Waveform Equalization Method (WEM) - Application to Assess Reduced Displacement Potential (RDP) of Nuclear Explosions

Abstract:

A new technique, called the "Waveform Equalization Method" has been developed to assess the relative sizes of seismic events. The method seeks to equalize waveforms by matching amplitude and phase of recorded from a cluster of closely-spaced events at common stations. The method convolves a theoretical source time function (TDSF) for an event (A) with the observed waveforms from event (B), then convolves a TDSF for event B with the observed waveforms of event A, and forms the difference waveforms between the two resulting waveforms. These differences are computed for all event pairs in the cluster and summed to construct an objective error function. A grid search is executed over the parameter space to minimize the error function. For two events, the WEM will often yield the relative event strengths, but investigations with synthetic seismograms indicate that when the technique is applied to to four of more events simultaneously, results tend to converge nearly to the absolute strengths. The TDSF is more sensitive to yield rather than to the depth of burial. It is possible, therefore, that an imprecise overburden value may still allow a reasonable yield. This study focusses on establishing the error bounds on the RDP calculated for the five DPRK nuclear explosions using waveform data from openly-available stations at all distances. Other factors such as attenuation or lateral earth structure do not affect the method.