Data gathered for offshore oil drilling gives the government a look at the Malibu Coastal and Anacapa-Dume quake faults.
By Hans Laetz / Special to The Malibu Times
A treasure trove of acoustical soundings gathered by offshore oil prospectors 20 years ago has been donated to federal seismologists, and the developing picture shows that Malibu sits atop a series of earthquake faults that is much larger and deeper than ever anticipated.
The developing picture is of a 125-mile-long fault system that extends from downtown Los Angeles west past Santa Barbara, with Malibu bracketed by parallel faults that have caused an offset of two to three miles over tens of thousands of years.
The faults caused a 5.3 magnitude quake at Point Dume in 1973, and have the potential for larger earth movements and tsunamis in the future, scientists said.
“We see for the first time that these are deep-penetration faults, with large displacement,” said study author Michael A. Fisher, a U.S. Geologic Survey marine geophysicist. “And these faults could trigger a magnitude 7 quake with substantial movement, and set up a tsunami that could go into Santa Monica Bay.”
In findings published in the latest Bulletin of the Seismological Society of America, the team of scientists concludes that Malibu sits atop and abreast of a series of major earthquake faults that have moved the mainland as much as three miles to the west past the seafloor and Channel Islands, over relatively recent millennia.
The study calls these faults “a serious potential earthquake hazard to nearby Los Angeles.”
The new study is based on oil-drilling data gathered by using very loud air blasts detonated in the ocean off Malibu during an exploration project by the oil services firm Western-Geco Inc. in 1985. Oil drillers used hydrophones to listen to the blasts reverberate, and developed a picture of undersea rock formations using that data.
“These explosions would not be possible now, because of a presumed harmful effect on marine mammals,” Fisher said in a telephone interview from his USGS office in Menlo Park.
Modern computers can perform much more detailed analysis using smaller sound sources, Fisher said, making the 1980s data obsolete.
“The oil industry has moved far beyond this technology, so for them the old data was trash.”
However, the data is valuable to scientists looking at broad faulting and earth formation trends. Also, the government has never had funding for any broad seismic soundings along the Pacific Coast.
Western-Geco kept its valuable data secret for 20 years, but then donated the echo soundings to the USGS two years ago.
“Until we got this data,” Fisher said, “the images we had of the sea floor were not deep enough to show the magnitude of these faults.
“When I first looked at this data, it was like opening the eyes and saying ‘wow, these are big faults’,” he said.
The mountains that Malibu sit on, Fisher explained, broke off 16 million years ago from others that are near present day San Diego. The Santa Monicas rotated 90 degrees to the left and stopped moving about 5 million years ago, when the San Andreas Fault began to develop as the boundary between the Pacific and North American tectonic plates.
Fisher said the Malibu Coastal Fault marks the result of a collision between the offshore California continental borderland, with its northwest-southeast running subsea mountains, and the east-west transverse ranges of Central California.
The transverse mountains, including the Santa Monicas, block the movement of the Pacific Plate, causing a series of hidden fault movements under the Malibu coast that have never been seen-until this oil industry data revealed them by taking pictures inside the subsea rock, Fisher said.
Other geologists with field experience in Malibu said Fisher’s findings confirm their suspicions that actual earth movement has moved offshore, as opposed to the Malibu Coastal Fault, which runs parallel to and north of Pacific Coast Highway through much of Malibu.
“It confirms my suspicions that the Malibu Coastal Fault itself has been inactive for 20,000 years,” said Eldon Gath, president of Earth Consultants International. “The active fault movement has to have shifted south, off the beach, and that there is a large fault system that has absorbed the movement off the coast from the Civic Center towards Point Dume.”
Another geologist, Christopher Sorlien, said he thinks the faults discovered in these latest findings are connected near Santa Monica to well-known faults that run east toward downtown Los Angeles.
Sorlien said the offshore Dume Fault described by Fisher “had not previously been accurately mapped, and had not been shown at depth on high quality industry seismic reflection data.”
In a study released a year ago, the USGS had estimated that the faults off Malibu could trigger a 7.5 magnitude quake. While the maximum forecast has been scaled back to a 7.0 quake, scientists said that movement may be more frequent than originally thought possible.
Tsunami possibilities have long been suspected for the Santa Monica Bay, and the city of Malibu has issued a tsunami awareness pamphlet to warn locals to seek high ground in the event of a strong local earthquake.
Fisher said the latest study should not scare people, but should make Malibu residents “aware that they live in an area where a very large quake or tsunami is comparatively very likely.”
“Look, I live in Menlo Park, not five miles from the San Andreas Fault,” the scientist said. “You can’t govern your life on ‘what ifs’ but you can balance the risk by storing some food and some water for the inevitable.”