The first earthquake, measuring magnitude 7.2, struck Venezuela on Wednesday evening with horrific force. But just 39 seconds after it started, an even more powerful magnitude 7.5 temblor struck, making a catastrophic disaster even worse.A seismic one-two punch, which seismologists call a “doublet,” has added to the unease over what is shaping up to be one of the Western Hemisphere’s worst quakes in years.Though less known to the general public, doublet earthquakes have long been studied by seismologists. Some have occurred in California, including in Imperial County and near Mammoth Mountain, as have big earthquakes followed by a substantial aftershock, as occurred in 1992. They prompted officials to issue an unprecedented public warning.“It’s obviously not the most common thing that happens with earthquakes, but it’s definitely not like an unusual physical phenomenon,” said Julian Lozos, an associate professor of geophysics at Cal State Northridge. The widespread destruction in Venezuela — with more than 900 dead and countless structures flattened — is expected to focus more attention on the seismic phenomenon and how to make buildings and infrastructure more resilient. Turkey and Syria were devastated just three years ago, where a magnitude 7.8 earthquake, rupturing 220 miles of the East Anatolian fault, was followed nine hours later by a magnitude 7.5 quake on the Cardak fault, rupturing an additional 100 miles, according to the U.S. Geological Survey. The twin earthquakes, plus a dizzying number of substantial aftershocks, were a generational catastrophe, killing more than 50,000 people. Scientists in California have been looking at what a devastating doublet sequence might look like. In Southern California, researchers with the USGS simulated a hypothetical doublet quake scenario, in which 52 miles of the Newport-Inglewood fault — which runs along the Orange County coast and then through Long Beach, South L.A. — would rupture in a magnitude 7.25 earthquake, which would then be followed several hours later by a magnitude 7.05 earthquake on the Palos Verdes fault. Doublets don’t necessarily need to stop at two earthquakes. There was a quadruplet quake sequence — all magnitude 6.3 quakes — in western Afghanistan in 2023. The first two temblors were separated by about four hours on Oct. 7, then followed by the additional quakes on Oct. 10 and Oct. 14. Many buildings collapsed, and 2,400 people died, according to scientists who detailed the quake sequence at an American Geophysical Union meeting in 2024. A Times front page with stories on the twin quakes in 1992, which rattled seismic experts. (Los Angeles Times) There isn’t a universally accepted definition on what makes a doublet. A relatively loose definition involves an earthquake followed by another temblor of a similar magnitude within minutes, hours or days after the first main shock. Seismologist Lucy Jones has a narrower definition, in which there are two earthquakes in the same seismic sequence within 0.4 units of magnitude of each other.“It’s a little bit semantic sometimes, like I know some people consider Ridgecrest a doublet, and some people don’t,” Lozos said, referring to the 2019 earthquake sequence that shook up L.A. from the Mojave Desert. In that sequence, a magnitude 6.4 on the morning of July 4 was followed about 34 hours later with a much more damaging magnitude 7.1 earthquake. The China Lake Naval Air Weapons Station, the Navy’s largest base for developing and testing weapons of warfare, suffered billions of dollars in damage, and the fact that the second quake was scarier than the first unnerved many Californians. Animation does not reflect actual speed. (Swetha Kannan/ Los Angeles Times) There are a few reasons why doublets can happen. “Fundamentally, it breaks down to how ready are all of the nearby faults to go,” Lozos said. One fault rupturing could prompt a nearby fault — almost already to rupture — to go off in a quake. Another example involves a system of faults all primed to rupture, but there’s somehow a delay in a second part of the fault rupturing.Doublets aren’t the only type of follow-up temblors that can raise anxiety or cause further destruction. A USGS report in 2008, called ShakeOut, said a plausible aftershock scenario after a magnitude 7.8 on the southern San Andreas fault, between the Salton Sea and Los Angeles County, included a magnitude 6.95 quake that would shake Sacramento and Modesto three days after the main shock, endangering the stability of the levees, which are crucial for maintaining flood control and water movement from the northern Sierra Nevada to cities across the state.Jones is working to update the ShakeOut scenario, and one plausible scenario she is planning to study is a magnitude 7.8 earthquake on the San Andreas followed by a big quake — perhaps around a magnitude 7 — on the Hollywood and Raymond faults, which cut under the heart of Hollywood and extend east into the San Gabriel Valley. It’s not certain that the USGS will stick to its initial declaration that the Venezuelan quakes involved a doublet, given how the gap between the start of the two quakes was less than a minute. And even then, the first magnitude 7.2 quake could have lasted for half a minute, meaning the first quake’s shaking waves may have been felt concurrently with the second quake. Zhongwen Zhan, director of the Caltech Seismological Laboratory, has said data from one seismic sensor in Venezuela indicate “a fairly continuous” quake. “The two earthquakes could basically get merged together, so we can expect potentially there is a reclassification later,” Zhan said. “I am sure the experience in Caracas was one really long earthquake. Maybe somebody who was able to be still enough to really notice what was going on would have been able to feel it slowing down for a few seconds,” Jones said. “If you ask people about how long Northridge lasted, you’ll find a lot of them who think it lasted for a really long time,” but that may be a reflection of not much of a break between the main shock and a big aftershock, which came less than a minute later. The fact that doublets exist is a reminder that people should be on guard that a big quake can be followed up by a subsequent quake. It’s worth noting, too, that a building damaged in an initial quake may not be as quake-resilient in a later temblor. “Caution should be taken that that building could get worse from an aftershock,” said Maria Mohammed, president of the Structural Engineers Assn. of Southern California. A damaged building “is not as strong as what it was before the earthquake.”It’s a warning that can be easily forgotten. A number of people died in Christchurch, New Zealand, from collapsing buildings resulting from a magnitude 6.1 earthquake in 2011, despite the structures having already been damaged in earlier, less damaging quakes. “It may look like, ‘OK, only minor damage,’ but whoever is inspecting it needs to think about, ‘Yeah, but what happens next?’” said David Cocke, a former president of the Earthquake Engineering Research Institute. “What is some more shaking going to do to this already damaged building?”Modest earthquakes can also lead to bigger ones. Moderate quakes of about magnitude 6.1 and 5.6 in Monterey County preceded the megaquake on the San Andreas fault in 1857, estimated to be a magnitude 7.9. (Raoul Rañoa / Los Angeles Times) That earthquakes can trigger other quakes has been demonstrated before. Scientists believe that the magnitude 6.1 Joshua Tree temblor of April 22, 1992, resulted in aftershocks that kept migrating north. They eventually triggered on June 28 the magnitude 7.3 Landers earthquake in the Mojave Desert — strong enough to cause shaking in Denver — and, hours later, a magnitude 6.3 earthquake in Big Bear.The state issued a strong warning urging residents to prepare for a large aftershock, and to stay off freeways if possible. Jones, then with the USGS, rigged up a “go to war scenario” where, if a magnitude 6.5 or greater quake occurred in a certain place on the San Andreas, it would trigger an automatic response that included calling out the National Guard. In the end, there was not another major quake that day.In 1987 in Imperial County, near the Mexican border, a magnitude 6.2 earthquake was followed 11 hours later by a magnitude 6.6 earthquake. Called the Superstition Hills earthquake sequence, the temblors injured 94 people and caused an estimated $4 million in damage. In Mexicali, a mother and a child fleeing a factory during the shaking were hit and killed by an automobile, the Red Cross said. And in 1980, there were four earthquakes — ranging between magnitude 6.0 and 6.2 — near Mammoth Lakes over three days, Jones said. The temblors were enough to break windows and water mains and down chimneys, and triggered widespread landslides and rockfalls, according to the California Geological Survey, and caused at least $2 million in damage. Two hikers in Yosemite Valley suffered severe injuries from falling rocks. Having occurred just a week after the deadly Mount St. Helens eruption in Washington state, the Mammoth earthquake sequence sparked worries there could be an eruption in the Mammoth Mountain area, the California Geological Survey said. That worry lasted for 10 to 15 years, Jones said, but no eruption happened.
