Do large earthquakes have a detectable preparation phase?
In plain words
Lab experiments show faults creep slowly before they snap, but it is unclear whether real large earthquakes do this in a way instruments can see. Some big quakes had small earthquakes or slow slip before them, others did not.
Precise statement
Rate-and-state friction predicts a quasi-static nucleation zone of size $L_c \sim G D_c/(\sigma (b - a))$ before dynamic rupture. Determine whether earthquakes with $M \ge 7$ are systematically preceded by aseismic slip or foreshock sequences distinguishable from ETAS background clustering (ETAS: a statistical model in which every earthquake can trigger later ones) at amplitudes above present geodetic and seismic detection thresholds. An answer is a statistically significant catalog-wide test.
What would settle it
A global test on dense geodetic and template-matched seismic catalogs comparing precursory activity before large events against ETAS-simulated catalogs.
Status in the literature
Unverified note
Precursory slow slip was reported before the 2011 Tohoku and 2014 Iquique earthquakes; whether precursors are systematic remains disputed as of 2025.