Quakes Serve as Reminder to Prepare for a Big One

Emergency officials warned Monday that the series of earthquakes over the weekend should be considered a wake-up call for Ventura County residents, and scientists cautioned that Ventura County may be the most hazardous urban area in quake-prone Southern California.

“These are all reminders to us that we need to be better prepared,” said Laura Hernandez, assistant director of the Sheriff’s Department’s Office of Emergency Services. “Even the Northridge earthquake was not the Big One. It was just a wake-up call.”

County emergency officials are better prepared now for a catastrophic earthquake than during the magnitude 6.7 Northridge quake in January 1994, having added stronger backup generators at their headquarters in Ventura, officials said. And they beefed up the emergency telephone and radio systems to avoid confusion in a crisis.

“Our biggest problem was our ability to coordinate with multiple agencies and jurisdictions,” said emergency services official Dale Carnathan, referring to the loss of telephone lines and radio difficulties that made communications chaotic in the early hours after the Northridge earthquake.

The strongest quake Monday measured only 2.4, which was 150 times smaller than the 5.0 and 4.9 quakes that rocked the area early Saturday and Sunday.

And while the weekend’s series of 13 earthquakes of at least 3.0 was not strong enough to test the county’s new emergency system, the quakes were powerful enough to shake most parts of the county and raise questions again about how soon Ventura County may be hit by a devastating earthquake.

Two scientists who study the Ventura Basin--an area from Santa Clarita to Ventura along the Santa Clara River--said Monday that in Southern California only the more isolated San Andreas fault is more likely to have a major quake than Ventura County.

The weekend’s earthquakes were centered near the Ventura Basin about two miles south of California 126 and seven miles northeast of Simi Valley. A 1994 study concluded that the Ventura Basin has a 75% chance of an earthquake of 6.0 or greater in the next 30 years.

“The Ventura Basin is certainly one of the most likely places for a large earthquake,” said Andrea Donnellan, a Caltech geophysicist who has studied seismic activity in the area for a decade. “A magnitude 6 or over is not at all unlikely.

“We think the hazard is significant and that the people there should be prepared for it,” Donnellan said. “You should make sure your buildings are up to code, that your bookcases are bolted to the walls and that you have adequate supplies of food and water. I can’t stress enough that earthquakes are going to happen at some point, and if we’re prepared for them they’re not as devastating.”

The weekend earthquakes revealed tensions caused by the movement of mountain ranges north and south of the narrow Santa Clara Valley, Donnellan said. “They’re definitely related to the movement across the basin. The valley is very narrow there and it’s being squeezed closed.”

Studies that include satellite photographs have shown movement of the Topatopa Mountains above Fillmore south toward the Santa Clara River and movement of the Oak Ridge range north to the river, she said. The speed is about one-third of an inch a year, very fast in geological terms.

All of the major earthquake faults in the area are fast moving--the Oak Ridge, San Cayetano and Santa Susana, she said.

Scientists are focusing on the long-active San Cayetano and Santa Susana fault system as the most likely cause of the weekend quakes.

Greg Lyzenga, a geophysicist at Harvey Mudd College in Claremont, helped set up seismic monitoring stations Monday near the epicenter of the weekend quakes to determine the relationship of the weekend quakes with the Northridge earthquake.

“What we really want to know is what direction the fault moved, and what relation it bears to the Northridge earthquake,” he said.

“The Northridge earthquake did not break all of the potential fault links in the Ventura Basin that still have strain on them. So there’s a great deal of interest in whether the Northridge quake could be one in a series that break continuously over decades to the west, eventually rupturing the whole Ventura Basin. We want to see if the activity is migrating to the west or staying put.”

Donnellan and Lyzenga have studied the Ventura County faults by satellite through the Global Positioning System and have come to suspect the same thing that Robert Yeats, a geologist at Oregon State University and former Ojai councilman, concluded during two decades studying the same faults.

“They’re looking at completely different data, the global positioning data, and coming to the same conclusion that we may be seeing a migration,” Yeats said.

Yeats said that the county’s major faults are all relatively active, but none has had a major rupture in at least 200 years, since Mission period history-keeping began. He said he expects a major quake--such as the Northridge or the 1971 quake in Sylmar--to relieve pressure at least once every 200 years.

“We basically have been releasing no strain through earthquakes in Ventura County since the time we’ve been keeping records, so the strain is building up.”

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Ventura County Faults

Scientists are focusing on two Ventura County faults, the San Cayetano and Santa Susana - for signs of new ruptures due to pressure from the 1994 Northridge earthquake. The San Cayetano fault, along with the Oak Ridge fault, is among the fastest-moving in the county, with rocks on either side grinding past each other at a rate of nearly one-fifth inch per year. The Santa Susana fault near Simi Valley moves even faster, at nearly one-quarter inch per year. The Santa Cruz fault has the potential to cause earthquakes measured at magnitude 7.4.

Source: 1994 Working Group on the Probabilities of Future Large Earthquakes in Southern California.

Assembling Your Quake Kit

Part of being prepared entails putting together an emergency supply kit to use in the days following a quake, when utilities may be out and stores closed. Three separate kits--for home, car and work--can be assembled. A rule of thumb is to check or replenish supplies when clocks are changed to or from daylight-saving time, twice a year. Items can be stored in a container such as an airtight trash barrel. Experts recommend that a home kit have a one-week supply of food per person and one gallon of water per person per day for a week.

Supplies:

Water

Water purification tablets

Instant food (cereal, crackers and cookies)

Dry food (pasta, rice)

Canned food

Flashlight

Portable radio

Batteries and tester

Manual can opener

Food and water for pets

Extra eyeglasses

Extra clothing, shoes

Tarpaulin, blanket

Work gloves

Camping stove and fuel

First aid kit: antibiotic ointment; bandages; gauze and tape; aspirin, scissors; tweezers; rubbing alcohol; cotton balls; instruction booklet; extra prescription medication

Sources: “Earthquake Preparedness,” by Libby Lafferty; “Putting Down Roots in Earthquake Country,” by the Southern California Earthquake Center.