TOKYO, Oct. 16 -- Imagine your phone suddenly blaring a warning that a catastrophic magnitude 9.9 earthquake is about to strike.

That's exactly the sort of alarm that sounded across parts of Japan this afternoon, when the country's sophisticated earthquake early warning system mistakenly forecast one of the most powerful tremors in recorded history.

The only problem? The earthquake was nowhere near as powerful as predicted.

The Japan Meteorological Agency (JMA) issued an emergency alert at 12.40pm local time, warning of potentially violent shaking across northern, eastern and central Japan.

But just eight minutes later, the warning was withdrawn after officials discovered that their system had dramatically overestimated the earthquake...