Paleoseismic features as indicators of earthquake hazards in North Coastal, San Diego County, California, USA

Abstract

New road cut and mass-grading excavations in the north coastal area of San Diego County, California expose heretofore generally unrecognized, probable late Holocene tsunami deposits and paleoseismically deformed sediments. Remnant tsunami deposits occur up to 100 + m in elevation around the margins of modern coastal lagoons and estuaries and, combined with local mima mounds of possible sand blow origin, provide indirect but compelling evidence for the late Quaternary activity of onshore and offshore faults in the immediate study area. Probable paleoliquefaction features are regionally widespread and range from fissures filled with sediments derived from overlying marine terrace sand and soil, to the more traditional sand-filled injection dikes and sills, lateral spreads, and filled craterlets. The source of most liquefied sediment is underlying Tertiary “bedrock sand” and local, Quaternary marine-terrace deposits. A paleoseismic liquefaction origin rather than soft-sediment loading is deduced for these features based on morphology, internal stratigraphy, field setting, and near proximity to known seismogenic sources. Some paleoseismic events impacted late Holocene Indian middens and burial sites. The last seismic event probably occurred within the past 1 to 3 ka, and possibly even records the historic earthquakes of either November 22, 1800 or May 27, 1862. The liquefaction features also affect marine terrace sediments tens of meters above modern regional water levels, inferentially “recording” paleoseismic events in this Mediterranean-type climatic region during winter rains when high-level, perched water saturates the several meter thick source sediments. Based on their regional extent, the paleoseismic features were likely caused by M ∼ 7+ tectonic events inferentially generated by the nearby offshore Newport–Inglewood/Rose Canyon fault system, or possibly by smaller, recently exposed, related and localized faults. Accordingly, the seismic hazard of the north coastal area of San Diego County may be substantially higher than previously assumed, and hence of concern owing to the rapid ongoing and projected population increase.