Why California's Next Big Quake Will Trap Seismic Waves

California. California. California. California. What if I told you that two neighborhoods in Los Angeles could experience the same major earthquake but shake very differently? In August 2026, scientists at Caltech revealed the most detailed three-dimensional picture yet of what lies beneath the Los Angeles basin. To build it, researchers deployed 273 seismic sensors across the region. What they found was extraordinary. Beneath parts of Los Angeles, soft sediment extends downward for nearly 10km, or more than 6 miles. The city beneath the city. Los Angeles looks like a city built on solid ground. Geologically, much of it isn't. Beneath the streets, freeways, and skyscrapers lies an enormous accumulation of sediment deposited over millions of years. This is the Los Angeles Basin. Think of it as an enormous geological bowl. At the surface are the neighbourhoods millions of people recognise. Downtown Los Angeles, Long Beach, communities stretching toward Orange County and the coast. But underneath them, layers of sediment descend thousands of metres, before reaching older, harder basement rock. Scientists have known about this basin for decades. Oil exploration, wells, seismic surveys, and earthquake studies have gradually revealed its general structure. But there was a problem. For earthquake scientists, knowing that a basin exists isn't enough. They need to know its exact shape. How deep is it here? How shallow is it there? Where are the edges? Where does sediment meet bedrock? Because seismic waves respond to to all of those differences, and until recently, Scientists didn't have the level of detail they wanted. The 273 sensors. In fall 2022, researchers carried out an unusually dense experiment across Los Angeles. They installed 273 temporary seismic stations. The instruments were spaced roughly one to two kilometers apart. They stretched from Pasadena to Long Beach and from Santa Monica to Orange County. Some were installed in residential neighborhoods. Then scientists waited. The sensors recorded seismic waves passing through the crust. Researchers led by Caltech graduate student Valeria Villa Working with seismologist Robert Clayton developed a method that combined those seismic observations with gravity measurements. That combination allowed them to identify underground boundaries that had previously been difficult to resolve. The result was published in the Journal of Geophysical Research, Solid Earth, on August 4th, 2026, and it revealed the hidden shape of Los Angeles in three dimensions. The 10-kilometer basin, The deepest part of the newly mapped basin approaches 10 kilometers. That's approximately 6.2 miles of sediment. To understand that scale, imagine placing an ordinary city on top of a geological structure deeper than many mountains are tall. Downtown Los Angeles sits above exceptionally deep sediments, but travel toward other parts of Southern California, And the underground geometry changes. The San Gabriel and San Bernardino valleys, for example, contain much shallower sedimentary basins in the areas compared by the Caltech researchers. That difference matters, because seismic waves don't travel through soft sediment the same way they travel through hard rock. And when earthquake energy enters a deep sedimentary basin, strange things can happen. The jello effect. Caltech used an unusually simple analogy. During simulations of a major San Andreas earthquake, the ground beneath downtown Los Angeles can shake almost like jello. The reason is wave speed. Seismic waves generally move more slowly through soft sediment than through hard rock. When waves enter the basin, they can slow down. Their energy can become trapped. They can reflect, they can interfere with other waves, and the geometry of the basin can concentrate that energy in particular locations. Instead of experiencing one short pulse of shaking, parts of Los Angeles can continue reverberating. That means the important question isn't simply, how far are you from the fault? It is also, what are you standing on? Distance isn't everything. This is one of the most important lessons earthquake scientists have learned from previous disasters. The closest location to an earthquake isn't automatically the location that experiences the most damaging ground motion. Local geology matters. Soft soils can amplify shaking. Deep sedimentary basins can trap energy. Basin edges can focus seismic waves. Topography can alter ground motion. And individual structures respond differently depending on the frequencies contained in that shaking. That's why two locations at similar distances from an earthquake can experience very different effects. For Los Angeles, this creates an unusual problem. The southern San Andreas Fault does not run directly beneath downtown. Yet, simulations show earthquake energy can travel from the fault toward the metropolitan region and become amplified inside the sedimentary basins. The fault generates the waves, but the underground structure helps determine what happens when those waves arrive. The big one, simulation. One of the most studied California earthquake scenarios begins near the Salton Sea. In the USGS Shakeout scenario, a magnitude 7.8 earthquake ruptures approximately 300 kilometers of the southern San Andreas Fault. The rupture travels northwest. Seismic waves spread across Southern California. Eventually, enormous amounts of energy reach the Los Angeles region. But they don't simply pass underneath the city and disappear. Computer models show the basins affecting the waves. Some energy becomes concentrated, some reverberates, and shaking can continue far longer than people might expect based only on their distance from the San Andreas. In the original ShakeOut modeling, pockets of very strong shaking with durations of approximately 45 to 60 seconds appeared in parts of the San Gabriel Valley and East Los Angeles. The new 2026 underground map can help scientists improve models like these, because they now have a much clearer picture of the structure through which those waves must travel. The Long Wave Problem Depth doesn't only change how strong shaking becomes, it can change the type of shaking. Caltech researchers explain that deeper sediments tend to produce longer period ground motions. Think about sound. A high-pitched note vibrates rapidly. A low, musical note has a much longer wavelength. Earthquake shaking behaves somewhat similarly. Short period shaking can be especially important for smaller, stiffer structures. Long period shaking becomes particularly important for taller buildings, and a deep basin can favour those longer period motions. That makes downtown Los Angeles especially interesting. It combines very deep sediment with a skyline containing large structures. Previous USGS-supported research has found that Los Angeles basin resonance can occur They occur at periods of roughly 3-7 seconds. While earthquake simulations have predicted strong amplification at periods similar to the natural response of some tall buildings, this doesn't mean every skyscraper will fail during the next major earthquake. Building design, age, construction materials, and engineering all matter enormously, but it means the geology beneath those buildings is part of the equation. The basin edge, Then, there are the edges. Imagine shaking a bowl filled with gelatin. The response isn't identical everywhere. Geometry controls how motion develops. A sedimentary basin behaves far more complicatedly than gelatin, but the basic idea helps explain why scientists care about its three-dimensional shape. Where the basin suddenly becomes deeper or shallower, seismic waves can behave differently. Energy can become focused near basin margins. Waves can reflect from boundaries. Multiple waves can arrive at the same location from different directions. This is known broadly as the basin edge effect. And Los Angeles has a complicated basin geometry. That's one reason the new 3D map matters. Older models could capture the general structure. The new measurements give researchers a sharper picture of those transitions, and eventually that could help identify smaller areas where unusual amplification is possible. What you can't see could change everything. Some of the most dangerous things on Earth are completely invisible, until scientists find a way to expose them. Ground can appear perfectly stable, while enormous forces build miles below. A familiar coastline can hide evidence of past disasters. A quiet mountain can contain systems capable of changing an entire landscape. And sometimes, a discovery forces scientists to reconsider what they thought they already understood. That's the world we investigate on DiscoverTube. Across more than 1,000 videos, we've followed incredible discoveries, unexplained geological activity, extreme natural events, and new research revealing the hidden forces shaping America and the rest of our planet. More than 231,000 subscribers have already joined us. So if you want to see what scientists are discovering before it becomes common knowledge, subscribe to DiscoverTube. Because the places that look the safest, quietest, and most ordinary can sometimes hide the biggest surprises, block by block. The new map is remarkably detailed, but Caltech researchers say it still isn't detailed enough to answer every question. That's because the shallowest part of the ground can vary over surprisingly short distances. Clayton explained that soil conditions can change from block to block. That matters for houses and buildings. A structure on relatively firm ground may respond differently from an otherwise similar structure built over softer material nearby. Researchers therefore want better information about the uppermost kilometre beneath Los Angeles. This is where very local geological differences become increasingly important. Eventually, scientists want to combine the deep basin map with detailed near-surface information. Deep structure tells them how earthquake energy enters and reverberates through the basin. Shallow structure helps reveal what happens to that energy just before it reaches buildings. Put those together and earthquake simulations become far more realistic. Downtown Los Angeles This brings us to one of the areas that immediately stands out. Downtown Los Angeles. Caltech's new map shows that the sediment beneath downtown reaches extraordinary depths, approaching the study's maximum of roughly 10 kilometers. That doesn't mean downtown will automatically suffer the greatest damage in every future earthquake The earthquake's location, rupture direction, magnitude, and frequency content all matter. So do individual building designs. But deep sediment creates conditions favourable for prolonged, long-period shaking. And that's exactly the kind of motion that becomes particularly relevant for large structures. Scientists, therefore, aren't trying to produce a simple map saying This neighborhood is safe, that neighborhood is doomed. Earthquake physics doesn't work that way. They're trying to identify how the underground structure changes the hazard, and downtown sits above one of the most extreme parts of that structure, East Los Angeles and the San Gabriel Valley. Another important region appears when scientists simulate a large southern San Andreas earthquake. The USGS shakeout scenario found pockets of especially strong and prolonged ground motion in East Los Angeles and the San Gabriel Valley. Some modelled locations experienced very strong shaking for 45 to 60 seconds. Why can earthquake energy become so intense far from the initial rupture? Part of the answer involves the network of sedimentary basins east of Los Angeles. Previous simulations suggest the San Bernardino, Chino and San Gabriel basins can help guide seismic energy westward toward metropolitan Los Angeles. Scientists sometimes describe this as a waveguide effect. Instead of energy spreading evenly in every direction, the geological structure can channel part of it. This means the path between the San Andreas and downtown Los Angeles is itself important. The waves have to travel through a complicated underground landscape before they ever reach the city's deepest basin. The older ShakeOut simulation gives an example of just how uneven modelled motion can become. For a point in Watts, the scenario produced maximum ground displacement of approximately 1.2 meters, around 4 feet, and maximum ground velocity around 1.2 meters per second. Compare that with Woodland Hills in the same scenario. There, the modeled point reached approximately 0.4 meters of displacement and 0.3 meters per second of velocity. Those numbers are from one hypothetical magnitude 7.8 scenario. They are not predictions for the next earthquake. A different rupture could produce a different pattern, but they demonstrate the principle. The same regional earthquake can produce dramatically different motion across the metropolitan area, and scientists now have better underground information to understand why. Long Beach Then there is Long Beach. Its importance comes from a different lesson. California's next damaging Los Angeles earthquake does not have to originate on the San Andreas Fault. On March 10, 1933, a magnitude 6.4 earthquake struck on the Newport-Inglewood Fault. The Long Beach earthquake caused widespread damage. Soft sediment contributed to the severity of shaking, while inadequate construction made many buildings particularly vulnerable. The disaster became an important turning point for California building standards. Caltech specifically points to the event when explaining why scientists shouldn't focus exclusively on a distant San Andreas rupture. Los Angeles contains faults much closer to its population centres. A smaller earthquake directly beneath or near the basin can still be devastating. And some faults aren't obvious at the surface at all. The hidden fault problem. Southern California contains a network of active faults. Los Angeles has also experienced destructive earthquakes on blind thrust faults. Faults that don't necessarily rupture all the way to the surface. The 1994 Northridge earthquake demonstrated that danger. Its magnitude was 6.7, yet it caused enormous damage in the Los Angeles region. This creates two separate variables scientists have to model. First, where will the next major rupture occur? Second, how will the ground beneath each neighbourhood modify the waves? The new 3D basin model doesn't answer the first question. It helps answer the second, and that distinction is critical. The 2026 Basin Discovery The new Caltech work isn't the only 2026 research focusing on this problem. In February 2026, USGS researchers published new site-response models designed specifically to capture the three-dimensional geometry of Southern California sedimentary basins. Traditional ground-motion models often simplify local geology, using measurements such as average shear wave velocity near the surface. Those measurements are valuable, but a giant basin isn't one-dimensional. It has shape, edges, changing depth, changing velocities. The 2026 USGS work explicitly incorporated multi-dimensional properties of the Los Angeles basin to better estimate site response. That means two major research efforts published in the same year are moving in the same direction. Scientists want earthquake models that treat Southern California as the complicated three-dimensional structure it actually is. The Resonance Problem One word keeps appearing in this research. Resonance. Every structure has natural frequencies at which it responds particularly strongly to vibration. Buildings do. Bridges do. Sedimentary basins do. If earthquake shaking contains energy near a structure's natural frequency, motion can become amplified. Deep basins favor longer period motion. Tall buildings also tend to respond more strongly at longer periods than small, stiff structures. That's why scientists care about the depth beneath downtown Los Angeles. A basin approaching 10 kilometers isn't simply deep. Its depth affects the frequencies and duration of the shaking moving through it. Previous simulations of large southern San Andreas earthquakes have repeatedly predicted strong amplification inside the Los Angeles basin. At periods relevant to tall structures, the new 3D map gives researchers a better foundation for testing exactly where and how those effects develop. The map isn't finished. Perhaps the most interesting part of the 2026 discovery is what scientists still can't see clearly enough. The new basin model reaches kilometers underground. But researchers want denser information near the surface. They want better measurements of the sediment-bedrock interface. They want more precise seismic velocities. And they want enough observations to understand differences approaching neighborhood, and potentially block scale. One possibility Caltech researchers mentioned is using community information such as Did You Feel It? reports to supplement instrumental observations. Imagine millions of individual earthquake experiences becoming additional data points. A person feels weak shaking here, strong shaking two neighbourhoods away. Scientists compare those observations with their underground model. Over time, the map improves. The next earthquake becomes another experiment. What the map cannot tell us. There is one major misconception worth eliminating. This map does not predict where California's next earthquake will happen. It does not show that a magnitude 7.8 San Andreas earthquake is imminent. And a 10km deep basin does not mean every location above it will experience catastrophic damage. Earthquake risk depends on both hazard and vulnerability. The rupture matters. Distance matters. Ground conditions matter. Building engineering matters. Infrastructure matters. Even the direction in which a fault rupture propagates can dramatically alter shaking. What the new map does is reduce one major uncertainty, the structure beneath Los Angeles. And that can make future simulations much more realistic. The Next Major Earthquake Scientists cannot tell Californians exactly where the next damaging earthquake will begin. They cannot give a reliable date for the next major San Andreas rupture. But they are becoming much better at answering another question. What happens after the rupture starts? The August 2026 Caltech study represents an important step. Researchers used 273 sensors. They mapped an underground structure approaching 10 kilometers deep. They revealed major variations across the Los Angeles region. And they're now working toward even finer measurements of the uppermost kilometer beneath the city. Combine that with new USGS basin response models, decades of earthquake simulations, and detailed information about Southern California's fault systems, and scientists can increasingly model how seismic energy might behave neighborhood by neighborhood. The result is a different way of thinking about California's earthquake threat. The fault may determine the magnitude, but the ground beneath you can help determine how that earthquake feels. And in Los Angeles, that ground is anything but uniform. For DiscoverTube, that's what makes the new map so important. Scientists aren't claiming to know exactly where the next building will fall. They're revealing why the same earthquake can create radically different outcomes across one enormous city. Some areas sit above relatively shallow sediment. Others lie above a geological basin extending miles downward. Some structures may experience short, sharp shaking. Others could be subjected to longer period motion, lasting far longer. And now, for the first time, researchers can see the shape of that hidden world in extraordinary three-dimensional detail. So here's the question. If California's next major earthquake struck tomorrow, what would the ground beneath your neighborhood do to the waves before they reached you? Tell us what you think in the comments below, and subscribe to DiscoverTube for more scientific discoveries, geological mysteries, and investigations beneath the United States.