Abstract
Structural analyses of the angles of frictional ‘lock-up’ for fault sets that have become progressively misoriented, together with field observations from seismology, geomorphology, and borehole stress measurements, suggest that Byerlee friction coefficients (0.6<μ<0.85) are widely applicable to natural sliding surfaces with displacements of up to a few kilometres in the upper crust, from the surface of the earth to seismogenic depths. Extensional normal faults operating under presumed vertical trajectories of extreme compressive stress provide some of the best evidence for frictional lock-up followed by the initiation of new favorably oriented faults, but similar lock-up phenomena also occur in thrust and strike-slip fault systems.
However, extensional detachments which appear to have formed and remained active at very low dips (<15°) lie well outside the dip range of currently active normal faults, requiring stress trajectories that deviate significantly from the vertical and horizontal during their initiation and perhaps also during their continued reactivation. Other conspicuous exceptions to the pattern of frictional lock-up expected from Byerlee friction are major transform structures, such as the San Andreas fault in California, which remain active though oriented at high angles to the maximum principal compression. On the basis of the evidence afforded by the lower displacement faults, the apparent weakness of such structures seems most likely to arise from locally elevated fluid pressure, rather than from the presence of anomalously low-friction material within the fault zones.
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