The Pacific Tsunami Warning Center (PTWC) is one of two tsunami warning centers that are operated by NOAA in the United States. Headquartered in 'Ewa Beach, Hawaii, the PTWC is part of an international tsunami warning system (TWS) program and serves as the operational center for TWS of the Pacific issuing bulletins and warnings to participating members and other nations in the Pacific Ocean area of responsibility. It is also the regional (local) warning center for the State of Hawaii. The other tsunami warning center is the National Tsunami Warning Center (NTWC) in Palmer, Alaska, serving all coastal regions of Canada and the United States except Hawaii, the Caribbean Sea and the Gulf of Mexico.
The PTWC was established in 1949, following the 1946 Aleutian Island earthquake and a tsunami that resulted in 165 casualties in Hawaii and Alaska. After the 2004 Indian Ocean earthquake and tsunami, PTWC has extended its warning guidance to include the Indian Ocean, Caribbean and adjacent regions until regional capability is in place for these areas. These regional systems will form a global tsunami warning system once they are in operation.
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. There are never false alarms—if the PTWC issues a tsunami warning for a particular area, the tsunami is already on its way and will hit. As it takes more time for tsunamis to travel trans-oceanic distances, the PTWC can afford to take the time to make sure of its forecasts.
Depending on the seismic data, PTWC will issue the following types of bulletins:
These stations give detailed information about tsunamis while they are still far off shore. Each station consists of a sea-bed bottom pressure recorder (at a depth of 1000–6000 m) which detects the passage of a tsunami and transmits the data to a surface buoy via acoustic modem. The surface buoy then radios the information to the PTWC via the GOES satellite system. The bottom pressure recorder lasts for two years while the surface buoy is replaced every year. The system has considerably improved the forecasting and warning of tsunamis in the Pacific Ocean.
Local populations in the United States of America receive tsunami information through radio and television receivers connected to the Emergency Alert System, and in some places (such as Hawaii) civil defense sirens and roving loudspeaker broadcasts from police vehicles. The public can subscribe to the RSS feed or email alerts from the PTWC web site, and the UNESCO site. Email and text messages are also available from the USGS Earthquake Notification Service which includes tsunami alerts.