A seismic tomographic study of the Mojave region and geophysical constraints on thrust belt structure in eastern Tennessee

Julia Laws Whitelaw, University of Texas at El Paso

Abstract

This study employs a tomographic inversion using P wave first arrival times from earthquakes and explosions recorded on the Southern California Seismic Network, the Northern California Seismic Network, and the Southern Great Basin Seismic Network to determine the velocity structure and its effect on wave propagation through the Mojave block and southern Basin and Range.^ The tomography algorithm used in this study incorporates a priori information to add stability to the inversion, accounts for anisotropy, and includes data and model uncertainties. The actual equation solved in this inversion is unique because it includes the actual data, the a priori model, both data and model uncertainties, and the residual matrix between the observed and predicted travel times, which is minimized. The computer time used for this technique is significantly less than that of traditional error analysis methods used in geophysics. The information density coverage differs significantly from the hit density, indicating that the hit count does not provide a good estimate of solution uncertainty.^ Crustal velocities of the western Mojave are similar to those of the southern Basin and Range, in areas resolved with raypath coverage. Basin and Range low velocities correlate well with areas of high heat flow; however, the Mojave Block has lower heat flow than the Basin and Range in areas with low velocities. High velocities in the eastern Mojave exist in an area containing the highest heat flow values. The Garlock Fault appears to separate areas of lower velocities of the southern Basin and Range from higher velocities of the Mojave block at 35-40 km depth for the one-dimensional model and 30-40 km for the a priori model, suggesting that it may be a deep-seated feature. The distributions of velocities at these depths correlates well with heat flow. These velocity observations in the mantle reinforce the suggestions by other researchers that the Basin and Range is more active than the Mojave Block.^ The second part of the dissertation integrates gravity data, seismic reflection profiles, and surface geology in order to study the thrust-sheet geometry in the Valley and Ridge Province of eastern Tennessee and test the hypothesis that the integration of seismic reflection and gravity data will lead to a more reliable interpretation of the subsurface.^ Balancing cross-sections adds subsurface constraints to an interpretation by limiting the suite of possible structures along the line of sections.^ Seismic reflection data provide excellent constraints in areas with good data quality. "Crooked line" thrust belt acquisition geometry often yields the best reflections in areas oblique to the dip direction, providing valuable constraints that can be interpolated into a straight-line profile to balance. Data acquired in this geometry may also allow a 3-dimensional look at the lateral characteristics and continuity of a thrust sheet.^ Gravity data/modelling was a significant asset in helping constrain thrust belt geometry and unit thicknesses, when used in conjuction with balanced sections and seismic data. This method is strengthened when good density control is available. (Abstract shortened by UMI.) ^

Subject Area

Geophysics

Recommended Citation

Whitelaw, Julia Laws, "A seismic tomographic study of the Mojave region and geophysical constraints on thrust belt structure in eastern Tennessee" (1996). ETD Collection for University of Texas, El Paso. AAI9718118.
http://digitalcommons.utep.edu/dissertations/AAI9718118

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