EGU General Assembly 2008
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  Information - TS2 Are faults weak or strong? Theoretical, experimental and geological approaches (including Outstanding Young Scientist Award Lecture)

Event Information
In situ stress measurements indicate that in general, the crust is strong existing in a state of failure equilibrium as predicted by Coulomb theory and laboratory-derived coefficients of friction of 0.6-0.7. However repeated localization of displacements along faults and shear zones, often over very long timescales, strongly suggests that faults are weak relative to their surrounding wall rocks and that some faults may be weak in an absolute sense (coefficients of friction <0.3).

Our understanding of fault zone structure and mechanical behaviour relies on the integration of data from three main sources of information: 1) geological studies of natural fault zones and their deformation products (fault rocks); 2) seismological, geophysical and neotectonic studies of currently active natural fault systems; and 3) laboratory-based deformation experiments using rocks or rock-analogue materials. Whilst these provide us with a basic understanding of brittle faulting in the upper crust our knowledge of fault behaviour under the high strain, long-term loading conditions typical of geological fault zones remains poor. There are significant issues of scaling concerning problems such as how can grain-scale weakening mechanisms transmit their effects upscale to influence the rheological behaviour of a crustal-scale fault zones? And how can geophysicists measure and predict such weakening effects using techniques that typically average measurements into a sample volume of ~1km3?

These challenges have once again moved centre-stage with the apparent confirmation during SAFOD that the San Andreas Fault is a weak fault imbedded in a strong crust. The recent recovery of exceptionally well-preserved, foliated fault rocks from the cores at approximately 2.7km depth reveal zones of strain localization and probable weakening. In the light of this major new development, this session brings together an international group of Earth Scientists working on the problem of fault zone rheology. We hope to revisit some of the old controversies and discuss new ways to understand how and why fault zones exist and mechanically persist throughout the geological record on planet Earth.

Preliminary List of Solicited Speakers
C. Spiers: “Experimental insights into the rheology of weak faults”
J. Suppe: "Pore Fluid Pressure and Crustal Strength"
G. Di Toro "Geological evidence for strong faults"
D. Faulkner "Slip on weak faults by the rotation of regional stress in the fracture damage zone"
L Chiaraluce: "Microseismicity along a weak low-angle normal fault"

Note: Titles are provisional


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