Particle dynamics simulations of fault and gouge
evolution and mechanics
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Principal Investigator: |
Juli Morgan (Rice University) |
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Collaborator: |
Chris Marone (Penn State Univ.) |
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Graduate Student: |
Yonggui Guo (now postdoc) |
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Undergraduate Students: |
Adam Halpert |
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Funding Sources: |
NSF EAR (Geophysics/Tectonics) |
The study of controls, processes, and deformation
behaviors of large fault zones, especially along active plate margins, has been
the focus of much recent lab, field, and theoretical work. These shear zones are often
characterized by broad zones of deformation, anastamosing fault strands, damage
zones, and thick accummulations of gouge. My recent research interests are on
the gouge, which separates the fault blocks and softens the fault zones. Gouge derives from the disaggregation
and alteration of wall rocks, and appears to evolve through time. Distinctive textures and fabrics (such
as grain size and PSD) in the gouge preserve a record of this evolution, and
probably have strong influence on the strength of fault zone, the deformation
behavior, and most likely, the seismogenic potential. Despite numerous lab and field studies focusing on
correlating gouge properties and structure with strength and behavior, we still
have very little understanding of the respective controls.
Due to the discrete nature of fault gouge, fault zone
properties and processes tend to be very heterogeneous, and marked by distinct
discontinuities, i.e., zones of localized slip. Such complexity is difficult to capture using typical
continuum theories or numerical techniques. Particle dynamics simulations offer a unique numerical
approach to study these granular shear zones, preserving the discrete character
of the material and enabling direct correlations among granular micromechanics
and shear zone behavior. Our group
has carried out DEM simulations of granular shear zones, exploring controls on
deformational fabrics, shear zone strength, and frictional response.
Our initial work in this area focused on simulations
of granular shear, and correlations between the transient internal deformation
and shear strength and frictional sliding behavior. Four animations can be viewed or downloaded from HERE.
Our recent numerical studies of fault processes using
particle dynamics techniques (i.e., discrete element method, DEM) have focused
on wall rock damage, gouge production, comminution, and deformation, and the
influences on fault zone strength and frictional behavior [Guo and Morgan,
2006, in revision-a, in revision-b].
For example, two cohesive blocks constructed of bonded particles and
separated by a predefined fault surface, are displaced past each other under a
constant normal stress (Figure 1).
The shear resistance to fault slip results in fracture of the wall
rocks, plucking of grains from the walls, and the gradual accumulation of
granular fault gouge which undergoes continuing comminution with shear
strain.
With increasing shear deformation, the gouge zone
increases in width, and undergoes changes in particle size distribution as a
function of the applied normal stress, the bond strengths, and the shear
displacement (Figure 2). By 200%
shear strain, moderate strength blocks exposed to high normal stress of 100 MPa
have produced a narrow zone of highly comminuted fault gouge (Figure 2a),
whereas the same strength blocks under normal stress of only 25 MPa, yields a
wider zone of course, poorly sorted fault gouge (Figure 2b). Increased and decreased bond strength
blocks sheared under 25 MPa produce comparatively courser and finer grained
fault gouge respectively (Figures 2c and 2d, respectively).
Figure
2
Additionally, fault friction and friction fluctuations
are strongly influenced by all of these parameters, showing pronounced changes
with increasing shear strain (Figure 3).
Generally, fault zones composed of higher strength rocks or deformed
under lower normal stresses exhibit increasing peak friction and higher
friction fluctuations with increasing slip, whereas those composed of lower
strength rocks, or deformed under higher normal stresses exhibit decreasing
peak friction and reduced friction fluctuations with displacement.
Figure 3
The varying frictional behavior of these systems
reflects the evolving grain size distribution and gouge thickness, and their
effects on fault zone dilation and grain and fault surface interlocking. These results demonstrate that the full
deformation history of fault zones influences final gouge configuration.
Figure
4
Another avenue of research involves implementing
rate- and state-dependent contact laws in particle dynamics simulations in
order to better capture the time- and velocity-varying behavior of fault zones
[Morgan, 2004]. Preliminary
investigations on granular (non-cohesive) assemblages have demonstrated that
such contact laws increase the tendency for persistent strain localization
within fault gouge, due to progressive strengthening of contacts that have not
slipped. These laws also reproduce
time-dependent strengthening of the assemblage during periods without slip
(Figure 4), as well as direct and evolving changes in shear strength (i.e., friction)
in response to steps in sliding velocity (Figure 5).
Figure 5
Papers:
Guo, Y, and J.K. Morgan, in revision-a, Fault gouge evolution and its dependence on normal stress and rock strength: Results of discrete element simulations 1. Gouge zone properties, J. Geophys. Res.
Guo, Y, and J.K. Morgan, in revision-b, Fault gouge evolution and its dependence on normal stress and rock strength: Results of discrete element simulations 2. Gouge zone micromechanics, J. Geophys. Res.
Guo, Y., and J.K. Morgan, 2006, The frictional and micromechanical effects of grain comminution in fault gouge from distinct element simulations, J. Geophys. Res., 111, B12406, doi:10.1029/2005JB004049.
Morgan, J.K., Guo, Y., and Marone, C.J., 2005, Comparative Laboratory and numerical simulations of shearing granular fault gouge, in 4th ACES Workshop Proceedings, edited by X.C. Yin, et al., APEC Cooperation for Earthquake Simulation.
Guo, Y., and Morgan, J.K., 2005, Role of grain fracture in sliding friction and fault strength: Results of DEM simulations, in 4th ACES Workshop Proceedings, edited by X.C. Yin, et al., APEC Cooperation for Earthquake Simulation.
Guo, Y., and J.K. Morgan, 2004, Influence of normal stress and grain shape on granular friction: Results of discrete element simulations, J. Geophys. Res., 109, doi:10.1029/2004JP003044.
Morgan, J.K., 2004, Particle dynamics simulations of rate and state dependent frictional sliding of granular fault gouge, Pure Appl. Geophys., 161, 1877-1891.
Morgan, J.K., 2002, Capturing physical phenomena in particle dynamics simulations of granular fault gouge, in 3rd ACES Workshop Proceedings, edited by A. Donnelan, M. MatsuĠura, and P. Mora, APEC Cooperation for Earthquake Simulation, p. 23-30.
Morgan, J.K., 2001, Distinct element simulations of granular shear zones: Micromechanics of localization and frictional behavior, in 2nd ACES Workshop Proceedings, edited by M. MatsuĠura, K. Nakajima, and P. Mora, APEC Cooperation for Earthquake Simulation, p. 83-90.
Morgan, J.K. and M.S. Boettcher, 1999, Numerical simulations of granular shear zones using the distinct element method: I. Shear zone kinematics and micromechanics of localization, J. Geophys. Res. B., 104, 2703-2719.
Morgan, J.K., 1999, Numerical simulations of granular shear zones using the distinct element method: II. The effect of particle size distribution and interparticle friction on mechanical behavior, J. Geophys. Res. B., 104, 2721-2732.
Abstracts:
Guo, Y., and Morgan, J.K., Geometrical effects of fault bends on fault frictional and mechanical behavior: insights from Distinct Element simulations, AGU, 87, Fall Meet. Suppl., Abstract T21B-0400.
Guo, Y., and Morgan, J.K., Fault gouge evolution and its dependence on normal stress and rock strength: Results of discrete element simulations, 2006 SCEC Annual Meeting, Southern California Earthquake Center, 16, 104.
Halpert, A., and J.K. Morgan, 2005, Granular Friction and Interparticle Force Networks; Effects of Particle Size, Size Distribution, and Strain Rate, EOS Trans. AGU, 86, Fall Meet. Suppl., Abstract T21B-477.
Morgan, J.K., 2005, Strain localization, gouge evolution, and granular friction in discrete element simulations of fault zones, SIAM Conference on Mathematical and Computational Issues in the Geosciences, Avignon, France, June 7-10, 2005.
Morgan, J.K., C.J. Marone, Y. Guo, J.L. Anthony, and M.W. Knuth, 2004, Comparative laboratory and numerical simulations of shearing granular fault gouge: Micromechanical processes, EOS Trans. AGU, 85, Fall Meet. Suppl., Abstract S31D-04.
Guo, Y., and J.K. Morgan, 2004, Effects of gouge zone evolution on frictional and mechanical behavior of fault zones: insights from Distinct element simulations, EOS Trans. AGU, 85, Fall Meet. Suppl., Abstract S41A-0927.
Morgan, J. K., Marone, C.J., Guo, Y., Anthony, J., and Knuth, M., 2004, Comparative laboratory and numerical simulations of shearing granular gouge: Micromechanical Processes, Proceedings and Abstracts, 2004 SCEC Annual Meeting, Southern California Earthquake Center, 14, 58.
Morgan, J. K., and Marone, C.J., 2004, Comparative laboratory and numerical simulations of shearing granular gouge, Proceedings and Abstracts, 4th ACES Workshop, Beijing, China, 4.
Guo, Y., and J.K. Morgan, 2004, Role of grain comminution in fault zone deformation, Proceedings and Abstracts, 4th ACES Workshop, Beijing, China, 7.
Guo, Y., and J.K. Morgan, 2004, Influences of grain comminution on the frictional properties of a simulated fault gouge, EOS Trans. AGU, 85, Joint Assembl. Suppl., Abstract T31A-02.
Morgan, J.K. and C.J. Marone , 2004, Comparative laboratory and numerical simulations of shearing granular fault gouge, EOS Trans. AGU, 85, Joint Assembl. Suppl., Abstract T13A-03
Guo, Y., and J.K. Morgan, 2003, Study of rock friction using refined DEM, EOS Trans. AGU, 84, Fall Meet. Suppl., Abstract S51B-01.
Morgan, J.K., 2002, Particle dynamics simulations of rate and state dependent frictional sliding of granular fault gouge, EOS Trans. AGU, 83, Fall Meet. Suppl., Abstract T11F-04.
Guo, Y, and J.K. Morgan, 2002, Influence of grain fracture and shape evolution on fault gouge strength in particle dynamics simulations, EOS Trans. AGU, 83, Fall Meet. Suppl., Abstract T21B-1093.
Morgan, J.K., 2002, Capturing physical phenomena in
numerical simulations of granular gouge: Effects of contact laws, particle size
distribution, and the 3rd dimension, Workshop Abstracts, 3rd
ACES International Workshop, Maui,
APEC Cooperation for Earthquake Simulation, 44.
Morgan, J.K., 2000, Alternative interparticle contact
laws in distinct element simulations of granular shear zones, EOS Trans.
AGU, 81, Fall Meet. Suppl., 1133.
Morgan, J.K., 2000, Distinct element simulations of
granular shear zones: Micromechanics of localization and frictional behavior, Workshop
Abstracts, 2nd ACES Workshop, Tokyo and Hakone, Japan, APEC Cooperation for Earthquake Simulation, 58.
Morgan,
J.K.,1999, The Micromechanics of localization, dilation, and
velocity-dependent friction in granular shear zones revealed by distinct
element simulations, EOS Trans. AGU, 80, Fall Meeting Suppl., 689.
Morgan, J.K., 1998, The Micromechanics of
localization and dilation in granular shear zones revealed by distinct element
simulations, EOS Trans. AGU, 79, Spring Meet. Suppl., 222.
Boettcher, M.S. and J.K. Morgan, 1997, Micromechanics
of deformation in granular shear zones revealed by distinct element
simulations: The role of particle
size distribution and interparticle friction., EOS Trans. AGU, 78 Fall
Meeting Suppl., 475.
Page last modified on 2-January-2007