By KENNETH REICH, Times Staff Writer
Research
by earthquake scientists suggests that parts of Ventura County, especially the
area around Piru in the Santa Clara River Valley, are among the most likely sites
in California for a major earthquake, possibly exceeding magnitude 7.
The
two sites near Piru that are the subject of greatest concern are the San Cayetano
and the Oak Ridge faults. An eastern extension of the Oak Ridge into the San Fernando
Valley is believed by some scientists to be the origin of the magnitude 6.7 Northridge
quake of Jan. 17, 1994.
A 200-foot-long, 15-foot-deep trench dug in 1999
along the Piru strand of the San Cayetano fault, two miles southwest of Piru,
uncovered evidence of a quake as great as magnitude 7.5 in the vicinity sometime
after 1660.
Another, secondary area of concern is along the Red Mountain
fault just north and west of the city of Ventura, where the rate of movement has
been high.
The "slip rate," or the average annual movement, of the San
Cayetano fault over thousands of years has been measured at higher levels than
those almost anywhere else in California, according to James F. Dolan, a USC quake
scientist who oversaw the trench study with Tom Rockwell of San Diego State.
Meanwhile,
delineations of the earth's shape show a sizable compression, or shortening of
terrain, between the north and south sides of the East Ventura Basin near Piru.
At a quarter of an inch a year, the so-called geodetic movement leads scientists
to believe the accumulating strain will bring forth periodic major quakes in the
vicinity.
Andrea Donnellan of the Jet Propulsion Laboratory, who directed
the geodetic studies, said the data also show accumulating strain along what are
known as the transverse mountain ranges extending from Santa Barbara east through
Ventura and Los Angeles counties to Cucamonga in San Bernardino County.
The
quake risk is considerable for communities all along those ranges, including the
San Fernando Valley, Sierra Madre and Upland. Other areas in California with a
high quake risk include the San Francisco Bay Area, the North Coast in Humboldt
County and the Imperial Valley.
Slip rates are measurements of how fast
specific faults have moved during periodic earthquakes. Around Piru, both the
slip rates and the geodetic measurements indicate that future big quakes are likely.
But the data give no indication of when.
In his new book, "Living With
Earthquakes in California--A Survivor's Guide," Robert S. Yeats, a geoscientist
at Oregon State University and an expert on the seismicity of the Ventura County
region, writes of the San Cayetano, Oak Ridge and Red Mountain faults:
"It
seems likely to me that one of these three fast-moving faults is likely to sustain
an earthquake in the near future, but I can't be specific about how long the 'near
future' is likely to be, or which one of the faults will go first."
Yeats'
hesitance is common in earthquake science. The scientists can use existing research
tools to identify the areas of greatest strain and fastest slip as those where
big quakes are ultimately likely.
But they cannot say precisely when quakes
will strike or where they will be centered.
The uncertainty extends even
to where and when quakes have occurred in the past.
For instance, Yeats
and Dolan, the leaders of the recent quake research for Ventura County, disagree
over the location of the epicenter of a magnitude 8 quake known to have occurred
Dec. 21, 1812.
Dolan said he believes slippage from that quake is found
in the trench near Piru. But Yeats said he believes the 1812 quake was centered
farther west, in the Santa Barbara Channel, and would have been felt more strongly
in Santa Barbara than in Ventura County.
The locale of the 1812 quake
could affect how soon enough strain accumulates to cause the next big quake in
Ventura County. If Dolan is right, it might take longer.
But Yeats and
Dolan do agree there is a substantial Ventura County risk. And a wide range of
other quake scientists in Southern California agree with them.
For instance,
Thomas Heaton, earthquake engineering professor at Caltech, says the area from
the San Fernando Valley through Ventura County to Santa Barbara has "one of the
fastest uplifts of coastal deposits anywhere in the world."
"The slip
rates are high. The geodetic measurements show accumulating strain," Heaton said.
"All this leads to the conclusion that there is a relatively high risk."
But
the scientists caution this does not necessarily mean Ventura County or nearby
areas are next up for a big earthquake.
Destructive earthquakes, such
as the one in Kobe, Japan, in 1995, often seem to occur in areas where they are
not expected.
So any statement that Ventura County is a particularly risky
area for large quakes does not necessarily mean the next big one in California
will occur there.
It does mean, however, that over the next few thousand
years, Ventura County will probably be the site of more big quakes than most other
counties in California and the Western states.
In his book, Yeats recognizes
that over the last two centuries, Ventura County has not been the center of major
quakes.
"Earthquakes are not part of the culture of Ventura," he writes.
"The great earthquake of 1812 was too far back in time to have any meaning to
the people who live there now." But, he observes, two centuries is very little
time when it comes to earthquakes, certainly too little for anyone to assume their
relative absence means no big ones will occur.
On Shaky Ground
Recent earthquake science suggests that faults near Piru in eastern Ventura
County are among the most likely in California to cause a major earthquake. Scientists
say accumulating strain along the San Cayetano and Oak Ridge faults, and compression
between the two, strongly suggests that an earthquake exceeding magnitude 7 could
strike in the vicinity.
But they can't say when. Recent trench studies
confirming past quakes and observation that the two faults are slowly moving closer
mean only that quakes are more likely to occur there than along other major faults.
Scientists have also noted a 200-year gap in significant quakes in Ventura
County compared with neighboring counties, an absence some say increases the likelihood
that Ventura County is due for one.
Significant quakes near Ventura County
1. San Andreas Fault
Wrightwood, San Bernardino County, Dec. 8, 1812.
Magnitude 6.9, ruptured between Palmdale and Cajon Pass.
2. Santa Barbara
County
Between Gaviota and Santa Barbara, Dec. 21, 1812. Magnitude about 8.0;
some scientists believe this was centered near Piru in Ventura County.
3.
San Andreas Fault
Fort Tejon, Jan. 9, 1857.
Magnitude 7.8, ruptured from
Cholame to Wrightwood.
4. Santa Barbara County
June 29, 1925. Magnitude
6.3, killed 20. 5.
Santa Barbara County
Off Point Arguello, Nov. 4, 1927.
Magnitude 7.5,
caused small tsunami, damaged Lompoc.
6. Santa Barbara County
June
30, 1941. Magnitude 5.9.
7. Los Angeles County
San Fernando, Feb.
9, 1971.
Magnitude 6.6, killed 58.
8. Santa Barbara County
Aug.
13, 1978. Magnitude 5.7, injured 65.
9. Los Angeles County
Northridge,
Jan. 17, 1994.
Magnitude 6.7, killed 57. |
