|Riverside County Earthquake - 6.5 - December 4, 1948|
Riverside County Earthquake, 1948: The earthquake probably was caused by displacement on the Mission Creek fault, one of the major branches of the San Andreas fault system in southern California. The highest intensities in the area were reported from the upper Coachella Valley from Thousand Palms to White Water, which also was the most densely populated area near the epicenter.
Considerable structural damage and slight cracks in the ground were observed in Desert Hot Springs. Some minor structural damage also occurred at Palm Springs. At Willis Palms, cracks formed in the ground and cliffs, riverbanks slumped, and springs increased in flow. Landslides and cracks in the ground were reported in the Indio Hills. Felt throughout southern California and at a few towns in western Arizona, southwest Nevada, and northern Baja California. About 72 aftershocks were accurately located in a zone 18 kilometers long, parallel to (but 5 kilometers north of) the trace of the Mission Creek fault. (from: United States Geological Survey Professional Paper 1527: Seismicity of the United States, 1568-1989, (revised), by C.W. Stover and J.L. Coffman, 1993, 418 pages) Larger map
|Santa Cruz Mountains Earthquake - 6.9 - October 17, 1989|
Santa Cruz Mountains Earthquake, 1989: This major earthquake caused 63 deaths, 3,757 injuries, and an estimated $6 billion in property damage. It was the largest earthquake to occur on the San Andreas fault since the great San Francisco earthquake in April 1906.
The most severe property damage occurred in Oakland and San Francisco, about 100 km north of the fault segment that slipped on the San Andreas. MM intensity IX was assigned to San Francisco's Marina District, where several houses collapsed, and to four areas in Oakland and San Francisco, where reinforced-concrete viaducts collapsed: Nimitz Freeway (Interstate 880) in Oakland, and Embarcadero Freeway, Highway 101, and Interstate 280 in San Francisco. Communities sustaining heavy damage in the epicentral area included Los Gatos, Santa Cruz, and Watsonville.
Liquefaction, as evidenced by sand boils, lateral spreading, settling, and slumping, occurred as far as 110 kilometers from the epicenter. It caused severe damage to buildings in San Francisco's Marina district as well as along the coastal areas of Oakland and Alameda in the east San Francisco Bay shore area. Liquefaction also contributed significantly to the property damage in the Santa Cruz and Monterey Bay areas, which lie near the epicentral zone. Structures damaged by liquefaction include buildings, bridges, highways, pipelines, port facilities, airport runways, and levees. Subsurface soil conditions, which amplified accelerations in the San Francisco Bay area, strongly influenced structural damage patterns and probably contributed to liquefaction problems in loose, sandy fills underlain by deep, cohesive soil deposits.
Engineered buildings, including those near the epicenter, performed well during the earthquake. Hospital buildings in the region sustained only minor system and cosmetic damage, and operational interruptions did not occur. Only five schools sustained severe damage, estimated at $81 million.
Most of the spectacular damage to buildings was sustained by unreinforced masonry buildings constructed of wood-frame roof and floor systems supported by unreinforced brick walls. These structures failed in areas near the epicenter as well as in areas far from the epicenter, at San Francisco and Monterey. The severe shaking near Santa Cruz caused heavy damage to the unreinforced masonry buildings in that area, particularly in the Santa Cruz Pacific Garden Mall, which consisted of several blocks of unreinforced masonry store buildings.
More than 80 of the 1,500 bridges in the area sustained minor damage, 10 required temporary supports, and 10 were closed owing to major structural damage. One or more spans collapsed on three bridges. The most severe damage occurred to older structures on poor ground, such as the Cypress Street Viaduct (41 deaths) and the San Francisco-Oakland Bay Bridge (one death). Damage to the transportation system was estimated at $1.8 billion.
Most of the more than 1,000 landslides and rockfalls occurred in the epicentral zone in the Santa Cruz Mountains. One slide, on State Highway 17, disrupted traffic for about 1 month.
The earthquake produced a pattern of northwest-trending extensional fractures in the north end of the aftershock zone northwest of the epicenter, but throughgoing right-lateral surface faulting was not found above the rupture defined by the main shock and its aftershocks. Six feet of right-lateral strike-slip and 4 feet of reverse-slip was inferred from geodetic data. The only surface fracturing that might be attributed to primary tectonic faulting occurred along a trace of the San Andreas near Mount Madonna Road in the Corralitos area, where en echelon cracks showed 2 centimeters of right-lateral displacement.
Extensional fractures (maximum net displacement of 92 centimeters) were observed about 12 kilometers northwest of the epicenter, in the Summit Road-Skyland Ridge area, east of State Highway 17, whereas zones of compressional deformation were found along the northeast foot of the Santa Cruz Mountains between Blossom Hill and Palo Alto. In Los Altos and Los Gatos, ground deformation appeared to be associated closely with zones of heavy structural damage and broken underground utility lines.
Other towns in the area that also experienced severe property damage include Boulder Creek, Corralitos, Hollister, Moss Landing, and several smaller communities in the Santa Cruz Mountains.
This earthquake was felt over most of central California and in part of western Nevada. The rate of aftershock activity decreased rapidly with time, but the total number of aftershocks was less than that expected from a generic California earthquake of similar magnitude. Fifty-one aftershocks of magnitude 3.0 and larger occurred during the first day after the main shock, and 16 occurred during the second day. After 3 weeks, 87 magnitude 3.0 and larger aftershocks had occurred. (from: United States Geological Survey Professional Paper 1527: Seismicity of the United States, 1568-1989, (revised), by C.W. Stover and J.L. Coffman, 1993, 418 pages) Larger map
|San Bernardino Earthquake - 6.4 - April 10, 1947|
San Bernardino Earthquake, 1947: This moderate shock was strongest in the Newberry Springs area, about 40 kilometers east of Barstow. One schoolhouse was condemned at Newberry Springs, and three adobe and brick houses were damaged severely. Minor damage, including one toppled chimney, fallen walls, cracks in chimneys and concrete, and cracked and slumped highways, were reported in the area. Also, cracks formed in the banks of the Mojave River. Felt over most of the southern half of California, a small part of southwest Nevada, and at several towns in western Arizona. Several light aftershocks occurred. (from: United States Geological Survey Professional Paper 1527: Seismicity of the United States, 1568-1989, (revised), by C.W. Stover and J.L. Coffman, 1993, 418 pages) Larger map