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THE NEXT DESTRUCTIVE TSUNAMI: ITS NOT IF, BUT WHEN.
Tsunamis are large, potentially destructive sea waves, most of which are formed as a result of submarine earthquakes, but which may also result from the eruption or collapse of island or coastal volcanoes and the formation of giant landslides on marine margins
a tsunami is a wave train, or series of
waves, generated in a body of water by an
impulsive disturbance that vertically
displaces the water
column
How a tsunami wave forms• A tsunami begins with an undersea earthquake.• The force jolts the ocean floor, which displaced
water, and produces waves. • Slowly, as they enter shallow water, the waves
begin to pile up until they break on shore.
TSUNAMI PHENOMENA
• SERIES OF OCEAN WAVES GENERATED BY A DISPLACEMENT OF SEAWATER
• MOST GENERATED BY EARTHQUAKES
• 5 TO 60 MINUTES BETWEEN WAVE CRESTS
• WAVELENGTHS ARE MUCH LONGER THAN THE OCEAN DEPTH
• SPEED DEPENDS ON WATER DEPTH
Coastal effects of tsunami
• If trough arrives first, appear as a strong withdrawal of water (similar to an extreme and suddenly-occurring low tide)
• If crest arrives first, appear as a strong surge of water that can raise sea level many meters and flood inland areas
• Tsunami often occur as a series of surges and withdrawals
MOST TSUNAMIS ARE GENERATED AT CONVERGENT PLATE BOUNDARIES, WHERE ONE PLATE IS SUBDUCTING BENEATH THE OTHER AND THE GREATEST EARTHQUAKES OCCUR.
DIFFERENT TYPES OF OCEAN WAVES
TYPE CAUSE TIME / CYCLE
SEA & WIND 2 - 25 SEC SWELL
TSUNAMIS RAPID OCEAN 5 - 60 MIN DISPLACEMENT TIDES ASTRONOMICAL > 12 HRS CYCLES
NUMERICAL MODEL OF THE 1896 SANRIKU, JAPAN TSUNAMI
Wave speed = ( g x h ) ½
where, g = acceleration of gravity = 10 meters / second 2
h = water depth
For a typical deep ocean depth of 5500 m,
speed = ( 10 x 5500 m 2 / s 2 ) ½
= 234 m / s = 524 miles / hour = 842 km / hour
Wave Energy The energy in a wave is proportional to the length (the distance between two crests of the wave) and to the square of the height (the distance
between the trough and the crest).
EstimatedTsunami
TravelTimes
Hourly Contours
WHY IS A TSUNAMI A HAZARD?
• WAVE HEIGHTS GROW IN SHALLOW WATER
• BEST CASE - QUICKLY RISING TIDE• WORST CASE - WALL OF TURBULENT
WATER WITH ROCKS AND DEBRIS• RUNUPS OF 30 meters 100 FT OR MORE• DANGER CAN CONTINUE FOR MANY
HOURS• HIGH FATALITY HAZARD
TWO TSUNAMI THREATS• LOCAL / REGIONAL TSUNAMI– GENERATED NEARBY (EQ EFFECTS)– STRIKES SHORE QUICKLY– MAY BE NO TIME FOR OFFICIAL WARNING– HIGHEST RUNUPS– DAMAGES ONLY TO LIMITED AREA
• DISTANT TSUNAMI OR TELETSUNAMI– GENERATED BY EARTHQUAKE FAR AWAY– MORE TIME AVAILABLE FOR WARNING– MORE WIDESPREAD DAMAGE
TSUNAMI MITIGATION
• HAZARD ASSESSMENT• WARNING SYSTEMS• EDUCATIONAL PROGRAMS • LAND USE• BUILDING CODES• DEFENSE WORKS
Richard H. HagemeyerPACIFIC TSUNAMI WARNING CENTER Ewa Beach, Hawaii
US Department of Commerce / National Weather Service
TSUNAMI WARNING CENTER OPERATIONAL ACTIVITIES
• COLLECTION / ANALYSIS OF SEISMIC DATA
• COLLECTION / ANALYSIS OF SEA LEVEL DATA
• DECISION MAKING
• CREATION / DISSEMINATION OF MESSAGES
TSUNAMI WARNING SYSTEM SEISMIC DATA
TSUNAMI WARNING SYSTEM SEA LEVEL GAUGES
DEEP OCEAN, REAL TIMETSUNAMI REPORTING SYSTEMThe tsunami
signal is detected by a pressure sensor on the ocean floor. That signal is relayed by acoustic telemetry to the bouy. The buoy in turn transmits the signal via satellite back to the warning centers.
WE STRIVE TO IMPROVE
• SPEED• ACCURACY• RELIABILITY• EFFECTIVENESS
PTWC PACIFIC BULLETINS:ELAPSED TIME TO ISSUANCE
0
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01/03/93 01/03/94 01/03/95 01/03/96 01/02/97 01/02/98 01/02/99 01/02/00 01/01/01 01/01/02
DATE
MINUTES
PTWC HAWAII BULLETINS:ELAPSED TIME TO ISSUANCE
0
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1/1/95 1/2/96 1/2/97 1/3/98 1/4/99 1/5/00 1/5/01 1/6/02
DATE
ELAPSED MINUTES
PTWC EARTHQUAKE MESSAGES:ELAPSED TIME TO ISSUANCE
NATURAL TSUNAMI WARNING SIGNS
• If you feel shaking from an earthquake so strong that it is difficult to stand, and you are are near shore, evacuate immediately inland or to high ground. Don’t wait for an official warning.
• Monitor the sea for at least an hour after any smaller felt earthquake for signs of a tsunami.
• If the sea recedes unusually, don’t investigate. Evacuate immediately.
• Abnormally strong and unusual currents may be the sign of a tsunami. Evacuate immediately.
TSUNAMI SAFETY TIPS
• Don’t act on your curiosity and go to the sea to watch a tsunami. Tsunamis move very quickly and by the time you see it - it will be too late to escape.
• A tsunami is a series of waves -- the first is often not the largest.
• Each wave in a tsunami may last for several minutes to an hour. The danger from a tsunami can last for many hours.
TSUNAMI SAFETY TIPS
• Tsunamis wrap around islands and are sometimes largest on the shoreline away from the source.
• Even a small tsunami can produce strong and unusual currents that are a hazard to swimmers and boaters.
• Tsunamis can be very large. Runups as high as 50 feet have been measured in Hawaii. Inundation may extend several hundred feet from the coast.
TSUNAMI SAFETY TIPS
• Tsunamis can travel great distances inland up low lying valleys and in the channels of rivers and streams.
• The moving water of a tsunami has tremendous force, and it can carry objects weighing many tons.
• The debris carried by a tsunami can be more destructive than the water.
• Never think you can surf a tsunami!
THE END
