1. Liquefaction Liquefaction occurs where ground water is near the surface in soils composed of sands and silts. The soil temporarily loses strength and behaves as a viscous liquid. Structures can settle or tip in the liquefied soil or be ripped apart as the ground spreads laterally or flows. Flow failures can move over kilometers at speeds of tens of kilometers per hour. They usually develop in loose, saturated sand on slopes greater than five percent. When subsurface sand layers lose strength because of liquefaction, lateral spreading can occur in overlying sediments allowing them to move down even the gentlest slopes. Soils may lose shear strength allowing heavy structures to settle or tip and lightweight, buried structures to rise buoyantly. Cracking may result from movement along faults, differential compaction of the soil, or slides. Strong ground shaking has compacted loose cohesionless materials and caused differential ground settlements ranging from 5 cm to more than a meter. Quite possibly one of the most classic examples of liquefaction from the 1964 Nigaata, Japan earthquake. While the buildings themselves seemed to suffer no structural damage, the ground beneath these buildings liquefied and the structures simply sank and rotated into the underlying soil.

2. Sands and silts –undergo temporary loss of strength –behave as viscous fluid Seismic waves cause void collapse resulting in densification Drainage of pore water cannot be achieved rapidly enough resulting in excessive pore pressures End point is development of a QUICK condition –material behaves as heavy liquid with virtually no shear strength Void collapse Original sediment structure

3. Liquefaction effects from the 1977 Caucete earthquake. Sand boils occurred (center) as building settled. Ground failure (left and right) was very common as water saturated soils lost their strength. Liquefaction damage during the 1964 Great Alaska earthquake (center), 1964 Nigaata, Japan earthquake (left) and 1983 Nihonkai-Chubu earthquake (right)

4. The potential for liquefaction in an area is well understood, and is dependent upon height of the groundwater table and types of soil present. Once these are known, maps may be constructed for potential liquefaction. These maps do not guaranty liquefaction (green areas) will occur in areas, and not in others, they simply indicate areas of highest susceptibility. This map is of the Newport Beach area from the California Division of Mines and Geology (CDMG) Seismic Hazards Mapping Program. The large green areas are largely within the Santa Ana River floodplain, and coastal beach zones, both with high water tables.