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Recent Landslides in La
Conchita, California Belong to a Much Larger Prehistoric Slide,
Report Geologists
October 19, 2005
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Mudslides at La
Conchita |
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| (Salt Lake City, Utah) –
The deadly landslide that killed 10 people and destroyed
approximately 30 homes in La Conchita, California last January is
but a tiny part of a much larger slide, called the Rincon Mountain
slide, discovered by Larry D. Gurrola, geologist and graduate
student at the University of California, Santa Barbara. The slide
started many thousands of years ago and will continue generating
slides in the future, reported Gurrola at the national meeting of
the Geological Society of America today in Salt Lake City.
Prehistoric slides present at Rincon Mountain cover an area of
about 1,300 acres with a minimum volume of about 600 million cubic
yards, said Edward A. Keller, professor of earth science at UC Santa
Barbara. Keller analyzed the landslide complex with Gurrola and Tim
Tierney, UCSB research scientist. Geological consultant Ted Powers
also contributed. The La Conchita landslides that occurred in 1995
and 2005 form only a small percent of a much larger landslide
complex, according to the geologists. These recent landslides
spilled over U.S. Highway 101 in the Ventura County community that
is located 25 miles south of Santa Barbara.
"The slope that failed in 1995 and 2005 is a holocene paleosea
cliff and is near the seaward edge of an ancient landslide that has
produced prehistoric and historic slides, slumps, debris and mud
flows," said Gurrola. "The question is not if but when the next
landslide will impact the community of La Conchita. A combination of
factors makes future landslides inevitable. These are: active
faulting and folding; rapid tectonic uplift; very weak rocks; steep
topography; and, the presence of springs."
Keller and Gurrola explained that the triggering mechanism for
debris flows and mud flows appears to be prolonged, intense
precipitation. The larger, complex slides may increase in activity
months or even years after wet years and infiltration of rainwater
to the subsurface environment. An earthquake could also trigger a
slide.
"Landslides similar or larger than the 1995 and 2005 events may
occur next year or in coming decades, during or shortly after
intense rain," said Gurrola. "People tend to have short memories
when it comes to geologic hazards such as landslides. If people
continue to live in La Conchita, more lives will be lost in the
future and this is unacceptable."
Keller and Gurrola urged that property owners be fairly
compensated for their property, and proposed that the site of La
Conchita be made into a beach park. "For this to happen will take a
significant community fund-raising effort with assistance at local,
state and national levels in cooperation with organizations such as
the Land Trust Alliance," said Keller.
He mentioned that a warning system of sensors to detect slope
movement was installed in the slope following the slide of 1995.
"However, the 2005 slide evidently started above these sensors or
the slide was too fast for a warning," said Keller. "In hindsight,
notice of the duration and intensity of rainfall might have been
helpful in providing a warning, but additional research would be
necessary to test this hypothesis. We do have 'Red Flag' day
warnings in Southern California for wildfire, based on air
temperature and wind pattern. Perhaps the same could be done for La
Conchita, if people insist on living there. A combination of
instruments to detect movement with assessment of rainfall might be
the best approach."
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