Mountain-building, earthquakes and other expressions of continental tectonics depend fundamentally on how rocks flow in response to stress. Rock flow properties depend upon temperature, rock type and fluid content, none of which are easily measured at depth, thus limiting our fundamental understanding of tectonic processes. This project will combine gravity and topography data with new tools for seismic imaging and new deformation measurements and modeling tools to carefully measure mass density variations in the Earth and the rock flow that inevitably must accommodate them. By measuring how rock flow responds to large vertical stresses, or ?loads?, that result from piling of sediments or volcanic flows on the Earth?s surface, from intrusion of magmas into the crust, and from thermal and crustal thickness variations, we can better understand flow properties of rock and also determine how these flow properties change from one place to another. The project?s scientific objectives have potentially far-reaching implications for our fundamental understanding of earthquake physics, seismic hazard and mountain-building processes. Knowledge of rock flow properties has the potential to greatly improve our understanding of the earthquake cycle and evolution of stress on faults, and may help to inform studies of glacial melting and other climatological changes. Â This project develops an innovative approach to estimating rheological parameters (and effective flow viscosity) of the lithosphere from stochastic inversion of dynamical models of gravity, topography, surface heat flow and geodetic data, coupled with new analysis tools for seismic measurement products. A key innovation will be the circumvention of errors commonly introduced in modeling of seismic velocity fields by inverting seismic measurements (e.g. receiver function amplitude stacks) in combination with the other data for desired 3D fields of mass and temperature. These in turn will be used as inputs to dynamical models, which will employ stochastic methods to invert for stress, strain rate and 3D variations in rheological parameters at shallow (lithospheric) depths. Forward modeling of Earth deformation incorporating 3D viscosity heterogeneity at shallow (lithospheric) depths suggests that lateral variations in flow rheology exert a very fundamental control on horizontal velocities and strains at the Earth?s surface. Stochastic inversion approaches to estimating lithospheric flexural strength in continental interiors exhibit strong correlation of sharp gradients in strength with locations of intracontinental seismic belts and geodetic strain focusing. Stimulated in part by the wealth of new data accruing from the EarthScope Major Research Equipment initiative, as well as by recent revolutions in data analysis methodologies and computing power, the project will examine the fundamental question of whether rheology does in fact exert a first-order control on intraplate deformation and explore whether stochastically inverted estimates of lithospheric rheology may illuminate seismic hazard. Project research will also explore mechanisms for (and possible utility of) observed azimuthal anisotropy of isostatic response as well as possible reasons for a discrepancy in estimates of shallow viscosity from long-term isostatic response versus from postseismic and Pleistocene lake rebound studies. The principal scientific products will be new, fully three-dimensional estimates of shallow (lithospheric) mass density, temperature and flow rheological parameters that will be made available to the scientific community and can be used to constrain deformation modeling, or as a means of separating out solid-Earth viscoelastic signals that are intertwined with other desirable signals such as fault slip or ice mass loading histories.
