Tsunami The devastating impact of seismic sea waves

Tsunami The devastating impact of seismic sea waves

Tsunami (harbor wave) Seismic sea waves (NOT tidal waves) Caused by processes that abruptly move large volumes of ocean water: earthquake submarine volcanic eruption coastal/submarine landslide or rockfall extraterrestrial impact

How do EQ cause tsunami? http: //geology. com/articles/tsunami-geology. shtml

Tsunami causes § Unless there is an underwater landslide, strikeslip EQ WILL NOT cause tsunami § Most tsunami generated by subduction zones § § § Chile, Alaska, Japan, Cascadia, Philippines, New Zealand

Tsunami wavelength § Long wavelengths (over 100 km) § Periods longer than 1 hour 316, 800 ft = 60 miles

Tsunami wave speed § Travel at high speeds : 400 to 500 mph (~200 yards/sec) § § Alaska to CA 4 to 7 hrs Alaska to Hawaii 4 to 6 hrs Chile to Hawaii 14 to 15 hrs Chile to Japan 22 to 33 hrs

Tsunami wave speed v = speed ~ § g = acceleration of gravity (9. 8 m/sec 2) § d = depth of ocean (m) § deeper water means higher speed § For d = 4, 600 m, v = 763 km/hr (speed of jet plane)

What happens when tsunami gets near shore? § Tsunami slows down (shallower water) § Example: d = 100 m, v = 113 km/hr § Wave gets taller § λ gets shorter, T gets shorter

Tsunami nears shore § As wave gets into shallow water bottom of wave drags along ocean floor § Top of wave still moving fast: can cause cresting of wave, and breaking onto shore

Tsunami run-up § Run-up = measurement of height of water onshore observed above a reference sea level § Generally don’t get big gigantic wave § Water comes as a fast moving rise in tide that rapidly moves inland § NOT JUST ONE WAVE…multiple waves coming in about ½ hour or so apart § See tsunami wave simulator http: //www. seed. slb. com/en/scictr/watch/living_planet/tsunami_wave/index. htm

Energy in tsunami § Loss of energy in a wave is inversely proportional to λ § Since λ very long, little energy lost § Waves can travel great distances and still be very distructive

Damage due to tsunami § Waves often full of debris (trees, cars, pieces of wood etc. ) § As the wave recedes, the debris drags more stuff with it § Can recede as much as a km out to see, leaving shoreline empty with flopping fish, boats, etc. on the bottom

Detecting a tsunami § Pressure recorder on bottom of ocean § Buoy to communicate readings via satellite § Tsunami Warning Centers issue warning

Tsunami Warning Centers § Hawaii and Alaska § When EQ considered capable of generating tsunami, send warning with estimated arrival time § Once tsunami hits somewhere, tsunami watch established to monitor tide gauges and ocean buoys

Tsunami Warning § When warning is issued – low lying areas are evacuated § U. S. Coast guard issues warnings over marine frequencies § Some places have sirens

Tsunami How could you evaluate the level of risk due to tsunami?

Possible tsunami “run-up” zones

standing wave Seiche § Standing wave in an enclosed or partially enclosed body of water (similar to sloshing in a bath tub) § Generated by wind or seismic activity § Often swimming pools experience a seiche during EQ http: //earthquake. usgs. gov/learning/glossary. php? term=seiche

Seiche potential in Lake Tahoe § Scientists at UNR have determinsed the seiches have occurred on Lake Tahoe in the past § Low probability – on average once every 2000 to 3000 years § Good page on seiches w/great animation http: //earthguide. ucsd. edu/earthguide/diagram s/waves/swf/wave_seiche. html