A plethora of phases can be distinguished in the long-period travel-time curves (Figures 4-6). This is due to the high coherence of the long-period waveforms. Each travel-time image contains over a million picks from more than fifty thousand seismograms. Similar phases, imaged using automatic-gain-control stacks, can be found in Shearer (1991b). The time-distance coverage of the long-period data is more uniform than that of the short-period data (Figure 2); however, as with the short-period data, areas of low data coverage are characterized by a peppery appearance. Light areas following major phases occur because arrivals are often not registered when large amplitudes are contained in the LTA window.
The vertical-component pick-density plots are dominated by P energy
(Figure 4). We see strong P, PKP, PP, PPP arrivals and a set of
S-to-P surface conversions including SP, SPP, and SPPP. A faint
arrival seen following PP between 80
and 120 coincides with the
expected arrival time of P660sP. This phase is a PP phase in which one of
its legs between the 660 discontinuity and the surface has been converted to
SV energy [see Shearer, 1990]. A similar phase, P660sPP, can be seen
following PPP in the range 90 to 150. A weak arrival seen preceding
PPP in the range 135 to 147 is most likely PPp660p , the topside
multiple off the 660-km discontinuity which follows PP [see Shearer,
1991a]. Unlike the short-period data, PcP is barely imaged; this results from
the LTA window not being clear of P energy when PcP arrives and the small
amplitude of PcP arrivals on long-period records.
The radial- and vertical-component pick-density plots are similar; however,
the radial-component image (Figure 5) shows more S energy and slightly
less P energy. P-to-S surface conversions (PS, PSS, PSSS, PPS, PPSS,
PPSSS, etc.) are seen branching off the top of S, SS, and SSS. It is
interesting to note two precursors to SKS (near 90) resulting from
S-to-P conversions at the 410- and the 660-km discontinuities. These have
been noted in long-period data for some time [e.g., Faber and Müller,
1980, 1984; Baumgardt and Alexander, 1984; Bock, 1988] and are
imaged in long-period waveform stacks [e.g., Shearer, 1991a].
The transverse-component image (Figure 6) is dominated by
S, ScS, and their surface multiples (e.g., SS, ScSScS, SSS,
, etc.). The ScS multiples can be seen (faintly) extending
back in range through the Love wave. Sharp bends in the travel time curves
resulting from the upper mantle discontinuities can be seen in S near
22, SS near 45, SSS near 68, etc. Secondary arrivals from
the 660-km discontinuity are also imaged emerging from these bends.
Figure 6
picks up some of the large amplitude P/SV polarized phases, suggesting
leakage into the transverse component.
