- The fault system has the potential to
do great damage.
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- Scientists have discovered a previously
unkown fault system in the rock under Los Angeles which is capable of producing
huge earthquakes.
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- They believe it was probably responsible
for the Whittier Narrows earthquake which killed eight people and injured
200 others in 1987.
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- They warn it could produce even more
powerful quakes in the future.
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- The system, which is not visible on the
surface, runs for 40 km (25 miles) from downtown Los Angeles to the Coyote
Hills in northern Orange County and toward Brea in the east.
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- It covers at least 840 square km (324
square miles), John Shaw of Harvard University and Peter Shearer of the
Scripps Institute of Oceanography report in the journal Science.
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- Damaging events
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- Faults which appear on the surface of
the land, such as the San Andreas fault, are much easier to study. But
scientists would like to get much more information about so called "blind-thrust"
faults similar to the one now identified by Shaw and Shearer.
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- They have been responsible for a recent
string of very damaging events, including the 6.7 magnitude Northridge
earthquake in 1994.
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- Shaw and Shearer used a new seismic model
to map the fault system. It was based on data rarely obtained from the
oil industry.
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- They found three distinct segments to
the system - one under Los Angeles, another underneath Santa Fe Springs
and the third beneath Coyote Hills.
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- It is closest to the surface in the south
- about 3 km (1.8 miles) down - and reaches to a depth of about 17 km (10.5
miles) towards the north.
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- Unconsidered hazard
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- "This an important earthquake source
for Los Angeles," says John Shaw, "and one that we've been able
to establish beyond inference."
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- It is impossible to say how often the
newly mapped fault system has ruptured in the past, but the authors are
still concerned that it may pose a significant and "previously unconsidered
hazard."
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- It appears that the Whittier Narrows
event (6.0 magnitude) ruptured just 10 percent of the newly mapped fault
system. Further ruptures along the full length of any one of the three
segments identified "could generate 6.5 to 6.6 moment magnitude earthquakes,"
Shaw and Shearer say.
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- A much larger earthquake of at least
7.0 magnitude could occur if all three ruptured at the same time, or if
the new fault system turns out to extend below the region involved in the
Whittier Narrows quake.
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