Reflection ofP′P′ seismic waves from discontinuities in the mantle

Abstract

A systematic study of the travel times and apparent velocities of precursors of the seismic core phase PKPPKP indicate that these phases are reflections from the mantle. The strongest reflection is from a depth of 630 km. In order of confidence, other reflectors were found at depths of 280, 520, 940, 410 (very weak), and 1250 km (tentative). The weakne• of the 410-km reflection was surprising in view of the large velocity increase at this depth indicated by refraction and Love-wave studies. This transition region must be broader than the others or must involve a smaller density jump. Reflections were observed that were possibly from the top and bottom of the low-velocity zone at depths of 50 and 130 km, respectively. The above reflections are interpreted in terms of the following solid-solid phase changes, in order of increasing depth: pyroxene-garnet solid solution, olivine --) B spinel, B spinel --) spinel and pyroxene --) spinel -[-stishovite, spinel --) post-spinel, and garnet --) ilmenite or oxides. A spin-spin transition in Fe 2' may be responsible for one of the deeper discontinuities found by others. Recent models of the structure of the upper mantle, such as those of Anderson and ToksSz [1963], Niazi and Anderson [1965], Archambeau et al. [1969], Johnson [1967], Ibrahim and Nuttli [1967], Green and Hales [1968], Julian and Anderson [1968], and Anderson and Julian [1969] include regions of high velocity gradient in the upper mantle. The sharpness of these discontinuities has led to new interest in looking for reflections of seismic waves from structure in the upper mantle Whircomb and Anderson [1968], and Niazi [1969] found reflections from the upper surfaces of discontinuities in the upper mantle. Another reflection of interest is the reflection of P'P' (PKPPKP) from the underneath surface of discontinuities.This reflection has the decided advantage of being in a quiet part of the seismic record before the main P'P' phase. Gutenberg [1960] noted arrivals up to 30 sec before P'P', and Adams [1968] interpreted P'P' precursors as reflections arriving up to 70 sec before the main phase. stations for reading the precursors, he could not recognize that some of his readings for the deep earthquakes were probably SKKKP, which, as E•gdahl and Flinn [1969a] have pointed out, has a different apparent slowness. However, Adams' remaining readings indicated discontinuities near 65 to 70 km and 160 to 180 km in depth. Engdahl and Flinn [1969b] found a precursor with the proper apparent slowness corresponding to a reflection depth of 650 km. Our notation is similar to that used by Bolt et al. [1969], who designated a reflection of the PP phase at a depth d in kilometers as PdP, in that P'dP' is used to indicate a reflection of the P'P' phase at depth d in kilometers. Thus, a reflection at 650 km would be P'650P', and P'OP' is equivalent to P'P'. Seismic records were searched systematically for P'P' precursors up to five minutes before the main phase and were analyzed for reflectors at depth. Two geographic regions of...

P and S wave travel time inversions for subducting slab under the island arcs of the northwest Pacific

“…From this system to whole mantle convection on the other end, the degree of depression is expected to be smaller. Observationally, a sharp 670-km discontinuity has previously been suggested based on precursors to P'P', denoted by P'610P' [e.g., Engdahl and Flinn, 1969;Whitcomb and Anderson, 1970;Richards, 1972;Husebye et al, 1977;Nakanishi, 1988].…”

Self-gravity, self-consistency, and self-organization in geodynamics and geochemistry

“…The evidence for large-scale stratification of the mantle includes tomographic patterns and spectra [e.g., Tanimoto, 1990;Scrivner and Anderson, 1992;Ray and Anderson, 1994;Wen and Anderson, 1995;Gu et al, 2001;Anderson, 2002a;Becker and Boschi, 2002;Trampert et al, 2004], dynamic topography and evidence for slab flattening, and apparent pileups [Fukao et al, 1992[Fukao et al, , 2001. Support for a complication, discontinuity, or barrier near 900-km depth is widespread [Whitcomb and Anderson, 1970;Tanimoto, 1990;Revenaugh and Jordan, 1991;Kawakatsu and Niu, 1994;Ritzwoller and Lavely, 1995;Wen and Anderson, 1995;Forte and Mitrovica, 2001;Shen et al, 2003;Mattern et al, 2004]. The tomographic structure of the mantle above the Repetti discontinuity-the upper mantle proper-correlates with present and past plate tectonics [Wen and Anderson, 1995;Becker and Boschi, 2002] and behaves as expected for fluid with a moderate Rayleigh number: cooled from above and driven by the plates and lithospheric architecture.…”

The Characteristics of J-Array Seismograms.

“…5(b). The precursors to P'P' (P'dP': d=410 or 660), which are underneath reflections at velocity discontinuities in the mantle, offer valuable information on the velocity jump, exact depths, and thicknesses of the transition zones (e.g., Engdahl and Flinn, 1969;Whitcomb and Anderson, 1970). Nakanishi (1988) showed the precursors to P'P' from the 660 and 410 km discontinuities beneath the midAtlantic ridge using waveforms from two events (mb =6.4 and 6.8) occurring in the Fiji and Tonga regions, obtained at stations of Hokkaido University.…”

“…Furthermore, there are several studies that support the existence of steep velocity gradient or discontinuity at the depth of around 1,200 km. In the pioneering study on precursors to P'P' by Whitcomb and Anderson (1970), very weak reflections at the 1,250 km depth were observed at stations in the United States, from events occurring in South America. Stunff et al (1995) revealed reflections at the depths of 785 and 1,180 km beneath the southern African rift.…”