The conventional thinking has long been that the San Diego region faces less danger from a devastating earthquake than the Los Angeles or San Francisco areas.

But a new landmark study shows just how a fault running through the heart of San Diego poses a much more serious threat than believed a generation ago.

Researchers examined the effects of the Rose Canyon fault producing a plausible magnitude 6.9 earthquake, threatening the civic and financial center of California’s second largest city and the nation’s fourth biggest naval base, causing liquefaction and landslides.

Such a quake could damage 120,000 of San Diego County’s 700,000 structures and causing $38 billion in economic losses from just building and infrastructure damage and $5.2 billion in lost income from business interruptions, according to a new report released Wednesday by the Earthquake Engineering Research Institute’s San Diego chapter on the first day of the National Earthquake Conference at the Sheraton San Diego Hotel & Marina.

San Diego could be disrupted for years. Yet most people in San Diego either know nothing about the Rose Canyon fault or think it’s still not active, even though it has been 30 years since experts confirmed it was not dormant, said California Seismic Safety Commissioner Jorge Meneses, president of the institute’s local chapter and a geotechnical engineer.

“This type of mentality needs to change,” said Meneses, whose organization has been working on the report for five years. “We have a seismic source here, running through downtown.”

Particularly troubling are the many decades the San Diego area was built to lower seismic standards than the ones applied to L.A. or San Francisco, based on the belief that San Diego had a lower seismic risk.

It was only after the discovery of the Rose Canyon fault’s activity that the minimum building codes for this region were raised to Seismic Zone 4, the highest level, and the same as that of Los Angeles and San Francisco, said Heidi Tremayne, executive director of the Earthquake Engineering Research Institute, a nonprofit based in Oakland.

Rose Canyon fault scenario

Violent shaking would ripple through the Rose Canyon fault in a hypothetical magnitude 6.9 earthquake in the heart of San Diego.

(Earthquake Engineering Research Institute / USGS)

“Many older, more seismically vulnerable buildings constructed before modern seismic design provisions were in place, including several key City of San Diego facilities, may be severely damaged with multiple older buildings potentially suffering partial to total collapse,” the report said. It did not specify which buildings.

There could be many deaths, as the San Diego region has relatively weak local laws requiring retrofits of vulnerable buildings compared to cities like Los Angeles and Santa Monica. The San Diego region is estimated to have thousands of apartment buildings with flimsy ground floors, hundreds of potential brittle concrete buildings that can be particularly deadly if they collapse, and scores of possibly vulnerable steel-frame office and hotel buildings.

None of those buildings described above are required to be retrofitted in the city. And for another particularly deadly class of buildings, old brick buildings, San Diego only required limited partial retrofits, according to the report.

The authors expressed great concern that the collapse or damage of these old brick buildings — which have been ordered retrofitted or demolished in other cities like L.A. — would dramatically worsen emergency response. Several hundred of them are believed to remain in places like downtown San Diego, National City, Chula Vista, El Cajon, Solana Beach, Encinitas, Oceanside and unincorporated areas of the county.

Many of San Diego’s civic institutions may end up being crippled, including police and fire stations and city offices, as first responders are called to perhaps hundreds of fires. The researchers estimate that nearly half of county schools and hospitals will be running at partial capacity for days.

Military facilities around San Diego Bay would suffer from severe ground shaking and liquefaction. And a large share of San Diego’s housing would be damaged, worsening the affordability crisis.

And with land on the western side of the fault lurching to the northwest relative to the eastern side, many pipelines, cables, bridges and railroads could be severed or otherwise disrupted. Water, wastewater and gas lines serving west of the fault, from La Jolla through Coronado, may be cut off for months after the quake. Coronado firefighters could find themselves without functioning water pumps to fight fires.

San Diego International Airport could find itself hamstrung as land underneath it acts like quicksand when shaken, damaging the runway, taxiways and buildings. A western section of the fault passes directly under the runway, and a quake would render it temporarily inoperative.

Gas line breaks and a loss of water pressure would make firefighting even more difficult.

And while the Coronado Bay Bridge has been retrofitted to withstand collapse, the experts said they expect land on one side of the fault to lurch 2 to 3 feet from the other side. Damage could render the bridge unusable for weeks, months and possibly years.

For decades, there had been no scientific work that had been done demonstrating the Rose Canyon fault was active. Then, in 1985, the first hint appeared during an excavation at Broadway and 14th Street, where a section of active fault was discovered, said Tom Rockwell, professor of geology at San Diego State.

The big discovery came in 1990, when trenches were dug across the fault in Rose Canyon. It showed the land on the western side of the fault had lurched to the northwest 30 feet over various earthquakes in the last 8,000 years, convincing evidence that the fault was alive, Rockwell said.

Today, it’s believed the Rose Canyon fault ruptures in a big earthquake of something approaching a magnitude 7 on average about every 700 years — give or take 400 years or so. The last such major quake is believed to happened between 1700 and 1750, Rockwell said, before the Spanish founded their first California mission in San Diego in 1769.

Between those big quakes, quakes in the range of magnitude 6 can strike. Such a quake ruptured on the fault right through Old Town in 1862, causing what the Los Angeles Star declared the Day of Terror in San Diego, Rockwell said.

Today, it’s known that the Rose Canyon fault is actually the southern continuation of the Newport-Inglewood fault, which caused Southern California’s deadliest earthquake on record, the magnitude 6.4 Long Beach earthquake of 1933 that killed 120 people.

(It’s possible the Newport-Inglewood/Rose Canyon fault system could rupture in the same earthquake, from the Westside of L.A. through Long Beach to San Diego, in a single earthquake. The energy released from that quake would be rated magnitude 7.4.)

Without a major change in San Diego’s psyche about earthquakes, the city could end up facing the fate of the city of Christchurch, New Zealand.

Many people in Christchurch also thought of themselves as relatively safe from earthquakes, as the city was also quite a distance from the South Alpine fault, the South Island’s version of the San Andreas fault.

So when a magnitude 6.3 quake ruptured under the city in 2011, the damage was catastrophic: The central business district downtown was left in ruins and 185 people died, mostly from the collapse of unretrofitted brick buildings and two brittle concrete buildings.

“Having spent a decade working down there and knowing how people felt about the risk before and now, this is still such a shock to them,” said Laurie Johnson, the president of the research institute and an urban planner.

San Diego can avoid this future if there’s a concerted regional effort to retrofit vulnerable buildings and infrastructure before such a quake hits. The authors recommend a committee of government officials, earthquake experts, utilities and others to identify county seismic hazards and suggest actions.

“Without that advanced mitigation work, we are worried it could jeopardize the economic vibrancy of the region,” Tremayne said.

Strengthening the region against quakes is part of the bargain of living in San Diego.

“We owe a lot of the beauty of San Diego to the Rose Canyon fault,” from the fault pushing up Mt. Soledad by La Jolla to the creation of the bays of San Diego, Rockwell said, enabling it to be the principal home port of the Navy’s Pacific Fleet.

“If we did not have the Rose Canyon fault, then we would look like Oceanside. It’d be a long, linear coastline with not much going on,” Rockwell said. “Because the fault line comes on shore in San Diego, it produces the topography that makes San Diego unique.”

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