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NOUS41 KGYX 271122
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MEZ007>009-012>014-018>022-023>028-NHZ001>010-013-014-281200

PUBLIC INFORMATION STATEMENT
NATIONAL WEATHER SERVICE GRAY ME
722 AM EST WED MAR 27 2012

THE NATIONAL WEATHER SERVICE AND THE NATIONAL OCEANIC AND 
ATMOSPHERIC ADMINISTRATION HAVE DECLARED THE WEEK OF MARCH 
26TH THROUGH 30TH TSUNAMI PREPAREDNESS WEEK IN THE UNITED 
STATES. 

...MECHANICS OF A TSUNAMI WAVE...

TSUNAMI WAVES ARE GENERATED BY EVENTS THAT CAUSE SOME INITIAL 
DISPLACEMENT OF WATER.   THIS DISPLACEMENT CAN BE CAUSED BY 
THE MOVEMENT OF OCEAN FLOOR, BY UNDERWATER (OR AT THE 
SHORELINE) LANDSLIDES OR LAND SLUMPS, VOLCANIC ACTIVITY, OR 
LARGE RELEASES OF GASSES FROM THE OCEAN FLOOR.   ALTHOUGH VERY 
RARE, A TSUNAMI ALSO CAN BE INITIATED BY A LARGE METEORITE OR 
ASTEROID PLUNGING INTO THE OCEAN.

DISPLACEMENTS OF THE OCEAN FLOOR?

TSUNAMIS ARE USUALLY GENERATED ALONG FAULT LINES IN THE 
EARTH'S CRUST, TYPICALLY, IN AREAS WHERE THE CONTINENTAL AND 
OCEANIC PLATES ARE IN COMPRESSION.  AS A GENERAL RULE, THE 
CONTINENTAL PLATE RIDES OVER TOP OF THE OCEANIC PLATE.  WHILE 
THE EDGES OF BOTH PLATES ARE ENGAGED (NOT MOVING WITH RESPECT 
TO EACH OTHER), THE COMPRESSIVE FORCES THAT RESULT FROM THE 
OVERALL PLATE MOVEMENT CAUSES TENSION TO BUILD UP BETWEEN THE 
PLATES IN THE OVERLAPPING AREA (CALLED THE SUBDUCTION ZONE).  
EVENTUALLY, DUE TO INCREASING PRESSURE, THE EDGES OF THE 
PLATES DISENGAGE, ALLOWING THE PLATES TO SHIFT.  THIS 
FRACTURING AND SUBSEQUENT MOVEMENT OF THE PLATES CAUSES AN 
EARTHQUAKE AND MAY PRODUCE A TSUNAMI.  EARTHQUAKES CAN ALSO 
GENERATE TSUNAMIS BY CAUSING SUBMARINE LANDSLIDES AND LAND 
SLUMPS, ESPECIALLY NEAR THE EDGE OF THE CONTINENTAL SHELF.

MOVEMENT OF A TSUNAMI WAVES?   

ONCE GENERATED, TSUNAMIS CAN MOVE RAPIDLY ACROSS OCEANS.  IN 
OPEN OCEAN, THE SPEED AND THE HEIGHT OF THE TSUNAMI WAVES 
DEPEND ON THE DEPTH OF THE OCEAN FLOOR.  IN AREAS OF THE 
PACIFIC WHERE THE OCEAN DEPTH IS 20,000 FT, TSUNAMI WAVES ARE 
USUALLY LESS THAN A FOOT HIGH AND MOVE AT SPEEDS OF ABOUT 550 
MPH -- ABOUT THE SPEED OF A COMMERCIAL JET.  HOWEVER, AS A 
TSUNAMI WAVE ENCOUNTERS SHALLOWER WATER THE SPEED OF A WAVE 
SLOWS AND THE HEIGHT INCREASES.  IN ABOUT 300 FT OF WATER, A 
TSUNAMI WAVE WILL SLOW TO ABOUT 60 MPH AND IN 30 FT OF WATER 
THE WAVE SLOWS TO ABOUT 20 MPH. 

WHEN TSUNAMI WAVES REACH THE COAST?

AS A TSUNAMI WAVE APPROACHES THE COAST, THE SPEED OF THE WAVE 
SLOWS AND THE HEIGHT OF THE WAVE GROWS.  DEPENDING ON WHICH 
SIDE OF THE SUBDUCTION ZONE (OR SLIDE OR SLUMP ZONE) YOU ARE 
LOCATED, THE WATER ALONG THE COAST MAY INITIALLY EITHER RECEDE 
OUT TO THE SEA OR INCREASE TOWARD AND ONTO THE LAND.  THE 
RECESSION OF WATER TOWARD THE SEA COULD BE A CLUE THAT A 
POWERFUL TSUNAMI WAVE MAY FOLLOW.  IN EITHER CASE THOUGH, A 
DEADLY TSUNAMI WAVE MAY BE IMMINENT.  IN ADDITION, IT IS 
IMPORTANT TO REMEMBER THAT THE FIRST WAVE MAY NOT BE THE 
LARGEST.  TSUNAMIS TYPICALLY GENERATE A SERIES OF WAVES AND 
SOME OF THE LATER WAVES ARE OFTEN GREATER THAN THE FIRST.  
WHEN A TSUNAMI REACHES SHORE, IT MAY APPEAR AS A RAPIDLY 
RISING OR FALLING TIDE, A SERIES OF SURGING WAVES, OR EVEN A 
TIDAL BORE. REEFS, BAYS, ENTRANCES TO RIVERS, UNDERSEA 
FEATURES AND THE SLOPE OF THE BEACH ALL HELP TO MODIFY THE 
TSUNAMI AS IT APPROACHES THE SHORE.

WHILE CONSIDERABLE ATTENTION IS DIRECTED TOWARD THE FLOODING 
EFFECTS OF THE TSUNAMI, MUCH OF THE DAMAGE IS DUE TO THE FORCE 
GENERATED BY THE RAPID MOVEMENT OF WATER.  EVEN IN CASES WHERE 
NO FLOODING OF LAND OCCURS, THE RAPID MOVEMENT OF WATER IN 
CHANNELS, BAYS, AND HARBORS CAN CAUSE CONSIDERABLE DAMAGE TO 
BOATS, DOCKS AND MARINAS.  IN ADDITION, THE STRONG CURRENTS 
CAN BECOME DEADLY AND WASH PEOPLE OUT TO SEA.

$$

JENSENIUS
NATIONAL WEATHER SERVICE
GRAY, MAINE

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