New data indicates that a complex fault system along the Malibu coast is both active and capable of causing a major earthquake of magnitude 6 or 7, two geologists who have closely studied the area warned Wednesday.
The warning, backed up by findings from other geologists, may prompt the California Coastal Commission to ask the state to designate the Malibu coast a “special study zone” in which development would be tightly regulated because of the seismic danger.
After hearing the warning, one commissioner urged her colleagues to press for the designation, and commission Executive Director Peter Douglas said a letter already is being drafted to send to the state if the commission votes to move forward with the request.
Former State Geologist James E. Slosson told the commission at a special workshop in Marina del Rey that recent geologic findings show such a designation is appropriate for Malibu.
With one exception, the 10 geologists who also appeared before the commission agreed.
Slosson, who has studied the area for 30 years, said a major quake along the Malibu coast fault zone--previously thought to be inactive--could cause extensive property damage and threaten lives by triggering “very serious” landslides and rock falls along Pacific Coast Highway and Malibu Canyon Road. The rocks along the coastline, he said, are “highly shattered and susceptible to failure in a major earthquake.”
In an interview, Slosson likened the danger to that posed by the Newport-Inglewood fault, which runs north to south across Los Angeles and Orange counties and caused the devastating Long Beach earthquake in 1933.
The second geologist, Gary Greene of the U.S. Geological Survey, said that based on historical data the Malibu coast fault system is probably capable of a magnitude 6 earthquake and “it could be higher.”
An earthquake registering 5.7 and centered off Point Mugu caused more than $1-million damage in the Oxnard-Ventura area in February, 1973.
Previously, geologists had not focused on the potential magnitude of a temblor from the Malibu fault because it was thought to be inactive.
But the recent discoveries have lead Greene and others to the conclusion that “we are dealing with a fairly active seismic zone.”
Greene said recent evidence indicates there is more than one fault in the Malibu area. “My feeling is it is a system of . . . maybe a half-dozen faults.”
Geologists have known of the main Malibu fault for many years. But in the last few years, evidence has been found to indicate that it is active and more extensive than previously realized. Among the significant recent discoveries are two splays or splinter faults off the main Malibu fault that have been active within the last 11,000 years--the benchmark for what constitutes an “active” fault in California.
The first discovery of an active splinter fault in 1987, forced General Motors Corp. to abandon plans for construction of an $11-million advanced design center across Pacific Coast Highway from Pepperdine University.
And in recent months, digging at the site of a single-family home in Latigo Canyon also yielded evidence of an active fault.
Under provisions of the Alquist-Priolo Act, passed by the Legislature after the 1971 Sylmar earthquake, the state geologist is required to designate special zones along known earthquake faults in California. The law prevents construction of buildings for human occupancy within 50 feet of an active fault.
Coastal Commissioner Madelyn Glickfeld, who lives in Malibu, urged her colleagues to press for such a designation. The matter is expected to be considered at the commission’s meeting next month.
Joe Cobarrubias, geologist for the city of Los Angeles, said he has seen enough evidence to convince him that a special study zone is needed in Malibu. “The right thing to do would be to recommend to the state that the area be zoned as a special study zone,” he said.
Newport Beach private geologist Roy Shlemon said the new information “strongly suggests” that two splays of the Malibu coast fault zone are “probably active.”
But geologist Howard Spellman said he does not believe enough evidence has been collected to justify a special study zone. Instead, he suggested that the Coastal Commission deal with projects in Malibu on a case-by-case basis. “I don’t think it should be zoned indiscriminately,” he said.
In their testimony, the geologists told the commission that the Malibu fault zone runs parallel to the coastline, onshore in some areas and slightly offshore in others. Federal geologist Greene described it as a “thrust fault,” in which one section of the earth’s crust is moving vertically relative to the other. This contrasts with a “strike-slip fault,” such as the San Andreas, in which the two sections move laterally.
Greene said it is possible that the Malibu fault system ties in with the Santa Cruz Island fault, which runs beneath the ocean off the Santa Barbara County coast. If so, Greene suggested, the system could be “very significant as far as a serious hazard.”
He said the fault zone may be responsible for the numerous oil and gas seeps that occur in Santa Monica Bay. “They all line up on a trend to the fault,” he said.