New Evidence on the State of Stress of the San Andreas Fault System

Abstract

Contemporary in situ tectonic stress indicators along the San Andreas fault system in central California show northeast-directed horizontal compression that is nearly perpendicular to the strike of the fault. Such compression explains recent uplift of the Coast Ranges and the numerous active reverse faults and folds that trend nearly parallel to the San Andreas and that are otherwise unexplainable in terms of strike-slip deformation. Fault-normal crustal compression in central California is proposed to result from the extremely low shear strength of the San Andreas and the slightly convergent relative motion between the Pacific and North American plates. Preliminary in situ stress data from the Cajon Pass scientific drill hole (located 3.6 kilometers northeast of the San Andreas in southern California near San Bernardino, California) are also consistent with a weak fault, as they show no right-lateral shear stress at approximately 2-kilometer depth on planes parallel to the San Andreas fault.

Explanatory Notes

“…Under these conditions, the breakout orientation develops in the direction of the least principal horizontal stress. It has been previously demonstrated that stress orientations deduced from rock breakouts are consistent with other independent indicators (Bell and Gough, 1979;Zoback et al, 1988).…”

Seismotectonics of the Loma Prieta, California, region determined from three‐dimensional Vp, Vp/ Vs, and seismicity

“…The fault has changed its location, at times stranding slivers of material [Sims, 1991]. The plate motion changed slightly about 3.5 Ma so that there is a compressional component [Harbert and Cox, 1989], generally taken up off the San Andreas fault [Zoback et al, 1987].…”

Comparisons of water and argon permeability in natural clay‐bearing fault gouge under high pressure at 20°C

“…In addition, the flow of fluids within larger fault zones, also controlled by the fault products contained within them, has major implications for fault zone mechanics. If high fluid pressures within faults zones can be maintained by low-permeability fault rocks for significant periods of time, then this will affect the overall strength of the fault zone, perhaps explaining observations such as the apparent low strength of the San Andreas fault system as suggested by the stress/heat flow paradox [Zoback et al, 1987;Lachenbruch and Sass, 1980]. Clay-bearing fault gouges are common constituents of large fault zones [Rutter et al, 1986;Miller, 1996] be present at depth to perhaps 10 km in currently active large faults [Wintsch et al, 1995;Hacker, 1997].…”