Data and Results

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, $m_b=5.5$, depth=33 km; 2003.09.06.15:46:59.9, $m_b=5.6$, depth=33 km in the PDE catalog) at SSI is $\sim 137.8^o$. 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 $\sim 420km$ and $\sim 620 km$ above the CMB, respectively.