Geology of the City of Long Beach, California, United States of America

The City of Long Beach, California, once part of two large Spanish land grants, occupies the extreme southeast corner of Los Angeles County and borders the Pacific Ocean. Development of the city since its final incorporation in 1897 has altered the original coastal landscape to such a degree that little remains in its natural state. Dominant physiographic features within the city are present in the form of small hills and mesas associated with the Newport-Inglewood structural zone. Northwest-trending faults and folds passing through the city generally parallel the active Newport-Inglewood structural zone. This zone has been recognized as a ground-water barrier since 1905, and as a structural crude oil trap since 1921. Other buried faults and structures within the city include the Wilmington oil field structural complex, and the Wardlow-Airport, Richfield, and Los Alamitos faults. Major folds in the city include the buried Wilmington anticline and Long Beach (Signal Hill) anticline which lies along the Newport-Inglewood structural zone. The city lies within the Los Angeles sedimentary basin and rests on top of about 4,267 m (14,000 ft) of Miocene and Pliocene sediments and an undetermined thickness of pre-Miocene basement rocks. Pleistocene to Recent surficial geologic units overlie these oil-producing zones. These rock units include the San Pedro Formation, terrace deposits, the Palos Verdes sand, alluvial and coastal deposits, and made land. Foundation-related geotechnical characteristics of surficial geologic units in the city vary from stiff to hard consistency and low compressibility in terrace deposits, to generally low-strength and moderate compressibility in made land. Subsurface investigation techniques commonly used in the city include borehole, pit excavation, cone penetration and seismic refraction. Foundation types in use include conventional footings, mats, drilled piers, and piles. Standard laboratory tests are used to identify the engineering properties of soils, and include moisture content, dry density, shear, compression, consolidation, grain-size analysis, and Atterberg limits. Oil revenues, generated from the Wilmington oil field with seven producing zones, enabled Long Beach to modernize its port and coastal commercial enterprises. However, production has caused approximately 8.8 m (29 ft) of subsidence (now essentially controlled). As many as 13 potential geologic constraints have been identified within the city. The most important of these factors now affecting development of the city is seismicity, and such related constraints as fault identification, fault displacement and ground shaking. Liquefaction and settlement, land subsidence, tsunami and seiche activity, flooding, marsh and made-land areas, shallow ground water, expansive soils, and slope stability are additional geologic considerations. Recurrence intervals for these potential constraints can only be generalized with the present data base. Most mitigation measures take the form of proper land-use planning and sound engineering design. Long Beach is located in an area of known historic seismicity. The 1933 Long Beach earthquake (M = 6.3), which occurred offshore Newport Beach along the Newport-Inglewood structural zone, is the largest event recorded that has affected the city. Over 200 aftershocks followed this main event. In subsequent years most of the shocks have been less than magnitude 4. As a result of the damages caused by the 1933 earthquake, the Field and Riley Acts of the California State Code were passed. The former required seismic design of school buildings; the latter required seismic design of all other structures critical to human occupancy. Immediately after the earthquake a building rehabilitation program identified 1,700 earthquake-damaged buildings. Currently (July 1, 1979), 715 buildings with minor damage remain unrepaired; 38 have been repaired, and 154 have been demolished. Artesian wells and springs, inland from the Newport-Inglewood structural zone, first supplied all of the city9s water requirements, but with time, groundwater levels were drawn down such that pumping was required. At the present time, additional water supplies come from as far away as 640 km (400 mi). Excessive pumping of ground water from four major aquifers has caused salt-water intrusion inland from the Pacific Ocean. Injection programs, in force since 1970, have created a fresh-water barrier halting further chloride advance. Sewage and waste-water treatment facilities provide primary and secondary treatment capabilities. Some treated water is used to irrigate city parks; some treatment sludges are used in landfill construction. The southwestern shoreline of the city consists of the Port of Long Beach, constructed almost entirely of dredge-fill and armor-rock. The southeastern shoreline includes a wave-cut terrace, Alamitos Bay and Naples Island. Alamitos Bay has been enlarged by dredging, and Naples Island has been formed by infilling. The entire shoreline is protected by a 13.1 km (8.2 mi) long breakwater, started in 1899 and completed in 1949. Long Beach is essentially a fully developed area and contains numerous industrial, commercial and residential areas. Highrise structures coexist with residential housing in some localities. Port facilities exist within areas of oil production. There are also petroleum refineries and a steam generating power plant within the city. A new downtown “Mall” shopping center is presently under construction. The steamship R.M.S. Queen Mary, Howard Hughes9 amphibious airplane the HK-1 Hercules (Spruce Goose), and new convention and sports arena facilities are all part of the plan for continuing growth of the city.