The Chagai cluster is based on the code names used for two nuclear tests
carried out by Pakistan on May 28, 1998 (Chagai-I) and May 30, 1998
(Chagai-II). Chagai-I was conducted in the Ras Koh Hills of Chagai
District, Baluchistan. Chagai-I is believed to have consisted of 5
simultaneous shots, but they were fired nearly simultaneously and are
treated as a single event. Chagai-II was located about 115 km southwest,
in the Kharan Desert of Baluchistan. The cluster includes both tests as
well as a dozen well-observed earthquakes in the region and uses the
locations of the nuclear tests for an indirect calibration of the
cluster. The calibration epicenter for Chagai-I is taken from
assumes a shot point ~1.3 km above sealevel. The calibration epicenter
for Chagai-II is taken from
below the surface, close to sea-level. The cluster hypocentroid is first
estimated from teleseismic P arrivals between 30 and 90°, and then the
cluster is shifted to best match the calibration hypocenters. The
epicentral shift (15.7 km at an azimuth of 109°) provides a close match
to the calibration epicenters of the two tests, with residuals of ~1.0
km and 1.6 km for Chagai-I and Chagai-II, respectively.
The main uncertainty in calibration concerns the shot times, which have
not been officially announced to sufficient accuracy. Of Chagai-I
Pakistani sources reported at the time of the tests that the firing
switch was activated exactly on the minute of 3:16pm local time (1016
UTC) but that was followed by a lengthy computer-controlled process for
actually firing the emplaced charges that is claimed to have taken 30
seconds or more. An origin time of ~1016:30 for this shot is not
tenable, however as the travel times recorded at regional and
teleseismic distance require the origin time to be close to 1016:15. The
nearest station with any reported arrival times from the cluster (QUE,
Quetta) is ~250 km away, too far to use the arrival times for origin
time constraint. The approach used here to calibrate the origin times is
unique to the Chagai cluster. A version of the cluster that included
earthquakes closer to Quetta was used to make an initial estimate of the
origin times and this was refined by comparing the baseline offset of
teleseismic P arrivals from the shifted Chagai cluster with that of the
nearest calibrated cluster, Ziarat, whose origin times are calibrated
with near-source and local-distance readings at station QUE. The
baseline offset for teleseismic P in the Ziarat cluster is +1.226 s, and
the calibration origin times of the two nuclear tests in the Chagai
cluster have been set (1016:15.45 for Chagai-I, 0654:55.30 for
Chagai-II) such that nearly the same baseline offset is observed for
teleseismic P (+1.257 s) and the relative time of the relocated nuclear
tests agrees within 0.04 s. Other estimates of the shot time seconds
field for these tests range from 15.2-17.8 for Chagai-I and 54.9-57.1
for Chagai-II, but none of these estimates qualifies as calibrated,
being based on uncorrected teleseismic data. Because of the uncertainty
involved in this calibration strategy the uncertainty of the origin
times of calibration events (i.e., the two nuclear tests) are inflated
to 1.0 s. The relative time of Chagai-I is referenced to zero focal
depth, while the true shot point was ~1.3 km above that depth, so the
true origin time of Chagai-I is estimated to have been ~0.2 s earlier
than the calibration time used here. For Chagai-II the shot point is
thought to be very close to sealevel so no correction is needed.
The earthquakes in the Chagai cluster include a 7.2 Mw event on January
18, 2011 and a 6.0 mb event on June 4, 2000. The earthquakes are about
evenly divided between events at crustal depths (10-20 km) and events
associated with the subducted slab from the Makran subduction zone, with
depths of 40-80 km. All earthquakes in this cluster have depth
constraint from teleseismic depth phases.