On March 28, 2025, at just after noon local time, the ground in central Myanmar shook violently. Within moments, the shaking spread across the region, toppling buildings, destroying bridges, and cutting power and communications to millions. The magnitude 7.7 earthquake that struck that day would prove to be one of the most significant continental earthquakes ever recorded, not only for its human toll, but for what it revealed about how major faults can rupture.
The epicenter lay along the Sagaing Fault, a major right-lateral strike-slip fault running more than 700 miles from the Himalayas to the Andaman Sea. It is one of the primary boundaries between the Indian and Sunda tectonic plates, accommodating a slip rate of roughly an inch per year. In structural and mechanical terms, the Sagaing Fault is strikingly similar to California’s San Andreas.
The rupture extended for approximately 317 miles, making it the longest documented rupture on a strike-slip fault in recorded history. Only subduction zone megathrusts beneath the oceans have produced longer ruptures. In satellite images taken after the earthquake, the ground offsets were stark. In some locations, features such as roads and fences were displaced sideways by several meters. This extraordinary rupture length immediately caught the attention of seismologists worldwide, because it demonstrated a mode of fault failure few had thought possible for this type of setting.
In the conventional view, long continental faults like the San Andreas are divided into segments. These segments are separated by bends or step-overs that can act as barriers to rupture. The expectation has been that a large earthquake would generally be confined to a single segment. This is the basis for many of California’s hazard scenarios. The 1906 San Francisco earthquake ruptured the northern San Andreas for about 296 miles. The 1857 Fort Tejon earthquake ruptured 225 miles of the central section. The southern section, extending from the Cajon Pass to the Salton Sea, has not ruptured in over three centuries. Planners have assumed that the next “Big One” would most likely be a southern segment rupture in the range of magnitude 7.8.
The Myanmar earthquake has shown that these assumptions may underestimate what is possible. The rupture there began in a section that had not broken since 1839, but instead of stopping at the edge of that long-quiet zone, it crossed into adjacent sections that had slipped in large earthquakes in the twentieth century. Some of these had ruptured as recently as the 1950s. Standard recurrence models would have expected these sections to remain locked for many more decades.
One of the critical factors appears to have been the smoothness of the fault surface along large stretches of its length. A smoother fault offers less resistance to rupture propagation. In the Myanmar case, this allowed the rupture to accelerate to supershear speeds, moving faster than the seismic shear waves that typically govern how energy is distributed. Supershear ruptures are rare and are known to produce unusually intense shaking over long distances. When the rupture speed exceeds the shear wave velocity, the seismic energy is concentrated into a narrow band ahead of the rupture, amplifying the destruction.
If a similar process occurred on the San Andreas, the implications are sobering. There are long, relatively straight and smooth reaches, especially in the central and southern parts of the fault, where a rupture could gather speed and travel great distances. A multi-segment rupture involving the northern, central, and southern San Andreas in a single event could produce a magnitude exceeding 8.0. The shaking would not be confined to one region, but would strike multiple major cities within seconds. Los Angeles, San Bernardino, Riverside, Bakersfield, and potentially even the San Francisco Bay Area could experience severe shaking in the same event.
The potential damage from such a rupture would be unprecedented in California’s modern history. In the Los Angeles Basin, hundreds of thousands of buildings could be damaged or destroyed. Transportation corridors such as Interstate 5, Interstate 10, and Highway 101 could be severed in multiple locations. Water supply lines crossing the fault could rupture, cutting off service to millions. Electrical transmission lines from northern to southern California could be damaged, leading to widespread and prolonged blackouts. Fires ignited by broken gas lines could spread rapidly, especially if water pressure drops.
The Myanmar earthquake also underscored the danger of secondary effects. Liquefaction occurred in many areas, where water-saturated soils temporarily lost their strength and behaved like a liquid. Buildings tilted, pavements buckled, and underground utilities failed. In California, similar soil conditions exist in parts of the Los Angeles Basin, the Santa Clara Valley, the Sacramento-San Joaquin Delta, and along river channels throughout the state. A major San Andreas rupture could trigger liquefaction in these areas, compounding the direct shaking damage.
High-rise buildings in Myanmar’s cities, some hundreds of miles from the epicenter, experienced long-period shaking that caused structural damage. California’s skyline includes many tall structures built to varying seismic standards. Long-period waves from a supershear rupture could subject these buildings to oscillations lasting a minute or more, testing even modern designs.
Researchers at Caltech, including lead author Solene Antoine and co-author Jean-Philippe Avouac, are now developing simulations to model thousands of possible rupture scenarios for the San Andreas. These models aim to capture the complexity of fault geometry, roughness, and the redistribution of stress following each event. Early findings suggest that rupture patterns can vary widely, with no reliable repetition from one cycle to the next. This makes it impossible to forecast a specific sequence of events, but it does reveal that very large multi-segment ruptures are physically possible.
The southern San Andreas, in particular, is known to be under significant stress. Geological records from the Carrizo Plain and Coachella Valley show evidence of repeated major earthquakes over the past several thousand years. The interval since the last rupture in some stretches is already longer than the average recurrence time. While this does not predict immediate failure, it confirms that enough stress has accumulated to produce a very large event whenever rupture conditions align.
California’s official planning scenarios, such as the USGS ShakeOut model, are based on ruptures shorter than the Myanmar example. In the ShakeOut scenario, a 7.8 magnitude rupture of the southern San Andreas causes nearly two thousand deaths and two hundred billion dollars in losses. Extending that rupture hundreds of miles farther north, through the Los Angeles Basin and into central California, would multiply those figures and test the state’s emergency response capabilities far beyond current expectations.
The March 2025 Myanmar earthquake will be studied for years, but its most immediate lesson is already clear: long, mature strike-slip faults can rupture in ways that defy traditional segment boundaries. For California, this means that the next San Andreas earthquake may not look like any in the historical record. It could be larger, faster, and far more disruptive than even the worst scenarios currently used for planning.
When it does occur, the shaking will not pause for political borders or county lines. It will race along the fault, affecting millions of people in seconds. It will challenge the resilience of every community it touches. The only variable is when that day will come.
Source New Paper:
Antoine, S. L., Im, K., Avouac, J.-P., et al. (2025). The 2025 Mw 7.7 Mandalay, Myanmar, earthquake reveals a complex earthquake cycle with clustering and variable segmentation on the Sagaing Fault. Proceedings of the National Academy of Sciences, 122(29). https://doi.org/10.1073/pnas.2514378122
