Earthquake Processes:
Fluids and Fault Weakening The San Andreas Fault, California

Collaborators

(1) Y. Kharaka, W.C. Evans, and J. Thordsen, U.S. Geological Survey, Menlo Park, CA.

(2) D. J. DePaolo, Center for Isotope Geochemistry, Lawrence Berkeley National Laboratory and Dept. of Geology and Geophysics, University of California, Berkeley.

(3) D. Shuster, Center for Isotope Geochemistry, Lawrence Berkeley National Laboratory.

(4) E. Pili, Departement Analyse et Surveillance de l'Environnement, F-91680 Bruyeres-le-Chatel, France.

Fluids are suspected to play a major role in earthquake mechanics, especially in the case of the weak San Andreas fault (SAF). The SAF is a classic strike-slip fault that defines a collisional boundary between the North American and Pacific plates. The forces along the boundary are compressive, yet fault failure is by shear, as the Pacific plate slides steadily northward. Models developed to explain the weakness of the fault include either low-friction fault-zone materials or super-hydrostatic fluid pressures within the fault zone. The shear strength of fault zone materials, determined by laboratory friction measurements, suggests that considerably more shear stress than observed should be required for the fault to fail.

Figure 1: Aerial view of the Carrizo Plain segment of the San Andreas fault in Central California. Courtesy of the U.S. Geological Survey.

Models invoking high fluid pressures are similar but rely on different fluid sources. During the earthquake cycle fault zone fluid pressure increases to near lithostatic values and induces rupture. Dilation accompanies rupture, locally lowering the fault zone fluid pressures and the cycle begins again. Crustal fluids, connate or meteoric, may be drawn into the fault zone in response to fault rupture and become trapped by mineral reactions; the high fluid pressures required to weaken the fault are reestablished by compaction of the sealed fault-zone materials. In this model, the base of the seismogenic zone, defined by the brittle-ductile transition, is treated as an impermeable boundary. In an alternative model, fault-weakening fluid pressures are generated by a high flux of deep crustal or mantle fluids that are continually supplied to the seismogenic zone from the ductile lower crust at super-hydrostatic pressure. To investigate fluid source and influence on SAF dynamics, we conducted a chemical and helium isotopic study of groundwaters associated with the SAF and companion faults and found that the groundwaters contained elevated ³He/⁴He ratios, thus providing evidence for a geopressured mantle fluid source (Kennedy et al, 1997).

Figure 2: Helium isotopic composition (R) normalized to the ratio in air (Ra) plotted as a function of approximate distance from the main strike of the SAF. Associated fluid flow rates (q) and mantle contributions are also indicated.

Because mantle fluids must pass through the lower ductile crust, they enter the base of the fault zone in the brittle upper crust at or near lithostatic pressures. In transit, the ³He/⁴He ratios are diluted with radiogenic 4He produced locally in the crust, generating a vertical gradient in the fault zone helium isotopic composition that depends on the vertical rate of fluid flow. The calculated flow rates (q, Figure 1) vary from ~1-10 mm yr-1. Although these are sufficient to establish near lithostatic-fault-weakening fluid pressures at the shallower depths of the seismogenic zone, the more important question is whether these flow rates can re-establish fault-weakening pressures on time scales relevant to earthquake cycles.

It is likely that the mantle helium is associated with other more abundant mantle volatiles, certainly CO₂ and perhaps water. However, using the mantle CO₂/³He ratio, the vertical CO₂ flux (~3 x 10^-4 kg km^-2 sec^-1) inferred from the helium isotopic data is low and appears inadequate, by at least an order of magnitude, to re-establish fault-weakening fluid pressures on the relevant time scale (Figure 3). Apparently, an additional source of fluid (water, CO₂, etc.) is required.

Figure 3: Computed times required to re-establish fault weakening fluid pressures from hydrostatic values for different vertical CO₂ fluxes and crustal permeability. Assumed rock porosity (f) and compressibility (C_r) are indicated.

Our recent isotope studies of rocks and fluid inclusions from deformation zones and vein fillings within the fault zone confirms the presence of mantle helium in the fault zone and suggests that the mantle helium is accompanied by deep crustal or metamorphic water and CO₂. The infiltrated deformation zones, veins and host rocks show that fault zones in the San Andreas system maintain a higher permeability than adjacent regions. Noble gas, carbon and oxygen isotope compositions provide evidence for the involvement of mantle-derived fluids in faulting and that the mantle helium is accompanied by deep crustal or metamorphic water and CO₂. Some or all of the CO₂ may be of deep crustal origin.

Related Publications

Kennedy, B.M., Kharaka, Y.K., Evans, W.C., Ellwood, A., DePaolo, D.J., Thordsen, J., Ambats, G., and Mariner, R.H., Mantle fluids in the San Andreas fault system, Science, 278, 1278-1281, 1997.

Kharaka, Y.K., Thordsen, J.J., Evans, W.C., and Kennedy, B.M., Faults and subsurface fluid flow in the shallow crust, Geophysical Monograph Series 113, Am. Geophys. Un, 129-148, 1999.

Pili, E., Kennedy, B.M., Conrad, M.S., Gratier, J.-P., and Poitrasson, F., Isotope constraints on the involvement of fluids in the San Andreas Fault System, Califronia, Proc. Goldscmidt Conf, 1998.

Funding

This project was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Engineering, and Geosciences Division of the U.S. Department of Energy (http://www.er.doe.gov/production/bes/bes.html).

Related Web Sites

San Andreas Drilling Project: http://icdp.gfz-potsdam.de/ch/search/html/search.html

See also: http://www.dosecc.org/ or http://pangea.Stanford.EDU/~zoback/FZD/

U. S. Geological Survey: http://pubs.usgs.gov/gip/earthq3/index.html