GaijinPot has a pretty nice little article about those emergency alerts you might get on your mobile phone while in Japan. Mostly has to do with Earthquakes and Tsunamis. Having experienced the “big one” back in March, 2011, I wish we would have had smart phones and the like to get us through that.
One of the scariest alerts you’ll hear in Japan is hands down the infamous earthquake alarm. Blasted on cell phones throughout the potentially affected areas, the “Earthquake Early Warning” (緊急地震速報) notice has had my heart leap out of my chest a good number of times.
In the article they run down the messages you might see, and how to translate them. Pretty important stuff to know.
I highly recommend installing and configuring an app like Yurekuru Call, available for iOS and Android. It will alert you to earthquake activity, often before you feel it yourself. You can configure it to only alert to events over a certain level, within a certain range of your phone, etc.
Even if you’re not in Japan, you can still set it up and have it monitor for activity around a given location, such as your planned hotel or city you’ll be staying in. You can fine tune the configuration so its not constantly going off for level 1 events etc.