Geology topics
J.E. Vidale
Publications and source records attributed to J.E. Vidale.
Sharpness of upper-mantle discontinuities determined from high-frequency reflections
AN understanding of the nature of seismic discontinuities in the Earth's upper mantle is important for understanding mantle processes: in particular, the amplitude and sharpness of these discontinuities are critical for assessing models of upper-mantle phase changes and chemical layering. So far, seismic studies aimed at determining the thickness and lateral variability of upper-mantle discontinuities have yielded equivocal results, particularly for the discontinuity at 410km depth1,2. Here we present short-period (0.8-2.0 s) recordings of upper-mantle precursors to the seismic phase P???P??? (PKPPKP) from two South American earthquakes recorded by the ???700-station short-period array in California. Our results show that the 410- and 660-km discontinuities beneath the Indian Ocean are locally simple and sharp, corresponding to transition zones of 4 km or less. These observations pose problems for mineral physics models3-5, which predict a transitional thickness greater than 6 km for the peridotite to ??-spinel phase transition. In contrast to the results of long-period studies6,7, we observe no short-period arrivals from near 520 km depth.
Upper-mantle seismic discontinuities and the thermal structure of subduction zones
The precise depths at which seismic velocities change abruptly in the upper mantle are revealed by the analysis of data from hundreds of seismometers across the western United States. The boundary near 410km depth is locally elevated, that near 660 km depressed. The depths of these boundaries, which mark phase transitions, provide an in situ thermometer in subduction zones: the observed temperature contrasts require at least moderate thickening of the subducting slab near 660 km depth. In addition, a reflector near 210 km depth may mark the bottom of the aesthenosphere.
A sharp and flat section of the core-mantle boundary
The transition zone between the Earth's core and mantle plays an important role as a boundary layer for mantle and core convection 1 . This zone conducts a large amount of heat from the core to the mantle, and contains at least one thermal boundary layer 2,3 ; the proximity of reactive silicates and molten iron leads to the possibility of zones of intermediate composition 4 . Here we investigate one region of the core–mantle boundary using seismic waves that are converted from shear to compressional waves by reflection at the boundary. The use of this phase (known as ScP), the large number of receiving stations, and the large aperture of our array all provide higher resolution than has previously been possible 5–7 . For the 350-km-long section of the core–mantle boundary under the northeast Pacific sampled by the reflections, the local boundary topography has an amplitude of less than 500 m, no sharp radial gradients exist in the 400 km above the boundary, and the mantle-to-core transition occurs over less than 1 km. The simplicity of the structure near and above the core–mantle boundary argues against chemical heterogeneity at the base of the mantle in this location.