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CHAPTER 3B TSUNAMI WAVES The term tidal wave is deeply entrenched in the expressions of both the general public and the popular press Further, it is frequently and incorrectly applied to two large wave types that have nothing to do with the tides Tsunami: (seismic sea wave) Japanese for “tsu”: harbor “nami”: wave; name given to a seismic sea wave generated as an indirect effect of an undersea earthquake, landslide or volcanic eruption felt over great distances Fitting to use “harbor wave” for this phenomena since greater heights possible in the physically confined locations Interesting (and a bit confusing) that tsunami is also the Japanese word for tidal wave (1) If you are interested in experiencing a tsunami, the Hawaiian Islands are a good choice … the Islands rarely if ever experience an earthquake (did just have a 6.6 on 10/15/06), since 1819 have experienced about 40 tsunami episodes … location makes it a target These occurrences can generate a concentric ring of waves similar to those resulting from objects thrown into the water These seismographically-induced waves may be unnoticeable in the open sea, but upon reaching shallow waters they begin to grow to massive proportions – may reach 30 m (100’) of more They have awesome kinetic energy from the mass of the volumes of water that constitute them Aside A tsunami is not to be confused with a tidal wave … which if viewed as in common perception as a mountainous thundering wall of water – is a misnomer as nothing like it exists in nature, and if we are to take the term literally – the term itself means nothing Tidal wave is not to be confused with storm surge either (2) Storm surge: (the result of atmospheric low pressure) – a “dome” of water rising beneath low pressure cells (hurricanes / cyclones / typhoons) and which cause extensive coastal flooding when they make landfall … ex: Bangladesh 1970, a cyclone and accompanying storm surge killed and estimated 500,000 persons … we do storm surge when we do hurricanes Formation of Tsunamis Most tsunamis originate along the Pacific Ring of Fire region as a consequence of tectonic activity … so, at least theoretically, any seismic activity can result in a tsunami anywhere along its 24,000 mi length It is fitting to use tsunami for this class of seismic wave, because the Japanese Islands combine the two essentials for their production and for destruction (1) tectonically active (2) steeply sloping continental shelf Tsunamis originate in ocean regions where submarine volcanic, seismic or tectonic activities are frequent or where sudden ocean floor subsidence or collapse or mass movement of sedimentary material occur --- type of displacement determines whether the leading edge of the wave is a wave trough (floor collapse) or a wave crest (floor doming) Quakes which disturb the sea commonly generate sufficient displacement to create powerful waves Concentric rings of waves race across the open ocean at speeds up to 500 mph These waves are frequently unseen / unnoticed --- VERY long waves --- may surpass 100 mi [wave length] and 20 min crests [period] --- less than 3’ in height [open ocean, a wave of that size with a wavelength of 100 mi or more would not register … following pictures are 2’- 4’ waves] Because the sea is fluid [no pun intended] any disturbance displaces the entire water column and wave energy of a tsunami is distributed from sea surface to sea floor Once it begins to move into shallower water the energy it is carrying [a 2’ wave of < L/20 depth 100 mi long] is compacted and the wave has no where to go but up Aside … try this: in the open ocean with a depth of [say] 12,000’ the wave moves at 656’/sec … near shore the ocean depth decreases to 330’ and friction reduces wave speed to about 99’/sec … at 165’ of depth wave speed is reduced to about 72’/sec all the water piling behind has to go somewhere – up [or if you like, Tad is driving at 70 mph [just believe it] on I-40 when mutant orange barrels force him to slow to 40 mph… then 20 mph… what must happen to traffic following him? What beautiful strains of vocalization must follow him?] --- upon reaching shallow water they begin to grow rapidly (maybe to 200’) … ultimate height will be variable according to bottom structure and slope, configuration of shoreline, etc. Sometimes the only warning of an approaching tsunami is a sudden increase or decrease in ocean level Some Notable Tsunami Disasters Worldwide tsunamis occur on average at least once a year --- the Pacific alone had 270 damaging tsunamis between 279 A.D. and 1946 --- worldwide the average is one tsunami per year Possibly the most dramatic event occurred about 3,400 years ago – destruction of volcanic island – Stronghyle (also known as Thera) [present day island of Santorini, Agean Sea] leaving a caldera and 3 arc-shaped smaller islands ? It has been put forward to be Atlantis ? Direct factor in decline of Minoan (Crete) culture ? May be part of biblical history (the Flood) July 21, 365 A.D. an earthquake in the eastern Mediterranean likely caused a seafloor collapse --- record of receding waters around Sicily, Greece, and Egypt --- returning water inundated low-lying areas --- City of Alexandria was reported to be the most heavily damaged … 50,000 people died, fishing boats relocated miles inland … the famous 600’ lighthouse was destroyed June 1692, - Port Royal, Jamaica --- three tremors create tsunamis registered to 24’ --- 100’s drown; total causalities reach 1,600+ [would love to dive here; won’t let you] November 1, 1755 – Lisbon, Portugal --- quake; fire; tsunamis --- 50,000+ was killed August 27, 1883 – most famous – eruption and ultimate destruction of the volcanic island – Krakatoa (Krakatau) --- over a 3 mo period the island’s 3 volcanoes began to erupt --- collapse of the largest volcano (Rakata) after 19 straight days of eruption allowed seawater into its partially collapsed magma chamber --- massive explosives resulted in particles and steam columns to 83.000’ (25,300m) --- resulting tsunamis waves 20’-130’ overtopped areas of Sumatra and Java --- 36,000+ were killed; 200 towns and villages were destroyed --- water level recorders as far away as Cape Horn (7800 mi) and Panama (11, 400 mi) were triggered --- estimated that seismic waves moved in excess of 400 mph April 1, 1946 – Hilo, HI --- 160 persons killed; $25 mill damage 36’ waves on Oahu; 56’ waves on Hawaii --- waves came in 15 intervals --- a disturbance in the Aleutian Trench of Alaska produced a series of tsunamis [ waves destroyed the Scotch Gap Lighthouse (33’) and its radio mast (108’) in the Aleutians] --- waves not only struck the windward shores, but were refracted and diffracted into high waves on the leeward shores Irony Stranded in Hawaii during the April 1946 Hilo tsunami was Dr. Francis Shepherd, a marine geologist, who was part of a group of scientist on their way to Bikini for an atomic bomb test ? A social comment by God? Citizens of Islands were warned to evacuate 6 hrs before the waves arrived – few listened The Pacific Tsunami Warning System came about because of Hilo, but as early as 1923 seismologists at the Hawaiian Volcano Observatory were accurately predicting the arrival of these seismic waves • Not a “disaster” but noteworthy July 9, 1958 – Lituya Bay, Alaska --- a giant rockfall created a seiche [an oscillating wave in a lake or bay whose period can last minutes or several hours] --- sent a wall of water up alternating slopes of the bay to heights of 1,700’ (520m) … believed to be the highest known waves in history Storm Surge Origin of Storm Surge Storm surge (or storm tide) period of excessive high water along a coast associated with changes on atmospheric pressure and wind action on the surface of the ocean storm surge, especially in association with normal high tide conditions, can be disastrous to low-lying coastal areas Storms at sea (esp. cyclones and hurricanes) are centered around areas of intense low pressure Under such storms, lowest pressure at its center allows the sea surface to rise in comparison to the storm’s periphery --- this difference can result in a sea level difference of as much as 39” [13x denser than water a 3” decrease in mercury pressure will raise water levels 13x greater] … additionally, inwardly spiraling winds serve to “drag” the sea surface upward As the storm moves shore-ward, drag on the surface of the sea pulls this “dome” of water with the storm and piles the leading edge of the water against the increasingly shallow shore … this increases the height of the water and produces the surge Water continues to pile against the shore steepening the slope of the wave until the tendency of the water to flow downward and back to sea is equal to the force of the wind driving the water ashore Interesting If water along the shore is deep (a steep shore line), the surge will not reach the ocean bottom and some of its waters will escape downward and seaward --- decreasing the height of the surge wave If the water along the shore is shallow (shallow shore line), the shore-ward surge will reach to the ocean’s bottom, no water will escape, and the surge wave will grow higher * Surge waves may last for several hours until the storm at sea diminishes Storm Surge Disasters Bay of Bengal – bowl-shaped northern region of the Indian Ocean [India and Bangladesh] Storm waters in the Indian Ocean move to confined Bay of Bengal where they are forced to rise [esp. is they occur during monsoon conditions] Area is alluvial delta only 12’ in elevation in places 1876: 100,00 to 250,000 dead 1970: 500,000 dead 1985: 10,000 dead 1991: 139,000 dead Atlantic-Gulf Coastal Plain Shallow coastal waters surrounding the United States have caused major problems 1900: Galveston, TX, 13’ over the island, destroying the city and killing 5000 1969: Hurricane Camille, surge est.s as high are 25”+, reputed to have reversed the flow of the Mississippi River 1989: Hurricane Hugo, hit Charleston, SC with 16.5’ surge 1992: Hurricane Andrew, until Katrina the worst in terms of damage and with 135 mph winds had almost no surge – it apparently was moving too fast from Bahamas to FL and kept running over its surge North Sea (1953) A slow moving mid-latitude cyclone over the North Atlantic pushed water into the shallow North Sea In combination with high tide conditions and narrowing of the sea to the south, sea levels rose 6’ (eastern British Isles) to 11’ (Netherlands), brought widespread destruction and killed more than 1800 persons