Characterization of high-frequency strong ground motion

Abstract

Dense hydrous magnesium silicate (DHMS) phase A forms in cold subducted slabs after the breakdown of antigorite serpentine, and may play an important role in the transport of water within the upper mantle. In this paper we present acoustic velocities and the single-crystal elastic properties of Fe-bearing phase A, (Mg0.981Fe0.019)7Si2O8(OH)6, measured by Brillouin spectroscopy on a sample compressed to 12.4(2) GPa in a diamond anvil cell. A fit to the acoustic data using a 3rd order finite-strain EOS yields the following adiabatic bulk (KS) and shear (μ) moduli and their pressure derivatives: KS = 106(1) GPa, (∂KS/∂P)T0 = 5.8(3), μ = 61(1) GPa, (∂μ/∂P)T0 = 1.8(1). Within the experimental resolution, the pure longitudinal elastic constants, C11 and C33, and the off-diagonal C12 constants exhibit positive linear pressure dependence, whereas C44, C66 and C13 increase with a quadratic dependence on pressure. The axial compressibility of phase A remains highly anisotropic in the investigated pressure range, with the a-axis being 15% more compressible than the c-axis at 12.4(2) GPa.Compared to forsterite, the aggregate compressional (VP) and shear (VS) acoustic velocities of phase A are 7% slower at room pressure. Although the velocity contrast diminishes to 3.5% for VP, it is maintained for VS over the investigated pressure range. Phase A has high shear wave anisotropy (AS) and shear-wave polarization anisotropy (ASP0) of AS = 20% and ASP0=18%, and a more moderate compressional wave anisotropy AP = 12% at room pressure. The AP of phase A decreases to 8% at 12.4(2) GPa, remaining significantly lower than that of forsterite, whereas the shear anisotropy is nearly constant at ∼20% over the same pressure range and exceeds that of forsterite by 12% at 12.4(2) GPa. At upper mantle pressures, the shear wave splitting in phase A (ASP0) is 20% higher than in forsterite. The results of this study were used with thermoelastic data for other relevant minerals to compute the density, seismic velocities and VP/VS ratios of subducted garnet–harzbugite and moderately depleted harzburgite assemblages with various degrees of hydration as a function of pressure along a slab isotherm at 1073 K. The results suggest that the seismic velocities of dry and water-saturated harzburgites (44.5 vol% phase A) may be indistinguishable at upper mantle P–T conditions because of the increasing concentration of high-pressure orthopyroxene upon hydration. This phase displays high seismic velocities that offset the decrease in velocities due to phase A, rendering hydration difficult to detect (anelastic attenuation is not considered). Combined observations from the analysis of seismic parameters indicate that significant shear wave anisotropy, accompanied by high VP/VS and Poisson's ratios and pronounced shear wave splitting, could be major diagnostic features for identifying phase A-bearing assemblages at depth (180–350 km) in cold subducted slabs.