LIQUEFACTION AND RELATED GROUND FAILURES IN IMPERIAL COUNTY, CALIFORNIA, CAUSED BY THE APRIL 4, 2010 EL MAYOR-CUCAPAH EARTHQUAKE

Representatives of GEER, the U.S. Geological Survey and the California Geological Survey surveyed liquefaction occurrences in Imperial Valley within a week of the April 4 El Mayor – Cucapah earthquake. These reconnaissance-level field investigations concentrated on easily-accessed locations along major and minor roads, canals and drains, and the New and Alamo rivers. Liquefaction occurrences were most common in the southwest portion of Imperial Valley, though a significant exception was encountered northwest of the town of Holtville.

Most road closures were associated with crossings of the New River or its tributaries, and predominantly found to be caused by liquefaction of bridge approach fills and/or the soils underlying the fills. This was the case at Worthington, Drew, and Brockman roads where they cross the New River, and at Brockman Road and possibly Lyons Road where they cross the Greeson Drain. Where Lyons Road crosses the New River, liquefaction-induced lateral spreading occurred in natural soils and road fill 100 meters east and 200 meters west of the bridge, and the bridge structure and approach fills were undamaged.

Most damage to irrigation canals appears concentrated on the Westside Main Canal, from its diversion from the All American Canal northeast to Fites Road, approximately 6 km southwest of Brawley. Liquefaction of relatively fine-grained soils was observed at the foundation of the All American Canal aqueduct over the New River. Liquefaction accompanied by lateral spreading on both sides of the Rosita Canal northwest of Holtville disrupted the canal and allowed seepage onto adjacent agricultural fields.

Other notable liquefaction-related damage includes: liquefaction and lateral displacement of the dam embankment and adjoining Drew Road at the western end of Sunbeam Lake south of Seeley; a large lateral spread into the Rice Drain No. 3 north of Evan Hewes Highway west of El Centro; liquefaction-induced slump of a levee allowed overtopping of the levee likely by seiche waves generated in Fig Lagoon; lateral spreading of the access road and sludge ponds at the Calexico Waste Water Treatment Plant. Notable sites that liquefied during the 1979 Imperial Valley earthquake did not re-liquefy on April 4th, including the Wildlife liquefaction array and the Heber Road site.