The Los Angeles earthquake hit a fault system that could pose a massive threat. Here’s what to know.
"The 4.4 magnitude earthquake that jolted the Los Angeles area this week hit in the area of a fault system that could pose an even greater threat to parts of the city than the notorious San Andreas fault. The Monday quake and recent smaller earthquakes in the vicinity have placed renewed attention on the Puente Hills fault and the dangers it could bring.
The Puente Hills thrust fault is a broad underground fault that runs through Los Angeles and Orange counties, including under downtown Los Angeles — and scientists warn that it could one day produce a massive magnitude 7.5 temblor, possibly rupturing an area from San Gabriel east of Los Angeles to Hollywood nearly 20 miles away. According to some models, that kind of earthquake could lead to some 18,000 deaths.
“We have an incredibly dense concentration of vulnerable buildings right on top of the Puente Hills fault, so that’s what makes it so particularly dangerous,” said Lucy Jones, a seismologist and a research associate at Caltech.
Of serious concern in this area are concrete structures built in the 1950s and ’60s that are common in commercial real estate in Los Angeles; two-thirds of them still need to be retrofitted to make them safer for earthquakes.
“Concrete is heavy — it kills a lot of people,” Jones said.
The 1987 Whittier Narrows earthquake, which struck east of downtown Los Angeles on a branch of the Puente Hills, was a magnitude 5.9 that killed eight people and caused $358 million in property damage, according to the Southern California Earthquake Center.
What to know about the Puente Hills fault
The Puente Hills fault lurks underground — buried under layers of rock and sediment. It’s part of a system of stacked and inclined faults, and while scientists aren’t sure exactly which one moved during Monday’s quake, the Puente Hills is a good candidate, according to Michael Oskin, a professor of geology at the University of California at Davis.
Known as “blind” thrust faults because they can’t be seen at the surface, they are difficult to study and may remain unknown to scientists until they produce an earthquake. A thrust fault is a type of fracture in which one block of rock moves up and over another at a shallow angle. The 1994 Northridge earthquake — a magnitude 6.7 — also occurred on a blind thrust fault, and it spurred research that has given scientists a good understanding of their general layout in the Los Angeles basin, Oskin said.
“Northridge highlighted how destructive they could be because it was so much larger than Whittier Narrows,” he said.
Oskin said it’s not unusual to have several quakes on these systems, as Los Angeles has seen this summer.
“The important lesson is that it’s a reminder that these faults are active, and we should be prepared,” he said.
Comparing the Puente Hills fault to the San Andreas
Southern California is rife with active faults — including many blind thrust faults between Los Angeles and Santa Barbara — and a few hundred of them are capable of producing a damaging earthquake, Jones said.
Many of them form in an area known as the “Big Bend” of the San Andreas fault, where the Pacific and North American plates collide rather than slip past each other, building up the mountains that surround Los Angeles, and breaking up the land underground.
Jones said that while the Puente Hills fault is more dangerous, the San Andreas fault moves faster and ruptures more often.
“Summed up over the next few thousand years, the San Andreas is going to do more to us than the Puente Hills because the Puente Hills will move once and the San Andreas is going to move 20 times,” Jones said.
The San Andreas is the main plate boundary between the Pacific and North American plates and is capable of producing the largest earthquakes because it tends to rupture over long distances. One of the largest faults in the world, it runs from the Salton Sea in the Southern California desert to Cape Mendocino in coastal Northern California. It hasn’t ruptured in Southern California since the 1857 Fort Tejon quake, an estimated magnitude 7.9 temblor that shook a region that was sparsely populated at the time, killing one person and heavily damaging an army post at Fort Tejon.
A similar-sized quake today on the San Andreas would result in major impacts across the Los Angeles region, especially to the now highly populated Inland Empire, the metropolitan area that includes Riverside and San Bernardino. Such a quake is expected to produce two minutes of strong shaking and 1,800 deaths, according to researchers.
Is California due for a major quake?
Experts say a major destructive quake is coming to California — and that it’s not a matter of if but when — but the next one may not necessarily strike on the San Andreas.
Jones said Southern California earthquakes have been unusually quiet over the past 20 years, recording just five or six quakes per year that are magnitude 4 or greater. The long-term average is closer to 10 or 12 per year. This year, however, has already seen 13 quakes of magnitude 4 or higher.
“We really are earthquake country and we’ve been lulled into a sense of complacency because we’ve had this quiet time,” she said. “We clearly aren’t quiet this year.”
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