SMS limitations
The most useful channel for relaying such information is the mobile phone. Even in developing countries, mobile phones are the most ubiquitous communications channel and subscription levels continue to grow exponentially. In addition, as mobile phones can support text messages, they have the ability to convey the level of detail needed during public warning alerts. SMS text messaging is the most widely used messaging system at present, but it is limited as a medium for public warning.
SMS is a point-to-point technology, meaning that an individual message needs to be sent to each device. This slows down or stops the process of sending messages to a required audience, and speed is essential for reducing casualties during an emergency. Moreover, as messages are sent to many people in the same location, the network can get congested by the volume of SMS messages being sent, effectively blocking any communication. Finally, SMS messages are only sent once, meaning that they cannot send appropriate guidance as people migrate into another area.
There are also privacy concerns involved with SMS as the technology relies on users registering their phone numbers with the authorities. The only way a government would be able to send text messages to all users within a specific location would be to track their movements. Besides the enormous technical challenges and related infrastructure investments required to make SMS location specific, this is obviously far too 'Big Brother' to work in practice.
The Japanese system has already been extended to mobile as the Earthquake Tsunami Warning System (ETWS) using a much more appropriate technology: Cell Broadcast.