How engineering can predict the next big earthquake

A new published study could shed new light on why some fault boundaries cause earthquakes and others do not. The secret could be all to do with geometry.

This study, published by Brown University-led researchers in the journal Nature, challenges many long-held beliefs about the cause of earthquakes. It also presents a new model of earthquake formation that does not rely entirely on friction, as is commonly believed.

The research team believes that geometry (or the shape of faults) plays a role more than just friction. The study can also help us predict where and how strong an earthquake is likely based on its shape.

“Our paper paints this very different kind of picture about why earthquakes happen,” said Victor Tsai, a geophysicist at Brown University and one of the paper's lead authors. “This has very important implications for where earthquakes can be expected to occur, and where earthquakes cannot be expected, and also for predicting where earthquakes will be the most damaging,” he adds.

If these findings are correct, this could be a watershed study in seismology, helping us better prepare for and predict devastating earthquakes long before they happen.

What causes earthquakes?

Current thinking is that earthquakes result from sudden movement along faults in the Earth. This movement releases large amounts of stored “elastic strain” energy, which forms seismic waves in the Earth.

These waves then propagate across the Earth, eventually causing the Earth's surface to shake violently. Movements on faults are generally thought to be a response to long-term deformation and stress buildup.

In case you don't know, fault lines are small or large cracks that, when large enough, can form visible boundaries on the planet's surface between tectonic plates in the Earth's crust. As the name suggests, they are large parts found on the outer “crust” of the Earth.

You can liken it to the cracks in the shell of a hard-boiled egg when you tap on it to peel the shell. On Earth, these plates are in constant motion thanks to movements deep in the Earth, causing them to constantly collide with each other.

This constant friction between these plates releases energy as faults slide and break each other. This is called slippage.

This movement along them is very slow but contains incredible amounts of energy. So when things go wrong, the results can be devastating to anything on the surface.

“Creep faults have a very slow movement (think inches per year), releasing the pressure that builds up (also slowly), whereas for earthquake-induced faults, which we find more pronounced, the pressure that builds up slowly is released very quickly, “Within seconds, during an earthquake,” Cai explained to IE in an email exchange.

Fault misalignment and surface creep rates along major faults at ca. Source: Jaesuk Lee et al. 2024. A new wrinkle to an old idea

Until now, rapid slip and the intense ground movements that follow have been thought to result from unstable friction at faults. However, the new study suggests that when friction is constant, plates can move against each other slowly without causing an earthquake.

This continuous, smooth movement is also known as crawling.

“People try to measure these frictional properties, like whether a fault zone has unstable friction or stable friction, and then, based on laboratory measurements of that, they try to predict whether or not an earthquake will occur there,” Cai explains. .

“Our findings suggest that it may be more important to look at the geometry of faults in these fault networks, because it may be the complex geometry of the structures around those boundaries that creates this unstable versus stable behavior,” he adds.

In this sense, geometry is related to the nature of the underground fault. When a fault zone is complex (sometimes called a “fault swarm”), it can include a series of bends, gaps, and transgressions. Less complex, simple faults, which resemble a long, straight fault (called “flat” or “linear” faults), will not usually create an earthquake.

Source: Jaeseok Lee et al 2024. Rock types are not important

Another element of the new model is that it is also a paradox of the rock type. This means that the types of rocks in which the faults are found are minor, even trivial, components.

“The main finding of our research is that it is primarily geometry, rather than rock types, that we suggest makes faults ‘sticky’ (when geometries are complex) versus ‘less sticky’ (when geometries are simple). “Whether the faults are aligned (simple) or misaligned (complex) certainly depends on geological history in a way that our study did not delve into, but it may be complex,” Cao told IE.

The new study expands on previous research that has examined why certain earthquakes produce more ground motion than others in different regions of the world, even when they have similar magnitude. These revealed that the collision of blocks within the fault zone during an earthquake contributes significantly to the generation of high-frequency vibrations.

However, the new study suggests that subsurface geometric complexity likely also plays an important factor in the occurrence and location of earthquakes. To test their idea, the researchers analyzed data from faults in California, including the well-known San Andreas Fault.

They found that fault zones with complex geometry underneath, where the structures were not parallel, had stronger ground motions than fault zones with less geometric complexity. This indicates that some of these areas will have stronger earthquakes, some will have weaker earthquakes, and some will not have earthquakes.

Increase the pressure

When speaking to IE, Tsai agreed that this mechanic can be likened to a ratchet-and-click system. In this analogy, the slow buildup of energy is like someone trying to ratchet between the “teeth” of a ratchet.

As the pawl reaches the tip of each tooth, it quickly falls into the next tooth, releasing massive amounts of energy into the crust. This energy manifests itself in the form of an earthquake on the Earth's surface.

A ratchet and pawl will do the same for less complex crack areas, except with much smaller “teeth” or no “teeth” at all. This would result in less sudden energy release and, more importantly, reduce the “risk” of earthquakes.

“Every slide down the canyon is an earthquake, and without the ratchet teeth, you have a very smooth turning wheel without any sudden movements. [I think] “That's a good analogy,” Cao told IE.

“Our idea is a fairly intuitive one that stuck defects are just like a saw with large teeth at large angles, while defects with smaller, smoother teeth don't build up stress as easily. But this simple idea contradicts the paradoxical idea It has long been argued that the type of rock present on faults determines whether friction is unstable or stable.

Hammer and ratchet on an old piece of equipment. Source: MWCPhoto/iStock More work to be done

“Understanding how faults behave as a system is essential to understanding why and how earthquakes occur,” explains Jaesuk Lee, a graduate student who led this work. “Our research suggests that the complexity of the error network architecture is the key factor and creates meaningful connections between sets of independent observations and integrates them into a new framework,” he added.

The researchers stated that further validation of the model is necessary, but preliminary results indicate promise. This is especially important because measuring misalignment or misalignment is easier than measuring misalignment properties. If this work is confirmed, it could eventually be incorporated into earthquake prediction models.

This goal is still a long way off as researchers plan to expand the study.

“The most obvious thing that comes next is to try to go beyond California and see how this model holds up,” Tsai said. “This is potentially a new way of understanding how earthquakes occur,” he added.

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About the editor

Christopher McFadden Christopher graduated from Cardiff University in 2004 with a Master's degree in Geology. Since then, he has worked exclusively in the built environment, occupational health and safety and environmental consulting sectors. He is a qualified and certified energy consultant, green deal evaluator and practicing member of IEMA. Chris's main interests range from science, engineering, military and ancient history, politics and philosophy.

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