Reflections of P'P' from upper mantle discontinuities beneath the Mid-Atlantic Ridge

• Published in: Geophysical Journal International
• Main Author: Nakanishi, Ichiro
• Format: Text
• Language: English
• Published: Oxford University Press 1988
• Subjects: Articles; Mid-Atlantic Ridge; Antarc*; Antarctica
• Online Access: http://gji.oxfordjournals.org/cgi/content/short/93/2/335; https://doi.org/10.1111/j.1365-246X.1988.tb02006.x

Precursors to P′P′ observed in Japan from a large deep earthquake ( m b = 6.4) in the Fiji Islands are analysed to investigate reflection properties of upper mantle discontinuities beneath an area close to the Mid-Atlantic Ridge (MAR) in the equatorial Atlantic Ocean. Traveltime analysis indicates that the precursors are interpreted as underside reflections of P′P′ from discontinuities at depths of 655 and 415 km in the mantle. The latter reflection is much weaker and more intermittent than the former. Slant-stacking and beam-forming are attempted to measure the slowness and incidence azimuth of P′P′ and its precursors. The obtained slownesses are 3.4 and 2.8 s deg−1 for P′P′ and P′ 655 P′, respectively. Little deviations of the ray incidence off the diametral azimuth are found for both phases. Amplitude ratio P′ d P′/P′P′ (here d = 655 or 415) is studied as a function of the wave period and the distance of reflection point of P′P′ to the axis of MAR. The overall trend of the spectral amplitude ratio for P′ 655 P′ is consistent with the theoretical value for the 670 km discontinuity of the PREM model (Dziewonski & Anderson 1981). P′ 415 P′ signals were very weak and it was difficult to obtain an unbiased estimate of P′ 415 P′/P′P′. The amplitude ratios P′ 655 P′/P′P′ of filtered seismograms with a passband of 0.5-4.0 s have a mean value of 0.13 and a standard deviation of 0.03. This amplitude ratio is in good agreement with those of previous studies on the reflections from the 650-km discontinuity beneath Antarctica and surrounding seas. When the reflection points of P′P′ are calculated under the assumption of a spherically symmetric earth, the observed P′ 655 P′/P′P′ seems to decrease as the points become close to the axis of MAR. Thus it could be inferred that some changes in the reflectivity of the discontinuity might occur at or near the spreading center. Another interpretation may also be possible. The low velocity beneath the MAR region would cause focussing of P′P′ rays.