The threat of a catastrophic earthquake in California is real
This week’s catastrophic earthquake in Turkey and Syria is just the latest warning of potential risks to California and other seismically active areas.
Some California cities retrofitted or demolished problematic buildings in the wake of earthquakes in the 1980s and 1990s. But many buildings across the state didn’t suffer the same kind of intense shaking experienced in Turkey and Syria.
The 7.8-magnitude earthquake struck southeastern Turkey at 4:17 a.m. local time on Monday on the Eastern Anatolia Rift. Aftershocks have spread across the southeast of the country and into Syria. A powerful 7.5 magnitude aftershock on another fault struck nine hours later, with an epicenter 60 miles to the northeast, producing another round of devastation.
The San Andreas Fault is capable of similar activity.
“We have had 7.8 earthquakes in our historical past. We had a great run without them, but it’s important to be prepared for these possibilities in the future,” said US Geological Survey research geophysicist Kate Scharer.
Two of these occurred on the San Andreas: the 1906 earthquake that destroyed much of San Francisco and the 1857 earthquake that severed a length of the Monterey County Fault through the Los Angeles County and the Cajon Pass.
“There will be 7.8 seconds in our future. Absolutely. We have the faults, we’ve seen it in the past, it will happen again,” said seismologist Lucy Jones, a research associate at Caltech. “When they unfold, as far as we can tell, is random. And there’s no way to tell when that’s going to happen…. Relative to the long-term average, we’ve been quiet for a while.
The scale of building collapses in Turkey and Syria, some of which have been captured on video, could be attributed to a number of factors. Some of the structures may have been built before the advent of modern building codes. The collapses could also be due to corruption in safety inspections or incompetence in design practices – problems that have arisen in Mexico, Taiwan and New Zealand.
But structural engineers said a large earthquake in California would be equally devastating, if not of the same magnitude. They have long warned of the risk of flimsy concrete buildings collapsing, as happened in the 1971 Sylmar and 1994 Northridge earthquakes. When the administration’s concrete hospital San Fernando Veterans’ Center collapsed in the 1971 earthquake, 49 people were killed.
Minimum building requirements were tightened in the years following the Sylmar earthquake, but these rules only applied to new construction. More concrete buildings suffered significant damage in the 1994 Northridge earthquake.
David Cocke, president of the Earthquake Engineering Research Institute and a structural engineer with Gardena-based Structural Focus, said some of the collapsed buildings he has seen in news footage in Turkey appear to have been constructed from non-ductile concrete, in which inadequate steel reinforcing bars allow the concrete to explode from the columns when shaken.
Similar videos emerged after a 7.1 magnitude earthquake hit Mexico in 2017. One showed a building wobbling, followed by concrete falling from a ground floor column; the columns then gave way and the upper floors collapsed, sinking into a cloud of dust.
A concrete school in Mexico City that collapsed in this earthquake killed 19 students and seven adults.
Videos and photos from Turkey and Syria show the collapse of buildings from different eras – some ancient, some modern. But they also show that many others survived the tremor. Experts say new buildings in Turkey — when properly constructed according to local codes — are comparable to California standards.
Some of the collapses in Turkey occurred several hours after the main dawn shock. The 7.5 magnitude aftershock occurred around 1:24 p.m.
Whether it’s one earthquake or two, “the longer the duration [of shaking]the better the chance of a building collapsing,” Cocke said.
Scharer visited the site of a 2011 magnitude 7.1 earthquake in eastern Turkey that produced intense shaking beneath the city of Van; a subsequent, less powerful earthquake caused further damage.
“We often call them a doublet,” Scharer said. The first signs of building weakness can be seen when large diagonal cracks, resembling the letter X, are visible in the building.
“So the building is weakened by the first earthquake,” she said. “And then when you have a significant aftershock, they actually crumbled. So it’s sort of a one-two punch.
In California, an earthquake of magnitude 7.8 would produce damage far more widespread than that caused by the tremors of the last century. A US Geological Survey simulation of a 7.8 magnitude earthquake in Southern California has led researchers to determine it is plausible that such an earthquake could cause nearly 1,800 deaths and 50,000 injuries. , and destroying major utilities carrying fuel, electricity and water. In northern California, a simulation of a magnitude 7 earthquake on the Hayward Fault east of San Francisco showed there could be at least 800 quake deaths, as well as hundreds others set fires afterwards.
Either scenario would result in the deadliest earthquakes to hit California in over 100 years. A magnitude 7.5 quake on the Puente Hills Fault – which passes under heavily populated areas of LA and Orange counties – could kill 3,000 to 18,000 people, according to the USGS and the Southern California Earthquake Center.
Among the world’s active seismic zones, California and Turkey, as well as New Zealand, are in a class of their own. All three of these areas have long mature faults and are on land, unlike Japan, where the largest faults are under the ocean.
In Turkey, the eastern Anatolian and northern Anatolian faults are of a similar type to the San Andreas faults – relatively more likely to rupture in the lifetime of a single human than others.
The land beneath central Turkey is seismically active because it’s wedged between the Arabian Plate, which pushes northward, and Europe, said Ross Stein, professor of geophysics at Stanford University and chief executive of Temblor. , which produces seismic hazard models. “And so he’s pressed west.”
Eastern Anatolia, like San Andreas, is a strike-slip fault – a fault that spans vertically and the ground moves laterally during rupture.
“You get very strong shaking along the fault, much stronger than you can even see from too far away,” Jones said.
The fact that the epicenters of the two main earthquakes in Turkey were 60 miles apart shows that follow-ups can occur at a distance from the main shock.
“A lot of aftershocks are happening on other faults,” Jones said.
Los Angeles Times