Can Large Strains Be Accommodated by Small Faults: “Brittle Flow of Rocks” Revised

Brittle deformation in the upper crust is thought to occur primarily via faulting. The fault length-frequency distribution determines how much deformation is accommodated by numerous small faults versus a few large ones. To evaluate the amount of deformation due to small faults, we analyze the fault length distribution using high-quality fault maps spanning a wide range of spatial scales from a laboratory sample to an outcrop to a tectonic domain. We find that the cumulative fault length distribution is well approximated by a power law with a negative exponent close to 2. This is in agreement with the earthquake magnitude-frequency distribution (the Gutenberg-Richter law with b-value of 1), at least for faults smaller than the thickness of the seismogenic zone. It follows that faulting is a self-similar process, and a substantial fraction of tectonic strain can be accommodated by faults that don't cut through the entire seismogenic zone, consistent with inferences of “hidden strain” from natural and laboratory observations. A continued accumulation of tectonic strain may eventually result in a transition from distributed fault networks to localized mature faults.