Former Res.

The following list of former projects illustrates some of my interests that my students and I are not currently investigating. However, there is lots of interesting work still to be done in these areas and they represent opportunities for future work. Although a complete list of references for this work exists in my list of publications, selected references relevant to the work mentioned below is listed at the end of each topic for convenience.

The topics discussed below are listed in the following crudely chronological list of hyperlinks:

Thermodynamics of nonhydrostatic stress
The origin of slaty cleavage and schistosity
The driving force of plate tectonics
Measurement of stress in the Earth
Fault roughness
Fault permeability
Earthquake prediction

	Thermodynamics of nonhydrostatic stress. Systems under nonhydrostatic stress have more complex descriptions than those for which the pressure or mean stress is a complete description of the stress state. The PV work that is adequate for conventional thermodynamics descriptions of systems is no longer satisfactory and a number of interesting situations arise that requites careful treatment. This is a topic that was first treated by Gibbs and is of importance in many situations. One of these is the role that nonhydrostatic stress might play in the creation of crystallographic preferred orientations and whether or not equilibrium thermodynamics under nonhydrostatic stress is an important consideration in their origin. To this day, the only case in which such considerations are clearly responsible for the development of a preferred orientation is in the case of that produced by Dauphine twinning of quartz as found in the experiments conducted by Jan Tullis that we analyzed in the paper below. Tullis, J. and Tullis, T. E., 1972, Preferred orientation of quartz produced by mechanical Dauphine twinning: thermodynamics and axial experiments in H. Heard et al., eds., Flow and Fracture of Rocks, Am. Geophys. Union Monograph 16, 67-82.
	The origin of slaty cleavage and schistosity. The processes that cause micas to become oriented during deformation are still not well understood. Rotation of grains due to their shape seems to be able to explain the relationship between the intensity of the preferred orientation and the finite strain for natural slates (Tullis and Wood, 1975), and this process seemed to be able to explain the preferred orientations I produced during syntectonic recrystallization in high-temperature lab experiments (Tullis, 1976), study of carefully selected field samples showed that some other mechanism involving recrystallization must be important (Holeywell and Tullis, 1975). My current thoughts are that the orientation mechanism may involve the interaction of anisotropic growth and dissolution rates with a non hydrostatic stress field (Tullis, 1989), but more work needs to be done to pursue this idea. Tullis, T. E. and Wood, D. S., 1975, Correlation of finite strain from both reduction bodies and preferred orientation of mica in slate from Wales, Geol. Sci. Am. Bull., 86, 632-638. Holeywell, R. C. and Tullis, T. E., 1975, Mineral reorientation and slaty cleavage in the Martinsburg Formation, Lehigh Gap, Pennsylvania, Geol. Soc. Am. Bull., 86, 1296-1304. Tullis, T. E., 1976, Experiments on the origin of slaty cleavage and schistosity, Geol. Soc. Am Bull., 87, 745-753. Tullis, T. E., 1989, Development of preferred orientation due to anisotropic dissolution/growth rates during solution-transfer creep, Eos Trans. AGU, 70, 457-458.
	The driving forces of plate tectonics. Plate tectonics is the surface manifestation of thermal convection in the Earth. However, it is still unclear exactly how this process works in detail. Does flow in the asthenosphere and mesosphere go faster than the lithospheric plates, carrying them along, or are the forces mostly localized in the plates themselves, such as sliding down the elevated topography at ridges and being pulled by cold, dense subducting slabs? Such questions were addressed by Bill Chapple and me in a paper that used the variety of observed plate motions and variety of plate areas and boundary types to invert for the relative sizes of driving forces. We concluded that the concluded that the primary forces are slab pull, ridge push and drag under continents. Thus, our result suggests that the plates themselves are the sites of the primary forces driving them and that thermal convection in the underlying mantle is not important in making the plates move. Chapple, W. M. and Tullis, T. E., 1977, Evaluation of the forces that drive the plates, J. Geophys. Rev., 82, 1967-1984.
	Measurement of stress in the Earth. Knowledge of the magnitude and direction of stresses in the Earth is important for understanding many important problems in structural geology, tectonics and geophysics. A well-known example of these problems is the so-called heat-flow paradox on the San Andreas fault, namely the fact that the fault does not seem to generate as much frictional heat as we would expect from its long-term slip rate and the high shear stress we would expect it to have, based on typical values of the friction coefficient for rocks, and expected values for the normal effective stress if the pore pressure follows a hydrostatic gradient. One of the ways of resolving this question is to make in situ stress measurements. Those that I report in the paper below were made near the Earth's surface, so their magnitudes are not relevant to values at depth, but the directions may be similar to those at depth. The directions of the well determined maximum principal compressive stress close to the fault suggest that the shear stress on the fault may not be unusually low. Tullis, T. E., 1981, Stress measurements via shallow overcoring near the San Andreas Fault, Am. Geophys. Union Mon., 24, 199-213.
	Fault Roughness. A fascinating study of the roughness of fault surfaces was the topic of the Ph.D. research of Bill Power about 10 years ago. He found that over a wide range of scales the roughness of fault surfaces is self-similar, namely that the amplitude to wavelength ratio is independent of the wavelength. The value of this ratio is about 0.01 to 0.001. This can explain previous observations that the width of gouge in fault zones is proportional to the total slip, by noting that larger and larger slip requires higher and higher amplitude irregularities to be worn off and so contribute to increasing the thickness of the gouge. In addition, the roughness of fault surfaces in the slip direction is about ten times lower in the slip direction than at right angles to it, again regardless of scale. Thus, scratch-type slickensides on the small scale have their equivalence in slip-parallel grooves at all larger scales studied. Another interesting result is that fault surfaces are well mated at all scales studied. Thus, positive irregularities on one wall of the fault match negative irregularities on the other wall of the fault. This was true to quite fine scales. It opens up the question as to what processes allow the walls of the fault to accommodate one another and whether this accommodation occurs on the time scale of tectonic slip, or whether the matedness occurs during the post seismic period. This result has important implications for the value that Dc in the rate and state friction constitutive description would have for natural faults. More about continuing work on this can be seen in the section on Ongoing Research. Power, W. L., Tullis, T. E., Brown, S. R., Boitnott, G. N., and Scholz, C. H., 1987, Roughness of natural fault surfaces, Geophys. Res. Lett., 14, 29-32. Power, W. L., Tullis, T. E., and Weeks, J. D., 1988, Roughness and wear during brittle faulting, J. Geophys. Res., 93, 15268-15278. Power, W. L. and Tullis, T. E., 1989, The relationship between slickenside surfaces in fine-grained quartz and the seismic cycle, J. Structural Geology, 11, 879-893. Power, W. L. and Tullis, T. E., 1991, Euclidean and fractal models for the description of rock surface roughness, J. Geophys. Res., 96, 415-424. Power, W. L. and Tullis, T. E., The contact between opposing fault surfaces at Dixie Valley, Nevada and implications for fault mechanics, J. Geophys. Res, 97, 15425-15435, 1992. Power, W. L. and Tullis, T. E., A review of the fractal character of natural fault surfaces with implications for friction and the evolution of fault zones, in Fractals in The Earth Sciences, Paul Lapointe and Chris Barton, Eds., Plenum, New York, 89-105, 1995.
	Fault Permeability. We conducted a series of experiments on the permeability of experimental faults. We looked at the evolution of permeability and its anisotropy as a function of slip displacement. The permeability decreases by about three orders of magnitude as a function of slip to about 200 mm of slip. This is due to the reduction in grain size and hence pore dimension as a result of comminution. In quartz gouge, anisotropy of abut a factor of 10 develops, due to the fact that a localized slip zone of fine grained material develops in the fault zone that impedes flow across the fault. In mica gouge with an initial preferred orientation, an initial anisotropy of permeability of a factor of about 10 exists, and this is actually reduced to only about a half an order of magnitude because the shearing disrupts the preferred orientation. We also looked at the influence of shear displacement on the pressure sensitivity of permeability. We find that the pressure sensitivity of mica gouge is reduced with displacement. This has implications for one popular explanation for the apparent weakness of the San Andreas fault. The model of Jim Rice depends on high pore pressure that results from fluid flowing up from depth. It depends on a decrease of permeability with depth as a result of increasing pressure with depth. This explanation is less likely with the low values we have measured for the pressure dependence of permeability. The first part of our USGS Annual Report for FY98 more fully explains this and summarizes the results of our work on permeability. Zhang, Shuqing, and T.E. Tullis, The effect of fault slip on permeability and permeability anisotropy in quartz gouge, Tectonophysics, 295, 41-52 1998. Zhang, S., T.E. Tullis, and V.J. Scruggs, Permeability anisotropy and pressure dependency of permeability in experimentally sheared gouge materials, J. Structural Geol., 21, 795-806, 1999. Zhang, S., T.E. Tullis, and V.J. Scruggs, Implications of permeability and its anisotropy in a mica gouge for pore pressures in fault zones, Tectonophysics, The Hobbs Volume, in press, 2000.
	Earthquake Prediction. Although much of our computer modeling described under Ongoing Research is focused on trying to understand processes during earthquake nucleation that might help predict earthquakes, I obtained a grant to support graduate student Scott Costello who examined whether a controversial proposed earthquake prediction method actually worked. A summary of the results can be found by clicking here. Unfortunately the method did not pass the test, but the triggering of microseismicity that might help predict earthquakes still is a viable idea. As discussed under Future Research, I am considering some further studies of this type. Costello, S., and T. E. Tullis, Can free oscillations trigger foreshocks that allow earthquake prediction?, Geophys. Res. Lett., 26, 891-894, 1999.