By Yimou Lee and Angie Teo

TAIPEI (Reuters) - Earthquake warning smartphone apps have surged in popularity in Taiwan due to high demand from people eager to get a few extra seconds to take cover after more than 1,300 aftershocks rattled the island in the past month following a large temblor.

Taiwan's east coast was struck by a 7.2 magnitude quake on April 3, killing 17 people.

The frequent aftershocks since then - including more than 200 tremors in a 24-hour period starting April 22 - have unnerved many.

While the government has an automated quake warning system, sent in the form of an text message accompanied by a loud alert sound meant to sound a few seconds before the shaking starts, it can be patchy. In the capital Taipei it did not sound before the April 3 quake, prompting criticism of the system.

That, along with the rising demand to stay more informed about forthcoming earthquakes, have helped a surge in popularity in privately-developed quake warning apps.

"What we offer is customised earthquake early warning. You get to set your own threshold," said Kuo Chen-yu, the 20-year-old co-founder of Disaster Prevention Information Platform, whose users jumped to nearly 370,000 from around 3,000 in recent weeks.

Lin Ruei, another co-founder of the app launched in 2022, said their app tracks fast-moving seismic waves from more than 130 seismic sensors the team installed across the island to give users early warnings. For instance, he said, the app could give users in more populated western Taiwan up to 30 seconds warning for a quake from the remote eastern coast.

"Some people might feel quite panic with just an earthquake of intensity 2, then they can download our app," said Lin, a 17-year-old student.

"If you get a 10-second early warning and you move to a secure spot, maybe it would be a life-saving tool for you."

The official, government-run warning system only goes to phone users in areas where major earthquakes with estimated intensity of 4 or above are expected, which authorities say are powerful enough to move some furniture, cause wall cracks or disrupt power or water supplies.

Wu Chien-fu, Director of the Central Weather Administration's Seismological Center, told Reuters they encourage development of alternative tools which provide innovative means for disaster response, adding authorities were aware of calls to lower thresholds for official alerts, a system designed for bigger tremors that are more likely to bring damage.

"We will consider whether to issue warnings to the whole of Taiwan for big earthquakes," Wu said.

Still, many are drawn to the privately-developed apps extra functionalities, such as automatic flashlight switch-on before the shaking starts, a countdown timer, or alerts for smaller quakes that did not trigger the government warnings - a reassurance for those who wish to stay more alerted for the frequent tremors.

"The 30-second countdown gave me enough time to mentally prepare myself, grab my survival kit, and dash to my child's room," one user wrote in the review of Taiwan Earthquake Quick Alert, currently one of the most-downloaded apps for iPhones in Taiwan which issues a countdown to the first shock waves.