FReeper “American In Tokyo” had quite a descriptive firsthand post of the quake. Said that the authorities issued a warning a full two minutes before it struck.
From here:
"The vibration of an earthquake is essentially a form of sound wave. The fastest waves from an earthquake, the P-waves, travel at 3-5 miles/s (5-7km/sec). The slower S-waves travel at about 2-3 miles per second. Clearly much faster than sound through air. The fact that these vibrations travel at different speeds depending on what they are travelling through has allowed us to map the interior of the earth by noting the time of arrival of the same earthquake vibration at seismographs located all over the planet and comparing the data."
The earthquake was 240 miles from Tokyo. If the waves traveled 2 miles/sec, then it would take 120 seconds for them to reach Tokyo, hence the opportunity for a warning.