorological Depa rtment. The intrinsic differences in their spectral characteristics are con firmed by this comparative study. We note that energy from the Pokhran even t peaked in the frequency range of 3.5 to 6 Hz compared to a range of 1 to 3 Hz for the Chaghai explosion.