1971 San Fernando earthquake
|Date||February 9, 1971|
|Depth||8.4 kilometers (5.2 mi)|
|Countries or regions||USA
The 1971 San Fernando earthquake (also known as the Sylmar earthquake) struck the San Fernando Valley near Sylmar, California at 6:00:41 am PST on February 9, 1971, with a magnitude of 6.6. The earthquake lasted approximately 60 seconds and caused 65 fatalities.
There are various names for this earthquake. The USGS and seismologists call it the San Fernando earthquake. "Sylmar Quake" or "Sylmar Earthquake" is the name initially given to the event by local media outlets, because the worst damage was to the Olive View Medical Center, located in Sylmar.
The Southern California Earthquake Data Center reports that the epicenter was on the other side of the San Gabriel Mountains from Sylmar, as indicated on this Google Map. This Instrumental Intensity Map identifies the same location using a star shaped symbol "centered in Iron Canyon, in the Sand Canyon area of Canyon Country". These maps help explain why the seemingly distant cities of Sylmar and San Fernando received the brunt of the damage: the quake energy was focused within the sparsely populated San Gabriel Mountains, extending slightly into the northeastern end of the densely populated San Fernando Valley.
The earthquake ruptured a segment of the San Fernando fault zone, a set of north-dipping, high-angled reverse faults along the southeastern margin of the San Gabriel Mountains.
It caused more than 10 miles of discontinuous surface ruptures with average displacements of about 3 feet both horizontally and vertically. A strong aftershock sequence followed the main shock and included four quakes in the Magnitude 5 range.
The quake claimed 65 lives and caused more than half a billion dollars in damage, including the destruction of two hospitals, two freeway interchanges and the Lower Van Norman Dam. Damage to the earthen bulwark dam raised concerns of a partial or total collapse. Much confusion ensued as various agencies declared a need for the mandatory evacuation of 40,000 people, or voluntary evacuations of various portions of the San Fernando Valley below the dam. This depended on which agency was consulted, and often the evacuees were not able to be informed of the status of an evacuation in a timely manner, often returning home just as the police arrived to notify them of a new evacuation order, or evacuating at a moment when officials decided not to evacuate. Communication was made difficult by disruption of telephone, water, and electrical service.
The most spectacular damage included the collapse of structures at Olive View Hospital (which had opened just a month prior to the earthquake) and at the Veterans Administration Hospital, both in Sylmar. The earthquake pushed Olive View Medical Center 1 foot (0.30 m) off its foundation, causing the first floor to collapse, killing three patients and a hospital worker. Forty-nine people died in the VA Hospital. Twelve overpass bridges fell into freeway lanes, including the freeway overpass connecting the Interstate 5 freeway and the Foothill Freeway that resulted in the death of at least two people. The recently completed Newhall Pass interchange connecting Interstate 5 and the Antelope Valley Freeway was destroyed as well. This interchange was rebuilt and reopened in 1973, but collapsed again 21 years later during the 1994 Northridge earthquake, killing one.
Besides damaging freeways and buildings, the Sylmar Quake also was reported to have damaged hundreds of schools all over the Los Angeles County area. It severely damaged one of Los Angeles' oldest elementary schools, Atwater Avenue Elementary School, and an historic high school in the Silver Lake region of Los Angeles, John Marshall High School. Its main building, in Collegiate Gothic style, was refurbished, but the cafeteria and other portions were razed.
Landslides were widespread and caused extensive damage throughout the San Gabriel Mountains.
In response to the quake, building codes were strengthened and the Alquist Priolo Special Studies Zone Act was passed in 1972. The purpose of this act is to prohibit the location of most structures for human occupancy across the traces of active faults and to mitigate thereby the hazard of fault rupture.
See also 
- Southern California Earthquake Data Center
- Historic Earthquakes
- "Feb. 9, 1971 'Sylmar' Earthquake Instrumental Intensity Map". Santa Clarita Valley Historical Society. Archived from the original on 19 June 2004. Retrieved July 4, 2004.
- "Day of Disaster". Los Angeles Times. February 10, 1971.
- "Historic Earthquakes." USGS. U.S. Dept. of the Interior. Web. Retrieved 12 Feb. 2012.
- "Veterans Memorial Community Regional Park." Parks and Recreation. lacounty.gov. Web. Retrieved 12 Feb. 2012.
- "Historic Schoolshere of the Los Angeles Unified School District." laschools.org. PDF. Retrieved 25 Feb 2012.
- "School History." John Marshall High School. Retrieved 24 Feb. 2012.
- SCEDC | San Fernando Earthquake (1971)
- USDA-produced documentary from 1973 about the earthquake
- The San Fernando, California, Earthquake of February 9, 1971: USGS Professional Paper 733
- The short film Earthquake (1972) is available for free download at the Internet Archive [more]