Which dynamic weakening mechanism controls friction during earthquake slip?
In plain words
When fault walls slide at about a meter per second, friction drops sharply. Several processes could cause this and it is unknown which one dominates deep in real faults.
Precise statement
At slip rates near $1e2\,\mathrm{cm}/\mathrm{s}$, laboratory friction falls to coefficients of about 0.1 to 0.3. Identify which mechanism (flash heating at asperity contacts, thermal pressurization of pore fluid, melt lubrication, nanopowder or silica-gel lubrication) controls coseismic shear stress at seismogenic depths on natural faults, and its dependence on slip and slip rate. An answer is a constitutive law validated against natural fault-zone observations.
What would settle it
Post-earthquake fault drilling with temperature measurement plus high-velocity friction experiments under in situ pressure and fluid conditions.
Status in the literature
Rapid-response drilling after the 2011 Tohoku earthquake measured a very low coseismic friction coefficient, but the controlling mechanism is still debated.