Abstract
This second paper (Part 2) pertaining to optimized site-specific threshold
monitoring addresses the application of the method to regions covered
by a teleseismic or a combined regional-teleseismic network. In the
first paper (Part 1) we developed the method for the general case,
and demonstrated its application to an area well-covered by a regional
network (the Novaya Zemlya nuclear test site). In the present paper,
we apply the method to the Indian and Pakistani nuclear test sites,
and show results during the periods of nuclear testing by these two
countries in May 1998. Since the coverage by regional stations in
these areas is poor, an optimized approach requires the use of selected,
high-quality stations at teleseismic distances. To optimize the threshold
monitoring of these test sites, we use as calibration events either
one of the nuclear explosions or a nearby earthquake. From analysis
of the calibration events we derive values for array beamforming
steering delays, filter bands, short-term averages (STA) lengths,
phase travel times (P waves), and amplitude-magnitude relationships
for each station. By applying these parameters, we obtain a monitoring
capability of both test sites ranging from mb 2.8-3.0 using teleseismic
stations only. When including the nearby Nilore station to monitor
the Indian tests, we show that the threshold can be reduced by about
0.4 magnitude units. In particular, we demonstrate that the Indian
tests on 13 May, 1998, which were not detected by any known seismic
station, must have corresponded to a magnitude (mb) of less than
2.4. We also discuss the effect of a nearby aftershock sequence on
the monitoring capability for the Pakistani test sites. Such an aftershock
sequence occurred in fact on the day of the last Pakistani test (30
May, 1998), following a large (mb 5.5) earthquake in Afghanistan
located about 1100 km from the test site. We show that the threshold
monitoring technique has sufficient resolution to suppress the signals
from these interfering aftershocks without significantly affecting
the true peak of the nuclear explosion on the threshold trace.
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