Sylmar earthquake

From Encyclopedia Jr, free information reference for Kids

The Sylmar earthquake struck at 6:00:55 a.m. PST on the morning of February 9, 1971. The earthquake that rocked the northern San Fernando Valley, near Sylmar measured magnitude 6.6 on the Richter magnitude scale.

There are various names for this earthquake. 'Sylmar Quake' 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. Local veterans of 'the Sylmar Quake' commonly refer to this seismic event as the 'February Ninth' quake. Seismologists call it the San Fernando earthquake. USGS

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, a freeway interchange and the Lower Van Norman Dam. Damage to the dam caused concern that the dam, of the Earthen Bulwark type, might collapse, in whole or in part.

Much confusion ensued as various agencies declared a need for mandatory evacuations, 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 in Sylmar (which had opened just a month prior to the earthquake) and at the Veterans Administration Hospital at San Fernando, where 49 people died. The earthquake pushed Olive View Medical Center a foot off its foundation, causing the first floor to collapse, killing three patients and a hospital worker. A freeway overpass connecting the Interstate 5 freeway and the Antelope Valley Freeway also collapsed, which resulted in the death of at least two people. (This interchange was rebuilt and reopened in 1973, but collapsed again 23 years later during the 1994 Northridge Earthquake, killing one.)

Landslides were widespread and caused extensive damage throughout the San Gabriel Mountains.

The Sylmar Earthquake disrupted several motion pictures being filmed at the time in the Los Angeles area, including The Omega Man. The quake also served as the inspiration for the 1974 film, Earthquake.