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San Fernando Valley California Earthquakes of 1971 and 1994 |
This slide set compares two earthquakes that were separated by a distance
of 10 miles and a time of 23 years. Disproving the notion that once an earthquake
has occurred, an area is safe from future earthquakes, these events affected much
of the same area and even some of the same structures. These two events were the largest
of 17 moderate-sized main shock/aftershock sequences that have occurred in the Los
Angeles area since 1920. The 1971 shock is referred to in the scientific literature
as the San Fernando earthquake. The 1994 shock (also in the San Fernando Valley) is
called the Northridge earthquake. This slide set compares the two earthquakes. The
pictures show the same type of damage in some cases, and effects at the same site
in other cases.The EarthquakesOn February 9, 1971, 14:01 UTC (6:01 A.M. local time),
a magnitude 6.5 earthquake occurred. The epicenter was located at 34 degrees 25' N,
118 degrees 24' W, 20 miles (32 km) north of downtown Los Angeles, at a depth of 8.4
km (five miles).Twenty-three years later on January 17, 1994, 12:31 UTC (4:31 A.M.
local time), a magnitude 6.8 earthquake occurred. The epicenter was located at 34
degrees 13' N, 118 degrees 32' W, 20 miles west-northwest of downtown Los Angeles,
at a depth of 12 miles (20 km). The 1994 earthquake was centered about 10 miles (16
km) southwest of the 1971 earthquake. Was history repeating itself?The aftershock
zones of the two earthquakes overlap, suggesting that some of the aftershocks may
be occurring along the same fault systems. Both earthquakes were the result of thrust
faulting. However, in 1971 the fault dipped toward the north, away from the population
and under the mountains. In 1994, the fault plane dipped toward the south, into the
densely populated valley. The 1994 sequence also started deeper (12 miles) as compared
to the 1971 sequence, which started at a depth of five miles. The main shock beneath
the Northridge suburb occurred on a shallowly-dipping, previously-unknown thrust fault.
The focal mechanism of the main shock from both first motions and teleseisms shows
a N 60 degrees west dipping and 35 degrees to 45 degrees south dipping plane. The
distribution of aftershocks in the San Fernando earthquake suggests southward thrusting
along a disc-shaped fault surface, and aftershock depths suggest that the thrust surface
dips about 35 degrees N toward N 20 degrees E.The Damage - Although the 1994 earthquake
was only slightly larger than the 1971 earthquake, it was much more damaging because
of its location beneath the San Fernando Valley and its closer proximity to other
communities in the Los Angeles basin. Northridge statistics include 56 dead, 25,000
dwellings uninhabitable, and $10 billion in damage. The 1971 San Fernando earthquake
was felt over an area of 80,000 square miles. The statistics include 58 dead and $497.8
million in damage. In the 1971 earthquake, several major hospital complexes were damaged,
including the Veterans Administration Hospital, the Olive View Hospital, the Holy
Cross Hospital, and the Pacoima Memorial Lutheran Hospital. In the 1994 earthquake,
20 medical facilities had at least one building tagged yellow or red. Spectacular
damage occurred to freeways in both earthquakes. In both earthquakes, the I-5 and
SR-14 interchange incurred major damage. In the 1971 earthquake, a major fault line
crossed the main and siding railroad tracks at Sylmar. In the 1994 earthquake, a 64-car
Southern Pacific train was derailed near the earthquake epicenter, spilling 5,000
gallons of sulfuric acid and 3,000 gallons of diesel fuel. In the 1971 event, more
than 750 homes and 100 businesses sustained major damage. In the Northridge earthquake
(23 years later), companies closed plants and offices, and more than forty retailers
reported heavy damage. Apartment complexes all over the San Fernando Valley were severely
damaged. Power outages occurred throughout the San Fernando Valley in both events,
and gas, water, and telephone services were disrupted. These two events in the San
Fernando Valley clearly dispel the myth that an earthquake in an area reduces the
threat of future earthquakes.
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