Why so little ruin from monstrous recent temblors?

LOS ANGELES—After major temblors on July 4 and 5, structural engineers descended on Ridgecrest expecting to study destruction from the largest earthquake to hit Southern California in nearly 20 years. They found relatively little. Yes, mobile homes were torn off foundations, chimneys fell, gas lines leaked and some homes caught fire. But overall, most buildings did fine—and many businesses were up and running within a day or two of the biggest shock, a magnitude 7.1.

“Ridgecrest, I’m just amazed,” California Earthquake Authority structural engineer Janiele Maffei said of the light damage.

But the outcome in Ridgecrest shouldn’t provide solace to California’s biggest cities.

The Mojave Desert town remained largely unscathed because its building stock was relatively new and remarkably resilient. Many homes are one or two stories, built in the 1980s. It lacks the kind of structures that experts say are most vulnerable in a big quake— unreinforced masonry, brittle concrete, so-called “soft story” apartments and single-family homes not bolted to their foundations.

As a result, Ridgecrest suffered far less damage than cities hit by less powerful quakes in recent years, including Napa and Paso Robles, where older buildings in the downtown areas crumbled amid the shaking.

Experts were quick to point out that last week’s quakes would have proved far more devastating had they been located near bigger cities filled with more susceptible buildings.

“You take a 7.1 and put it into the Hollywood fault or Newport-Inglewood fault in Long Beach—we’re going to see substantially different levels of damage,” said Ken O’Dell, president of the Structural Engineers Association of Southern California. “Ridgecrest did a very good job surviving this particular 7.1.”

Keith Porter, a nationally renowned earthquake engineer and research professor at the University of Colorado Boulder, said Ridgecrest’s result should not be seen as a “victory lap.”

“We still have dangerous buildings, and we still have a building code that is not optimal and doesn’t protect society as well as it could,” he said. “Instead of a dozen collapsed manufactured homes, hundreds or thousands of collapsed manufactured homes. Instead of four or so building fires, hundreds of building fires.”

Progress has been made by cities—Los Angeles and San Francisco among them—to require some building retrofits. But even those large population centers have not mandated retrofits of all of the types of structures engineers worry about. And authorities in many suburban areas—including Silicon Valley, San Mateo County and the beach cities of Los Angeles County’s South Bay—haven’t ordered that flimsy apartment buildings be strengthened.

Riverside and San Bernardino counties haven’t required fixes to brick buildings, a vulnerability Californians have known about for a century.

A U.S. Geological Survey simulation said a plausible magnitude 7.1 earthquake on the Hayward fault in the San Francisco Bay Area could kill 800 people, burn the equivalent of 52,000 single-family homes and displace 400,000 people, worsening the region’s housing crisis.

And a hypothetical magnitude 7.8 earthquake that would send violent shaking waves along a 186-mile section of the southern San Andreas fault could kill 1,800 people, leave 50,000 injured and cause lasting harm to Southern California’s economy.

Such a direct hit “would take days or weeks to get to the place we are (at in Ridgecrest)—gearing up toward restoration and early recovery,” said Laurie Johnson, president of the Earthquake Engineering Research Institute.

There are a number of reasons why Ridgecrest was largely spared.

The town, which began growing up around the Naval Air Weapons Station China Lake during World War II, does not have a stock of unretrofitted brick buildings such as those constructed before the 1933 Long Beach earthquake, said USGS seismologist Susan Hough. Unretrofitted brick buildings are a major killer in quakes, causing at least five to die in San Francisco during the 1989 magnitude 6.9 Loma Prieta earthquake and two fatalities in the 2003 magnitude 6.5 Paso Robles earthquake.

There are also very few soft story apartments with weak ground floors built to accommodate parking—likely, Hough said, a result of “having enough room to not ever need high-density housing.” A soft-story apartment collapse killed 16 people in the 6.7 Northridge earthquake in 1994.

And because they are newer, the single-family homes in Ridgecrest lacked the vulnerability of many Southern California and Bay Area pre-1980 wood-frame houses built with a handful of steps above the ground. Sharp shaking can snap the wood supports connecting such homes to their foundations. A retrofit to brace and bolt the structure can cost several thousand dollars—but repairing the problem after a quake can cost hundreds of thousands of dollars.