Seismic velocities, anisotropy and elastic properties of crystalline rocks and implications for interpretation of seismic data

The knowledge of seismic and elastic properties of polyphase crystalline rocks under high pressure and temperature conditions is fundamental in interpreting in-situ seismic data (e.g., reflections, refractions, received functions, and tomography). These data may be understood in terms of lithology, mineralogy, and physical state and as such they allow establishing lithospheric structure and composition models of continents. The present study aims to better understand how the seismic and elastic properties [e.g., compressional- and shear-wave velocities (Vp and Vs), anisotropy, and elastic parameters] under confining pressure are affected by modal and chemical compositions, microstructures, lattice preferred orientations (LPO) of rocks and by the geometry and state of microcracks.;This thesis consists of six chapters. Chapter 1 addresses the framework of rock seismic property studies, focusing on the concepts of elasticity and mixture rules, and providing an overview on results of statistical analysis on previous laboratory-measured seismic data of different lithologies and rock-forming minerals, and on the seismic anisotropy of the continental crust and upper mantle.;Chapter 2 deals with seismic and elastic properties measured at hydrostatic pressures up to 800 MPa for 12 representative samples from the Longmen Shan complex in which the great 2008 Wenchuan earthquake took place. These allowed understanding how coseismic ruptures nucleated and propagated. This study also offers necessary information for broadband simulations of strong ground motions in the assessment and forecast of earthquake hazards in the region. Furthermore, the study, which yields a moment magnitude of 7.9-8.0 given the variation in the dip of the coseismic ruptures and the uncertainty in the depth to which the coseismic rupture may propagate downwards below the depth of the mainshock hypocenter, presents the first accurate quantification of the 2008 Wenchuan earthquake’s size.;Chapter 3 is the strength of this thesis, which provides a detailed study of seismic properties (P- and S-wave velocities, hysteresis, anisotropy and shear wave splitting) on a unique suite of deep borehole core samples from the Chinese Continental Scientific Drilling (CCSD) project. The data show that the velocity-pressure data can successfully provide important hints about the preferred orientation of microcracks that causes P-wave velocity anisotropy and shear wave splitting in cracked rocks, and that the effect of compression on the Vp/Vs ratios is negligible for crack-free compacted rocks. The seismic velocities of equivalent isotropic (fabric-free) and crack-free crystalline aggregates calculated from room pressure single crystal elastic constants using the Voigt average are in agreement with laboratory data at ∼200 MPa. Comparison of the seismic reflection image from the vicinity of the borehole with the normal-incidence reflection coefficient profile computed from the laboratory-measured velocities and densities infers that the seismic reflections originate from mafic (eclogite and retrograde eclogite) or ultramafic units within dominantly felsic rocks.;Chapter 4 is devoted to the characterization of Lamé constants for common types of crystalline rocks in the Earth's crust and upper mantle and their variations with pressure (P), temperature (T) and mineralogical composition. The analysis was based on the equivalent isotropic elastic data of 475 natural rocks, reported in the literature. Lamé parameter (λ) and shear modulus (μ) are the most important, intrinsic, elastic properties of rocks. When no partial melting, metamorphic reaction, dehydration or phase transformation occurs, λ of a crystalline rock as a function of P and T can be described by λ=a+(dλ/dP)P- cexp(-kP)-(dλ/dT)T, where a is the projected λ value at zero pressure if microcracks were fully closed; dλ/dP is the pressure derivative in the linear elastic regime; c is the initial λ drop caused by the presence of microcracks at zero pressure; k is a decay constant of the λ drop in the nonlinear poro-elastic regime; and dλ/dT is the temperature derivative. λ increases nonlinearly and linearly with increasing pressure at low (∼300 MPa) pressures, respectively. (Abstract shortened by UMI.).