Y. Gitterman et al., Spectral discrimination analysis of Eurasian nuclear tests and earthquakesrecorded by the Israel Seismic Network and the NORESS array, PHYS E PLAN, 113(1-4), 1999, pp. 111-129
The energy spectral ratio and the innovative spectral semblance discriminan
ts, successfully performed previously on a local Israeli events, were verif
ied on teleseismic short-period recordings. The events tested include 29 nu
clear explosions and 41 earthquakes (m(b) = 5.2-6.5), mainly from China and
Kazakhstan, recorded by the Israel Seismic Network (ISN) and the NORESS ar
ray. A 15-s window comprising P- and P-coda waves was selected for the anal
ysis. The 'semblance' statistic commonly used in seismic prospecting for ph
ase correlation in the time domain was modified and utilized as a measure o
f coherency of the smoothed spectra across the network/array. The semblance
and the average spectral ratio of low-to-high frequency energy were evalua
ted, using a subset of 7-10 stations for a given event. Semblance and spect
ral ratio values, calculated from ISN seismograms, were found to be higher
for earthquakes, where the analyzed waves are richer in low-frequency energ
y and have more coherent spectral shapes than explosions. These observation
s are contrary to those observed for local events. The best performance is
provided in the frequency bands (0.6-1 Hz)/(1-3 Hz) (for the ratio) and (0.
6-2 Hz) (for the semblance). Joint application of the two discriminants sho
wed almost full separation (95%) between the two populations. Some explosio
ns exhibited pronounced minima (nulls) near 1 Hz which could be interpreted
in terms of interference of P- and pP-waves from a source at the depth of
several hundreds of meters. Nevertheless, this feature could not be utilize
d as a discriminant: many explosions revealed strong variability of this mi
nimum across the ISN network and some earthquakes also distinctly exhibited
this feature. The ISN and NORESS discrimination performances were compared
. The latter records showed the same (as the ISN) relation between spectral
ratio values for earthquakes and explosions, whereas the character of semb
lance was reversed. The ratios in the frequency bands (0.6-0.8 Hz)/(0.6-10
Hz) yield full separation of the two populations. In general, the spectral
ratio performance appears to be better for NORESS recordings! whereas the s
pectral semblance is better for the ISN data. (C) 1999 Elsevier Science B.V
. All rights reserved.