Why can earthquake shaking trigger a lateral spread?

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landslide This 1995 landslide at La Conchita, a coastal town in California, swept away a hillside road and destroyed a number of houses.

Earthquake shaking triggers lateral spreading primarily through the process of soil liquefaction.

When seismic waves pass through water-saturated, unconsolidated granular soils, the vibrations cause the soil's pore spaces to collapse. This collapse forces a rapid increase in water pressure between individual soil grains, effectively pushing the grains apart and allowing them to move freely within the watery matrix.1

As the soil loses its internal strength and takes on the characteristics of a viscous liquid, it loses its resistance to shear stress. On even gentle slopes or near free faces (such as riverbanks), this liquefied mass can no longer maintain its position and begins to flow or displace laterally. This movement, often categorized as a type of landslide, can cause significant ground deformation and structural failure.1