What decides whether a fault slips slowly or in earthquakes?

Slow slip events and tectonic tremor at subduction zones move at slip rates of order $10^{-6} \text{ to } 10^{-5}\,\mathrm{cm/s}$, roughly $10 \text{ to } 100$ times the plate rate and about $10^{7} \text{ to } 10^{8}$ times slower than earthquake slip (near $10^{2}\,\mathrm{cm/s}$). Identify the physical control: near-neutral rate-and-state frictional stability ($a - b \text{ near } 0$) with high pore-fluid pressure, dilatancy strengthening, mixed frictional-viscous rheology, or fault-zone heterogeneity, and whether and when slow slip triggers large earthquakes. An answer is a constitutive model, tested against laboratory data, that reproduces the observed moment-duration scaling, recurrence and tremor of slow slip (review: Burgmann, EPSL 2018, https://doi.org/10.1016/j.epsl.2018.04.062).

Laboratory friction experiments on natural fault-zone materials under in situ fluid pressure that reproduce slow slip, matched to geodetic and tremor catalogs.