When the Ground Becomes Liquid
Liquefaction is one of the most dramatic and destructive earthquake phenomena. When seismic waves shake loose, saturated sand, the cyclic loading progressively transfers stress from the soil skeleton to the pore water. When pore water pressure equals the overburden stress, the sand grains float in suspension and the ground behaves like quicksand. Buildings tilt and sink, buried tanks and manholes float upward, and pressurized sand erupts through fissures to form sand boils on the surface.
The Simplified Procedure
The standard method for evaluating liquefaction potential compares the earthquake-induced cyclic stress ratio (CSR) against the soil's cyclic resistance ratio (CRR). CSR depends on peak ground acceleration, depth, and total vs. effective overburden stress. CRR depends on soil density, measured by Standard Penetration Test (SPT) blow count or Cone Penetration Test (CPT) tip resistance. When CSR exceeds CRR (FS < 1), liquefaction is predicted.
Paleoliquefaction Features
Ancient liquefaction leaves distinctive signatures in the geological record: sand dikes (vertical tabular intrusions of clean sand cutting through clay layers), sand sills (horizontal intrusions along stratigraphic contacts), and sand blows (conical deposits of erupted sand on the paleo-ground surface). These features can be preserved for thousands of years, providing evidence of strong shaking in areas with no written earthquake history. By dating the host sediments, paleoseismologists constrain the timing of past events.
Magnitude from Liquefaction
The size and spatial extent of liquefaction features constrain the magnitude of the causative earthquake. Larger earthquakes produce stronger shaking over wider areas, generating bigger sand dikes at greater distances from the fault. Empirical relations between maximum liquefaction distance and magnitude allow back-calculation of paleomagnitude. This simulation lets you explore the physical conditions that trigger liquefaction and the features that would be preserved in the geological record.