San Diego's Quake Risk Is Big, But Not 'Big One' Big - Voice of San Diego

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San Diego's Quake Risk Is Big, But Not 'Big One' Big

Everything you ever wanted to know about our shaky past and future

Downtown San Diego / Photo by Sam Hodgson

On the earthquake front, San Diego County has been been sitting pretty — so far.

Throughout our recorded history, quakes here have done fairly little damage and only taken a single life. Just about every other part of Southern California from El Centro to Santa Barbara has seen much more destruction and death. Still, we should be rattled about seismic safety here.

It’s quite possible for San Diego to be struck by a homegrown quake as big as the 7.1 magnitude one near Ridgecrest that startled many of us Friday night. And while the San Andreas Fault is too far away to shake us much, the dreaded Big One could still pack a punch here.

Here are some questions and answers.

How do we stack up when it comes to quake risk?

“In any other state, San Diego would be considered a high-risk city. You only look like a low-risk city becomes of the comparison with Los Angeles and San Francisco,” said retired U.S. Geological Survey seismologist Lucy Jones in a 2018 interview with Voice of San Diego.

In fact, Jones believes we’re in the “worst of both worlds” because the risk isn’t high enough to scare people into taking action to protect themselves and not low enough to ignore. (Jones is known as the “Earthquake Lady” for her many media appearances after Southern California earthquakes, and she updated reporters and the public immediately following the quakes last week.)

Why are we in danger?

As Jones noted, San Diego’s Rose Canyon Fault runs smack through the middle of the city. Indeed, the fault roughly follows I-5 from downtown to La Jolla and Highway 52.

“The Rose Canyon Fault is certainly capable of upwards of a magnitude 7 quake,” San Diego State seismologist and geologist Thomas K. Rockwell told Voice of San Diego Sunday. “Fortunately, they’re not that frequent. On the Rose Canyon Fault, the average recurrence is every 700-800 years, and the last one was about 300 years ago.”

Still, he said, “there’s certainly enough strain to have a magnitude 6 earthquake, which could be quite destructive.”

How might a big quake on the Rose Canyon Fault affect us?

Under one scenario produced by scientists, a 6.9 magnitude quake on the fault would be the second-most devastating and deadly earthquake in U.S. history. (The 1906 quake in San Francisco would remain the worst). Up to 2,000 people could die here, about half in a tsunami caused by the quake, according to a preliminary projection released in 2017. Damage could reach $60 billion.

What about other local faults?

There are plenty of other earthquake faults in this region. Most notably, the San Jacinto Fault and the Elsinore Fault — which have set off major quakes in the recent past — run through remote East County.

Unknown faults can also — surprise! — set off nasty quakes. The first big Ridgecrest quake last week, the 6.4 magnitude tremor that struck on July 4, “was on a fault that’s much smaller than the Rose Canyon Fault,” Rockwell said. “It’s on a fault that wasn’t even named, and it’s not clear it was even mapped. The quake destabilized a larger fault, which produced the 7 magnitude quake on Friday night.”

When’s the Big One coming?

Nobody knows. Scientists don’t believe that the large Ridgecrest quakes had any effect on the San Andreas fault, which remains locked and loaded. It’s expected to release tension through the “Big One” — a quake of magnitude 8.0 or higher.

“The last big earthquake to hit the L.A. segment of the San Andreas Fault was 1680. That’s over 300 years ago,” physicist Michio Kaku told CBS News last week. “But the cycle time for breaks and earthquakes on the San Andreas Fault is 130 years, so we are way overdue. In any given year, the probability of the Big One is 3 percent.”

Are we in big trouble here when the Big One hits?

Sort of. Back in 2008, researchers projected what would happen to Southern California if a 7.8 magnitude earthquake struck the southern section of the San Andreas fault.

Researchers expect the quake here would feel much like the Friday night Ridgecrest quake did. In others words, some of us wouldn’t even notice it while others would rush outside to see trees sway and pools slosh from side to side. Only a handful of casualties are expected in San Diego County, which would be spared serious damage. But an estimated 48,000 people would suffer injuries and 1,800 would die in SoCal. The scenario also envisions power and water outages, fires, broken freeways and lots of chaos.

The scenario doesn’t say where hundreds of thousands of newly homeless people will go, but it seems likely that plenty would head south — to here.

So the Big One is the worst-case scenario?

Not by itself. If you want to dip into total doomsday mode, imagine a Big One that erupts during a Santa Ana weather condition, tears into remote power lines and sparks mammoth wildfires across Southern California when many roads are already impassable.

Or picture an 8 magnitude quake that hits during a “megaflood” — “The Other Big One” — caused by a catastrophic series of storms. As we reported in 2017, “researchers think a megaflood is just as likely to hit the state as a 7.8 magnitude earthquake, and has the potential to cause three times as much damage.” There’s nothing to stop both disasters from striking at the same time. (See Law, Murphy’s.)

You’re going to tell me it could get even worse than that, aren’t you?

Absolutely! “When things start unleashing, they could unleash for years,” U.S. Geological Survey seismologist Ned Field told Smithsonian Magazine.

And, of course, other faults don’t have to wait politely to set off earthquakes after the San Andreas does its thing. Quakes could come right one after another.

Can you give me a reason for hope?

The same rule applies to investments and earthquakes: Past performance is no guarantee of future results. But San Diego has been very fortunate over the past few centuries.

Yes, earthquakes have broken windows, destroyed buildings and freaked out locals here.

On a “day of terror” in 1862, back when we were a little Civil War-era town that wasn’t impressed by Abraham Lincoln, a quake estimated around magnitude 6.0 cracked the tower of the Point Loma Lighthouse. The aftershocks from the quake, thought to have occurred on the Rose Canyon Fault, scared residents into sleeping outside for days. And in 1986, a quake off Oceanside injured 29 people and killed a hoarder in a tiny downtown San Diego hotel room when his thousands of books fell on him.

But overall, we’ve avoided seismic catastrophe. And, as Rockwell noted, our shaky ground has been mighty good to us. “We have a fantastic harbor and bay,” he said, “and they’re there because of the Rose Canyon Fault. It’s given San Diego much of its unique character.”

But, to borrow a phrase, while the fault giveth, the fault can also taketh away.

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