[ppt]powerpoint presentation - san jose state university · web viewthe devastating impact of...
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TsunamiTsunami
The devastating impact The devastating impact of seismic sea wavesof seismic sea waves
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Tsunami (harbor wave)Seismic sea waves (NOT tidal waves)
Caused by processes that abruptly move large volumes of ocean water:
earthquakesubmarine volcanic eruptioncoastal/submarine landslide or
rockfallextraterrestrial
impact
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How do EQ cause tsunami?How do EQ cause tsunami?
http://geology.com/articles/tsunami-geology.shtml
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Tsunami causesTsunami causes Unless there is an underwater landslide, strike-Unless there is an underwater landslide, strike-
slip EQ slip EQ WILL NOTWILL NOT cause tsunami cause tsunami Most tsunami generated by subduction zonesMost tsunami generated by subduction zones
Chile, Chile, Alaska, Alaska, Japan, Japan, Cascadia, Cascadia, Philippines, Philippines, New ZealandNew Zealand
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Tsunami wavelengthTsunami wavelength Long wavelengths (over 100 km)Long wavelengths (over 100 km) Periods longer than 1 hourPeriods longer than 1 hour
316,800 ft = 60 miles
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Tsunami wave speedTsunami wave speed Travel at high speeds :Travel at high speeds :
400 to 500 mph (~200 yards/sec)400 to 500 mph (~200 yards/sec)
Alaska to CA 4 to 7 hrsAlaska to CA 4 to 7 hrs Alaska to Hawaii 4 to 6 hrsAlaska to Hawaii 4 to 6 hrs Chile to Hawaii 14 to 15 hrsChile to Hawaii 14 to 15 hrs Chile to Japan 22 to 33 hrsChile to Japan 22 to 33 hrs
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gd
Tsunami wave speedTsunami wave speed
v = speedv = speed ~~
g = acceleration of gravity (9.8 m/secg = acceleration of gravity (9.8 m/sec22)) d = depth of ocean (m)d = depth of ocean (m)
deeper water means higher speeddeeper water means higher speed
For d = 4,600 m, v = 763 km/hrFor d = 4,600 m, v = 763 km/hr (speed of jet plane) (speed of jet plane)
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What happens when What happens when tsunami gets near shore?tsunami gets near shore?
Tsunami slows down (shallower water)Tsunami slows down (shallower water) Example: d = 100 m, v = 113 km/hrExample: d = 100 m, v = 113 km/hr
Wave gets taller Wave gets taller λλ gets shorter, T gets shorter gets shorter, T gets shorter
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Tsunami nears shoreTsunami nears shore As wave gets into shallow water bottom of As wave gets into shallow water bottom of
wave drags along ocean floorwave drags along ocean floor Top of wave still moving fast: can cause Top of wave still moving fast: can cause
cresting of wave, and breaking onto shorecresting of wave, and breaking onto shore
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Tsunami run-upTsunami run-up Run-up Run-up = measurement of height of water = measurement of height of water
onshore observed above a reference sea level onshore observed above a reference sea level Generally don’t get big gigantic waveGenerally don’t get big gigantic wave Water comes as a fast moving rise in tide that Water comes as a fast moving rise in tide that
rapidly moves inlandrapidly moves inland NOT JUST ONE WAVE…NOT JUST ONE WAVE…multiple waves multiple waves
coming in about ½ hour or so apartcoming in about ½ hour or so apart See tsunami wave simulator See tsunami wave simulator
http://www.seed.slb.com/en/scictr/watch/living_planet/tsunami_wave/index.htmhttp://www.seed.slb.com/en/scictr/watch/living_planet/tsunami_wave/index.htm
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Energy in tsunamiEnergy in tsunami Loss of energy in a wave is inversely Loss of energy in a wave is inversely
proportional to proportional to λλ Since Since λλ very long, little energy lost very long, little energy lost Waves can travel great distances and Waves can travel great distances and
still be very distructivestill be very distructive
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Damage due to tsunamiDamage due to tsunami Waves often full of debris (trees, cars, Waves often full of debris (trees, cars,
pieces of wood etc.)pieces of wood etc.) As the wave recedes, the debris drags As the wave recedes, the debris drags
more stuff with itmore stuff with it Can recede as much as a km out to see, Can recede as much as a km out to see,
leaving shoreline empty with flopping leaving shoreline empty with flopping fish, boats, etc. on the bottomfish, boats, etc. on the bottom
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Detecting a tsunamiDetecting a tsunami Pressure recorder on bottom of oceanPressure recorder on bottom of ocean Buoy to communicate readings via Buoy to communicate readings via
satellitesatellite Tsunami Warning Centers issue Tsunami Warning Centers issue
warningwarning
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Tsunami Warning CentersTsunami Warning Centers Hawaii and AlaskaHawaii and Alaska When EQ considered capable of When EQ considered capable of
generating tsunami, send warning with generating tsunami, send warning with estimated arrival timeestimated arrival time
Once tsunami hits Once tsunami hits somewhere, tsunami somewhere, tsunami watch established to watch established to monitor tide gauges monitor tide gauges and ocean buoysand ocean buoys
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Tsunami WarningTsunami Warning
When warning is issued – low lying When warning is issued – low lying areas are evacuatedareas are evacuated
U.S. Coast guard issues warnings over U.S. Coast guard issues warnings over marine frequenciesmarine frequencies
Some places have sirensSome places have sirens
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Tsunami
How could you evaluate the level of risk
due to tsunami?
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Possible tsunami“run-up” zones
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SeicheSeiche SStanding wavetanding wave in an in an enclosed or partially enclosed or partially
enclosedenclosed body of water (similar to sloshing in a body of water (similar to sloshing in a bath tub)bath tub)
http://earthquake.usgs.gov/learning/glossary.php?term=seiche
Generated by wind Generated by wind or seismic activityor seismic activity
Often swimming Often swimming pools experience a pools experience a seiche during EQseiche during EQ
standing wave
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Seiche potential in Lake Seiche potential in Lake TahoeTahoe
Scientists at UNR have determinsed the Scientists at UNR have determinsed the seiches have occurred on Lake Tahoe in the seiches have occurred on Lake Tahoe in the pastpast
Low probability – on average once every 2000 Low probability – on average once every 2000 to 3000 yearsto 3000 years
Good page on seiches w/great animation Good page on seiches w/great animation
http://earthguide.ucsd.edu/earthguide/diagramhttp://earthguide.ucsd.edu/earthguide/diagrams/waves/swf/wave_seiche.htmls/waves/swf/wave_seiche.html