Reflection properties of phase transition and compositional change models of the 670-km Discontinuity

S U M M A R YA generalized inversion of a new data set, consisting of surface wave phase velocities and S, , traveltimes, has resulted in a new shear velocity and density model for the west-European platform (WEPL2). The surface wave phase velocity data set consists of fundamental and higher mode (mode 1-6) measurements from 25 to 70 mHz, using the Network of Autonomously Recording Stations (NARS) array, which was specifically designed for higher mode surface wave measurements. The S, , traveltime data set consists of ISC readings for selected European events. Major features of WEPL2 are a high-velocity lid of 80-140km depth, underlain by a pronounced low-velocity zone between 160 and 220 km depth. The transition zone (400-650 km depth) is characterized by high shear velocities with respect to PREM. The density shows a positive gradient from 60 to 140km depth, followed by an approximately constant value down to 400 km depth. A model for the Baltic shield (SCSH2) is obtained by combination of Nolet's (1977) higher mode phase velocity data set €or western Europe including the Baltic shield and the present phase velocities for the platform. This shield model shows an absence of the low-velocity layer and low shear velocities in the top of the transition layer with respect to WEPL2. Density, although less well resolved, shows lower values for the shield structure between 200 and 350 km depth.

Section: Absolute Velocity 300-400 Krn Depthcontrasting

confidence: 63%

Upper Mantle Structure Under Western Europe From Fundamental and Higher Mode Surface Waves Using the Nars Array

Dost

1990

“…This result contradicts seismological studies of P'P' precursors (Lees, Bukowinski & Jeanloz 1983;Benz & Vidale 1993), which suggest a simple and sharp 410 km discontinuity. The velocity discontinuity at 660 km depth corresponds to a phase change over 5 km from y-spinel to perovskite and magnesiowustite (Helffrich & Bina 1994).…”

Section: Introductioncontrasting

confidence: 88%

Velocity structure of upper-mantle transition zones beneath central eurasia from seismic inversion using genetic algorithms

Neves¹,

Singh²

1996

S U M M A R YWe present velocity constraints for the upper-mantle transition zones beneath Central Siberia based on observations of the 1982 RIFT Deep Seismic Sounding (DSS) profile. The data consist of seismic recordings of a nuclear explosion in north-western Siberia along a 2600 km long seismic profile extending from the Yamal Peninsula to Lake Baikal. We invert seismic data from the mantle transition zones using a non-linear inversion scheme using a genetic algorithm for optimization and the WKBJ method to compute the synthetic seismograms. A statistical error analysis using a graphbinning technique was performed to provide uncertainty values in the velocity models.Our best model for the upper-mantle velocity discontinuity near 410 km depth has a two-stage velocity-gradient structure, with velocities increasing from 8.70-9.25 km s-' over a depth range of 400-415 km, a gradient of 0.0433 s -', and from 9.25-9.60 km s-' over a depth range of 415-435 km, a gradient of 0.0175 SKI. This derived model is consistent with other seismological observations and mineral-physics models. The model for the velocity discontinuity near 660 km depth is simple, sharp and includes velocities increasing from 10.15 km s-l at 655 km depth to 10.70 km s-' at 660 km depth, a gradient of 0.055 s-'.

“…Calculating the PP reflection coefficient of the transition zone structure, Richards (1972) showed that a transition zone width of less than 4km is required to generate an observable P&,l" signal. Lees, Bukowinski & Jeanloz (1983) suggested that a discontinuous change in phase and chemical composition is required to explain P:,,P'/P'P' of about 0.1. In the present paper, we analyse the precursors to P'P' observed by a local seismic network in Hokkaido region, Japan from a large deep earthquake in the Fiji Islands.…”

Section: Introductionmentioning

confidence: 99%

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

Nakanishi

1988

SUMMARY Precursors to P'P’ observed in Japan from a large deep earthquake (mb= 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'P', respectively. Little deviations of the ray incidence off the diametral azimuth are found for both phases. Amplitude ratio P'dP'/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'655P’ is consistent with the theoretical value for the 670 km discontinuity of the PREM model (Dziewonski & Anderson 1981). P'415P’ signals were very weak and it was difficult to obtain an unbiased estimate of P'415P'/P'P'. The amplitude ratios P'655P'/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'655P'/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.