The Big Impact of Small Quakes: Shaking Up the Rhythm of Tectonic Tremors

When you think of an earthquake, you probably imagine sudden, violent shaking. But not all fault movement is loud and jarring. Deep beneath our feet, some faults move in near silence – a slow, steady slip accompanied by a faint seismic “hum” called tectonic tremor. These slow-motion slip events don’t shake buildings or grab headlines, yet they fascinate geologists because they offer clues to how stress builds up and releases on major faults. In fact, studying these quiet tremors is like eavesdropping on the Earth’s whispering before it shouts.

Recently, a team of geophysicists at UC Santa Cruz made a surprising discovery about these tremors. In a new study (Science Advances, May 2025), Gaspard Farge and Emily Brodsky show that even tiny earthquakes can have an outsized effect on the fault’s quiet slipping rhythm. Imagine a delicate dance where everyone moves in sync – until a mischievous toddler keeps jumping onto the dance floor, throwing off the timing. In the world of faults, those “toddler” disruptions are small, local quakes, and the delicate dance is the synchronized cycle of tectonic tremor. The research reveals that small quakes can disturb the fault’s natural rhythm, explaining why some fault segments slip like clockwork while others dance to a messier beat. This finding challenges the old notion that only big forces matter and suggests that even little earthquakes can ripple through a fault’s system in a big way.

Let’s break down the basic science of these slow-slip tremors, then dive into how “small fry” quakes manage to shake up the synchronization of fault movement. We’ll tour some real-world subduction zones – from the Pacific Northwest to Japan – to see this phenomenon in action and explore the clever model the researchers used (hint: it involves seeing the fault as a bunch of rhythmical oscillators, not unlike a crowd of synchronizing fireflies). Finally, we’ll look at why this discovery matters for understanding earthquake behaviour and possibly even improving forecasts of big quakes.

Tectonic Tremors: Earth’s Quiet, Slow Earthquakes

Most of the earthquakes we hear about occur when tectonic plates get stuck and then suddenly lurch past each other, releasing energy in a burst of shaking. Tectonic tremors, on the other hand, happen when faults slip slowly. These are sometimes called slow slip events (SSEs) or “slow earthquakes.” Instead of a jolt lasting seconds, a slow slip can unfold over days or weeks, usually so deep underground (often 20–40 km down) that we don’t feel any shaking on the surface. The only sign is that faint seismic noise – the tremor – detectable by sensitive instruments. It’s as if the fault is creeping along, sighing instead of snapping.

Such slow and silent fault motions were only recognized in the early 2000s, first in places like the subduction zone under Vancouver Island and Washington State. Researchers noticed that GPS stations would drift slightly, indicating the ground was moving slowly, coincident with bursts of tremor-like seismic signals. This was a revelation: a whole new mode of fault slip that doesn’t produce a classic quake. Since then, tremor and slow slip have been found in many subduction zones (where one tectonic plate slides under another) and even along some other faults. They tend to happen in the “transition zone” of a fault – just below the region that generates big quakes. That makes them especially interesting, because they occur adjacent to the locked patches that eventually break in damaging earthquakes. In other words, tremors are happening in the neighbourhood of megaquake territory. No, tremors won’t topple buildings, but they might reveal how stress is accumulating or being released on the deeper parts of a fault that later produce giant quakes.

Here’s an analogy: Think of a fault as a giant stuck door. A regular earthquake is like someone suddenly kicking the door open – a lot of noise and energy release all at once. A slow slip event is like someone leaning on that door and slowly pushing it open over a long time – quiet, but still movement. The tremor is the faint creaking of the hinges as the door opens gradually. Importantly, even though it’s quiet, that door is moving – which means stress that had been building up is getting relieved without a big bang. This might sound like a good thing (and in some sense it is, since some pent-up energy is dissipated harmlessly), but scientists also suspect these slow slips could interact with locked sections of the fault above. Studying tremor may give insights into how close a fault is to letting loose a real earthquake.

Synchronized vs. Chaotic: Faults That Keep a Beat… and Faults That Don’t

One of the puzzling observations about tectonic tremor activity is how different it can look from one place to another. In some fault zones, tremors come in regular, predictable episodes, almost like clockwork. We call these “synchronized” or periodic tremor episodes. For example, a segment of a subduction zone might experience a tremor (and accompanying slow slip) every 14 months, nearly on schedule, and over a broad area all at once. It’s as if a hundreds-of-kilometres-long stretch of the fault decides to slip together in one big, coordinated event. Many cycles can repeat with surprisingly consistent timing and coverage. In other areas, though, tremor is a lot more erratic – little bursts here and there, with no clear periodicity or coordination. These “chaotic” segments have tremor popping off in a disorganized, asynchronous way.

Think of an audience clapping after a performance. In some cases, the clapping becomes synchronized – a steady clap-clap-clap in unison across the whole crowd. In other cases, everyone’s clapping at their own pace – a messy, overlapping noise. Fault tremors are similar: some fault segments manage to get their “audience” of slip patches to clap together on a steady beat, while other segments are more like a chaotic applause with no unison.

Scientists have been aware of this contrast for a while. It’s not just random: often, distinct segments along the same plate boundary will each have their own tremor personality – one segment might be orderly and periodic, next one down the line might be disorderly. This kind of segmentation is very clear in certain regions. It naturally raises the question: why? What makes one section of a fault behave like a well-synchronized marching band and another like free-form jazz?

Initially, researchers figured the reasons must lie in the local geology and fault conditions. After all, faults are not uniform; the rocks vary, temperature changes with depth, fluid pressure can differ, etc. These structural factors (like rock type, how “sticky” or smooth the fault surfaces are, the amount of water in the fault, and so on) surely play a big role in how slow slip happens. And indeed, these factors do matter. Some patches of a fault might be strong and stuck, others weaker and prone to creep. The surprising twist from the new study, however, is that geology isn’t the whole story. Farge and Brodsky found evidence that the rhythmic or chaotic behaviour of tremor might also depend on what’s happening around the fault, not just within it.

In particular, they homed in on an external influence that hadn’t been fully appreciated: the pitter-patter of small earthquakes in the neighbourhood. It turns out those little quakes can act like unruly kids disrupting a choir performance – the choir being the synchronized tremor slip. If the kids are quiet, the choir can keep a steady tempo. If the kids keep making noise, the singers lose timing. In fault terms: fault segments free of little seismic disturbances can maintain large, periodic tremor episodes, whereas segments bombarded by many small quakes end up with tremor that’s scattered and irregular. This was a major clue that something beyond just the internal fault structure was at play.

Little Earthquakes, Big Disturbances: How Small Quakes Throw Off the Fault’s Rhythm

We usually think of big earthquakes as the ones that influence other seismic activity – for instance, a large distant quake’s waves can trigger tremor or even small quakes elsewhere. What Farge and Brodsky have shown is that even small local earthquakes (we’re talking magnitude 2-ish, basically tiny on the scale of things) can disrupt the timing of tremors on a nearby major fault. It’s like learning that not only do cannon blasts make ripples, but even popping bubble wrap can disturb a sensitive system.

Here’s what the researchers found: when a patch of the deep fault begins to slip slowly (starting a tremor episode), normally it nudges its neighbouring patches to slip as well – kind of a chain reaction that can lead to a big synchronized tremor covering a wide area. This is the fault’s natural tendency to synchronize – the deep fault patches are all gently pushing on each other, trying to all join the slow slip party in unison. However, if a small earthquake happens nearby around the same time, its seismic waves send a jolt through the area. That little jolt can reset the timing for some patches or kick them out of the ongoing slip, effectively throwing off the coordination. Depending on exactly when and where these small quakes hit, they might speed up a tremor that was about to happen or delay one that was building up. And crucially, because small quakes occur far more frequently than large ones, they represent a constant source of tiny “noise” in the system, continually poking and prodding the fault segments.

Over months to years, this constant jostling prevents the fault from settling into a steady rhythm. Instead of one big well-timed tremor every so often, the segment breaks up its slip into smaller, jittery episodes – hence the chaotic, irregular tremor pattern. In essence, small earthquakes are acting like spoilers to the fault’s regular schedule. If they’re absent or rare, the fault can slip in a more orderly, combined fashion; if they’re frequent, the fault’s slow slips get fragmented.

The researchers described this as a competition between internal synchronization and external perturbation. The fault wants to slip in a large synchronized way (that’s driven by its internal dynamics and the fact that one slipping patch encourages others), but the small earthquakes are outside perturbations constantly trying to throw it off. It’s a bit like a tug-of-war between order and chaos on the fault. Their observations implied that this tug-of-war outcome – orderly vs. chaotic tremor – depends strongly on the rate of those small quake perturbations. As Farge and Brodsky put it, tremor synchronization in space is limited by the activity of small nearby crustal and intraslab earthquakes. If the small quake rate is low, synchronization wins and the fault can break in larger, coherent segments; if the small quake rate is high, the perturbations win and the fault breaks in smaller, fragmented segments.

This idea was backed up by data across different regions. The team systematically measured how large an area would light up together during tremor episodes (the “synchronous segment size”) and checked how that correlates with the background level of small earthquakes in that area. The result: the more small earthquakes buzzing around, the smaller the synchronized tremor patches tended to be. It’s a real-world correlation that suggests those little quakes are slicing and dicing the tremor zone into bits, preventing it from uniting into one big family reunion of a tremor event.

A Worldwide Pattern: From Cascadia to Japan (and Beyond)

This phenomenon isn’t just theoretical – it shows up in multiple subduction zones and fault regions around the world. Farge and Brodsky examined tremor and earthquake data from several well-known tectonic zones: the Cascadia subduction zone in North America, the Nankai Trough in southwest Japan, the Japan Trench region (off Tohoku in northeast Japan), the Hikurangi subduction zone in New Zealand, and even a section of the San Andreas Fault in California (the Parkfield region, known for its slow-slip and tremor activity). Despite the differences in geology and tectonic setting, the same general trend appeared in all these places. Wherever small earthquakes were relatively scarce, the tremor episodes tended to organize into large, periodic, synchronized bursts covering long stretches of the fault. Wherever small quakes were abundant, tremor was more desultory and scattered.

For instance, take Cascadia, which stretches from Northern California up through Oregon and Washington into British Columbia. Cascadia is famous (or perhaps infamous) for its massive megathrust quakes every few centuries, but in between those, it’s humming along with regular tremor and slow slip every year or two. The new study highlighted a stark contrast within Cascadia: across most of Oregon and Washington, the deep subduction fault is almost eerily quiet in terms of small earthquakes, and consequently the slow slip tremor events there occur like clockwork – about every 14 to 18 months, a huge section of the fault (hundreds of kilometres long) slips in unison. It’s a smooth, periodic cycle, almost like the fault is on schedule. But farther south, in Northern California near the Mendocino area, there’s a lot more small earthquake activity jostling the edge of the subduction zone, and here the tremor episodes break down into fitful, small bursts with no regular timing. In other words, the fault behaviour goes from orderly to chaotic as you move into the zone with more little quakes. This aligns perfectly with the idea that the small quakes are the culprits disrupting the coherence.

Similarly, in the Nankai Trough in Japan, different segments show different tremor characteristics. Parts of the Nankai subduction zone exhibit very regular slow earthquakes (some segments historically slip every few years predictably), whereas other parts are less regular. The study found that these differences correlate with local seismicity: the segments with the most regular, large-scale tremor episodes are the ones with lower background micro-earthquake activity, and vice versa.

Even along the Japan Trench (the plate boundary off northeastern Japan), where slow earthquakes are a bit less well-known than in Nankai, the same kind of relationship appears to hold. After the massive 2011 Tohoku earthquake, scientists discovered various slow earthquake phenomena in that region too. Farge and Brodsky’s work suggests that portions of the Japan Trench fault that are getting peppered by small seismic events have trouble generating big, synchronized tremor episodes, whereas quieter sections can synchronize more easily.

Over in New Zealand’s Hikurangi subduction zone, there are well-documented slow slip events that recur at different intervals along the plate interface. Again, the new research indicates that where the Hikurangi subduction zone is relatively free of little quakes, it’s more likely to have the tremor and slow slip line up into a larger event, whereas sections with lots of tiny quakes nearby tend to have more disjointed tremor activity.

And it’s not just subduction zones. The Parkfield segment of the San Andreas Fault in California – famous for its repeat magnitude ~6 quakes and lots of small seismic creep events – also shows tremor behavior. The analysis hints that even there, the degree of tremor synchronization (or lack thereof) ties in with how much small earthquake “noise” is in the system locally.

All told, the global survey by Farge and Brodsky showed a consistent picture: small earthquakes are a significant factor in shaping how tremor (and by extension slow slip) is segmented along a fault. In their data, the rate of local quake perturbations could statistically account for about 10–20% of the variation in how synchronized the tremor was from place to place. That may not sound like a huge percentage at first, but in terms of natural geological systems, it’s quite notable – especially considering that many other factors (like rock type, fault stress, etc.) are also in play. In areas with really high small-earthquake activity, this factor outweighed other known factors in explaining why the tremor comes in broken-up bursts.

The takeaway is that the “personality” of a slow-slipping fault segment – whether it slips in big smooth bouts or little sputtery ones – isn’t just a built-in trait of the rocks, but is partly molded by its noisy neighbors (small quakes). Fault segments are not isolated; they’re listening to the commotion around them. If the surroundings are calm, the segment can gather itself and slip in a grand event; if the surroundings are constantly rattling, the segment stays jittery.

Modeling a Fault’s Rhythm with Pulse-Coupled Oscillators (or, Faults as Fireflies)

Okay, so how do scientists actually explain and test this competition between synchronization and perturbation on a fault? In the Farge & Brodsky study, they turned to a conceptual model known as a system of pulse-coupled oscillators. That might sound like a mouthful, but it’s a fancy term borrowed from mathematics and physics to describe things that naturally oscillate (like pendulums, heart cells, or even flashing fireflies) which can interact via pulses. A classic example: fireflies flashing in synchrony – each firefly is an oscillator that emits a pulse of light, and seeing a neighbor’s flash can tweak the timing of its own rhythm. Given the right conditions, they all lock onto the same beat. Now imagine occasionally poking a few fireflies so they flash off-beat – that would disturb the whole synchronization. This is essentially the idea of pulse-coupled oscillators with some added disturbances.

In the context of fault tremors, you can picture each segment of the fault’s tremor zone as an oscillator that has a natural cycle (it wants to slip every so-many months) and it also tends to influence its neighbors (when one patch slips, it nudges the adjacent patch to slip – a pulse of stress transmitted along the fault). Left alone, this kind of system might synchronize – much like metronomes on a shared board will eventually tick in unison. However, now add in occasional “pulses” from outside – the seismic waves from those small earthquakes – hitting some of the oscillators at random times. That’s like someone randomly tapping some of the metronomes or fireflies out of phase. The result? It breaks the perfect synchronization; some oscillators reset or change their phase, and the group can’t maintain a single steady rhythm.

The researchers built a simple model along these lines, essentially simulating a fault segment as a network of pulse-coupled oscillators that are also subject to random external pulses (the small quakes). By tuning the rate of those external pulses, they could see how the system’s behavior changed. And it mirrored the real data: with frequent disturbances, the oscillators (tremor patches) struggled to sync up, whereas with fewer disturbances, they fell into a synchronized pattern more easily. In fact, the model reproduced the same kind of anti-correlation observed in nature between perturbation rate and synchronization intensity – essentially confirming that this simple mechanism is enough to explain the key observation.

One neat aspect of using this pulse-coupled oscillator approach is that it provides a conceptual bridge to many other synchronized systems in science. It tells us that the fault’s tremor behaviour might be understood in the same framework as, say, flashing fireflies or firing neurons – systems where the timing of pulses can lead to order or chaos. It’s not every day that you get to compare an earthquake fault to a row of fireflies, so that’s a fun insight in itself! The informal way to think of it: the fault wants to slip in rhythm (it has an internal groove), but little quakes keep improv-jamming on the side, messing up the groove. The model formalizes that idea, and it matches what we see in tremor patterns around the world.

Why It Matters: Reading Fault “Heartbeats” and Improving Forecasts

At this point you might wonder: this is all very interesting for geophysics nerds, but what are the broader implications? It turns out, understanding how small quakes perturb slow slip rhythms could help us in the quest to forecast seismic hazards. Here’s why.

First, this study underscores that faults are sensitive to even small stress changes. If tiny earthquakes can disrupt slow slip timing, it means the state of stress on these deep fault segments is near a tipping point – nudges matter. This sensitivity is actually useful. Think of the fault as a kind of heart monitor for stress: by observing tremor behavior (the “heartbeat”) and how it responds to little stress perturbations (the “stress tests”), we might infer the condition of the fault and how close it is to failure in a larger quake. Emily Brodsky, one of the authors, emphasized that by tracking how tremor responds to small stress nudges, scientists might be able to “read the stress landscape of a fault – offering clues about where and when it might rupture in a big way”. In other words, the chaotic or smooth nature of tremor could tell us something about the buildup of stress on the dangerous, quake-generating parts of the fault above.

Secondly, this work challenges the assumption that only big triggers (like large distant quakes or slow loading from plate motions) affect fault behavior. We now have to consider that even the background buzz of microquakes is an important part of the story. This opens a new perspective: by paying attention to these “little guys” we might glean insights on the segmentation of faults – basically, understanding which parts of a fault are likely to rupture together versus remain separate. That is crucial for assessing earthquake potential. For instance, a fault that breaks in a synchronized 200-km slow slip might also be capable of a larger, more coherent earthquake rupture, whereas if it’s always fragmented, maybe big ruptures are less likely to span the whole area. This is speculative but plausible. As the authors put it, their results “imply previously unrecognized interactions across subduction systems, in which earthquake activity far from the fault influences whether it breaks in small or large segments”. In plainer terms, if we want to know how a future earthquake might propagate along a fault, we might need to account for how small surrounding quakes have been either keeping it segmented or allowing it to unify.

Finally, by incorporating small quake effects, we might improve models for earthquake forecasting or hazard assessment. Currently, a lot of focus is on mapping out locked vs. creeping sections of faults (from geological and geodetic data) to guess where big quakes will start or stop. This research suggests we should also map out where the little earthquakes are happening around a fault, because those could mark boundaries of synchronized tremor zones and perhaps boundaries of future large ruptures. It gives geologists another factor to consider. Gaspard Farge hinted at this future application, saying that understanding these perturbation effects could help us pinpoint “where earthquakes should be expected to be regular, and where not” – essentially highlighting which fault sections are in a stable cycle and which are unpredictable.

The big-picture message comes down to this: even the Earth’s quietest rumbles can speak volumes about its mightiest quakes. By listening carefully to the pattern of tremors (and how they change when poked by small quakes), scientists gain a new tool for probing the deep workings of faults. It’s a reminder that in Earth science, small things can make a big difference. Just as a tiny flutter can alter a heartbeat, a magnitude 2 quake in the right spot can alter the “heartbeat” of a fault’s slow slip cycle.

In an era where improving earthquake forecasts is a holy grail (and admittedly a very challenging one), findings like these add a piece to the puzzle. They show how complex and interconnected seismic systems are: the big and the small, the fast and the slow, all dancing together. It’s a fascinating glimpse of our planet’s inner rhythms – a reminder that the Earth’s crust isn’t just lurching in big jumps, but also swaying to a subtle tune, a tune that can be remixed by the tiniest of quakes. And if we learn the melody well enough, we just might get better at anticipating the grand finales.

References:

Gaspard Farge, Emily E. Brodsky. The big impact of small quakes on tectonic tremor synchronization. Science Advances, 2025; 11 (20) DOI: 10.1126/sciadv.adu7173