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 part of the fault located in the Old Town neighborhood determined that the system — which before 1990 was thought to be inactive — generates a magnitude 6.5 to 6.8 earthquake about once every 700 years.

Seismologist Tom Rockwell said earlier analysis 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, then up Rose Canyon through Mt. Soledad and finally heading offshore at La Jolla.

Living on the fault line (USGS)


In its offshore portion, the fault is known to extend as far north as Oceanside.

“A powerful quake in the mid- to upper 6s could cause liquefaction around San Diego 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.

Drake Singleton, his doctoral student, said: “We could see the history of ruptures in the soil of Old Town, and that told the story.”

The research team said it also found evidence at a dig site in Old Town that the Rose Canyon 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,” 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 that wasn’t previously known.

Last fall, Singleton and Rockwell dug a 160-foot long, 3-foot wide trench on the Presidio Hills Golf Course and spent months studying the orange-gray sediment for traces of past quakes. The effort led them to discover that a major quake had occurred before the founding of Mission San Diego de Alcalá in 1769.

San Diego State University doctoral student Drake Singleton, and seismologist Tom Rockwell, stand inside the trench used to study the Rose Canyon fault in Old Town (Rockwell Lab, SDSU)


“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. The paper added to long held beliefs by scientists that California’s offshore faults can be as perilous as those that are on shore.


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