HEATQUAKES

Climate change isn't just heating the planet, it's shaking the Earth's foundations

Astonishingly, earthquakes shake the US state of California almost 9,000 times a year, on average – that’s about once every hour. California’s official nickname is the golden state, harking back to the mid-19^th century gold rush that saw its population explode, in just four years, from 14,000 to a quarter of a million. But if you have ever been lucky enough to visit, and felt the ground move beneath your feet, I am sure you will agree that ‘the earthquake state’ is a far better fit. None of this should really be a surprise given that California hosts the great San Andreas Fault and its many offshoots, marking the contact between two of the world’s great tectonic plates – the North American to the east and the Pacific to the west.

Rather than for its earthquakes, California has attracted the world’s attention, in recent years, more for its devastating wildfires, biblical deluges, and flash floods, all super-charged by global heating and its disruption of our once stable climate. The news, then, that such extreme weather can also promote earthquake activity is far from welcome in one of the planet’s biggest seismic hotspots.

When we think about climate change, if we think about it at all, it is usually in terms of how the atmosphere and oceans are heating up. The idea that it can also affect the solid Earth beneath out feet seems almost laughable. Nonetheless, it’s true. I have been researching for decades how the climate can drive deadly geological phenomena, like earthquakes, tsunamis and volcanic eruptions, and the evidence, both from the present day and the distant past, is absolutely clear. The latest piece of research, by the Swiss Seismological Service (SSS), and published this summer, links swarms of small tremors beneath the Mont Blanc massif in the European Alps, to rapid thawing of ice and snow during a heatwave in 2015. Percolating downwards, the extra water eventually found its way into a major fault zone that slices through the 12km-long Mont Blanc Road Tunnel, lubricating it and causing it to shift sufficiently to trigger a burst of low-level seismic activity. The occurrence of small tremors has since remained elevated, substantially hiking the risk of bigger future quakes. As global heating continues to drive longer and more intense heatwaves, meltwater sourced by accelerated glacier melting and the thawing of permafrost, can be expected to drive up seismic activity across the world’s high mountain ranges, and across the great tracts of permafrost in Canada and Siberia.

As well as raising concerns amongst those who live in the Mont Blanc region, the Swiss research also holds lessons for any town or city underpinned by geological faults that have spawned big quakes in the past; think Tokyo in Japan, and San Francisco and Los Angeles in California. None of these are in mountainous terrain, and therefore meltwater is not an issue, but there are other ways and means by which a changing climate can increase the rate of water infiltration into the faults that threaten these cities. Tokyo, for example, is under growing threat from typhoons that climate change is making slower and wetter. As a consequence, storms now dump more rainfall along their path, increasing the amount available for percolating into the ground and, ultimately into active fault zones.

Tokyo is awaiting a ‘big one’ with some trepidation. The chance of a serious quake hitting the Japanese capital in the next 30 years or so, is thought to be as high as 70 percent, so anything that can increase this probability is a pretty big deal. The big worry is not, as in the Alps, that lubricating water will trigger swarms of little quakes, but that the infiltration of water into a fault that is teetering on the edge of rupturing will set off the ‘big one’. I have a seismologist colleague who is fond of warning that all that is needed to trigger a major earthquake, at a fault that is locked and loaded, is the pressure of a handshake. The tiny force exerted by the percolation of water into a fault zone may just be sufficient to provide this.

California is awaiting the next ‘big one’ too – both in the north, in the San Francisco Bay area – and further south in the vicinity of Los Angeles. The USGS (US Geological Survey) reckons that there is a more than a 70 percent chance that a quake of magnitude 6.7 or bigger, capable of causing widespread damage, will strike in the Bay Area in just the next 18 years; the Hayward Fault posing the biggest threat. Southern California has an even greater level of earthquake risk, in fact the highest in the entire country, and more than 300 faults in the region have the potential to trigger serious quakes. Here, the San Andreas itself, presents the greatest threat, with the likelihood of a magnitude 7.5 quake in the next three decades estimated at more than 1 in 3. Whenever it happens, such an event is predicted to take around 2,000 lives and cost a colossal $200 billion in damages.

Given that a number of faults in the state are pretty much primed and ready to go, the concern is that climate change might provide the extra impetus that advances the timing of the next big quake. In recent years, California has taken a real hammering from what are known as atmospheric ‘rivers’. Sometimes called ‘rivers in the sky’, these are weather phenomena thousands of kilometres long and a few hundred kilometres wide, which carry prodigious volumes of water vapour, adding up to 15 times the flow of the Mississippi River. Atmospheric rivers are not uncommon across California, including the so-called Pineapple Express that draws moisture all the way from Hawaii, but in recent years they have been suped-up, dumping enormous amounts of rain and snow across the state. Climate change is also making such ‘rivers’ bigger and wetter, while the strongest – known as category fives – are becoming more common. In February 2024, an atmospheric river brought 48 hours of continuous rainfall to southern California, causing landslides and mudflows, knocking out power to almost a million people, and leading to flash flooding across Los Angeles, where a state of emergency was declared. This February too, saw a barrage from more atmospheric rivers across the region, bringing flooding and loss of life.

A mix of rainstorms and active faults on the edge of rupture is not exactly a recipe for seismic catastrophe, but the conspiracy certainly has the potential to elicit a major earthquake through fault lubrication. Furthermore, there is now convincing evidence – from the other side of the world – linking rainfall and big seismic shocks. Like California, Taiwan is no stranger to earthquakes, so the magnitude 7.6 Chi-Chi quake that struck central parts of the country in September 1999 was hardly a surprise. The event was both devastating and lethal, destroying more than 50,000 buildings and taking 2,500 lives. It also happened just three years after Super-typhoon Herb dropped a couple of metres of rain across the region. On its own, this could simply be coincidence, but at least four other big Taiwan quakes in the last half-century followed in the wake of especially wet storms. On top of this, it seems that earthquakes of magnitude 6 or higher, that have affected the country, are five times more likely to happen within four years of a major storm than otherwise.

There is, however, a twist to this story. Because of the big time lag between storms and quakes, it is unlikely that quickly percolating rainwater lubricating faults is the link. Instead, it has to be something slower acting. One proposal is that the triggering of thousands of landslides by the torrential rainfall, and the removal of the debris by erosion, causes the weight bearing down on faults below to be diminished, allowing them to slip more easily. The weight unloading is tiny, but it’s all about that pressure of a handshake thing again. A similar connection between quake and rainstorm is recognised on the Caribbean island of Haiti, where a devastating earthquake in 2010, which took at least 160,000 lives, followed in the wake of a succession of four very wet hurricanes a couple of years earlier.

California is in a very similar situation too, with the torrential rains brought by a procession of atmospheric rivers promoting landslide and mudflow formation across the state; the situation aggravated by the huge areas of bare ground exposed by the catastrophic wildfires of recent years, which are also driven by global heating. As such, any fault that is ready to rupture faces a double whammy of percolating rainwater and unloading due to landslides and erosion, either or both, of which, could set it off. The thing is, however, when the next big one does happen – either beneath the Bay area or LA – we will likely never know whether one or other was a contributing cause or not; whether climate change played a role or whether the timing of the quake was entirely down to geological processes.

And this raises an important point about the general influence of a changing climate on seismic activity. Broadly-speaking, it doesn’t have the capacity to cause additional quakes, just the potential – via that extra little nudge – to bring forward the timing of an event that was going to happen at some point anyway. So, don’t expect to see a massive up-tick in the numbers of big earthquakes, or even any increase that can be distinguished from the normal background. Notwithstanding this, however, there are some parts of the world where the seismic response of the solid Earth to our rapidly changing climate is clear and present, and this is where major ice masses are undergoing wholesale melting. Alaska is one such region, where an increase in seismic activity has followed the loss, in places, of a vertical kilometre of ice in the last century or so – the unloading of this enormous weight from the crust allowing faults below to slip more easily.

Looking ahead, Greenland is by far the biggest concern in this regard. Seismically-speaking, the planet’s biggest island is ominously quiet; not because there are no faults there, but because the huge weight of the ice above is stopping them slipping. But this is unlikely to last as the 3km-thick Greenland Ice Sheet (GIS) – which holds sufficient water to raise global sea level by fully seven metres – melts ever faster. Since the early 1990s, the GIS has lost an astonishing six trillion tonnes of ice, generating so much meltwater that it would cover the entirety of the United States to a depth of half a metre. As melting continues to cause the load on the faults below to diminish, so it would be no surprise at all if the shaking started. Any faults beneath the ice would have been accumulating strain and winding themselves up for many thousands of years, so we are talking about the possibility of some serious earthquakes on the horizon.

A little less than 11,000 years ago, the loss of Greenland ice mass at the end of the last ice age triggered a quake as big as magnitude 8.3 off the southern tip of the island, modelling of the event suggesting that it generated a major tsunami that sent waves up to seven metres high crashing into the coastlines of Canada and the UK. Something similar happened a bit more than 8,000 years back, when a massive quake in Norway, triggered by the melting of the Scandinavian Ice Sheet, promoted one of the world’s biggest submarine landslides that, in turn, send a huge tsunami into the North Atlantic, this time with waves more than 20m high.

Atlantic tsunamis on the scale of those we see today only within the Pacific Ring of Fire is a shocking and unlooked for consequence of global heating. The reality, however, is that the all-pervasive nature of resulting climate breakdown means that nothing and nowhere is immune, and that includes the Earth beneath our feet.

A version of this article was published in the September 2025 issue of BBC Science Focus magazine.