Can We Predict Volcanic Eruptions?

What on Earth Is Shear-Wave Splitting? (And Why Should We Care?)

Imagine shouting into a canyon and hearing your voice split into two echoes that arrive at slightly different times. Weird, right? That’s a bit like what happens deep in the Earth when shear waves – a type of seismic vibration – travel through cracked rock. In normal, uncracked rock, a shear wave just zips through with a single clean “voice.” But in rock full of tiny cracks or aligned mineral grains, that single wave’s vibration splits in two, each part shaking in a different direction. One part (the “fast” wave) slips through more easily and arrives first; the other part (“slow” wave) lags behind. This phenomenon is called shear-wave splitting (fancy term: seismic birefringence), and it’s like listening to the Earth in stereo.

Why does this splitting happen? Think of the Earth’s crust as a piece of wood with a grain. If you try to split wood along the grain, it cracks open fast and easy. But swing the axe across the grain, and it’s tougher and slower. Similarly, when the ground has many parallel cracks (the “grain”), a shear wave vibrating along those cracks cruises through quickly, while one vibrating perpendicular to them gets dragged down by having to break through more barriers. So the wave effectively splits into two paths – one aligned with the cracks (zoom!), and one against them (slowpoke). By measuring the tiny time gap between the fast and slow waves, and the direction each wave is vibrating, scientists get a kind of X-ray of the rock’s internal cracks. It’s as if the Earth’s crust tells us, “Psst – my cracks are mostly pointing this way, and there are this many of them.”

Now, why should we care about cracks and split waves? Because volcanoes care – a lot. Volcanoes are built on cracks and fractures in the Earth. When magma (molten rock) starts moving and pressure builds under a volcano, it changes the stress on the surrounding rocks. Think of stress like the pressure you’d feel in a squeezed balloon. The rocks respond by opening cracks in certain orientations (and maybe closing some others). The result? The “grain” of the rock – those crack patterns that shear waves react to – can shift or intensify. If we’re listening with our seismometers, we might hear the volcano’s whisper change tone: the split shear waves arrive with different timing or a new orientation. In short, the way shear waves split can tip us off that a volcano is feeling “stressed out” and getting ready to do something big. It’s a subtle signal, but it could be a lifesaver for eruption forecasting.

Cracks as Clues: How Anisotropy Reveals Volcano Stress

The term seismic anisotropy sounds technical, but it just means that seismic waves travel faster in some directions than others through a material – exactly what happens with aligned cracks. You can picture the rock under a volcano like a crowd of tiny cracks acting in unison. Under normal conditions, those cracks might be oriented randomly or just following the regional stress (like how tectonic plates push). But as a volcano’s magma chamber inflates or a magma dike (a finger of magma) pushes its way through rock, the stress pattern changes. It’s a bit like wind blowing through a field of grass – the blades (cracks) all bend and align in the wind’s direction. In the rocks, cracks tend to align perpendicular to the strongest squeezing force (or along the direction of stretching). This reorientation and opening of cracks is the volcano’s way of relieving pressure. And when those cracks realign, our shear waves suddenly get a new “preferred path.” The fast wave might rotate to follow the newly aligned cracks, and the time gap between fast and slow waves might grow if the cracks open wider or multiply (more gaps to slow the slow-wave down).

By tracking shear-wave splitting over time – say, day after day at a restless volcano – scientists are essentially eavesdropping on the crust’s changing stress. It’s not unlike listening to a building creak when it’s under strain. Before an eruption, magma and hot fluids moving underground exert pressure on the rocks, sometimes creating swarms of tiny earthquakes. Those quakes are the “voices” that send out shear waves for us to analyze. If we notice that, over weeks or even just hours, the splitting pattern shifts – perhaps the fast wave’s direction twists a bit, or the delay between fast and slow grows – it’s a strong hint that the stress regime is evolving. The cracks might be opening up more (signaling increased pressure) or swiveling into a new orientation (perhaps pointing toward where magma is headed). This is like the volcano’s heartbeat or breathing pattern changing. It provides insight beyond the usual counting of earthquakes or measuring ground deformation: it tells us about the alignment of the cracks and the stress build-up deep below.

Crucially, scientists have found that it’s often changes in anisotropy (those crack-driven wave speed differences) that matter most. A stable anisotropy might mean the volcano’s stress is steady, even if it’s high. But a sudden jump or rotation in anisotropy can be the telltale sign of a volcano that’s crossed a critical threshold – its internal “grain” is being rewoven by pressure, perhaps foreshadowing that rock is about to break and magma is about to erupt. In other words, shear-wave splitting gives us a direct line on the orientation of stress and cracks, which are the architecture of how a volcano prepares to erupt.

Let’s make it more tangible with two real Icelandic volcano stories – one that merely whispered, and one that practically shouted, through their crack patterns.

Askja 2007: The Quiet Whisper of a Restless Volcano

In early 2007, the Askja volcano in central Iceland gave a murmur of activity. Askja is a brooding caldera volcano – remote, often snow-covered, with a cold blue lake in its crater. It’s famous for a colossal eruption back in 1875, but in 2007 it wasn’t trying to repeat that drama. Instead, deep under Askja, magma was on the move without ever making it to the surface. Think of it like an attempted heist that never quite broke into the daylight – magma sneaking upward, cracking rock as it went, then stalling out before erupting. This kind of underground magma injection is called an intrusion, and it often shows up as swarms of small earthquakes. Sure enough, in 2007, seismometers detected a flurry of tremors under and around Askja, especially near a spot east of the volcano called Upptyppingar. It was as if Askja cleared its throat, but then went quiet again.

So what did the shear-wave splitting “ears” hear during this quiet unrest? Surprisingly, not much change. Monitoring stations around Askja didn’t record any dramatic shifts in the splitting of shear waves during that period. The fast waves kept going in more or less the same direction they always had, and the delay between fast and slow stayed about constant. In plain language, the cracks underground hadn’t reoriented significantly or grown dramatically during this episode. It was a seismic whisper, not a shout.

Why would that be? Scientists suspect it’s because the stress change was modest. The magma might have been opening some cracks, but perhaps not enough of them, or not over a wide enough area, to alter the overall crack pattern that the seismic waves were sampling. It’s a bit like only a few people in a crowd started to move – not enough for the whole crowd’s pattern to shift. Or consider a small gust of wind that rustles some grass but doesn’t bend the whole field. Askja’s 2007 intrusion was likely below the threshold needed to flip the seismic anisotropy in a big way. And indeed, since no eruption actually happened, it seems the volcano relieved its pressure gently. The takeaway is powerful: if shear-wave splitting stays steady, the volcano might just be letting off steam (or magma) quietly, with no major eruption on the immediate horizon.

This isn’t to say Askja was totally quiet – it did produce earthquakes, and other instruments may have seen slight ground deformation. But in terms of the shear-wave splitting “language,” Askja spoke in the same tone as usual. In fact, volcanologists can take some comfort (or caution) from that: a lack of anisotropy change could mean the volcano’s plumbing is just gurgling normally, not undergoing a major rupture. In 2007, Askja’s cracks remained more or less calmly aligned in their old pattern, and the volcano went back to sleep. It was a precursor without punch. From a monitoring perspective, this was an important baseline case: a little volcanic unrest that did not produce the cracking signature of an impending eruption.

Bárðarbunga 2014: When the Earth Shouted in Two Voices

Now contrast that with the saga of Bárðarbunga in 2014 – a volcano that decided it was time for a big show. Bárðarbunga is a huge volcano buried under Iceland’s Vatnajökull glacier. In August 2014, it started to rumble in a way no one could ignore. Thousands of earthquakes started shaking the region, and within days it became clear that magma was forcing its way out of the caldera, ripping open a 48-kilometer-long crack (a dyke) through the crust toward the northeast. This underground magma highway eventually burst open at the surface beyond the glacier, producing the spectacular Holuhraun fissure eruption – the largest Icelandic eruption in over two centuries. It was as if Bárðarbunga let out a roar that echoed across the highlands.

So, what did the shear-wave splitting technique reveal during this dramatic buildup? Here, the volcano’s whisper turned into a shout. As the dyke propagated underground – essentially prying the crust apart like a giant crowbar – the pattern of cracks in the crust changed on a large scale. Right before and during the eruption’s onset, seismologists observed that the splitting of shear waves intensified significantly. In fact, the difference in arrival times between the fast and slow waves roughly doubled compared to the calm before all this started. That’s a huge change – it means the crust became markedly more anisotropic (more “directional”) as the eruption approached. It’s like the Earth’s internal “grain” got stronger or more pronounced; many cracks were opening wider or new cracks were forming, all aligned by the stress of the intruding magma.

Not only did the time delay grow, but the orientation of the fast waves – remember, they align with the crack direction – pivoted to a new direction, pointing toward the path of the magma intrusion. Basically, as the magma blade cut northeast, the surrounding rock’s stress field also pointed northeast, and cracks aligned accordingly. The shear waves obligingly followed suit: their fast component lined up with those NE-SW cracks, almost as if drawing an arrow on our seismographs showing which way the magma was going. This was an amazing validation of the method: even before lava hit the surface, the Earth’s crust was telegraphing the magma’s movement through these subtle seismic signals. It’s the kind of thing that gives geologists goosebumps – the planet was talking to us, if we knew how to listen.

To put it in metaphor, during the 2014 crisis Bárðarbunga’s seismic anisotropy was like a choir suddenly singing in a new key. The “notes” (waves) stretched farther apart in time and reoriented in unison, reflecting the huge stress of magma forcing its way out. Contrast this with Askja 2007’s choir humming the same old tune despite a bit of commotion. In Bárðarbunga’s case, the changes in shear-wave splitting were not only detectable – they were dramatic. And importantly, they occurred in the lead-up to the big eruption, providing a potential early heads-up. Indeed, scientists noted these splitting changes just before and during the eruption’s onset. It was as if the volcano, under immense pressure, cracked enough to fundamentally change its “voice” – a clear signal that an extraordinary event was underway.

Tuning In for Better Volcano Forecasts

These two Icelandic episodes – the quiet Askja whisper and the loud Bárðarbunga shout – highlight why shear-wave splitting is so exciting for volcanologists. Traditional monitoring methods like counting earthquakes, measuring ground swelling (inflation), and checking gas emissions are all crucial, but they don’t always capture the full picture of what’s happening with stress and cracks underground. Shear-wave splitting offers a fresh perspective, almost like adding a new sense to our volcano monitoring toolkit – the ability to feel which way the stress is pushing and how intensely.

The potential of this method for eruption forecasting is huge. Imagine being able to detect not just that “something’s rumbling” beneath a volcano, but also that “the internal cracks just realigned dramatically north-south and opened up wider in the last 24 hours.” That kind of information could tell us, for example, that magma is pushing in a certain direction or that pressure has reached a tipping point. It’s a bit like hearing the creaking of a dam right before it breaks – a qualitative change in sound that signals an imminent release. In practical terms, if monitors around Bárðarbunga had real-time shear-wave splitting analyses in 2014, they might have caught the crack signature of the dyke propagation as it happened, possibly improving warnings about where the eruption would occur (since the cracks “pointed” northeast) and how intense it was likely to be (since the splitting intensified so much).

Scientists are now working on making this technique more automated and real-time. The idea is to have computers constantly listening to local earthquakes around a volcano and flagging any changes in the splitting pattern. Of course, it’s not a magic crystal ball – volcanoes are noisy, complicated systems. Sometimes cracks might change due to other factors (like rainfall, or small tectonic faults slipping), and not every eruption will have a neat splitting signal beforehand. False alarms and missed signals are challenges to overcome, just as they are with any monitoring method. But the evidence so far – including the stark contrast between Askja’s minor 2007 episode and Bárðarbunga’s major 2014 eruption – suggests that when a volcano is really gearing up for a significant eruption, the shear-wave splitting signal can be a giveaway. It’s especially promising for catching those big, dangerous eruptions that we most want advance warning for. In the 2014 Iceland event, the fact that anisotropy doubled and the crack orientation swung so clearly was a neon sign that “this is the big one.” If we can reliably catch such signs in the future, scientists could raise the alert earlier or with more confidence.

Perhaps one of the most exciting aspects is how accessible this listening technique is. It doesn’t require new fancy satellites or expensive new drills into the volcano – it leverages existing seismic networks (the same seismometers that detect earthquakes) and clever data analysis. It’s like discovering you had a hidden feature on your phone all along. By simply “tuning” our analysis a bit differently, we gain a whole new channel of information. And as computing power and algorithms improve, our ability to discern these subtle crackly whispers will only get better.

In the end, shear-wave splitting is letting scientists listen to the language of cracks inside a volcano. It turns out that language can be incredibly rich in information. In Iceland, it translated to understanding when a volcano was simply murmuring versus when it was about to scream. For communities living near volatile volcanoes, that difference is everything. A volcano’s whisper, as captured by split seismic waves, might soon become a key part of a chorus of warnings that help us act in time – proof that sometimes the smallest signals can speak loudest when it comes to the restless power of the Earth.

Journal Reference:

1. Michael Kendall, Toshiko Terakawa, Martha Savage, Tom Kettlety, Daniel Minifie, Haruhisa Nakamichi, Andreas Wuestefeld. Changes in seismic anisotropy at Ontake volcano: a tale of two eruptions. Seismica, 2025; 4 (1) DOI: 10.26443/seismica.v4i1.1101