<P> The PTWC was established in 1949, following the 1946 Aleutian Island earthquake and a tsunami that resulted in 165 casualties in Hawaii and Alaska . </P> <P> The PTWC uses seismic data as its starting point, but then takes into account oceanographic data when calculating possible threats . Tide gauges in the area of the earthquake are checked to establish if a tsunami has formed . The center then forecasts the future of the tsunami, issuing warnings to at - risk areas all around the Pacific basin if needed . </P> <P> Depending on the seismic data, PTWC will issue the following types of bulletins: </P> <Dl> <Dt> Tsunami Information Bulletin </Dt> <Dd> At this time, though a threat exists, there is no evidence that a tsunami is making its way across the Pacific . </Dd> <Dt> Tsunami Watch </Dt> <Dd> PTWC has determined the earthquake may very likely have created a tsunami and is advising parties to be alert as PTWC awaits tide data to support tsunami generation . A watch may be upgraded to an advisory or a warning . </Dd> <Dt> Tsunami Advisory </Dt> <Dd> PTWC has determined the earthquake has created a tsunami with strong currents or waves dangerous to those in or very near the water . </Dd> <Dt> Tsunami Warning </Dt> <Dd> PTWC finds conditions serious enough to issue immediate concern to parts of the Pacific . The message will include approximate arrival times for various parts of the Pacific . </Dd> </Dl>

What magnitude of an earthquake will trigger the pacific tsunami warning system