Abstract:

This paper identifies the origin of 1 Hz scattered waves in the vicinity of PKKP by comparing measurements of slowness and onset time to ray-theoretical predictions. The measurements are obtained from slant stacks of Large Aperture Seismic Array (LASA) data from 36 earthquakes and 6 explosions in the range 30 to 116 degrees. Three types of scattered waves explain the main features seen in the stacks including: P scattered to PKP near the Earth's surface (P.PKP), PKKP scattered near its core-mantle-boundary (CMB) reflection point (PK.KP), and SKKP scattered near its CMB reflection point (SK.KP). The LASA stacks image the scattered waves' amplitude and slowness variations with time. They also show where these waves can be detected, and where they are free from contaminating arrivals. SK.KP waves rise above the noise ~100 s before onset time of the main SKKP arrival near 113 degrees. Observations of PK.KP span 30 degrees to 100 degrees. However, at distances greater than 50 degrees they suffer from P.PKP contamination. At distances less than 40 degrees the PK.KP last for ~280 s. This is ~130 s longer than the maximum ray-theoretical prediction for waves scattered at the CMB, indicating a possible combination of near-surface scattering and contributions from the overlying mantle.