California 2019 desert earthquakes have raised risks of a 'big one' that could devastate Los Angeles: Report
Two big earthquakes that occurred in the remote Southern California desert in July last year have increased the possibility of a major quake striking the San Andreas Fault, that runs more than 800 miles through California. Such a major earthquake – which is being referred to as the “big one” – could devastate Los Angeles and its surrounding communities.
A sequence of earthquakes rattled Searles Valley near Ridgecrest, California, in July 2019. The two dominant events, a magnitude-6.5 on July 4 followed 34 hours later by a magnitude-7.1 event on July 5, were felt over most of southern California and caused surface ruptures with minor damage to the nearby towns of Ridgecrest and Trona. These earthquakes occurred within the Eastern California Shear Zone (ECZS), a zone of faulting and seismicity that runs east of the San Andreas Fault (SAF) system and joins with it in the Salton Trough. In all, there were more than 100,000 aftershocks stemming from the twin earthquakes. The sequence rattled most of Southern California, but the strongest shaking occurred about 200 kilometers north of Los Angeles.
According to the new analysis, published in Bulletin of the Seismological Society of America, the 2019 Ridgecrest earthquakes shifted underground stresses, making the San Andreas fault three to five times more likely to rupture. The research team includes Professor Shinji Toda, International Research Institute of Disaster Science (IRIDeS), Tohoku University, Japan, and Ross S. Stein, a former United States Geological Survey (USGS) geophysicist who now runs Temblor, Inc., a catastrophe modeling company in California.
The new threat comes by way of the 160-mile-long Garlock fault, which runs along the northern edge of the Mojave Desert, and connects the site of the Ridgecrest quakes to the larger San Andreas fault. In their analysis, researchers say that the Ridgecrest quakes changed stresses along the Garlock, and there is now a 2.3% chance of an earthquake of magnitude-7.7 occurring on a section of the fault in the next 12 months. There is also a 1.15% chance of a similar quake hitting San Andreas -- which is 3 to 5 times higher than earlier forecasts, the researchers explain. Currently, the USGS forecasts a 31% probability that an earthquake measuring magnitude 7.5 will occur in the Los Angeles region in the next 30 years.
“When we sum up the earthquake probability along the entire length of the Garlock fault, the main fault that runs perpendicular to the Ridgecrest faults, we estimate a 2.3% chance of a magnitude-7.7 Garlock Fault rupture in the next year — in other words, one chance in 43. This is 100 times higher than its annual chances in the ‘UCERF3’ benchmark model for California, which is jointly issued by the USGS, the Southern California Earthquake Center, and the California Geological Survey,” the authors write in a blog post.
After the 2019 events, a very shallow portion of the Garlock slipped slowly, a process known as ‘creep,’ perhaps in response to the stress imparted by the Ridgecrest earthquakes, or perhaps due to the ground shaking. “Could the Garlock fault produce a very large rupture? The fault appears to have ruptured in large earthquakes during the past 600 years, and so could presumably do so again,” says the team.
“If a Garlock Fault ruptured to within about 30 miles (45 kilometers) of its junction with the San Andreas Fault, we calculate it would raise the probability of a San Andreas rupture extending to the southeast, on the so-called ‘Mojave section,’ by a factor of about 150, they explain. That translates into a 50/50 chance of a San Andreas Mojave section rupture (with a range, 25%-67%), either immediately following a Garlock quake or after some delay, say experts. “We thus estimate the net chance of a large San Andreas earthquake in the next 12 months to be 1.15% or 1 chance in 87. While small, this probability is 3.5-5.0 times higher than the annual chance of a large San Andreas earthquake in the ‘UCERF3’ benchmark model for California,” they explain.
According to the authors, while a 1.15% chance of a great San Andreas rupture in the next year is low, it is the same as saying that there is a 98.9% chance it will not rupture. “So, no one should panic. But, as we have seen during the Covid-19 pandemic, low probability chain-reaction events do occur in nature, and when they do, their consequences can be great. It is the responsibility of scientists to assess the likelihood of such events, and then to bring them to the attention of the public and decision-makers,” warns the team.
A report earlier this year had warned that the strain placed on the Garlock Fault by July's earthquake activity triggered it to start slowly moving, and that the fault has slipped 0.8 inches or 2 centimeters since July 2019. A study last year also found that the Ridgecrest earthquakes had increased stress on the Garlock Fault, which has been dormant for at least a century. The team estimated that the stressed section could be capable of producing an earthquake between a magnitude 6.7 to a magnitude 7.0 if it ruptured.