Slow earthquakes release energy over hours to months, not seconds. Their statistical signature is distinct: smaller events dominate, moment release is gradual, and the frequency-magnitude distribution differs from the Gutenberg-Richter law that governs fast earthquakes. The geological explanations invoke complex fault zone properties — fluid pressure, mineral transitions, rate-and-state friction with specific parameters.
A tabletop experiment reproduces the statistics. Soft hydrogel particles floating on lubricating fluid, sheared in a Couette cell, produce slip events with slow earthquake statistics. No geological complexity. No fault mineralogy. No kilometer-scale structure. Just soft particles and low friction.
The mechanism: slip size is determined by competition between two effects. Shear localization tries to grow the rupture — once particles start sliding, neighbors are more likely to follow. Porosity-dependent pressure enhancement arrests it — as particles compact during sliding, pressure increases and resists further motion. In hard, high-friction materials, localization wins decisively, producing fast, large ruptures. In soft, low-friction materials, the two effects compete at comparable scales, producing the gradual, small-event-dominated statistics of slow earthquakes.
The structural point: slow earthquake behavior is not a geological property but a material property. Softness and low friction are sufficient. The complex geological explanations may be correct about what produces softness and low friction in fault zones, but the statistical signature requires only those two material parameters, not the specific geology. The tabletop system is minimal but complete.