When we think of earthquakes, we imagine sudden, violent shaking. But deep beneath the Earth’s surface, some faults move in near silence. These slow, shuffling slips and their accompanying hum — called tremors — don’t shake buildings or make headlines. But scientists believe they can serve as useful analogs of how major earthquakes begin and behave.
A new study by geophysicists at UC Santa Cruz and supported by the National Science Foundation Earth Science Program explains how some of these tremor events can yield insights into how stress builds up on the dangerous faults above where major earthquakes occur. The study, published on May 14 in the journal Science Advances, was led by Gaspard Farge, a postdoctoral researcher in the university’s Seismo Lab, and Earth and planetary sciences professor Emily Brodsky, the lab’s principal investigator.
When faults where tectonic plates meet slip fast past each other, earthquakes result. Tremors are produced when this happens slowly, usually tens of miles underground — often in subduction zones, where one plate dives beneath another. Tremors don’t pose immediate danger, but they also shouldn’t be ignored because they often happen in the vicinity of where the world’s biggest earthquakes eventually occur, say the study’s authors.
“We find that the faults that produce tremor are more sensitive and connected to their surroundings than previously thought,” said Farge, who researches what processes shape minute seismic activity. “Even small, frequent earthquakes can affect how a major fault behaves.”
Chaotic effect of small quakes
Farge and Brodsky discovered that small earthquakes, even those tens of kilometers away from the main fault, can disturb a tremor’s natural rhythm. As a patch of the fault begins to slip, it usually nudges its neighbors along for the ride — leading to large, synchronized tremor episodes. But when small quakes send seismic waves rippling through the area, they can throw off that coordination.
These outside disturbances can either speed up or delay tremor activity, depending on timing and location. And because small earthquakes happen far more often than large ones, they may constantly jostle the system out of synchrony.
Over time, this could explain why some segments of a fault show highly regular tremor patterns — slipping in coordinated episodes — while others remain chaotic. The segments aren’t just shaped by the rocks underground, a marble here, granite there; they also adapt to the constant perturbation from nearby seismic activity.
The dynamic Northwest
This pattern is evident in the Cascadia subduction zone, which extends from Northern California, through Oregon and Washington, to British Columbia. The zone produces extensive tremor activity and very large earthquakes on a 400-year basis. Across Oregon, the subduction is almost silent — and without perturbation from earthquakes — the plate slips like a clock, every year and a half in a section hundreds of kilometers long.
In Northern California, however, the activity of small earthquakes near Cape Mendocino disturbs the regularity of the fault, and the tremor is produced in small, disorganized episodes.
Scientists have known that the shape and makeup of a fault zone — the rock types, temperature, water content, and even the slope of the sinking plate — all help define how and where a tremor happens. These are called structural factors, and they affect how sticky the fault is and how easily it slips.
But this new study introduces a twist: Dynamic factors, like the stress waves from small earthquakes nearby, may also shape when and where tremor happens — and whether it occurs in a smooth, predictable way or in a scattered, messy fashion.
“These findings go beyond tremors. By showing how small earthquakes can affect the timing and behavior of slow fault movements, this discovery opens up new ways to understand the buildup to large, damaging earthquakes,” said Brodsky, a leading earthquake physicist. “If we can track how a tremor responds to these small stress nudges, it may be possible to read the stress landscape of a fault — offering clues about where and when it might rupture in a big way.”
Quake magnitude isn’t everything
This study shifts our understanding of a common assumption: that only large forces shape the behavior of major earthquake faults. In fact, tiny, nearby quakes — usually considered too small to matter — may play an outsized role in defining where and how the Earth’s plates slip past one another. That means that by listening to the Earth’s quietest rumbles, we may be able to learn how to better anticipate its loudest ones.
“Ultimately,” Brodsky said, “this study proposes a way to measure the elusive dynamic factors that influence how fault slips — the stress landscape that informs how stress is built up on these dangerous faults.”
“The fact that we can measure and understand the effects of earthquakes’ perturbation on slow fault ruptures gives us hope that we could use the same logic to understand where earthquakes should be expected to be regular, and where not,” Farge concludes.
