California Geological Survey Zones of Required Investigation for Liquefaction – Livermore Valley, California

Official Seismic Hazard Zone Maps of Dublin, Livermore, and Altamont 7.5-minute quadrangles (1:24,000 scale) show areas that may be susceptible to earthquake-induced liquefaction. The maps are the result of the three-dimensional integration of geotechnical and geological data. Approximately 450 geotechnical borehole logs were collected, analyzed, entered into a geotechnical Geographic Information System database, correlated with surficial geologic units, and evaluated for the susceptibility of deposits to liquefaction. In addition, a map showing depth to historically highest groundwater level was prepared. The continuous relocation over the years of temporary water-filled pits associated with gravel mining in central Livermore Valley has caused localized changes to permeability, resulting in variable groundwater conditions. The boundary for the Zone of Required Investigation (ZORI) is in most places defined by the contact of Holocene deposits with late Pleistocene deposits or with bedrock, and extends along the base of the foothills surrounding the Livermore Valley. Near Hacienda Drive and Central Parkway, sediment mapped as late Pleistocene to Holocene alluvial fan deposits (Qf) is included in the ZORI. Although the age of the unit suggests that the sediment has had sufficient time to consolidate, thus rendering it unlikely to liquefy, subsurface data indicate the deposit includes a greater abundance of silt, and lower penetration resistance, compared to occurrences of Qf mapped in other portions of the western margin of the Livermore Valley. Near the intersection of Hopyard Road and Arroyo Mocho, an area once known as “Willow Swamp” is excluded from the ZORI. Groundwater is within 10 to 20 feet of the ground surface throughout much of this area; however, California Geological Survey Zones of Required Investigation for Liquefaction – Livermore Valley, California