  Earthquakes at seismogenic plate boundaries are a response to the
differential motions of tectonic blocks embedded within a geometrically complex
network of branching and coalescing faults. Elastic strain is accumulated at a
slow strain rate of the order of $10^{-15}$ s$^{-1}$, and released
intermittently at intervals $>100$ years, in the form of rapid (seconds to
minutes) coseismic ruptures. The development of macroscopic models of
quasi-static planar tectonic dynamics at these plate boundaries has remained
challenging due to uncertainty with regard to the spatial and kinematic
complexity of fault system behaviors. In particular, the characteristic length
scale of kinematically distinct tectonic structures is poorly constrained. Here
we analyze fluctuations in GPS recordings of interseismic velocities from the
southern California plate boundary, identifying heavy-tailed scaling behavior.
This suggests that the plate boundary can be understood as a densely packed
granular medium near the jamming transition, with a characteristic length scale
of $91 \pm 20$ km. In this picture fault and block systems may rapidly
rearrange the distribution of forces within them, driving a mixture of
transient and intermittent fault slip behaviors over tectonic time scales.
