Fault more dangerous than believed
San Diego’s Rose Canyon fault produces powerful earthquakes more frequently than once believed, but a major temblor isn’t imminent, according to researchers from San Diego State University.
SDSU scientists who studied the fault in Old Town determined that the system — which before about 1990 was thought to be inactive — generates a magnitude 6.5 to 6.8 earthquake about once every 700 years.
Seismologist Tom Rockwell said that earlier work indicated that such quakes occur every 1,000 to 1,500 years on the 40 mile-long fault, which extends from San Diego Bay, through Old Town and across Mission Valley, and up Rose Canyon through Mt. Soledad, to the offshore at La Jolla.
The fault is known to extend as far north as Oceanside in the offshore.
“A powerful quake in the mid-to-upper 6s could cause liquefaction around San Di- ego and Mission Bays and locally in Mission Valley, and cause the land to be offset across the fault, which would damage buildings,” said Rockwell, one of California’s most experienced seismologists.
His doctoral student, Drake Singleton, said, “We could see the history of ruptures in the soil of Old Town, and that told the story.”
The research team said they also found evidence at a dig site in Old Town that the strike-slip fault has produced at least two additional quakes in the magnitude 5.0 to 6.0 range in recent centuries— shaking referred to as background seismicity.
“A 6.0 quake likely wouldn’t break the surface of the ground, but it could cause liquefaction in San Diego and Mission Bays,” said Rockwell, who has dug trenches on faults across Southern California.
Even so, Singleton’s work shows that the 1862 earthquake in San Diego, estimated at magnitude 6, did produce minor ground breakage in Old Town, which was not previously known.
Late last fall, Singleton and Rockwell dug a 160foot long, 3-foot wide trench on the Presidio Hills Golf Course and spent months studying the orange-gray sediment for traces of past quakes, leading them to discover that a major quake had occurred before the founding of Mission San Diego de Alcalá in 1769.
“If we had dug in a place where there were no faults, the sediment wouldn’t be very disrupted at all,” Singleton said. “But in the Rose Canyon trench, things are very chaotic. You can see the past.
The initial results were submitted as an abstract to the Geological Society of America.
The new finding comes two months after UC San Diego’s Scripps Institution of Oceanography released a study saying that offshore segments of the Rose Canyon and Newport-Inglewood faults could rupture and produce a 7.3 quake, damaging large areas of the Southern California coastline.