Tsunami warning system

Crescent City, Calif.,where 11 died in a tsunami in 1964, was hit last week by a wavecaused by a quake off the Russian coast. This time no one died andthere was little damage when a surge estimated at about 5 feet flowedinto the harbor. The earthquake was monitored by the Deep OceanAssessment and Report of Tsunamis, or DART, a system of sensors andbuoys that monitors earthquake and tsunami activity. An alert wasissued last week, but it was canceled more than five hours before thetsunami struck. DART is part of the U.S. contribution to the GlobalEarth Observation System of Systems, designed to give early warning oftsunamis in the Pacific and Caribbean basins. There are 15 suchmonitoring stations operating, with 24 more to be online by mid-2007.

1. Quakestrikes, forcing a water surge: When a subduction-zone earthquake occurs, onetectonic plate forces its way under another, with the upper platerising violently. When this occurs on or near the ocean floor, thewater column above rises and moves outward as a tsunami. | 2. Sensor records surge: As the water column moves through the ocean, its pressure isrecorded by a sensor on the seafloor as an abnormal event. | 3. Data are sent to a buoy,then on to a satellite: Data are transferred by acousticmodem to a surface buoy floating nearby. | 4. Satellite links with ground: Pressure data aretransferred to a geostationary satellite linked to the DART warningsystem. | 5. Ground centers notified: Data are sent to tsunami warningcenters in Alaska and Hawaii, which notify a network of operations centers. | 6. Public warning issued: Local emergency preparedness agencies mobilize as awarning is issued for low-lying coastal areas that a large ocean surgeis imminent.

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Seafloor pressure recorder: Pressure data on the seafloor are reported every15 minutes. When the sensor detects an event, data are transmittedevery 15 seconds initially and then at longer intervals.n Transducer converts electronic data into anacoustic signalfor transferto the buoy.| Surface buoy: A buoy is anchored loosely to the seafloor with aswivel-and-chain system that allows it to move with the swell andtides.It can be deployedin very deep water.n Each buoy contains transducers that solicit and receive data fromthe pressure recorder. Buoys are linked to satellites that maintain a constant position.| The network: There are now 15 units in operation -- including five in the Gulf of Mexico-Caribbean-Atlantic region -- with 24 more planned. Each unit iscomposed of a bottom pressure recorder and a surface buoy. They arearrayed along the edge of tectonic plates where seismic activity iscommon.n Under separate programs, sensors are being deployed in the Indian Ocean and off Chile.

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Recent regional explainer graphics are available atlatimes.com/localgraphics

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*--* Run-up Date Location in ft.* Location Magnitude April 1, 1946 Half Moon Bay 10 Gulf of Alaska 8.1 May 22, 1960 Santa Monica 4.6 Chile 9.5 Nov. 29, 1965 Catalina 4.6 Hawaii 7.2 Island March 28, 1964 Crescent City 20 Gulf of Alaska 9.2 Nov. 4, 2000 Pt. Arguello** 23 unknown - Dec. 26, 2004 Santa Monica 1.4 Sumatra, 9 Indonesia Nov. 15, 2006 Crescent City 5 Kuril Islands, 8.1 Russia

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*Water height above sea level. Some are estimates. **Northern Santa Barbara County

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Sources: National Oceanic and Atmospheric Administration, U.S.Geological Survey, UNESCO