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On April 26, 1981, an earthquake (Ms=6.0) occurred 6 km north of Westmorland in the Imperial Valley of California. Although there was no surface faulting associated with this earthquake, there were many surface expressions of liquefaction, mostly sand boils. Similar effects occurred in the area after earthquakes in 1930, 1950, and 1957. Sand boils were concentrated in areas of late-Holocene fluvial-deltaic deposits. These deposits originated from the surrounding mountains and from the Colorado Plateau via the Colorado River. To investigate the liquefaction sites from the 1981 and previous earthquakes, soundings were made with a cone penetrometer to define sediment profiles. Following cone testing, samples and standard penetration data were taken from selected intervals. Continuous auger samples were taken to obtain large disturbed samples and to retrieve a complete section of sediment. Sediment was tested in the laboratory to determine grain-size distribution and water content-properties. The penetration data and sediment properties were used to classify sediment and develop sediment profiles at 68 sounding sites. These sounding sites are grouped into 11 study areas, of which five are described in detail. The five principle study areas are (1) Wildlife Management area, (2) Vail Canal, (3) Kornbloom Road, (4) Radio Tower, and (5) McKim Ranch. These sites havd also been selected for additional studies by Purdue University, Stanford University and the University of Texas. Field studies and laboratory tests show that liquefaction occurred in loose very fine sand and loose silt that were deposited in channel, flood plain, and deltaic environments during the late-Holocene. Although the sediments that liquefied are presumed to be similar in age, the sediments originated in different depositional environments and show very limited lateral continuity. The only stratigraphic unit that did show lateral continuity at all sites is a lacustrine deposit underlying sediment that liquefied.

Geotechnical investigation of liquefaction sites, Imperial Valley, California