California’s most dangerous faults are not only defined by the headline grabbing magnitude 7 shocks that topple freeways and rupture gas lines. Deep below the surface, swarms of tiny earthquakes, some so small they barely register on standard instruments, are sketching a far more intricate and unsettling picture of how the state’s tectonic engine really works. Those microquakes are now exposing hidden structures and behaviors that could shape the next “Big One” in ways residents and planners can no longer afford to ignore.
By tracking clusters of these almost invisible events, scientists are mapping buried faults, rethinking how major fault systems connect, and probing why some segments seem primed for unusually fast and destructive ruptures. The emerging view is of a fault network that is more fragmented, more fluid driven, and in some cases more capable of extreme shaking than traditional models suggested.
Microquakes that redraw the map under Northern California
In Northern California, researchers have turned to swarms of earthquakes around magnitude zero, thousands of times smaller than the quakes most people feel, to illuminate structures that never break the surface. By stacking and precisely locating these tiny events, they have traced a complex web of faults around The Mendocino Triple Junction, where the Pacific, North American and Gorda plates meet, revealing that the region’s tectonics are far more segmented than a simple three plate boundary. One study describes a new model with five moving pieces, including two blocks that are entirely hidden from view at the surface, a configuration that helps explain why this junction is one of the most seismically active corners of the state and a key driver of stress along the northern San Andreas system, as detailed in work on hidden faults.
Those same microquakes have also exposed what one team calls a “hidden earthquake world” beneath Northern California, a realm where faults slip quietly and repeatedly without producing damaging shocks but still transfer stress into the broader system. By closely monitoring clusters of these Invisible events, scientists have identified narrow zones of weakness that link deeper plate interactions to the shallower crust, suggesting that small shifts at depth can reorganize stress on faults that cut directly beneath communities from Eureka to the Bay Area. The work, led by researchers including Andy Fell and colleagues, shows that the Earth beneath the region is partitioned into more blocks and boundaries than surface geology alone would suggest, a conclusion underscored by a separate analysis that emphasizes the five piece model and its support from the National Science Foundation, as described in a report on a new tectonic model.
San Ramon, Calaveras Fault swarms and the role of fluids
Farther south, in the East Bay suburb of San Ramon, residents have grown used to flurries of small quakes that rattle dishes but rarely cause damage. San Ramon sits atop the Calaveras Fault, a major branch of the broader San Andreas system, and for reasons scientists are still working to pin down, it is a hotspot for earthquake swarms that can involve dozens of minor events over days or weeks. Detailed analyses suggest that in this area, water moving through the crust in an unsteady way may be triggering repeated slip on small fault patches, a pattern that helps explain why the San Ramon swarms are so persistent and why they sometimes include several slightly larger quakes embedded in a cloud of tiny ones, as highlighted in research on The San Ramon activity.
These fluid driven swarms are not confined to the Bay Area. Similar patterns have been documented near geothermal fields and along other fault strands where deep circulation of water or gas can weaken rock and promote slip. In San Ramon, the repeated swarms offer a rare laboratory for watching how the Calaveras Fault behaves between larger events, and they raise uncomfortable questions about how stress might be migrating toward more heavily loaded segments. Local coverage has emphasized that San Ramon sits squarely atop the Calaveras Fault and that, for reasons that remain partly mysterious, tiny earthquakes happen there with unusual frequency, a reality captured in reports on San Ramon and its uneasy relationship with the fault beneath it.
From Holtville to the Mendocino Triple Junction, a statewide swarm pattern
When Californians hear about earthquake swarms, they often think of the Imperial Valley, where clusters of small quakes have long rattled communities near the Mexican border. Around the agricultural town of Holtville, near the southern end of the San Andreas system, swarms have periodically lit up seismic networks, reminding residents that the plate boundary does not end at the Salton Sea but continues as a complex tangle of faults into the Gulf of California. The region around Holtville is a vivid example of how swarms can cluster along step overs and secondary faults that may, in turn, influence how a major rupture on the southern San Andreas might start or stop.
Statewide, scientists now see swarms as a recurring feature of life along California’s giant faults rather than an oddity. In many parts of California, a flurry of minor earthquakes is simply how the crust relieves stress in small increments, and most of these sequences never lead to a large event. Reporting on why earthquake swarms happen notes that an earthquake swarm is defined as a series of quakes in a small area over a short time without a single dominant mainshock, and that in many regions these swarms are driven by fluid movement or slow fault slip rather than the classic mainshock aftershock pattern, a point emphasized in explanations of why swarms happen.
Do swarms mean a big quake is coming?
Whenever a swarm lights up social media, the first question people ask is whether it signals a looming major earthquake. Seismologists are careful on this point. Analyses of past sequences show that swarms can slightly increase the statistical odds of a larger event nearby, but the absolute risk usually remains low, and in many cases the swarm ends without any significant mainshock. Scientists stress that while swarms can sometimes precede larger quakes, they can also redistribute stress in ways that make a big earthquake elsewhere less likely, a nuanced message that has been underscored in coverage explaining that this is the question people most often ask and that the answer is usually no, as summarized in guidance from Scientists.
That nuance can be hard to square with long term forecasts that sound stark. One recent analysis of an earthquake swarm that jolted California highlighted that there is a 95 percent chance of a major disaster level earthquake by 2043, a figure rooted in statewide probability models that account for slip rates, fault lengths and historical activity. In that context, swarms are best seen as one more reminder that the system is active and evolving rather than as a precise countdown clock. Researchers mentioned three specific fault zones in that discussion and warned that the potential for stronger shaking in some scenarios remains underappreciated, a warning captured in coverage of an earthquake swarm and its broader implications.
Supershear ruptures and the hidden risk of extreme shaking
While microquakes and swarms help map where faults are and how they creep, another line of research is probing how fast those faults can rupture when they finally let go. Supershear earthquakes, in which the rupture front outruns its own shear waves, can produce unusually intense and focused shaking, more akin to a sonic boom than a typical rolling quake. Work by USC researchers has warned that California’s next big one could be faster and far more destructive if it takes the form of a Supershear rupture, with shaking that propagates along the fault in a way that concentrates energy on certain corridors, as described in studies of Supershear earthquakes.
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