Public Information Statement
Issued by NWS Portland, ME

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PUBLIC INFORMATION STATEMENT
NATIONAL WEATHER SERVICE GRAY ME
700 AM EST TUE MAR 29 2016

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 AND NEW
HAMPSHIRE, AS WELL AS IN THE REMAINDER OF THE COASTAL 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
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 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 OCCURRING.  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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