Fine Scale Structure of the Upper Mantle Using Array Technologies

Arizona State University, Scottsdale AZ

Investigators

Abstract

The internal boundaries (seismic discontinuities) of Earth's upper mantle down to depths of the mantle transition zone at 700 km contain important information about the thermal, compositional and dynamical state of the Earth's interior. High resolution studies of this region of the Earth provide important constraints on the internal works of this planet. Most upper mantle and transition zone studies image structure at intermediate to long wavelength (500 - 1000 km), thus lacking resolution to study structure at shorter scales (10's km or less). This proposal focuses on using seismic array data and array processing technologies to improve fine scale resolution of this region. A seismic array is a seismometer network that permits analysis of Earth structure though time series stacking to enhance signal to noise ratios of coherent arrivals over incoherent energy. Improved signal to noise ratios allows studying Earth structure at short scale lengths. Arrays permit measurement of the incidence angle of the wavefield, therefore allowing better phase characterization and identification. Using data from two arrays located in Australia (Warramunga Array) and India (Gauribidanur Array) we target three main objectives: 1) Resolving fine scale structure of the 410-km and 660-km discontinuities through array analysis of the Pwave triplication beneath Australia and India; 2) Studying the nature of the 210-km discontinuity beneath oceanic regions using PP scattering; 3) Detection, characterization and modeling of reflectors in the uppermost lower mantle through Pto- P and S-to-P reflections and conversions. Using a large dataset including hundreds of earthquakes recorded at the two arrays will give us an unprecedented resolution beneath northern Australia and beneath north and northeast India. The higher resolution will facilitate a more direct comparison to mineral physical and geodynamical implications including the distribution of water in the mantle, key features of the mantle transitions zone discontinuities (thickness, seismic velocity gradient, and discontinuity sharpness), the presence or absence of an intermediate depth discontinuity at 520-km and the existence of partially molten material above 410-km depth.