The function of earthquake warning on smartphones is available on most smartphones in China, but users may have to manually turn them on, service workers of various phone brands said.

This function was activated for many people in North China on Sunday morning when a magnitude-5.5 earthquake hit Pingyuan county of Dezhou, Shandong province. The quake led to minor injuries of nearly 30 people and disruption in the county's utility supply and transportation.

Many people said on social media that they were awakened from their sleep by sharp sounds or light signals from their smartphones, which they found was warnings on the epicenter, depth, magnitude of the quake and the intensity they would felt at their own place. The alert also contained a countdown of when the seismic wave is expected to hit. As the countdown came to zero, tremors kicked in and lasted less than a minute.

There are also many people who simply haven't felt the tremor and slept normally through the night. They were left surprised and baffled when they saw the news, and "mobile phone earthquake alerts" became a trending topic on various social media platforms.

The earthquake warning function works by taking advantage of the time gap between two types of seismic waves. The system receives the faster, less-damaging vertical seismic wave, calculates relative parameters and sends an instant alert to users, who may find shelter or brace for tremors before the slower, more-destructive horizontal wave arrives. The time gap, usually several seconds to about a minute, depends on the distance between the receiver's location and the epicenter.

Domestic brands using original or modified Android systems, such as Xiaomi, Huawei, Oppo and Vivo, all have the alert as a built-in function as they were in cooperation with the system developed by the Institute of Care-Life in Chengdu, Sichuan province. But sometimes they were not activated by default, so users may have to manually activate the function in the settings.

Users of iPhones, on the other hand, have to download an earthquake alert app in the app store to use the function, as the built-in function in iPhones isn't hooked with data providers.