Shear Attenuation Beneath the Central Pacific and Implications for Anelasticity and Hydration in the Oceanic Upper Mantle

Session: Advances in Seismic Imaging of Earth’s Mantle and Core and Implications for Convective Processes
Type: Oral
Date: 4/29/2020
Time: 11:15 AM
Room: 120 + 130
Description: 

Seismic attenuation in the mantle occurs as a consequence of transient grain-scale dissipative processes driven by the stress changes imparted by the passage of the seismic waves. Constraining the transient behavior is important for estimating temperature and the presence of water or partial melt. It can also illuminate the grain-scale deformation mechanisms that are responsible for the transient behavior. We have developed the first regional-scale model of upper-mantle shear attenuation (1/Q) in the middle of an oceanic plate. We measure long-period Rayleigh waves for 125 earthquakes recorded by the NoMelt array of ocean-bottom seismometers, which was located on 70-Myr Pacific seafloor in 2012. Attenuation and azimuthally anisotropic phase velocity are determined by approximating the wavefield as the interference of two plane waves. Q in the NoMelt lithosphere is higher than values from global attenuation models and, when compared with 3-Hz lithospheric Q shows very weak frequency dependence, revising previous interpretations. The effect of anelasticity on shear velocity (VS) is estimated from the ratio of observed velocity to predicted anharmonic velocity. We use laboratory-based parameters to predict the attenuation and velocity spectra that result from the superposition of a weakly frequency dependent high-temperature background and an absorption peak. We test a large range of frequencies for the position of the absorption peak (fe) and determine, at each depth, which values of fe predict Q and VS that can fit the NoMelt Q and VS values simultaneously. We show that between depths of 60 and 80 km the seismic models require an increase in fe by at least 3-4 orders of magnitude. Under the assumption that the absorption peak is caused by elastically accommodated grain-boundary sliding, this increase in fe reflects a decrease in grain-boundary viscosity of 3-4 orders of magnitude. A likely explanation is an increase in the water content of the mantle, with the base of the dehydrated lid located at ~70-km depth.

Presenting Author: Colleen A. Dalton

Authors