2.1 The Ssds phase
A shear-wave reverberation beneath the receiver is abbreviated here as “Ssds”, following the notation of SB19. Ssds is a shear wave that follows a similar path in the mantle as the direct S wave and reflects off the free surface and off the top of a reflector at depth d before it is recorded by a seismometer on the surface (Figure 1). The arrival time of Ssds after S depends primarily on d and the shear-wave speed above the reflecting layer. For PREM (Dziewonski & Anderson, 1981), an earthquake at the surface, and an epicentral distance of 80°, Ss410s and Ss660s arrive 159.6 s and 242.2 s after S, respectively. Ssds can interfere with SS precursors but the two phases have different slownesses and are distinguishable in waveforms recorded over a wide epicentral distance range. The top-side reflection sdsS near the source has the same traveltime as Ssds at any distance for a 1-D velocity structure (Figure 1). For stations at similar azimuths, source-side reflection points are virtually identical whereas the Ssds reflection points are separated beneath the arrays of stations. Therefore, variations in the Ssds traveltime are primarily due to seismic structure in the upper mantle beneath the seismic stations. There is no source-side and receiver-side ambiguity if the analysis is limited to earthquakes deeper than the reflecting boundaries of interest (Liu & Shearer, 2021) but the data set would be significantly smaller.