Near-Fault Deformation
From UNC Tectonic Geomorphology
Not all crustal deformation is concentrated on faults. Processes of distributed deformation constitute an important sink of strain energy and could amount to significant uncertainty when comparing geologic deformation rates to geodesy. This project seeks to quantify the amount and activity of distributed deformation near active faults. One view of this deformation is that it is relict from initial fault formation, and that active strain accumulation is overwhelmingly localized onto faults. Our observations challenge this view. We find that distributed strain adjacent to active faults can be a significant proportion of total fault slip (up to 25 percent). It appears that this deformation scales with fault slip, and thus it is likely active. Secondary faults with evidence of late Quaternary slip and compilation of observations of historic earthquake ruptures further supports that fault activity occurs in a zone of deformation that may exceed 1 km in width. The majority of deformation is localized onto the main fault trace, but significant deformation via parallel faults and micro-block rotation occurs in the surrounding rock mass.
We find evidence that this zone may be further partitioned into two belts: an outer zone of distributed shear that steadily increases towards the fault, and an inner zone approximately 100 to 200 m in width where distributed shear is much more intense. This partitioning could be a result of different distributed deformation processes: an outer zone dominated by secondary faults, and an inner zone where the slip-pulse from the main fault pervasively shatters and shears the surrounding rocks.
