Quakes Shift Attention to Long-Dormant Fault

A series of minor earthquakes over the past two months in the Oxnard-Port Hueneme area indicates a long-dormant fault has suddenly become active, geologists said Tuesday.

There is no evidence, however, that the recent activity presages more--or larger--quakes than have already occurred, experts say.

Monday afternoon brought the latest rumbling on the Oxnard Plain, when an earthquake of magnitude 2.6 occurred three miles southeast of Port Hueneme. There were no reports of damage or injuries.

That was the fourth quake in that area in recent weeks. Ground movement ranging in magnitude from 2.3 to 2.6 was recorded on Nov. 29, Dec. 28 and Jan. 12. Only the most recent two quakes were felt.

Although all the quakes have been minor, one geologist says the fault has the potential to cause a larger quake, possibly in the magnitude of 5.0 or above.

“People should be aware there is a hazard, but the fault could be quiet for the next 100 years and I wouldn’t be surprised,” said Craig Nicholson, a research geophysicist at UC Santa Barbara. Still, he added, “The fault could rupture tomorrow, and I wouldn’t be surprised.”

Nicholson said the last major seismic event in the area was a 1957 earthquake of magnitude 5.0 in Port Hueneme.

“Since then, the activity has been fairly low on the fault,” he said, adding that residents and even scientists had forgotten about the fault’s existence until the recent activity. The fault does not appear on maps of Ventura County’s faults and has not been named.

Nicholson said existence of the Camarillo hills had previously led geologists to believe a significant fault line lies beneath the Oxnard Plain.

“For hills to develop on the flood plain like that, there usually has to be a physical explanation,” Nicholson said.

Nicholson said because the small quake Monday was felt and described by some as a rolling motion, the fault may not be far beneath the surface.

Most California faults lie about nine miles underground, but the Oxnard fault could lie only a mile below the surface, experts say.

“When earthquakes are very shallow, they are more likely to produce surface waves and produce that rolling motion,” Nicholson said.

The 1957 Port Hueneme quake, Nicholson said, is further evidence that the fault is shallow. According to reports, he said, movement and damage were greater than would be expected for an earthquake that size.

“We can pretty safely say, based on the activity that has occurred in the past, there is a fault there that’s certainly capable of producing magnitude 5.0 earthquakes, since it has in the past,” Nicholson said.

At that magnitude, ground-shaking would knock items off shelves and cause some structural damage.

What the recent activity along this fault portends is difficult to predict, Nicholson said. Some geologists believe when a fault becomes active after years of inactivity, a larger quake is coming. Others worry when faults are “suspiciously quiet.”

Caltech geophysicist Andrea Donnellan, who has researched the Ventura basin, said the area is being compressed from north to south.

“It’s not surprising that there are earthquakes occurring,” Donnellan said. “I’m not sure that means that fault will do anything substantial in the future.”

Donnellan said a series of small quakes, often called swarms, rarely result in large seismic events.

Ventura County’s major active faults include the Oak Ridge and San Cayetano faults, where the plates are grinding across each other and moving at a yearly rate of nearly one-fifth of an inch near Santa Paula and Fillmore. The Santa Susana fault near Simi Valley moves even faster, nearly a quarter inch per year.

The Santa Cruz fault beneath the Channel Islands has the potential to cause earthquakes of magnitude 7.4, according to a 1994 report on the probability of future large quakes.

Donnellan said she recently gathered data she hopes will prove whether the 1994 Northridge earthquake--magnitude 6.8--put added stress on any of Ventura County’s major active faults.