Collaborative Research: Seismic Attenuation and Anelasticity in the Upper Mantle: The Effect of Continuous Far-field Dislocation Creep
Thermal convection in the Earth's mantle drives plate tectonics, at the origin of numerous hazards such as earthquakes and volcanic eruptions. The upper mantle, which lies beneath the crust to depths of 410 km, is largely unreachable. Seismology is, thus, a major tool when investigating mantle features. Seismic-wave energy can be absorbed by the materials through which they pass, a process called seismic attenuation. This allows to identify structures at depth like the presence of melt. Each attenuation process must first be characterized in the laboratory. Wave amplitude can be attenuated by back-and-forth motions of dislocations, which are linear defects shearing minerals during deformation. Here, the team measures experimentally the effects of rocks' microstructure, such as dislocations and grain orientations, on the attenuation of seismic waves. The researchers use water ice as analog for mantle rocks because ice physical properties are well known. Ice can also be deformed to high strain at modest pressure and stress conditions. During deformation, ice specimens are here exposed to low-amplitude oscillating stress like those induced by seismic waves, while the attenuation is quantified. Results from this research provide critical insights to understand glaciers' and ice-sheet behavior and the role of rocks microstructures on seismic attenuation. The project has direct implications in Seismology and broader impacts in Material Sciences and Planetary Science (icy planetary bodies). It also provides support for an early-career female scientist, training for a post-doctoral associate in the field of Rocks Physics and outreach to high-school students.
The team expands on their previous studies of ice behavior by exploring seismic-wave attenuation as a function of strain in polycrystalline ice. The study is designed to explore upper mantle conditions in terms of microstructure: dislocation density, sub-grain size and crystal preferred orientation. The goal is to measure the attenuation signature for materials experiencing a high background stress in the dislocation creep regime. Preliminary laboratory studies have identified an increase in attenuation in highly strained samples. Yet, the controlling parameters and the nature of how dislocation damping scales to the mantle is not known. Here, samples are pre-deformed to high strain in either shear torsion or compression in the high-pressure cryogenic deformation apparatus at University of Pennsylvania. Their microstructure is characterized by light microscopy and back scatter diffraction in a cryo-scanning electron microscope. Specimens are then tested for attenuation over a broad frequency range in the ambient-pressure cryogenic apparatus at Lamont-Doherty Earth Observatory (Columbia University). The results are used to estimate the effect of both mantle stress magnitude and fabric strength on seismic attenuation. They will also have applications to glaciers and ice sheets on Earth and icy planetary bodies experiencing tidal forcing.