HR: 16:30h
AN: S44A-02 INVITED     [Abstracts]
TI: A Combination of Passive- and Active-Source Seismological Studies are Required to Unravel Crustal Seismic Anisotropy Produced by Tectonic Deformation
AU: * Okaya, D
EM: okaya@usc.edu
AF: Univ. Southern California, Dept Earth Sciences, USC, Los Angeles, CA 90089-0740 United States
AU: Wilson, C
EM: wilsonck@ldeo.columbia.edu
AF: Lamont Doherty Earth Observatory, PO. Box 1000, Palisades, NY 10964-1000 United States
AB: Physical processes that produce rock fabrics during deformation are shearing and rotation, ductile flattening, recrystallization, and chemical solution and precipitation. These fabrics possess material anisotropy and can be localized or be pervasive for tens to hundreds of kilometers. This material anisotropy is caused by regional fractures and cracks, isotropic heterogeneity or layering, and aligned material composition and textural properties. Because the fabric-forming processes are associated with tectonics and metamorphism, the orientation and magnitude of anisotropy may serve as proxies for crustal deformation such as extensional lateral flow, channel flow, extrusion and exhumation, and obduction of accreted (schistose) terranes. Recent studies exist which have identified intracrustal anisotropic zones using shear wave splitting, traveltime delays, receiver function back-azimuth effects, and orthogonal refraction profiles. The primary factor in the production of crustal seismic anisotropy is the relative angle between a seismic wave and the (dipping) symmetry axes representing the crustal material even as either change along the propagation raypath. Because anisotropy-producing material is truly three dimensional in shape and internal orientation within the crust, different directions of seismic raypath directions are required to provide enough observations sufficient for anisotropic analysis. This requires the combination of passive- and active-source seismic observations using teleseismic waves (near-vertical paths), local seismicity (oblique), seismic reflections (vertical), and/or seismic refraction (horizontal). Each seismological method by itself has advantages and disadvantages for seismic anisotropy; thus a combination of these methods is needed. We examine these strengths and weaknesses, plus seismological factors such as associated sources and generated phases, bandwidth with respect to the scale of the anisotropic features, propagation through geological 3D geometry and heterogeneity, bending of raypaths due to velocity gradients, and potential waveform alteration at the free surface. We also note that given structural geometries within the crust, full 360 degree azimuthal observations are often necessary, and petrophysical measurements are vital to be performed on rocks collected within the region of interest. Finally, many previous seismic experiments have had widely spaced receiver stations so that small magnitude anisotropy effects could be attributed to other factors (e.g., near-receiver station statics, sensor misalignment). However, the improvements to the IRIS PASSCAL instrument pool and the upcoming existence of the USArray facilities within EarthScope provides new and exciting opportunities to deploy dense, high-resolution seismic arrays to identify and use crustal anisotropy for tectonic mapping.
DE: 7203 Body waves
DE: 7205 Continental crust (1219)
DE: 7218 Lithosphere (1236)
DE: 8025 Mesoscopic fabrics
DE: 8110 Continental tectonics: general (0905)
SC: Seismology [S]
MN: Fall Meeting 2005