CONNECTIONS BETWEEN FAULT ZONE STRENGTH, FABRIC, AND THE MODE OF FRICTIONAL SLIDING: LESSONS FROM EARTHSCOPE
In this talk, we discuss connections between fault zone strength, shear fabric, and the mode of frictional sliding. We focus on recent works facilitated by the EarthScope program indicating that major tectonic faults are weak and that they tend to creep. We review evidence for the extreme weakness of the San Andreas Fault in central California, discuss recent laboratory measurements on core samples recovered during drilling of the San Andreas Fault Observatory at Depth (SAFOD), and review mechanisms for low friction, including nanocoatings of clay minerals within fault zones and the presence of pervasive, anastomosing shear fabric elements. We review data showing a clear relationship between frictional strength and slip stability. In the context of rate and state friction, these data indicate that velocity weakening friction and the necessary condition for unstable slip and earthquake nucleation are met only for strong faults, with friction exceeding ~ 0.5, whereas weak faults exhibit only velocity strengthening frictional behavior. Moreover, fault rocks recovered from the actively creeping SAF exhibit unusual frictional properties, such that ‘static’ friction is essentially independent of contact time; that is, the rate of frictional restrengthening in slide-hold-slide tests is nearly zero. These laboratory friction results are consistent with field-based constraints on fault strength inferred from heat flow and stress orientation data.
The successes of the EarthScope program drive home a fundamental truth about scientific progress: data drives science. EarthScope is a shining example of how science works best, with observations of natural phenomena motivating focused laboratory work, theoretical studies of coupled processes, and detailed field observations. We discuss successes of the EarthScope program in the context of recent progress in fault mechanics and earthquake physics.