what caused the 1971 san fernando earthquake
But the lessons learned from the great 1971 San Fernando earthquake remained strictly a local education. At 2 mi (3.2 km), Hubbard Street Elementary School was the closest school to Pacoima dam and was also less than a mile from the Veterans Hospital complex. Cracks formed in the rocks and a rock slide came within 15 feet (4.6 m) of the apparatus, and the foundation remained undamaged, but a small (half-degree) tilt of the unit was discovered that was apparently responsible for closing the horizontal pendulum contacts. The spans slipped off of their supports at either end due to lack of proper ties and insufficient space (a 14 in (360 mm) seat was provided) on the support columns. The 1971 San Fernando earthquake (also known as the Sylmar earthquake) occurred in the early morning of February 9 in the foothills of the San Gabriel Mountains in southern California.The unanticipated thrust earthquake had a moment magnitude of 6.5 or 6.7 (as determined by several independent institutions) and had a maximum Mercalli intensity of XI (Extreme). Southern California The San Gabriel Mountains are a 37.3 mi (60.0 km) long portion of the Transverse Ranges and are bordered on the north by the San Andreas Fault, on the south by the Cucamonga Fault, and on the southwest side by the Sierra Madre Fault. The event was one in a series that affected the Los Angeles area in the late … | Earthquake Faults and Faulting The earthen lip of the dam fell into the reservoir and brought with it the concrete lining, while what remained of the dam was just 5 feet (1.5 m) above the water level. The shaking surpassed building code requirements and exceeded what engineers had prepared for, and although most dwellings in the valley had been built in the prior two decades, even modern earthquake-resistant structures sustained serious damage. Nursing homes also were affected. The unincorporated districts of Newhall, Saugus, and Solemint Junction had moderate damage, even to newer buildings. (hra135), Cooperative Institute for Deep Earth Research (CIDER), Northern California Earthquake Data Center (NCEDC), : Destruction in the Eastern Aegean Sea, : An Explosion in Beirut heard all over the Middle East, : Quake in Turkey highlights the hazard in the East Bay. The quake happened at 6am Pacific Standard Time. [20], On the grounds, there were cracks in the pavement and soil, but no surface faulting. The area of surface breaks on the ground at the site was 900 ft (270 m) (at its widest) and stretched 4,000 ft (1,200 m) down a 1% grade slope towards the southwest. The main building was demolished at a cost of $127,000, and none of the various post-Field Act buildings were damaged at the site. The difference in rigidity at the second floor was proposed as a cause of the considerable damage to the lower levels. Two were due to power failure of life-support systems and one, that of an employee who was struck by part of the collapsing building as he or she tried to exit the building, was a direct result of the destruction. 100% (1/1) Los Angeles, California Los Angeles, CA Los Angeles, United States. After the M6.4 1933 Long Beach earthquake, the Field Act was passed the following month, and after the 1989 Loma Prieta earthquake, the Seismic Hazards Mapping Act and Senate Bill 1953 (hospital safety requirements) were established. As the plate shifted to the north, a portion of the terrane that was once parallel with the coast was rotated in a clockwise manner, which left it positioned in its east–west orientation. Both events had similar size, occurred on blind thrust faults beneath the densely populated San Fernando Valley of the Los Angeles metropolitan area, and hence offer a rare opportunity to compare the effects of the two earthquakes. Jet Propulsion Laboratory George Ellery Hale NASA Institute of Technology (United States) Southern California Intercollegiate Athletic Conference. Along this zone, many additional structures sitting on top of the fault zone were severely damaged directly by fault movement at the surface. The hospital buildings, the freeway overpasses, and the Sylmar Juvenile Hall were on coarse alluvium that overlay thousands of feet of loosely consolidated sedimentary material. Other sites throughout the Los Angeles area had been instrumented as a result of local ordinances, and an unprecedented amount of strong motion data was recorded, more so than any other event up until that time. The event affected a number of health-care facilities in Sylmar, San Fernando, and other densely populated areas north of central Los Angeles. [27][28], Introduced as Senate Bill 520 and signed into law in December 1972, this legislation was originally known as the Alquist-Priolo Geologic Hazard Zones Act, and had the goal of reducing damage and losses due to surface fault ruptures or fault creep. The red line is the fault on which the Northridge quake occurred in 1994. The area where the heaviest effects were present was limited by geographical features on the three remaining margins, with the Santa Susana Mountains on the west, the 2). [20], The Medical Treatment and Care Building included a basement that was exposed (above grade) on the east and south sides, mixed (above and below grade) on the west side, and below grade on the north side of the building, the variation being due to the shallow slope at the site. Review: April 2, 2021. Gas lines were broken and separation of the buildings' porches was due to lateral displacement of up to six inches. The one-story Foothill Nursing Home sat very close to a section of the fault that broke the surface and was raised up three feet higher than the street. Loss of life that was directly attributable to the earthquake amounted to 58 (a number of heart attacks and other health-related deaths were not included in this figure). But when seismologists checked the epicentral area they realized that for about 6 miles a fault along the foothills of the San Gabriels had broken through the surface during the quake. In a previous study of the distribution of red-tagged (‘unsafe’) buildings and of breaks in the water distribution system caused … The upper lake subsided 3 feet (0.91 m) and was displaced about 5 feet (1.5 m) as a result of the ground movement, and the dam's concrete lining cracked and slumped. In comparison, both the Loma Prieta (1989) and Northridge (1994) quakes measured “IX Violent” on the Mercalli Scale, two steps below this quake (the Sylmar Earthquake, 1971). A band of similarly intense damage further away near Ventura Boulevard at the southern end of the valley was also identified as having been related to soil type. More than once have dams failed or were overtopped due to the shaking of large earthquakes - with devastating consequences for the people living downstream. One unit would be installed within the structure and the second would be set up as a free-field unit located a short distance away from the facility. Even at that time, engineers were against the idea of constructing buildings to resist the high forces that were seen in the El Centro shock, but after a 1966 earthquake peaked at .5g, and a maximum of 1.25g was observed at the Pacoima Dam during the San Fernando event, a debate began as to whether that low requirement was sufficient. More than 400 portable classrooms and 53 wood frame pre-Field Act buildings were also in use. In the vicinity of the Sylmar Fault segment, there was a low possibility of landslides due to a lack of elevation change, but in the foothills and mountainous area a large amount of landslides occurred and more work was necessary to eliminate the possibility of misidentifying a feature. The buildings at both facilities were constructed with mixed styles, but engineers were unable to thoroughly study the buildings' responses because they were not outfitted with instruments for recording strong ground motion, and this prompted the Veterans Administration to later install seismometers at its high-risk sites. Some canals in the area of the dams were damaged and not usable, and dikes experienced slumping but these did not present a hazard. The Northridge earthquake was caused by dip slip on a thrust fault—the overriding movement of one crustal block up and over a second crustal block (see p. The individual faults in the zone move more slowly than the San Andreas fault and produce smaller earthquakes than those expected on the San … One hundred seventy-five of the recordings came from these buildings, another 30 were on hydraulic structures, and the remainder were from ground-based installations near faults, including an array of the units across the San Andreas Fault. — At 6 a.m. on February 9, 1971, a 6.6-magnitude earthquake struck the San Fernando Valley. The event was one in a series that affected the Los Angeles area in the late … All these buildings had been previously inspected with regard to the requirements of the Act, and many were reinforced or rebuilt at that time, but earthquake engineering experts recommended further immediate refurbishment or demolition after a separate evaluation was done after the February 1971 earthquake, and within a year and a half the district followed through with the direction with regard to about 100 structures. The 1971 San Fernando Earthquake also caused liquefaction, especially in the area of Lower Van Norman Dam. The greatest number of slides were centered to the southwest of the mainshock epicenter and close to the areas where surface faulting took place. Most dams are constructed well and they are built for eternity. The Transverse Ranges form the perimeter of a series of basins that begins with the Santa Barbara Channel on the west end. The two geologists Carl Wentworth and Robert Yerkes from the US Geological Survey in Pasadena later wrote that "the San Fernando fault was discovered to be active only when it ruptured the ground surface on February 9, 1971. Another result of the event involved the hundreds of various types of landslides that were documented in the San Gabriel Mountains. Photo: E. Leyendecker, USGS. Due to continuing impacts from the COVID-19 pandemic, the Lifelines 2021 Conference was rescheduled from February 2021 to February 7-11, 2022. San Fernando Earthquake Series, 1971' Focal Mechanisms and Tectonics JAMES H. WHITCOMB, CLARENCE R. ALLEN, JAN D. GARMANY AND JAMES A. HILEMAN Seismological Laboratory, California Institute of Technology Pasadena, California 91109 The largest events in the San Fernando earthquake series, initiated by the main shock at 14h 00m 41.8s UT on February 9, 1971, were chosen for analysis … Known as the San Fernando earthquake or the Sylmar earthquake, it caused 58 deaths and more than $500 million in damage. The success of the Imperial Valley event was especially pronounced because of a recently constructed and fully instrumented government building that was shaken to the point of failure. The unanticipated thrust earthquake had a magnitude of 6.5 on the M s scale, and a maximum Mercalli intensity of XI (Extreme).The event was one in a series that affected the Los Angeles area in the late … But it was generally accepted that none of these demarcation lines between two separate geologic units were active, meaning that they had not produced any earthquakes recently. This paper compares the distribution of damage from the San Fernando, 1971, and Northridge, 1994, earthquakes. Today in Earthquake History, February 9, 2017 In addition to the collapse of the stairways, the elevators were out of commission. The hospital was staffed by 98 employees and had 606 patients at the time of the earthquake; all three deaths that occurred at the Olive View complex were in this building. New legislation included the Alquist-Priolo Special Studies Zone Act and the development of the Strong Motion Instrumentation Program. Author irememberjfk_l6fq3c Posted on July 10, 2019 Categories The News. The quake took place at 6 am under the town of Sylmar. ", Figure 2: Earthquakes on two blind thrust faults caused severe damage in the San Fernando Valley. It caused hospitals and schools to collapse, brought down sections of elevated freeways and cost 64 people their lives. Casualties in these highly affected areas might have increased had the shock occurred later in the day. Sometimes they are very strong and other times you would hardly notice them. The slides ranged from 49–984 feet (15–300 m) in length, and could be further categorized as rock falls, soil falls, debris slides, avalanches, and slumps. [22], Differential ground motion and strong shaking (MMI VIII (Severe)) were responsible for serious damage to the Sylmar Juvenile Hall facility and the Sylmar Converter Station (both located close to the Upper Van Norman lake). This stipulation ultimately found its way into the Uniform Building Code as an appendix several years later. The frontal fault system at the base of the San Gabriel Mountains extends from the San Jacinto Fault Zone in the east to offshore Malibu in the west, and is defined primarily by moderate to shallow north-dipping faults, with a conservative vertical displacement estimated at 4,000–5,000 feet (1,200–1,500 m). The two blue lines mark the faults which ruptured in the San Fernando earthquake 46 years ago today. C. R. Allen and his co‐workers assigned the main shock parameters: 34°24.7′N, 118°24.0′W, focal depth h = 8.4 km, and local magnitude M L = 6.4. (The earthquake of 9-Feb-1971). The damage at the lower dam consisted of a landslide that dislocated a section of the embankment. Uplift and other effects affected private homes and businesses. However, when you live in earthquake country, you might want to give the question a second thought. Part of the reason there were so many stations to capture the event was a 1965 ordinance that required newly constructed buildings in Beverly Hills and Los Angeles over six stories in height to be outfitted with three of the instruments. If you can improve it further, please do so. Twenty-six buildings that were built prior to 1933 had been constructed following the local building codes and did not require seismic-resistant designs. Schools were affected, as they had been during the 1933 Long Beach earthquake, but this time amended construction styles improved the outcome for the thousands of school buildings in the Los Angeles area. The Epicenter Was NOT in Sylmar And The Actual Size Of The Quake Really Was...? The unanticipated thrust earthquake had a magnitude of 6.5 on the Ms scale and 6.6 on the Mw scale, and a maximum Mercalli intensity of XI (Extreme). The domain of the ranges stretches from the Channel Islands offshore to the Little San Bernardino Mountains, 300 miles (480 km) to the east. The estimate for repairs at the site was $485,000. The five-story, reinforced-concrete Medical Treatment and Care Building was one of three new additions to the complex (all three of which sustained damage), was assembled with earthquake-resistant construction techniques, and was completed in December 1970. Even more surprising was the location of the focus of the quake, which later came to be known as the San Fernando earthquake. Although the Richter magnitude of the tremblor was 6.6, ranking it as moderate to large, but not great, it shook a wide, heavily populated area, leaving death and destruction in its wake. The epicenter was located approximately 10 km northeast of LSFD at a focal depth of approximately 9 km. Soil liquefaction played a role within confined areas of the slide, but it was not responsible for all the motion at the site, and tectonic slip of faults in the area was also excluded as a cause. Since both occurred in urban and industrial areas and resulted in significant economic impairment, each event drew critical observation from planning authorities, and has been thoroughly studied in the scientific communities. | Ground displacement damaged sidewalks and roads, with cracks in the more rigid asphalt and concrete often exceeding the width of the shift in the underlying soil. Preparedness, Risks, and Hazards The 1971 San Fernando earthquake (also known as the Sylmar earthquake) occurred in the early morning of February 9 in the foothills of the San Gabriel Mountains in southern California. Though the reinforced concrete block structure was afflicted by the shock and uplift, the relatively good performance was in stark contrast to that of the Olive View and Veterans Hospital complexes. Ground displacement at the site was ruled out as a major cause of the failure, and in addition to the fallen sections and a crane that was struck during the collapse, other portions of the overpass were also damaged. It was the first strong quake to hit the area since the 1893 Pico Canyon quake, with an epicenter several miles west of Sylmar. The near total failure of the Lower Van Norman Dam resulted in the evacuation of tens of thousands of downstream residents, though an earlier decision to maintain the water at a lower level may have contributed to saving the dam from being overtopped. Itâ s pretty exciting for me as a professional that at the end of my career, now I can say we built this system that can do the things that we really needed in San Fernando. [24], While the Newhall Pass interchange was still under construction at the time of the earthquake, the requisite components of the overpass were complete. The event was one in a series that affected Los Angeles county in the late 20th century. [26], At Los Angeles High School (20 mi (32 km) from Pacoima Dam) where the exterior walls of the main pre-Field Act building (constructed 1917) were unreinforced brick masonry, long portions of the parapet and the associated brick veneer broke off and some fragments fell through the roof to a lower floor, while other material landed on an exit stairway and into a courtyard area. Most deaths occurred at the Veterans and Olive View hospital complexes, and the rest were located at private residences, the highway overpass collapses, and a ceiling collapse at the Midnight Mission in downtown Los Angeles. In the city of San Fernando, underground water, sewer, and gas systems suffered breaks too numerous to count, and some sections were so badly damaged that they were abandoned. Several instances of failure or collapse at the site took place and two men were killed while driving in a pickup truck as a result. Additional breaks occurred farther to the east that were in a more scattered fashion, while the western portion of the most affected area had less pronounced scarps, especially the detached Mission Wells segment. While lateral, transverse, and vertical motions were all observed, the largest individual component of movement was 5 ft 3 in (1.60 m) of left lateral slip near the middle of the Sylmar segment. The accelerometer was mounted on a concrete platform on a granite ridge just above one of the arch dam's abutments. It also occurred on a hitherto unknown fault, albeit a different one from the San Fernando fault (see Figure 2). 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