Around 15 minutes after the magnitude-9 Tohoku earthquake on 11 March 2011, almost the whole of Japan jumped half a centimetre to the east. This lurch resulted from an immensely powerful seismic wave that travelled 5800 kilometres to the planet’s core and then bounced back towards the surface.

In the context of the devastation caused by the earthquake, including localised land movements of many metres and 40-metre tsunami waves that led to the meltdown of three reactors at the Fukushima Daiichi nuclear plant, 5 millimetres may seem insignificant.

But this movement took place over a distance of 3000 kilometres, nearly seven times longer than the length of the earthquake’s main rupture line and longer than any slip ever recorded.

What also makes the case unusual is the timing and the pattern, says at the University of Chicago. “We see a small 5-millimetre eastward step that happens nearly simultaneously and with similar size across almost all of Japan, without any ordinary earthquake at that exact time.”

Not only was the shift immense in its north-south extent, but its width encompassed all of Japan and beyond, into the ocean.

“It is not just a narrow ‘edge’ that moved,” says Park. “The eastward step extends at least across the whole of Japan where we have GPS stations. If we had similarly dense instruments on the seafloor, we could say more precisely how far offshore this motion extends, but on land, the shift is observed pretty much everywhere across Japan.”

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By analysing extensive GPS and seismic data recorded during the catastrophe, Park and her colleagues have figured out how such a phenomenally vast movement was triggered and why the rupture took place 15 minutes after the main quake.

Earthquakes often generate waves that travel deep into Earth’s interior and reflect off the core, but they usually become quite weak by the time they have travelled to the planet’s centre and then back up.

In Tohoku’s case, the main shock was so large that the original wave, though weakened, remained powerful enough on its return to the surface to cause the nationwide lurch, as four adjoining tectonic plates moved in unison.

“We think the vigorous shaking from the original Tohoku earthquake might have already weakened the plate boundaries, making them more susceptible to be moved when the core-reflected wave came by,” says Park.

The event demonstrates there are previously unrecognised mechanisms of destruction that can follow earthquakes, says Park. “It shows that, after a big earthquake, we might also need to be aware of potential seismic hazards due to such deep-travelling wave arrivals that can trigger more events, and over very large distances.”

More research is now needed to understand the implications of this kind of movement for other parts of the world with similar faults, says at the University of Canterbury, New Zealand.

“It shows that large earthquakes can trigger widespread, delayed fault motion minutes later, and over much larger regions than expected,” says Lee.

A drying climate in East Africa reduced the amount of water in Kenya’s Lake Turkana over thousands of years, which unleashed earthquakes and volcanoes from underneath it. This hazard of climate change could eventually affect other bodies of water around the world as rain and drought patterns shift.

Lake Turkana is often called the cradle of humanity, as fossils up to 4.2 million years old have been found there from at least half a dozen hominin species, some of which appear to have co-existed. As the lake shrank over recent millennia, those human ancestors would have had to contend not only with a drying climate, but also with greater seismic activity.

“We postulate that there would have been more frequent earthquakes and more frequent volcanic eruptions during these time intervals,” says at Syracuse University in New York. “It would have compounded the already difficult conditions that can be observed today in that area.”

Lake Turkana is located between Kenya and Ethiopia in the Great Rift valley, a place where the continental plate is slowly splitting and spreading apart. It is the largest desert lake in the world, a body of greenish, salty water ringed by sandy shrublands and windy outcrops. But nine millennia ago, the lake was even bigger and surrounded by lush grasslands and pockets of forest.

Between 4000 and 6000 years ago, the climate became drier and the water levels in the lake dropped by 100 to 150 metres. Lower water levels create less pressure on the lakebed below, which can impact seismic activity. To determine the effects of this climate change, Scholz and his colleagues identified certain sediment layers corresponding to different time periods in cores that had previously been taken from the lakebed.

From a boat, they then performed sonar imaging at 27 faults on the lakebed to see how far the same sediment layers had been displaced from each other vertically on either side of each fault. They found as the climate dried, the sides of the faults began slipping past each other more quickly, increasing at an average rate of 0.17 millimetres per year.

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“The main process is literally sort of clamping or unclamping this deformation zone, the zone of slip which results in earthquakes,” Scholz says. “A drier system and lower lake load allows it to slip more readily.”

Computer modelling suggested the reduced water mass also let more magma flow up from below the lake. One of the three volcanic islands in Lake Turkana erupted in 1888.

Scientists previously found lower sea levels increased volcanism at ocean ridges. But this is the first clear evidence of that happening around a lake, says at the University of California, Santa Barbara. “It’s almost like loosening the cork on a champagne bottle,” he says. “As you decrease that pressure, the magma is more likely to rise up in the crust and erupt.”

While increased rainfall due to climate change is now raising water levels in Lake Turkana once again, it would take thousands of years for that to significantly suppress earthquakes and volcanoes.

But assessments of seismic hazards should start considering how the changing climate might affect water levels, according to the study authors. And governments should take earthquake risk into account before they build or remove dams.

“They should put [seismometers] in before they make any huge changes,” Macdonald says.

Your phone may be among the billions of devices worldwide that already serve as an earthquake early-warning system in dozens of countries.

Since launching in 2020, Google’s Android Earthquake Alerts system has expanded to enable 2.3 billion Android phone and smartwatch users to receive alerts about seismic shaking nearby, compared with 300 million people who may get such alerts from other sources, according to a new study by researchers at Google. But the phones aren’t just delivering warnings – their sensors also help detect earthquakes.

“Billions of Android devices work together and act as mini-seismometers to create the world’s largest earthquake-detection network,” says at the University of California, Berkeley, who is also a visiting researcher at Google.

The system developed by Allen and his colleagues analyses vibrations detected by accelerometers in Android phones and smartwatches. Together, this network of sensors can indicate how large an earthquake is and which phone users are close enough to danger to receive a warning message.

Google’s system notifies people when it detects an earthquake with a magnitude of 4.5 or greater. But the system “cannot detect all earthquakes” because it requires that a large enough number of phones will be close enough to the quake, says Allen. For example, it doesn’t detect earthquakes originating from most mid-ocean ridges, although it can detect seismic events occurring tens to hundreds of kilometres offshore.

One of the greatest challenges has been to quickly and accurately determine the magnitude of each seismic event. The researchers have improved the system’s earthquake-detection algorithm over the years by developing regional models to better represent local tectonic movements and accounting for varying sensor sensitivities among different Android phones.

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Google’s global system is now about as accurate as the ShakeAlert system that covers the US West Coast and the Japan Meteorological Society’s earthquake early-warning program, says Allen. He notes that Google’s project is intended to supplement, rather than replace, seismometer-based systems like these – it even incorporates and delivers ShakeAlert warnings for people on the West Coast. “But the reality is that many earthquake-prone regions don’t have the regional seismic networks necessary to provide warnings,” says Allen.

Google’s system provides a “unique source of information” for countries without earthquake early-warning systems, says at Western University in Canada, who isn’t involved in the initiative. It also reaches more people in total, even when other national or regional alert systems are available, he says.

The system currently provides alerts to , including the US but not the UK. “We’ve generally focused on countries that have larger historical seismic risk while not having an existing earthquake early-warning solution,” says at Google.

According to the new study, which analysed the program’s performance and accuracy, the system had issued alerts for 1279 seismic events as of March 2024, with just three false alerts. Two of those false alerts involved thunderstorms and one was triggered by an unrelated mass notification event that vibrated multiple phones. The team has since updated the detection algorithm to avoid such false triggers.

Most Android devices are opted in to participate in the phone-based seismometer network and receive alerts about nearby earthquakes by default, although users can change both of these settings. In a Google user survey, more than one third of the system’s participants received phone alerts before they felt any shaking – and most people who got alerts described them as very helpful.

If Android phone users do remain signed up for alerts, these come in two types. More urgent Take Action alerts are designed to spur people to take protective actions, such as “drop, cover and hold on”. But they often provide just a few seconds’ worth of early warning because they don’t go out until the system predicts strong shaking. In contrast, the less intrusive Be Aware alerts, which provide more general information, can arrive tens of seconds before phone users feel an earthquake.

“The physics of earthquakes dictates that there will be less warning prior to stronger shaking than weaker shaking,” says Stogaitis. “But we continue to explore modifying our alerting strategy to improve the warning times in future earthquakes.”