Mary Lou Zoback. Research Geophysicist at the U. S. Geological Survey, Menlo Park, CA. B.S. 1974, M.S. 1975, and Ph.D., 1978, all in Geophysics and all from Stanford University. National Research Council Post-Doc 1978-1979 with USGS Heat Flow Studies group, research scientist in the Earthquake Studies office at the USGS since 1979, presently in Seismology Branch. Major area of interest is active tectonics, with emphasis on the relationship of the in-situ tectonic stress field to earthquake deformation. Regions of interest range from the Basin and Range province, the San Andreas fault system, and intraplate regions. Leader, World Stress Map Project of the International Lithosphere Program (1986-1992), this project involved more than 40 scientists from over 30 different countries with the objective of compiling and interpreting geologic and geophysical data on the present day tectonic stress field to infer the relative magnitudes of the different forces acting on the lithosphere. Lead to a special issue of JGR, July 1992.
Past member of U. S. Geodynamics Committee (National Research Council); Editorial Board, GEOLOGY; NSF review panel for the Continental Dynamics program; and National Research Council Panel on Coupled Hydrologic/Tectonic/Hydrothermal Systems at Yucca Mountain. Past member of Geological Society of America Council and Executive Board; past-President, GSA Cordilleran Section; past-Chair, GSA Geophysics Division. Honors: AGU Macelwane Award (1987), Elected National Academy of Sciences (1995), USGS Gilbert Fellowship Award for a one year sabbatical in Karlsruhe, Germany (1990-1991), Fellow, GSA (1984), Fellow, AGU (1987).
(contributions from Mark Zoback, Lanbo Liu, Tony Crone, Mike Machette, and Randy Richardson)
During the past two decades, important constraints on the processes driving intraplate seismicity have been defined. In most intraplate areas, in situ stress measurements, bore-hole breakouts, and faulting styles inferred from earthquake focal mechanisms yield a relatively simple picture in which the seismicity results from a regionally uniform principal stress field that correlates with the stresses expected from plate-driving forces. Such a stress field is not likely to vary much with time except as a result of major plate reorganizations or changes in plate motions. In intraplate areas of moderate to high heat flow, the plate-driving forces are sufficiently large (and the lower crust and upper mantle may be sufficiently weak) that stress is transmitted mainly through the brittle upper crust. In areas of relatively low heat flow, such as shields (~40 mW/m2), the lower crust and upper mantle appear to be so strong that the cumulative strength of the lithosphere exceeds the plate-driving forces. Thus, the contrast between seismicity in shield areas and surrounding non-shield areas may largely result from whether the lower crust and mantle support an appreciable fraction of the plate-driving forces. In some places, local stress perturbations due to lateral variations in crustal structure may modify the stress field; thus, local stresses can help explain the common association of seismicity with ancient structures such as old continental rifts or suture zones.
Intraplate deformation from large earthquakes appears to be episodic, with active periods (of one or more moderate to large events) separated by lengthy periods of quiescence. In contrast to the regionally and temporally uniform intraplate stress field, geologic and geodetic strain rates indicate marked contrasts between short-term (<10,000 years?) and long-term (>1 million years?) deformation rates. Reoccupation of triangulation stations in the New Madrid seismic zone indicate a strain rate roughly one-third that of the San Andreas fault. Paleoliquefation and trenching studies of the New Madrid and Charleston earthquakes suggest multiple Holocene events, but there are no surface fault scarps or other evidence of comparably high, long-term deformation rates. Trenching and geologic studies of recent intraplate surface ruptures in Australia, India, Canada, and Oklahoma, (USA) indicate time intervals between active episodes of at least tens to hundreds of thousands of years, and possibly millions of years (Ungava, Canada).
A key to understanding intraplate seismicity is defining the self-limiting, feedback processes responsible for time-varying rates of deformation. One such process could be a slow pore-pressure build-up at depth over millions of years from grain boundary flow culminating in a period of active faulting. The resulting deformation and fracturing of the surrounding crust could permit relatively rapid dissipation of the high pore pressure through fracture-induced flow. Continued grain boundary fluid flow at depth could then geochemically seal the newly-opened fractures, permitting pore pressure to rebuild.
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