Public Information Statement
Issued by NWS Caribou, ME

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PUBLIC INFORMATION STATEMENT
NATIONAL WEATHER SERVICE CARIBOU ME
128 PM EDT TUE MAR 29 2016

...TSUNAMI PREPAREDNESS WEEK IN MAINE...

THE NATIONAL WEATHER SERVICE AND THE NATIONAL OCEANIC AND
ATMOSPHERIC ADMINISTRATION HAVE DECLARED THE WEEK OF MARCH 28TH
THROUGH APRIL 1ST TSUNAMI PREPAREDNESS WEEK IN MAINE, AS WELL AS IN
THE REMAINDER OF THE COASTAL UNITED STATES.

TODAYS TOPIC...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 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 TSUNAMI WAVES

ONCE GENERATED, TSUNAMIS CAN MOVE RAPIDLY ACROSS OCEANS.  IN THE
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 YOUR LOCATION WITH
RESPECT TO THE SUBDUCTION ZONE (OR SLIDE OR SLUMP ZONE), THE WATER
ALONG THE COAST MAY INITIALLY EITHER RECEDE OUT TO THE SEA OR SURGE
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 OR OCCURING.  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.

PLEASE STAYED TUNED-IN THIS WEEK AS WE PROVIDE ADDITIONAL
EDUCATIONAL MATERIAL ON THE TSUNAMI THREAT IN MAINE.

$$


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