Travel-time residuals of PP waves reflected under oceanic and continental platform regions

Variations in structure and tectonics along the Mid-Atlantic Ridge, 23 degress N and 26 degrees N

“…We fmd that the P delay for 26°N to be on average much smaller, by perhaps 1 s or more relative to the Herrin [1968] Earth model, than reported by other investigators [e.g., Girardin, 1980;Dorbath and Dorbath, 1981;Woodward and Masters, 1990]. Although the positiveS wave travel time delay indicates that lower S wave velocities than appropriate for the average Earth may be present, the small magnitude of the delay suggests that the ridge segment near 25°50'N may not be underlain by as large a low velocity anomaly as earlier studies have indicated for other regions of the MAR [e.g., Girardin and Poupinet, 1974;Weidner, 1974;Duschenes and Solomon, 1977;Steinmetz et al, 1977;Rowlett and Forsyth, 1979;Dorbath and Dorbath, 1981;Grand, 1989;Sheehan and Solomon, 1989].…”

“…By measuring the differential residual between the direct-arriving and reflected P wave arrival, several authors have found equivalent one-way, upper mantle P wave delays (-0.8 s to +1.0 s relative to the ill tables [Girardin, 1980;Dorbath and Dorbath, 1981]) which are more than 1.5 s larger than the -2.4 s m delay observed for 26°N ( Figure 11). …”

“…Differential body wave travel time residuals (PP-P, SS-S), interpreted as arising from the upper mantle in the vicinity of the bounce point of the surface-reflected phase, generally indicate positive residuals and thus lower upper mantle velocities relative to older oceanic lithosphere and to average Earth models [Dorbath and Dorbath,1981;Girardin, 1980;Kuo et al, 1987;Solomon, 1988, 1989;Masters, 1988, 1990]. An advantage of these studies is that travel time errors due to errors in the estimation of hypocentral parameters and to crustal and mantle heterogeneities beneath the source and receiver cancel because of the similarity of their ray paths, and thus the observed residual can be assumed to describe anomalies below the bounce point.…”

On ISCPPandpPdata and their use in delay-time tomography of the Caribbean region

“…The arrival times of the PP-waves should be determined by correlation of the attenuated and Hilbert transformed P-wavelet with the part of the seismogram containing the reflected PP phase (Girardin 1980;Woodward 1989). In this study, the objective is to incorporate pP and PP phase data in the tomographic inversions, together with some hundreds of thousands of P phases: hence, instead of relying on a few high-quality data, as many published data as possible are used, assuming that the overall effect of larger reading errors in individual data will be averaged out (see also Stewart 1976;Darragh 1985).…”

“…1) (Stewart 1976;Woodward & Masters 1987;Grand 1987;Woodward 1989) and by depth phases ( p P and S P ) (Engdahl & Billington 1986;Engdahl & Gubbins 1987). In studies using reflected phases, usually the spatial distribution of residual differential times is investigated (Stewart 1976;Dorbath & Dorbath 1981;Girardin 1980;Woodward & Masters 1987). However, this approach has several disadvantages which are discussed below.…”