The first sign of this morning's earthquake for some North Islanders wasn't shaking ground — it was a blaring alarm from their Android phone.
The shallow 5.9 magnitude earthquake struck about 5km south of Taumarunui at 4.51am on Saturday.
It was widely felt across the North Island and generated several aftershocks, but there were no immediate reports of serious injuries or significant damage.
On social media, many people said the alert sounded before they felt the quake, while others said it arrived as the shaking began or shortly afterwards.
Some Auckland residents described receiving the notification seconds before the ground started moving, with one calling the technology "crazy accurate".
But how does it work?
How Android phones detect earthquakes
The Android Earthquake Alerts system uses the accelerometers built into millions of smartphones to detect the first seismic waves generated by an earthquake.
Known as primary, or P-waves, these travel faster than the stronger secondary (S) waves that typically cause the most noticeable shaking and damage.
This data is automatically anonymised and reported in the background to Google, and once there are enough matching reports, an alert gets pinged out to all Android users who might be affected.
Depending on the estimated strength of the earthquake, users receive one of two alerts. A 'Be Aware' notification is sent for earthquakes below magnitude 4.5, while larger events trigger a louder 'Take Action' alert designed to grab people's attention, even if their phone is on silent.
The system is not an earthquake predictor, but rather rapidly detects an earthquake after it begins and uses the difference in speed between the initial P-waves and the slower, more destructive shaking to provide advance warning to people further from the epicentre.
The technology is now available in more than 90 countries, although its effectiveness depends on how many Android devices are nearby.
People closest to an earthquake may receive little or no warning because the shaking reaches them almost immediately, while those further away can receive several seconds' notice.
It can also be less accurate in sparsely populated areas or for earthquakes that occur offshore, where there are fewer phones available to detect the initial shaking.
Civil Defence Manawatū-Whanganui said while it was a helpful system, it still had its flaws.
"It does not have the accuracy of the equipment GeoNet uses. This means it can get things wrong."
The agency also explained why New Zealand's Emergency Mobile Alert system is generally not used for earthquakes.
"By the time we would have logged into the system, written a message and decided where to send the alert, the quake would have been and gone."
Instead, emergency mobile alerts are primarily used for hazards that develop over a longer period, such as tsunamis, severe weather and flooding, where there is enough time to warn people before the danger arrives.
Officials continue to remind people that regardless of whether they receive an alert, the safest action during an earthquake is to Drop, Cover and Hold.
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