Smartphone earthquake warnings have become a hot topic in China as people swarm to embrace mobile technologies for more timely safety alerts, after a magnitude-5.5 earthquake hit Pingyuan county in Dezhou, Shandong province, on Sunday morning.

In Apple's iOS China app store, for instance, multiple earthquake warning apps now rank among the top three most downloaded free apps.

Meanwhile, Chinese smartphone brands using original or modified Android systems, such as Xiaomi, Huawei, Oppo and Vivo, said the function of earthquake warnings is available on their smartphones, and users can directly turn them on, without the need to download an app.

The companies said the earthquake alert is a built-in function, and is a result of coordination with the Institute of Care-Life in Chengdu, Sichuan province, which has developed such a warning system for handsets.

The comments came after many people said on social media that they were awakened from their sleep by shrill sounds or light signals from their smartphones, as the temblor hit Pingyuan county.

Such sounds and flashes serve as warnings which indicate the epicenter, depth and magnitude of the quake along with the expected intensity smartphone owners could expect to feel at their current location. The alert also contains a countdown of when the seismic wave is expected to hit. As the countdown came to zero, tremors occurred which lasted less than a minute, they said.

Such a function first became available on Xiaomi's smartphones in June 2021 when the company partnered with the Institute of Care-Life to announce that China's first smartphone earthquake monitoring and warning network was available.

Wang Tun, head of the institute, said traditional earthquake warning networks install seismic monitoring devices, which are spaced about 15 kilometers apart, in earthquake-prone areas, providing warnings to the public several seconds to tens of seconds before potentially destructive seismic waves arrive. The mobile earthquake monitoring and warning network uses built-in sensors on mobile phones to replace traditional monitoring instruments for detection and warning.

Smartphones are equipped with built-in acceleration sensors that can sense vibrations. If the phone detects information that it believes may be an earthquake, it will send a message to the warning center server. The warning center server automatically determines whether an earthquake has occurred based on the information from many phones, and calculates the earthquake warning magnitude, epicenter location and time of occurrence in seconds when it is determined to be a temblor. It also sends warning messages to affected populations around the epicenter, Wang said.