Precursors to P‘P’ re-examined using broad-band data

Abstract

S U M M A R YObservations of underside P-wave reflections from discontinuities deep within the upper mantle which precede the P'P' phase have provided important constraints on the existence and physical properties of these dynamically critical features. In this study, we extend earlier work by examining for the first time, broad-band recordings of those earthquakes for which one would expect to observe P'P' and its associated precursors P'dP'. An exhaustive search of 20 yr of Carnegie data and 11 yr of Grafenberg data uncovered a number of fine P'P' observations, but in only one or perhaps two instances are there evidence of a precursor from near 670 km depth.These null results are consistent with what has been reported in earlier studies and provide us with an opportunity to estimate the variability of P velocity near 670 km or, alternatively, the scale length of discontinuity topography which acts to defocus the precursor. This has been done by using the reflectivity and Gaussian beam methods to compute synthetic seismograms for a suite of 1-D and 2-D earth models which vary slightly from PREM. We find that the null data may be satisfied either by (i) reducing the PREM impedance jump at 670 km from 8.5 to 4-5 per cent; (ii) smoothing this 8.5 per cent jump linearly over a zone 15-30km thick or (iii) imposing deformations in the 670 km discontinuity as small as 10 km in amplitude and 300 km in wavelength. For the one case of an observed precursor, the relative size of the underside reflection and main P'P' phase suggests that, in this instance, the precursor's amplitude is correctly predicted by PREM.

Gross errors in upper-mantle discontinuity topography from underside reflection data

“…also Davis et al 1989). Hence, the downwarped discontinuity is more easily observed in underside reflection data.…”

“…This diffraction is most pronounced for the depressed 670-km discontinuity, illustrating the fact that a downwarping of the discontinuity generates more efficient focusing of P,P energy than an upward deflection (see also Davis, Kind & Sacks 1989). The synthetics for mid-points about 8" away from the centre of the slab illustrate the sensitivity of PIP waves for offpath structure.…”

Topography of the 400 km discontinuity from observations of long-period P400P phases

“…This explains the character of the data in Fig. 3 and also explains the lack of well-developed P,P phases reported in the literature (Wajeman 1988;Davis et al 1989). This also renders this part of the seismogram unusable for waveform inversion, as the waveforms may be seriously distorted upon reflection at the discontinuity.…”

Deep Earth Structure: Seismic Scattering in the Deep Earth

“…In this case, however, short-period reflections from the 410 and 660 km discontinuities are much easier to observe, and this has become one of the best constraints on the sharpness of these features (e.g., Benz and Vidale, 1993;Davis et al, 1989;Engdahl and Flinn, 1969;Richards, 1972;Xu et al, 2003). In this case, however, short-period reflections from the 410 and 660 km discontinuities are much easier to observe, and this has become one of the best constraints on the sharpness of these features (e.g., Benz and Vidale, 1993;Davis et al, 1989;Engdahl and Flinn, 1969;Richards, 1972;Xu et al, 2003).…”