The high quality short-period Yellowknife Seismograph Array (YK) is a long-term primary array
in the International Monitoring System (IMS) seismic network, acting as
the backbone facility for nuclear explosion monitoring. The epicentral distance from
YK to the doublet (1993.12.01.00:59:01.2, , depth=33 km;
2003.09.06.15:46:59.9, , depth=33 km in the PDE catalog) at SSI is .
18 of all 19 stations at YK recorded very high signal-to-noise PKP precursors for both events.
In order to enhance the precursor signals, we filtered the original seismograms in the frequency
range of 1 to 2 Hz. Before stacking, we aligned traces with respect to PKIKP phases by
means of cross-correlation and performed
array-sided travel time corrections to remove the influence of heterogeneities just beneath the
seismic array. We applied two different stacking methods: linear stacking (Fig. 1a) and
Phase-Weighted Stacking (PWS) (Fig. 1b).
Based on ray tracing and the single-scattering assumption, we are able to locate
the scattering regions responsible for the individual PKP precursors,
using our precise measurements of slownesses,
back-azimuths, and differential arrival times.
Comprehensive consideration of the high quality differential arrival times, slowness,
and back-azimuth deviations enables us to locate the mantle scatterers for precursors A, B, D, and E (Fig. 2).
Precursors A and B are scattered at the CMB, while precursors D and E
are scattered at and above the CMB, respectively.
Berkeley Seismological Laboratory
215 McCone Hall, UC Berkeley, Berkeley, CA 94720-4760
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