TOKYO - In what has become a too-familiar experience in Tokyo, a crowded subway train screeches to a sudden halt. From around the car, mobile phones start to ring.

The sound is an emergency earthquake warning from an iPhone app that lets subscribers know when the quake will come, where the epicenter will be located and how strong the shaking will be. Often, seconds after the phone rings, the shaking starts.

Following the devastating March 11 earthquake and tsunami, which have left nearly 28,000 dead and missing, Japan has been hit by hundreds of aftershocks of varying strengths.

The app called Yure Kuru, or shaking coming, was developed by RC Solution Co, a Tokyo-based firm that specializes in providing disaster-related information services, such as relaying warnings or letting people confirm the safety of friends and family.

"Emergency quake warnings began several years ago, but the equipment to receive them was quite expensive," said Akira Kuriyama, president of RC Solution.

"We wondered if there wasn't a cheaper and easier way to provide this service, and we thought of smartphones."

The app, based on technology originally developed by Japan's Meteorological Agency and railway firms, was released in November 2010.

When the alarm is triggered, a window comes up on the phone screen saying that in approximately 10 seconds, there will be a quake of a certain strength. Users can decide for themselves at what strength of quake the phone will ring.

Prior to March 11, there were 100,000 subscribers, Kuriyama said. But now there are at least 1.5 million, and the company has stopped counting.

The app is symbolized by a cartoon catfish, which, according to Japanese folk belief, are said to warn of quakes.

"We're very happy to be of service," Kuriyama said. "Among other things, there still isn't a lot of understanding about the quake warning. If our app helps more people learn about this, and increases their safety, we'll be pleased."