Characterization of Small-scale Heterogeneity in the Deep Earth

University Of Connecticut, Storrs CT

Investigators

Abstract

Little is known about three-dimensional structures in the deep earth whose size is less than 100 km. These small-scale deep structures are important to understanding the historical and future evolution of earth's composition and its surface tectonics. They affect the shapes and amplitudes of seismic waves used to forecast the size of earthquake ground motions and to predict the differences between the seismic signatures of earthquakes and underground nuclear tests. Such small-scale structures cannot be imaged by conventiional seismic tomography, which processes travel times of seismic waves with methods similar to medical CAT scanning. Instead, this project employs a technique using seiismic recordings by many, closely spaced, seismometers to retrieve a characterisitic spectrum of scale lengths of small-scale structures. This specturm constrains the sizes, shapes, and composition of the heterogeneities. The project also trains graduate students in seismic data retrieval, processing, imaging, and computer programming. These are skills important in resource exploration and remediation of contaminated groundwater. Results will be incorporated in undergraduate courses in the physical sciences to enhance recruitment and training in both academic and industrial applications of geophysical research. The characterization of small-scale heterogeneity (<100 km) is important to understand the surface sampling of Earth's mantle chemistry and the time history and nature of chemical mixing controlled by convection and plate tectonics. Directionally dependent scattering of seismic waves by small-scale heterogeneity can affect estimates of viscoelastic attenuation and anisotropy, which in turn are important for estimating temperature, mineral composition, and mineral phase. Amplitude and phase coherences of body waves recorded by dense arrays of seismic stations will be inverted to image statistical realizations of small-scale heterogeneity. This process of stochastic tomography will determine the characteristic lenth scales and seismic velocity perturbatons of the small-scale chemical and thermal heterogeneity of the earth that cannot be resolved by conventional tomography. The resullts will and test fundamental assumptions of convective miixing predicted from geodynamic models.