A 4.4-magnitude earthquake near Highland Park on Monday, Aug. 12, brought back vivid memories for many in the L.A. area.
As the shaking rippled through the region on Monday – and after a flurry of quakes in recent weeks – it was a reminder of the prevalence of fault systems in Southern California.
And it was a reminder that even relatively itty-bitty faults can rattle nerves and, at times, do big damage.
The United States Geological Survey said the epicenter of Monday’s 12:20 p.m. earthquake was about 2.5 miles south of Highland Park.
Seismologist Dr. Lucy Jones said the Puente Hills Blind Thrust system was responsible for the quake. The same fault system caused the Whittier Narrows earthquake, a 5.9 temblor that occurred at 7:41 a.m., Oct. 1, 1987. A 5.5 magnitude aftershock occurred at 3:59 a.m. Oct. 4.
When the dust settled, eight deaths were attributed to the quakes, 9,000 people were displaced, along with hundreds of millions of dollars in damages.
The mechanics of fault systems is much more complicated than lines on a map.
The Puente Hills fault is located in northern Los Angeles Basin and runs for about 25 miles from downtown Los Angeles east to Brea in Orange County, according to a study in the Bulletin of the Seismological Society of America.
The system includes three segments that scientists call Los Angeles, Santa Fe Springs and Coyote Hills.
In total there are more than 350 faults in California.
Jones said Puente Hills is a complex stack of faults that scientists discovered thanks to tests that were funded in the wake of the 1994 Northridge earthquake, which was caused by a different blind thrust fault system.
These faults cannot be seen from the earth’s surface and could be discovered only through imaging studies similar to an ultrasound.
Jones said a common misconception about earthquakes is describing their sources as lines.
“You’ll never hear a scientist say that, because faults aren’t lines, they’re surfaces,” Jones said.
Unlike the infamous San Andreas fault, Puente Hills is a thrust fault, meaning blocks of rocks on either side of the fault move up and down unlike the strike-slip San Andreas, in which two crustal blocks move horizontally past one another, similar to two opposing lanes of traffic.
Jones described the San Andreas fault system as the fastest moving and longest in the state, meaning it has the potential for the biggest earthquakes most often.
The Whittier Theater as it is being demolished after the Whittier Narrows earthquake. (Courtesy Whittier Museum)
The more than 800-mile-long fault system makes Puente Hills seem tiny by comparison. San Andreas stretches from Northern California to Cajon Pass near San Bernardino and includes several branch faults including San Jacinto and Banning, according to a USGS report.
Puente Hills is one of many thrust faults in the Los Angeles Basin, including Elysian Park and Compton thrust faults, according to USGS Research Geologist Dr. Alex Hatem.
Jones noted on social media on Tuesday that 13 earthquakes have rattled Southern California this year with magnitudes of 4.0 or greater.
Historically, there have been about five earthquakes per year of 4.0 or greater in Southern California over the past 20 years. And since 1932, the average number of earthquakes with magnitudes of 4.0 or greater in Southern California is between 10 and 12, Jones posted. The number of earthquakes greater than 4.0 in 2024 is closer to the average recorded in Southern California over the past 90 years.
But even though the region has been shaken by several quakes recently, it hasn’t been a huge surprise to researchers. Despite Monday’s hefty shake, the Puente Hills fault is not showing any unusual activity.
Seismologist Lucy Jones talks during a news conference at the Caltech Seismological Laboratory in Pasadena, Calif., Thursday, July 4, 2019.(AP Photo/John Antczak)
“The faults of the L.A. Basin are indeed considered active and have been known about for many years,” Hatem said in an email. “We have not detected an increase in earthquakes from this fault.”
Hatem said with every California earthquake there is generally about a 5% chance that it could precede a larger earthquake.
Jones said ground vibrations do not vary much based on the type of earthquake. She compared the motion of a fault to fingers moving together when snapping. What people feel during an earthquake is the equivalent of the sound of a snap.
“Every one of us has active faults nearby. We don’t know which one’s going to go next.” — Dr. Lucy Jones
Those closer to the epicenter of the earthquake will feel more jerky motion, but farther away, as high frequency waves have dissipated, all that’s left are lower-frequency waves that result in the slower rolling motion feel.
While the Puente Hills Fault system is being singled out because of the recent earthquake, Jones said there are hundreds of similar faults all around the region.
Everyone in Southern California lives within five miles of an active fault, she said.
“Every one of us has active faults nearby,” Jones said. “We don’t know which one’s going to go next.”
Hatem said it’s hard to call a smaller earthquake good or bad based on the amount of energy it releases.
The 6.7-magnitude Northridge earthquake released nearly 2,800 times the energy as Monday’s, according to Hatem.
Based on USGS calculations, the 1987 Whittier Narrows quake was 31 times bigger than Monday’s Highland Park earthquake, releasing 177 times more energy.
Whittier Mayor Joe Vinatieri was driving on Monday, so he didn’t feel the earthquake, but hearing the location of the epicenter triggered memories of 1987.
“We always didn’t think that much about it growing up,” Vinatieri said. “We knew there was a fault line right here where we lived, but we found out how active it can be.”
This replica of Michaelangelo’s statue of David was originally installed at Forest Lawn, until it toppled and broke in the Whittier Narrows earthquake, and was subsequently donated to Cal State Fullerton. (Photo by Bruce Chambers, Orange County Register, 2002)
Just before 8 a.m. the morning of the earthquake, Vinatieri was in a fifth-floor office when it struck. He said it started slow and became more and more intense. Vinatieri described those evacuating the building as being like deer in headlights.
“It was a very scary time,” Vinatieri said.
On his drive home he noticed damaged buildings, and at home found his wife and children huddled inside terrified. The plaster of the house had cracked.
“It was like one of those surrealistic things that you see in a movie,” Vinatieri said.
Since Whittier Narrows and the 1994 Northridge earthquake, Vinatieri said, a generation has gone by without experiencing a significant quake, leaving people with their guard down.
“It’s a reminder for me also, for those of us who are in public office to say, hey wait a second. Where are we at on earthquake preparedness and what do we need to do,” Vinatieri said.
For more information, visit earthquake.usgs.gov.
Originally Published: August 17, 2024 at 6:00 a.m.
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Publish date : 2024-08-17 13:53:00
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