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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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