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Dynamics of the western United States

The full version of Dynamics of the Pacific-North American plate boundary zone in the western United States, Science, 287, 834-836, 2000. can be obtained in pdf format.

The vertically averaged deviatoric stress tensor field within the western United States was determined using topographic data, geoid data, recent GPS observations, and strain rate magnitudes and styles from Quaternary faults. Gravitational potential energy differences control the large fault-normal compression on the California coast. Deformation in the Basin and Range is driven, in part, by gravitational potential energy differences but extension directions there are modified by plate interaction stresses. The California shear zone has relatively low vertically averaged viscosity of about 10^21 Pascal seconds, while the Basin and Range has higher vertically averaged viscosity of 10^22Pascal seconds.














A self-consistent, continuous velocity field solution (black arrows) Shen-Tu et al. [1999] determined using GPS and VLBI data (red arrows) [Bennett et al., 1999; Ma and Ryan, 1998; Thatcher et al., 1999] Quaternary fault data [Jennings, 1994; Peterson and Wesnousky, 1994], and imposed NUVEL-1A plate motion [DeMets et al., 1994]. Ellipses represent a 95% confidence limit. Blue dots represent seismicity recorded from 1850-1998. (GV = Great Valley, B&R = Basin and Range, CP = Colorado Plateau, SAF = San Andreas Fault, SN = Sierra Nevada, GB = Great Basin). Longitude and latitude are given in degrees west and north.












The minimal root mean squared deviatoric stress field determined from GPE variations, calculated assuming Airy compensation of topography. Different colors represent delta GPE values, sigma zz, relative to a column of lithosphere at sea level. Tensional stress are open white principal axes and compressional stress are black principal axes.












Stress field boundary conditions. The analog motion (open arrows) associated with these boundary conditions has a PA-NA pole that is approximately 10 degrees west of the NUVEL-1A [DeMets et al., 1994] PA-NA pole. Tensional stress are open white principal axes and compressional stress are black principal axes.












The total vertically averaged (over L=100 km) deviatoric stress field that is the sum of stresses due to potential energy variations (above) and plate interaction (above).Tensional stress are open white principal axes and compressional stress are black principal axes.












The self-consistent flow field determined from strain rates calculated by scaling the total stress tensor field (above) by the inverse of viscosity (below) for all areas east of the San Andreas fault.












The total deviatoric stress field determined from the sum of stresses due to GPE variations estimated using the filtered GEOID96 and the corresponding best-fit stress field boundary conditions associated with PA-NA plate interaction. Tensional stress are open white principal axes and compressional stress are black principal axes.












The vertically averaged effective viscosity (over L=100 km) for the western US determined by dividing the magnitude of the total deviatoric stress (above) by the magnitude of the strain rate for each grid area determined from the self-consistent kinematic model Shen-Tu et al. [1999].

last edited 02/21/00 by L. Flesch