The slip-weakening distance, Dc, over which a fault weakens during seismogenic fault motion, is one of most important parameters controlling fault instability. A paradox over the last decade has been the source of the large difference between laboratory-measured values of Dc using conventional friction experiments (10−5–10−3 m) and seismically determined Dc values for natural earthquakes (10−1–100 m). Dc is a difficult quantity to estimate for natural faults, but this study attempts to determine the order of Dc values for natural faults containing pseudotachylytes based on a comparison between experimental and natural pseudotachylytes. Experiments at a constant slip velocity of 0.85 m/sec and a normal stress of 1.4 MPa on gabbro have revealed that the progressive growth of a molten layer causes marked slip weakening. Slip weakening (a reduction in frictional strength with displacement) and the growth of the molten layer (an increase in the thickness of the molten layer with displacement) are both characterized by exponential changes with nearly the same characteristic distances. The comparison suggests that the order of Dc can be estimated from the relationship between pseudotachylyte thickness along its generation fault and fault displacement. Sibson’s (1975) data from the Outer Hebrides exhibit a similar pseudotachylyte-thickness-versus-fault-displacement curve with a characteristic distance of 0.38 m. This is the same order of magnitude as seismically inferred values of Dc. The effect of frictional melting is a possible solution for the Dc paradox, although it is not the only possible solution, since frictional melting along large faults is rather uncommon.

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