§In the deep ocean the tsunami speed may exceed 600 km/s; thus, the need for rapid data handling and communication becomes obvious. Because of the time spent in collecting seismic and tidal data, the warnings issued by PTWC and ATWC (headquarters for international warnings) cannot protect areas against local tsunamis in the first hour after generation; for this purpose, regional warning systems have been established in some areas.

§The regional systems generally have data from a number of seismic and tide stations telemetered to a central headquarters. Nearby earthquakes are located, usually in 15 minutes or less, and a warning based on seismological evidence is released to the population of the area. Since the warning is issued on the basis of seismic data alone, watches or even warnings will occasionally be issued when tsunamis have not been generated. Since they are issued only to restricted area and confirmation of the existence or nonexistence of a tsunami is rapidly obtained, dislocations of populations are minimized. To limit the number of agencies to be contacted, warnings are generally issued to only one agency in each country, territory, or administrative area.

Citation source: http://www.drgeorgepc.com/TsunamiFAQ.html