The 1971 San Fernando earthquake (also known as the Sylmar earthquake ) occurred on the morning of February 9 in the foothills of the San Gabriel Mountains in southern California. The unexpected thrust earthquake has a magnitude of 6.5 on the scale, and the maximum Mercalli intensity XI ( Extreme ). This event is one of a series that affected the Los Angeles area at the end of the 20th century. Severe local damage in the northern San Fernando Valley and extensive surface damage to the south of the quake's epicenter in the mountains, as well as urban settings along city streets and neighborhoods. Encouragement and other effects affect home and personal business.
The incident affected a number of health care facilities in Sylmar, San Fernando, and other densely populated areas north of downtown Los Angeles. Veteran Olive View Medical Center and Hospital were severely damaged, and buildings collapsed in both locations, causing the majority of deaths to occur. Buildings in both facilities were built in a mixed style, but the engineers could not fully study the building's response because they were not equipped with instruments to record a strong ground motion, and this prompted the Veteran Administration to then install a seismometer at high risk. site. Other sites throughout the Los Angeles area have been documented as a result of local ordinances, and an incredible amount of powerful movement data has been recorded, more than any other event up to that point. Success in this field spurred the initiation of California's Strong Instrumentation Instrumentation Program.
Transportation around the Los Angeles area suffered severe road damage and partial collapse of several major highway intersections. The nearly total failure of the Lower Van Norman Dam resulted in the evacuation of tens of thousands of downstream residents, although earlier decisions to maintain water to a lesser extent may have contributed to saving the dam from overtopping. Schools were affected, as happened during the 1933 Long Beach earthquake, but this time the improved construction style improved the yield for thousands of school buildings in the Los Angeles area. Another result of the event involved hundreds of different types of landslides documented in the San Gabriel Mountains. As happened after another earthquake in California, laws relating to building codes were once again revised, with laws specifically addressing the construction of a home or business near a well-known fault zone.
Video 1971 San Fernando earthquake
Pengaturan tektonik
The San Gabriel Mountains is 37.3 mi (60.0 km) long section of the Transversal Lintang and is bordered to the north by the San Andreas Fault, to the south by the Cucamonga Fault, and on the southwest side by the Sierra Madre Fault. The San Bernardino Mountains, Santa Ynez, and Santa Monica are also part of the anomalous east-west Transverse mountains. Domain range extends from the offshore Channel Islands, to the Little San Bernardino Mountains, 300 miles (480 km) to the east. The frontal fault system at the base of the San Gabriel Mountains extends from the San Jacinto Fault Zone in the east to the offshore Malibu in the west, and is defined primarily by moderate to shallow north-dipping errors, with conservative vertical displacements estimated at 4,000 -5,000 feet (1,200-1,500 m).
Paleomagnetic evidence has shown that the Western Transverse Mountains are formed because the Pacific Plate moves northward relative to the North American Plate. As the plates shifted northward, a portion of the terrarium which was once parallel to the shore was rotated clockwise, leaving it positioned in its east-west orientation. The Transverse Ranges form the perimeter of a series of basins beginning with the Santa Barbara Channel at the western end. Moving east, there is the Ventura Valley, the San Fernando Valley, and the San Gabriel Basin, with an active upside down façade (San Cayetano, Red Mountain, Santa Susana and Sierra Madre) all lined up on the northern border. A small number of destructive events have occurred, with three in Santa Barbara (1812, 1925, and 1978) and two in San Fernando Valley (1971 and 1994), although other errors in basins that have high Quaternary slip rates have not produced what great earthquakes even.
Maps 1971 San Fernando earthquake
Earthquake
The San Fernando earthquake occurred on February 9 at 6:00 am: 41 am Pacific Standard Time (14:00:41 UTC) with a powerful ground motion of about 12 seconds. The origin of the error lies five miles north of the San Fernando Valley. Significant damage is seen in the localized parts of the valley and also in the foothills of the San Gabriel Mountains above the fault block. The error responsible for the movement is not one that is considered a threat, and this highlights the urgency to identify other similar faults in the Los Angeles metropolitan area. The shock exceeded the requirements of building codes and exceeded what the engineers had prepared, and although most of the dwellings in the valley had been built in the previous two decades, even modern earthquake-proof buildings were seriously damaged.
Some of the key attributes of the event were divided by the 1994 Northridge earthquake, as both are caused by a thrust fault in the northern mountains of Los Angeles, and each resulting earthquake is similar in magnitude, although no surface breakage occurred in 1994. Since both occur in urban and industrial areas and resulted in significant economic damage, each event attracted critical observations from planning authorities, and has been thoroughly studied in the scientific community.
Surface error
The prominent surface fault N72 Â ° W is observed along the San Fernando Fault Zone from the southern point of Sylmar, stretching almost continuously for 6 miles (9.7 km) east to Little Tujunga Canyon. Additional breaks occur further east which are in more dispersed mode, while the western part of the most affected areas has less prominent scars, especially the separate Mission Wells segment. Although the previously complete Sierra Madre Fault Zone has been mapped and classified by name into its constituent error, the fracture fault group provides a natural way of identifying and referring to each section. As categorized during intensive study immediately after the quake, they were labeled segments of Wells Mission, Sylmar segment, Tujunga segment, foothills area, and Veteran faults.
All segments share common elements of thrust fault with the left-lateral slip component, the east-west general strike and the northern slope, but they are not unified with respect to their relationship to the associated bedrock. The initial surveyors of the widespread faults in valleys, foothills, and mountains only reported tectonic damage, while excluding gaps and other features arising from the effects of compaction and landslides. In the vicinity of the Sylmar Fault segment, there is a low likelihood of landslides due to lack of altitude changes, but in the foothills and mountainous areas a large number of landslides occur and more work is needed to eliminate the possibility of misidentifying a feature.. Along the front lines of the Tujunga segment, some ambiguous formations are present because some scarps may have an effect of descending movement, but for the most part they are tectonic in nature.
In repeated measurements of different fault errors, the results remain consistent, leading to the belief that most of the slips have occurred during the mainshock. While lateral, transverse, and vertical movement are all observed, the largest individual component of the movement is 5 ft 3 in (1.60 m) from the lateral slip left near the middle of the Sylmar segment. The largest cumulative number of 6 feet 7 inch (2.01 m) slip occurs along the Sylmar and Tujunga segments. The overall displacement of the fault is summarized by the geologist Barclay Kamb and others as "an almost equal number of north-south compression, vertical lift (north side up), and left lateral slip and hence can be described as thrusting the northern block to the southwest. above the southern block, along the surface of the fault dipping about 45 ° north. "
aftershock
A three-week follow-up follow-up study was conducted that included events recorded by various permanent stations operated by the California Institute of Technology, USGS instruments stationed at Point Mugu, and the California Water Resources Department's seismometer at the Pyramids. Springs and Cedar Springs. The temporary seismometer formed in response to major shocks rises and runs from as fast as a few hours to several days after the main event and provides additional data until March 1st. The catalog of goods is largely complete and includes 200 3.0 or greater magnitude shocks and four magnitude 5.0 or greater shocks. During the first hour of activity, a larger aftershock overshadows smaller events.
The overall pattern of subsequent follow-up activity appears in symmetrical "U" form but with slightly more concentrated activity on the southwest side. Some smaller shocks are closer to the surface fault area, but for the most part, areas that experience the most severe shocks and damage (as a result of major shocks) have no subsequent activity. Pacoima Dam, with its very high peak acceleration readings, is located very close to the center of the earthquake-free zone.
Landslide
The USGS commissioned private companies and the United States Air Force to take aerial photographs of more than 97 m² (250 km km) from the mountainous area north of the San Fernando Valley. Analysis revealed that an earthquake triggered more than 1,000 landslides. Crushed stones are also documented along hilltops, and rocks (which continue for several days) are the result of initial shocks and aftershocks. Several slides released from the air are also observed from the ground. The largest number of slides is centered to the southwest of the mainshock epicenter and close to the area where surface damage occurs. Slide ranges from 49-984 feet (15-300 m) in length, and can be further categorized as falling rock, falling soil, slide debris, avalanche, and slump. The most common type is the slide (less than 3 feet (0.91 m) thick) slide debris and most often found in areas composed of sedimentary rocks.
Strong movement
In early 1971, the San Fernando Valley was the location of a solid-tissue powerful seismometer movement, which provided a total of 241 seismograms. This makes earthquakes the most documented event, at that time, in terms of the seismology of the powerful movement; by comparison, the 1964 Alaska earthquake did not provide a strong record of movement. Part of the reason why there are so many stations to catch that event is the 1965 law that requires newly built buildings in Beverly Hills and Los Angeles with six-story altitudes to be fitted with three instruments. This provision eventually found its way into the Uniform Building Code as an annex a few years later. One hundred and seventy-five recording came from these buildings, the remaining 30 were in the hydraulic structure, and the rest came from land-based installations near the fault, including the arrangement of units along the San Andreas Fault.
Instruments mounted on Dam Pacoima recorded a peak horizontal acceleration of 1.25 g , a value that is two times larger than anything ever seen from an earthquake. Extraordinarily high acceleration is just one part of the picture, given the duration and frequency of shaking also play a role in how much damage can occur. The accelerometer is mounted on a concrete platform above the granite back just above one of the arch arch abutments. The cracks formed on rocks and rock avalanches come within 15 feet (4.6 m) of the equipment, and the foundation remains undamaged, but a small (half-degree) slope of the unit is found that appears to be responsible for closing the horizontal pendulum contact. As a result of what was considered a lucrative accident, the machine continued to record for six minutes (until it ran out of paper) and provided scientists with additional data on 30 of the initial aftershocks.
Damage
The strongest affected areas are the remote communities north of Los Angeles bordered by the northern edge of the San Fernando Valley at the base of the San Gabriel Mountains. The unrelated districts of Newhall, Saugus, and Solemint Junction suffered moderate damage, even to newer buildings. The area where the heaviest effect is present is limited by geographical features on the remaining three margins, with the Santa Susana Mountains to the west, the Santa Monica Mountains and the Los Angeles River to the south, and along the Verdugo Mountains to the east. The loss of lives directly caused by an earthquake of 58 (a number of heart attacks and other health related deaths are not included in this figure). Most of the deaths occurred in the Veterans and Olive View hospital complexes, and the rest were in private homes, highways collapsed, and the ceiling collapsed at the Midnight Mission in downtown Los Angeles.
The damage is most severe near and to the north of the surface damage, and at the foot of the mountain. Hospital buildings, freeway passes, and Sylmar Juvenile Hall are in rugged alluvium covering thousands of feet of consolidated sedimentary material. In the city of San Fernando, underground water, sewers, and gas systems suffered too much damage to count, and some parts were badly damaged so they were abandoned. Earth removal destroys sidewalks and roads, with cracks in asphalt and rigid concrete often exceeding the width of the shift in the underlying soil. The severe damage near alluvium was documented during the investigation of the effects of the Santa Rosa earthquake of 1969. A band with similar damage far near Ventura Boulevard at the southern end of the valley was also identified as being associated with soil type.
Federal, regional, and private hospitals suffered varying degrees of damage, with four major facilities in San Fernando Valley suffering structural damage, and two of them collapsed. Indian Hills Medical Center, Foothill Medical Building, and Pacoima Lutheran Professional building were heavily damaged. Home care is also affected. The one-storey Foothill Feet House is very close to the breaking part that breaks the surface and is raised three feet higher than the road. The tramp ran along the sidewalk and across the property. The building is not used and remains standing. Although the reinforced concrete beam structure is struck by surprises and improvements, its relatively good performance is in stark contrast to the Olive Landscape Hospital and Veterans' Hospital complex.
Olive View Hospital
Most of the buildings in Los Angeles County's 880-bed hospital complex had been built prior to the adoption of new construction techniques that had been imposed after the Long Beach earthquake of 1933. The one-story building group 300 meters west of the new facility, and several other buildings, was not damaged. Destructible buildings are various structures of wood and stone frames. The five-storey concrete reinforced Concrete Care and Care Building is one of three new additions to the complex (the third of which is sustained damage), assembled with earthquake resistant construction techniques, and completed in December 1970. The hospital is managed by 98 employees and has 606 patients at the time of the earthquake; all three deaths in the Olive View complex are in this building. Two are caused by a life-support system power failure and one, that an employee being beaten by a part of a collapsed building as he tries to get out of the building, is a direct result of destruction.
Medical Treatment and Maintenance Buildings include underground spaces (above grade) on the east and south sides, mixed (upper and lower grade) on the west side, and below the class on the north side of the building, variations that are due to the shallow slopes at location. The complete structure, including four external staircases, can be considered five separate buildings, since the ladder tower is detached from the main building about four inches. The earthquake holder used in the second to the fifth is composed of a sliding wall, but the rarely slip slip technique used with concrete walls on the first floor prevents them from becoming part of the system. Damage to buildings, including ceiling tiles, telephone equipment, and elevator doors, excessive on the ground floor and first floor, with further minor damage. The difference in stiffness on the second floor is proposed as a cause of considerable damage at lower levels. Since the first floor nearly collapsed, the building leaned almost two feet north, and three of the four concrete staircases fell from the main building.
By reason, there are cracks in sidewalks and dirt, but there is no surface fracture. In addition to the collapse of the stairs, the lifts did not work. Electricity and communication failed at the hospital during the earthquake, but very few people occupied the lower floors and stairs in the early hours. Victims in these highly affected areas may have increased so that surprises occur in the future. The duration of strong ground motion at that location may be similar to the 12 seconds observed in the Pacoima Dam, and other seconds' shocks are considered sufficient to make the building collapse.
Veterans Administration Hospital
The Veterans Administration Hospital began operating as a tuberculosis hospital in 1926 and became a public hospital in the 1960s. In 1971, the facility comprised 45 individual buildings, all located within 5 km (3.1 m) of the fault fault in Sylmar, but structural damage was found to have occurred as a result of shocks rather than land transfers or fractures. Twenty-six buildings built before 1933 have been built following local building codes and do not require seismic design. These buildings suffered the most damage, with four buildings completely collapsed, resulting in the loss of lives at the facility. Most of the masonry and reinforced concrete buildings built after 1933 survived with shock and most did not collapse, but in 1972 a resolution appeared to leave the site and the remaining structures were later destroyed, the site became a city park.
Several powerful seismometer movement installations were present outside the western United States prior to the San Fernando earthquake but, on recommendation by the Earthquake and Wind Committee, the Veterans Administration signed an agreement with Seismological Field Service (later linked with NOAA) to install the instrument on all VA sites in the zone Uniform Building Code two and three. It has been established that these zones have a higher probability of having a strong soil acceleration, and plans are designed to complement selected VA hospitals with two instruments. One unit will be installed in the structure and the second will be established as a free field unit located not far from the facility. By 1973, some of the highest risks (26 completed in zone 3 only) sites that had been completed were in Seattle, Memphis, Charleston, and Boston.
Van Norman Dam
Both the Norman Upper and Lower Dam dam were heavily damaged by the earthquake. The lower dam is very close to the offense, and about 80,000 people are evacuated for four days while the water level in the reservoir is lowered. This is done as a precaution to accommodate further collapse due to strong aftershocks. Some of the canals in the dam area are damaged and unusable, and the dikes are declining but this does not pose a danger. Damage at the lower dam consists of a landslide sprained part of the embankment. The lips of the ground from the dam fall into the reservoir and bring along the concrete layer, while what remains of the dam is only 5 feet (1.5 m) above the water level. The upper lake drops 3 feet (0.91 m) and shifts about 5 feet (1.5 m) as a result of ground motion, and the dam's concrete layer cracks and slumps.
The upper dam was built in 1921 with a hydraulic filling process, three years after the larger lower dam, created using the same force. The lower dam inspection in 1964 paved the way for an arrangement between the State of California and the Los Angeles Department of Water and Electricity that would maintain a water level of the reservoir that was 10 feet lower than usual. Because the collapse of the weir decreases its overall height, the decision to reduce its capacity proves to be a bit of a valuable assurance.
Strong differential motion and strong vibrations (MMI VIII ( Severe )) are responsible for serious damage to the facilities of Sylmar Juvenile Hall and Sylmar Converter Station (both located close to the upper Van Norman lake). The Los Angeles Water and Electric Department, as well as the County of Los Angeles, investigates and verifies that local soil conditions contribute to land transfers and lead to destruction. The broken surface area on the ground at the site is 900Ã, ft (270 m) (at the widest) and stretches 4,000 ft (1,200 m) down the slope of the 1% class to the southwest. A total of 5Ã, ft (1.5 m) of lateral motion was observed on both ends of the slide, and trenches dug during the site inspection revealed that some gaps were up to 15 ft (4.6 m) deep. Two facilities, located near the Grapevine and Weldon valleys that deliver water and debris in the Sierra Madre Mountains, are bounded by steep mountains and have formed alluvial fans in their mouths. The narrow band of soil disturbance found has been the result of the deposition of soft soil in a descending motion. Soil liquefaction plays a role in a restricted area of ​​the slide, but is not responsible for all movements on the site, and the fault tectonic slip in that area is also excluded as the cause.
Transportation
Substantial disturbance to about 10 miles of highway in the northern San Fernando Valley takes place, with most of the damage occurring at the Foothill Freeway/Golden State Freeway junction, and along the Interstate 210-meter stretch of five miles. On Interstate 5, the most significant damage occurred between the junction of the Newhall Pass at the northern end and the I-5/I-405 intersection in the south, where the decline in the bridge approached and cracked and bend in the way made it unusable. Several landslides occurred between Balboa Boulevard and California State Route 14, but the most significant damage occurred at two major intersections. The Antelope Valley Freeway has been damaged from the Newhall Pass to the northeast, mainly from settlement and alignment issues, as well as debris and cracks in the Santa Clara Bridge and Solemint bridge.
- Golden State Freeway - Antelope Valley Freeway Interchange
While the Newhall Pass intersection is still under construction at the time of the earthquake, the required flyover component is complete. Vibration caused two of the 191-foot bridge sections to fall from a maximum height of 140 ft (43 m), along with one supporting pillar. The spans are dislodged from their ends at both ends due to lack of proper bonding and insufficient space (mid-size seating available (360 mm) in the supporting column.The displacement of the site is ruled out as the main cause of failure, and in addition to the falling part and the crane being hit during collapse, other parts of the bridge are also damaged.The shear fracture occurs in the column closest to the western abutment, and the soil in the same column base shows rotational evidence.
- Golden State Freeway - Foothill Freeway Interchange
This exchange is a vast complex of overpasses and bridges that are nearing completion at the time of the earthquake and not all parts are open to traffic. Some examples of failures or site collapse occurred and two people were killed while driving in a pickup truck as a result. I-210 west to south I-5, which is complete except for paving in the ramp section, collapses to the north, probably due to the vibrations that move the bridge from its supporters due to inadequate seats. Unlike the situation in Antelope Valley Interchange, permanent soil movement (defined as a few inches of left lateral displacement with possible thrust) was observed in the area. This movement contributed to heavy damage at the Sylmar Juvenile Hall facility, Sylmar Converter Station, and Metropolitan District Water Treatment Center, but its impact on the intersection was not fully understood in a 1971 report from the California Institute of Technology.
School
The large number of public school buildings in the Los Angeles area show mixed responses to shocks, and those built after enforcement of the Field Act clearly show the result of a reformed style of construction. The Field Act came into effect only one month after the devastating earthquake of Long Beach in March 1933 that destroyed many public school buildings in Long Beach, Compton, and Whittier. The Los Angeles Unified School District has 660 schools comprising 9,200 buildings at the time of the earthquake, with 110 stone buildings yet to be reinforced to meet the new standards. More than 400 portable classrooms and 53 wooden frames, Pre-Field Act buildings are also used. All these buildings have previously been checked in relation to the requirements of the Act, and many were reinforced or rebuilt at that time, but earthquake engineering experts recommended further repair or disassembly after separate evaluations were conducted after the February 1971 earthquake, and in one and a half the year of the district was followed by directions relating to about 100 structures.
At Los Angeles High School (20 miles (32 km) from Dam Pacoima) where the exterior wall of the main Pre-Field Act building (built 1917) is an un-reinforced brick, the length of the parapet and associated brick veneer are cut off and some fragments fall through the roof to the lower floor, while other materials land on the exit stairs and into the courtyard area. The main building was destroyed at a cost of $ 127,000, and none of the various post-field Law buildings were damaged on site. Except for the concrete gym, all buildings in Sylmar High School (3.75 mi (6.04 km) from Dam Pacoima) are Post-Field Act, one floor, wooden construction. Abundant cracks formed on the ground at the site, and several foundations and many sidewalks were also cracked. Estimates for improvements on the site are $ 485,000. At 2 mi (3.2 km), Hubbard Street Elementary School is the nearest school with the Pacoima dam, and also less than a mile from the Veterans Hospital complex. Wooden frame buildings (classrooms, multi-purpose buildings, and several bungalows) were built after the Field Law, and the cost of damage and cleaning there reached $ 42,000. The gas line is damaged and the separation of the hall is due to a lateral displacement of up to six inches.
Aftermath
After many major earthquakes in California, lawmakers acted quickly to develop legislation related to seismic security. After the Long Beach Earthquake M6.4 1933, the Field Act was passed the following month, and after the 1989 Loma Prieta earthquake, the Bill of Seismic Mapping and Senate Bill 1953 (hospital safety requirements) was established. After the San Fernando event, earthquake engineers and earthquake experts from established scientific organizations, as well as the recently formed Los Angeles County Quake Commission, stated their recommendations based on lessons learned. List of items that need to be repaired include building codes, dams and bridges that are made more earthquake resistant, hospitals designed to keep operating, and development restrictions near known fault zones. New laws include the Special Study Zone Law of Alquist-Priolo and the development of a Strong Motion Instrumentation Program.
Alquist-Priolo Special Studies Zone Act
Introduced as Senate Bill 520 and signed into law in December 1972, the law was originally known as the Alquist-Priolo Geologic Hazard Zones Act, and has the goal of reducing damage and loss due to surface fault or creep error. This action limits the construction of buildings designed for human occupancy across potentially active cesareans. Because it is suspected that surface rupture is likely to occur where past surface displacements have occurred, state geologists are given the responsibility to evaluate and map errors that have Holocene evidence broken, and create a regulatory zone around them called the Earthquake Zone. State and local agencies (as well as property owners) are then responsible for enforcing or complying with building restrictions.
California Power Movement Program
Prior to the San Fernando earthquake, some structural engineers have believed that the existing foundation for seismic design requires improvement. Although the instrument had recorded the strength of 0.33 g during the 1940 El Centro earthquake, the building code only required a structure to withstand lateral forces.1 g until the late 1960s.. Even then, engineers opposed the idea of ​​building buildings to withstand the high forces seen in El Centro's shock, but after the 1966 earthquake peaked, 5 g , and a maximum of 1.25 g observed in Dam Pacoima during the San Fernando event, the debate began whether the low requirements were sufficient.
Despite the attractive seismogram of the 1940 events at El Centro, stronger seismology motion was not explicitly sought until later events occurred - the San Fernando earthquake made it clear the need for more data for earthquake engineering applications. California's Robust Movement Program began in 1971 with the goal of maximizing the volume of data by providing and maintaining the instruments on selected lifestyle structures, buildings, and emergency response stations. By the late 1980s, the program had instructed more than 450 structures, bridges, dams, and power plants. The 1979 Imperial Valley and 1987 Whittier Narrows earthquakeakes were presented as favorable events recorded during that period, as they produced valuable data that increased knowledge of how moderate events affected buildings. The success of the Imperial Valley event is especially pronounced because the recently constructed and fully instructed government buildings are shaken to the point of failure.
See also
- California State Route 126
- Interstate 105 (California)
- List of earthquakes in 1971
- List of earthquakes in California
- Long Beach Search and Rescue
- San Gabriel Error
- Wadsworth Chapel
References
Source
External links
- San Fernando Earthquake - Southern California Earthquake Center
- Historic Earthquake - San Fernando, California - United States Geological Survey
- California Geological Survey - About CSMIP - California Conservation Department
- International Seismology Center has a bibliography and/or authoritative data for this event.
- Veterans Memorial Community Regional Park - Los Angeles County Parks and Recreation Department
Source of the article : Wikipedia
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