TsunamiTsunami

The devastating impact The devastating impact of seismic sea wavesof seismic sea waves

Tsunami (harbor wave)Seismic sea waves (NOT tidal waves)

Caused by processes that abruptly move large volumes of ocean water:

earthquakesubmarine volcanic eruption

coastal/submarine landslide or rockfall

extraterrestrial impact

How do EQ cause tsunami?How do EQ cause tsunami?

http://geology.com/articles/tsunami-geology.shtml

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

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

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

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)

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

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

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

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

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

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

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

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

Tsunami

How could you evaluate the level of risk

due to tsunami?

Possible tsunami“run-up” zones

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

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