WHAT LOCKS SUBDUCTION THRUSTS?

There are many paradigms to describe the initiation and propagation of fault rupture, but little research focuses on the restrengthening or locking of faults after earthquakes. Geodetic observations of post-seismic subduction zones have revealed the spatial distribution and timescales of fault locking on broad scales.

What makes faults strong? Locking can be geometric, in which case, roughness is the important factor and compaction is the process that engages the interlocking surfaces. This type of strength can only persist as long as slip on a geometrically locked fault is still favored over development or reactivation of another fault, so the strength that can be achieved this way is still limited by the structural context outside the fault. Mineralogy controls the cohesion and resistance to frictional sliding, so changes in mineral composition of fault rocks (or migration of faults into zones of weaker composition) can control whether a fault will creep or lock. Similarly, the arrangement of minerals (development and evolution of foliations) determine which minerals in a rock control its strength.

Geometry, mineralogy and fabrics in subduction thrusts vary with thermal structure, depth, and composition of the subducting plate, and presumably with the hanging wall composition and history. I’ll show a variety of observations from exhumed subduction-related faults from seismogenic depths and discuss constraints on earthquake rupture, recovery, and locking of subduction plate boundary faults.