As Typhoon Etau ripped northward across the central part of Japan this week, residents, office workers and pedestrians in Tokyo's Minato Ward had a surprise when their smartphones abruptly made an unfamiliar ringtone and the screen flashed an alert.

The alert, "evacuation preparation information regarding mudslide disaster," was issued Wednesday and the first of its kind for the ward, though Tokyoites may be more familiar with alerts for earthquakes or tsunami.

And it is part of a trend as mobile phone alerts against possible floods or mudslides in heavy rain are increasingly issued in various parts of the country, local government and industry sources said.