by George Curtis
The tsunami warning system was developed in 1947-48 by scientists of the U.S. Coast and Geodetic Survey and others. Since most tsunamis result from earthquakes near or under the ocean, the scientists combined seismographs, oceanography, tide gauges, observers, and communications into a system which has detected every Pacific-wide tsunami since 1948. When an earthquake occurs, seismic waves travel through the earth and are measured, and the quake is located by instruments at the Pacific Tsunami Warning Center (PTWC) at Ewa Beach, O'ahu, and other locations.
A tsunami is a series of waves 10 to 20 minutes apart generated by the heave and dip of the sea floor. As soon as the earthquake happens, tsunami waves begin to spread out at the speed of a jet liner, but are already being analyzed and queries sent to tide stations near the disturbance. (Some sea level gauges are telemetered to Honolulu and can be read directly at the Warning Center.) Since the speed of the tsunami waves can be easily calculated by knowing the depth of the ocean path they are crossing, the scientists at PTWC can check the observing stations at the estimated time of arrival of the first waves. The reports and data from the gauges are evaluated with consideration for the earthquake's magnitude and depth, the history of tsunamis from that area, and the proximity to settled coastlines. In a few minutes, the scientists in charge must make a difficult decision involving many human lives and millions of dollars, and it must be a "YES/NO" decision. If they expect waves over one meter (three feet) high to impact an inhabited coast, they will advise the civil authorities in that area; it is up to them to handle the evacuation of "low-lying areas". Because the waves travel about 500 miles an hour, they then advance the warning across the ocean to stay three or four hours ahead of the first wave, while evaluating the impact on the areas already affected to verify if the warning must be continued and expanded.
The PTWC is operated by the U.S. and involves 26 countries which may send gauge data and receive the watch and warning messages. A local system is maintained in Alaska and issues warnings on seismic data alone for events near Alaska; there is not enough time to verify the waves as evacuation must be almost immediate. The PTWC provides the same service for local events in Hawai'i. The Japanese maintain a center for events in their part of the ocean, as do the Russians, with the evaluation method dependent on the time available.
There are a few important things for Hawai'i residents and visitors to remember when there is a tsunami warning: if you hear Civil Defense sirens or see a TV message about a possible tsunami, turn on a radio and follow instructions. Don't use the telephone! Instead, check the maps in the front of the telephone book to see if you are in an evacuation zone. If you are at the beach and feel the ground shake hard, you just received nature's warning of a local tsunami. Run for high ground without waiting for sirens! If you want to read more about this application of science, see:
Dudley, Walter and Min Lee. 1998. Tsunami!. University of Hawaii Press. This book is available at the Pacific Tsunami Museum gift shop.
George Curtis is a tsunami research specialist formerly affiliated with the Joint Institute for Marine and Atmospheric Research at UH-Manoa. He lectures in Marine Science & Physics at UH-Hilo and elsewhere and is a member of the Museum's Scientific Advisory Council. George developed the tsunami evacuation maps that Civil Defense has placed in all Hawai'i telephone directories and is the Hawai'i County Tsunami Advisor.
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Last Revised August 2013