The 1971 San Fernando earthquake (also known as the 1971 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 magnitude of 6.5 on the scale and 6.6 on the scale, and a maximum Mercalli intensity of XI (Extreme). The event was one in a series that affected Los Angeles County during the late 20th century. Damage was locally severe in the northern San Fernando Valley and surface faulting was extensive to the south of the epicenter in the mountains, as well as urban settings along city streets and neighborhoods. Uplift and other effects affected private homes and businesses.
The event affected a number of health-care facilities in Sylmar, San Fernando, and other densely populated areas north of central Los Angeles. The Olive View Medical Center and Veterans Hospital both experienced very heavy damage, and buildings collapsed at both sites, causing the majority of deaths that occurred. 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; this prompted the Veterans Administration to later install seismometers at its high-risk sites. 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 success in this area spurred the initiation of California's Strong Motion Instrumentation Program.
Transportation around the Los Angeles area was severely afflicted with roadway failures and the partial collapse of several major freeway interchanges. All 4,084 square miles of Los Angeles County were declared a disaster area by California Governor Ronald Reagan. 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. 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. Another result of the event involved the hundreds of various types of landslides that were documented in the San Gabriel Mountains. As had happened following other earthquakes in California, legislation related to building codes was once again revised, with laws that specifically addressed the construction of homes or businesses near known active fault zones.
Tectonic setting
The San Gabriel Mountains are a 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 San Bernardino, Santa Ynez, and Santa Monica Mountains are also part of the anomalous east–west trending Transverse Ranges. The domain of the ranges stretches from the Channel Islands offshore to the Little San Bernardino Mountains, 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 offshore Malibu in the west, and is defined primarily by moderate to shallow north-dipping faults, with a conservative vertical displacement estimated at .
Paleomagnetic evidence has shown that the western Transverse Ranges were formed as the Pacific plate moved northward relative to the North American plate. 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. The Transverse Ranges form the perimeter of a series of basins that begins with the Santa Barbara Channel on the west end. Moving eastward, there is the Ventura Basin, the San Fernando Valley, and the San Gabriel Basin, with active reverse faults (San Cayetano, Red Mountain, Santa Susana, and Sierra Madre) all lining the north boundary. A small number of damaging events have occurred, with three in Santa Barbara (1812, 1925, and 1978) and two in the San Fernando Valley (1971 and 1994), though other faults in the basin that have high Quaternary slip rates have not produced any large earthquakes.
Earthquake
The San Fernando earthquake occurred on February 9, 1971, at 6:00:41 am Pacific Standard Time (14:00:41 UTC) with a strong ground motion duration of about 12 seconds as recorded by seismometers, although the whole event was reported to have lasted about 60 seconds. The origin of faulting was located five miles north of the San Fernando Valley. Considerable damage was seen in localized portions of the valley and also in the foothills of the San Gabriel Mountains above the fault block. The fault that was responsible for the movement was not one that had been considered a threat, and this highlighted the urgency to identify other similar faults in the Los Angeles metropolitan area. 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.
Several key attributes of the event were shared with the 1994 Northridge earthquake, considering both were brought about by thrust faults in the mountains north of Los Angeles, and each resulting earthquake being similar in magnitude, though no surface rupture occurred in 1994. 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.
Surface faulting
Prominent surface faulting trending N72°W was observed along the San Fernando Fault Zone from a point south of Sylmar, stretching nearly continuously for east to the Little Tujunga Canyon. 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. Although the complete Sierra Madre Fault Zone had previously been mapped and classified by name into its constituent faults, the clusters of fault breaks provided a natural way to identify and refer to each section. As categorized during the intensive studies immediately following the earthquake, they were labeled the Mission Wells segment, Sylmar segment, Tujunga segment, Foothills area, and the Veterans fault.
All segments shared the common elements of thrust faulting with a component of left-lateral slip, a general east–west strike, and a northward dip, but they were not unified with regard to their connection to the associated underlying bedrock. The initial surveyors of the extensive faulting in the valley, foothills, and mountains reported only tectonic faulting, while excluding fissures and other features that arose from the effects of compaction and landslides. 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. Along the hill fronts of the Tujunga segment, some ambiguous formations were present because some scarps may have had influence from downhill motion, but for the most part they were tectonic in nature.
In repeated measurements of the different fault breaks, the results remained consistent, leading to the belief that most of the slip had occurred during the mainshock. While lateral, transverse, and vertical motions were all observed, the largest individual component of movement was of left lateral slip near the middle of the Sylmar segment. The largest cumulative amount of slip of occurred along the Sylmar and Tujunga segments. The overall fault displacement was summarized by geologist Barclay Kamb and others as "nearly equal amounts of north–south compression, vertical uplift (north side up), and left lateral slip and hence may be described as a thrusting of a northern block to the southwest over a southern block, along a fault surface dipping about 45° north".
Main shock Mercalli intensities MMI Locations Sylmar San Fernando Kagel Canyon Granada Hills, Newhall Los Angeles, Northridge Malibu, Ontario Santa Barbara, San Diego, Barstow Las Vegas, Parkfield, Bishop Merced, Moss Landing, Palo Verde U.S. Earthquake Intensity Database, NGDC
Landslides
The USGS commissioned a private company and the United States Air Force to take aerial photographs over of the mountainous areas north of the San Fernando Valley. Analysis revealed that the earthquake triggered over 1,000 landslides. Highly shattered rock was also documented along the ridge tops, and rockfalls (which continued for several days) were the result of both the initial shock and the aftershocks. Few of the slides that were logged from the air were also observed from the ground. The greatest number of slides were centered to the southwest of the mainshock epicenter and close to the areas where surface faulting took place. The slides ranged from in length, and could be further categorized as rock falls, soil falls, debris slides, avalanches, and slumps. The most frequently encountered type of slide was the surficial (less than thick) debris slides and were most often encountered on terrain consisting of sedimentary rock.
Strong motion In early 1971, the San Fernando Valley was the scene of a dense network of strong-motion seismometers, which provided a total of 241 seismograms. This made the earthquake the most documented event, at the time, in terms of strong-motion seismology; by comparison, the 1964 Alaska earthquake did not provide any strong motion records. 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. This stipulation ultimately found its way into the Uniform Building Code as an appendix several years later. 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.
The instrument that was installed at the Pacoima Dam recorded a peak horizontal acceleration of 1.25 g, a value that was twice as large as anything ever seen from an earthquake. The extraordinarily high acceleration was just one part of the picture, considering that duration and frequency of shaking also play a role in how much damage can occur. The accelerometer was mounted on a concrete platform on a granite ridge just above one of the arch dam's abutments. Cracks formed in the rocks and a rock slide came within 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. As a result of what was considered a fortunate accident, the machine kept recording for six minutes (until it ran out of paper) and provided scientists with additional data on 30 of the initial aftershocks.
Damage