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Seismic Properties and Interpretation

The structure of the Earth's mantle interpreted through laboratory measurements of seismic wave speeds and attenuation: background and techniques

Seismic wave speeds typically increase with increasing depth in the Earth's mantle – the generally smooth variation evident in global average models being punctuated by discontinuous increases of 5-10% at depths near 410 and 660 km. Superimposed upon this radial variation is substantial lateral variability especially in the uppermost 300 km of the mantle and near the core-mantle boundary.

Are the discontinuities near 410 and 660 km depth simply the result of known pressure-induced changes in crystal structure or is the mantle sharply stratified in chemical composition? What causes the marked lateral variability of the shear wave speed and attenuation in the upper mantle? Variations in temperature? Compositional heterogeneity? Partial melting?

These questions are central to an understanding of the internal dynamical processes represented at the Earth's surface by continental drift and plate tectonics. Answers require measurements on appropriate materials performed under controlled laboratory conditions of pressure and temperature.

The variation of elastic wave speeds with pressure and temperature, like thermal expansion, arises from asymmetry of the interatomic potential energy. Such ‘anharmonic' variations of elastic wave speeds can be conveniently measured in the laboratory on mineral or rock specimens of ~0.1 mm to cm size at sufficiently high frequencies (MHz-GHz) by ultrasonic interferometry and opto-acoustic techniques, (see figure a. below). During the past decade, both single-crystal and coherent polycrystalline specimens of most of the major mantle minerals (including high-pressure phases) have been characterised with these high-frequency methods. Much has been learned about the pressure, and more recently temperature, dependence of their elastic wave speeds. However, substantial uncertainties remain – especially as regards the combined influence of pressure and temperature.



At the much lower frequencies of teleseismic wave propagation (< 1 Hz) the shear modulus and hence both shear and compressional wave speeds may be profoundly altered at high temperature, and in the presence of fluids, by viscoelastic relaxation. The stress-induced migration of crystal defects (vacancies, dislocations and grain boundaries) and/or redistribution of interstitial fluid results in additional strain and hence lower wave speeds accompanied by attenuation.

These effects have only recently become amenable to laboratory study through the methods of ‘mechanical spectroscopy' – the testing of cm-sized cylindrical specimens at mHz-Hz frequencies with sub-resonant torsional forced-oscillation and microcreep methods (see figure b. above).

The following are 2003 highlights of our ongoing long-term commitment to the development and application of both ultrasonic and forced-oscillation/microcreep methods.