But the mobile telephone network was quickly repaired, letting survivors and relief workers communicate easily. Japan has more than 100 million mobile telephones in use in a population of 127.7 million.

Telecom giant NTT Corp., whose assistance is often requested by the government even though it is a private firm, installed traditional phone booths in shelters for earthquake victims.

Its cellular wing NTT DoCoMo set up machines to charge mobile phones.

Japanese mobile operators also have free message boards -- accessible in two clicks during a disaster -- that let people share information.

"For example, you can write, 'Everything's okay,' or 'Get in touch with us,' or any other phrase for your family to see," said an official at operator Softbank Mobile.

For those who cannot use their thumbs or do not have a mobile, a similar voice-message service for disasters exists on all Japanese telephones -- cellular or stationary -- by dialing 171.

KDDI Corp., Japan's second largest mobile operator, also offers a service providing radio control and maps in disasters. The service guides the person to the safest route by assessing the region in real time.

"The system works even if the link with the cellular network is cut off as the locations are charted out through the GPS satellite system," a KDDI engineer said.

At the request of disaster authorities, all mobile telephones sold in Japan since April have come equipped with a GPS receiver, which lets rescue workers trace where a call is coming from even if the person does not know where he or she is.

The service was already on many telephones previously, meaning that roughly 30 percent of the mobiles used in Japan can be traced by GPS, or the Global Positioning System.

NTT DoCoMo also recently came up with a special high-tech helmet to be used in times of disaster.

The multimedia helmet has a high-speed connection and GPS receiver, along with a lamp, a camera in the front and a miniature solar panel to power it.

"This equipment would let a rescue worker look automatically and in real time at what's going on, while allowing complete freedom of movement," said a researcher of the Tanizawa firm that helped develop the helmet.

Japan experiences about 20 percent of the world's major earthquakes and has developed an infrastructure to deal with them, including an elaborate system of disaster alerts.

However, the latest earthquake caused alarm as it triggered a small radioactive leak at the world's biggest nuclear plant, situated just nine kilometres (five miles) from the tremor's epicentre.

Automakers also scaled back production due to damage to a company supplying auto parts.

Softbank suffered the worst damage among mobile operators, with 93 sites affected. NTT DoCoMo said a dozen antennas went out of service and KDDI said only three were knocked out.

But all three companies said most damage was repaired within days.

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