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IN WATER is quite similar. Most waves are created by the frictional drag of wind blowing across the water surface. A begins as Tsunami can be the most overwhelming of all waves, but a tiny ripple. Once formed, the side of a ripple increases the their origins and behaviors differ from those of the every­ day waves we see at the seashore or lakeshore. The familiar surface area of water, allowing the wind to push the ripple into waves are caused by wind blowing over the water surface. a higher and higher wave. As a wave gets bigger, more wind Our experience with these wind-caused waves misleads us energy is transferred to the wave. How tall a wave becomes in understanding tsunami. Let us first understand everyday, depends on (1) the velocity of the wind, (2) the duration of wind-caused waves and then contrast them with tsunami. time the wind blows, (3) the length of water surface (fetch) the wind blows across, and (4) the consistency of wind direction. Once waves are formed, their energy pulses can travel thou­ Wind-Caused Waves sands of kilometers away from the winds that created them. Waves transfer energy away from some . Waves moving through a water mass cause water particles to rotate in WHY A WIND-BLOWN WAVE BREAKS place, similar to the passage of seismic waves (figure 8.5; see Waves undergo changes when they move into shallow water­ figure 3.18). You can feel the orbital motion within waves by water with depths less than one-half their . Wave standing chest-deep in the . An incoming wave will pick friction on the floor of the shallow ocean interferes with the you up and carry you shoreward and then drop you downward orbital motions of water particles, so waves begin slowing and back as it passes. At the water surface, the diameter of the (figure 8.6). Friction with the bottom flattens the circular water-particle orbit is the same as the . The diameters motions of the water into elliptical and horizontal movements. of water orbits decrease rapidly as water deepens; wave orbital As waves slow down, their decrease, thus motion ceases at a depth of about one-half of the wavelength. concentrating water and energy into shorter lengths and Waves vary from tiny, wind-blown ripples to monster causing the waves to grow higher. When the wave height­ rogue waves, but the rotational motion of water within a wave to-wavelength ratio (H:L) reaches about 1:7, the wave front has grown too steep, and it topples forward as a breaker 1...--- Wavelength (L) ----. 1 (figures 8.7 and 8.8). Note that the 1:7 ratio is reached by Wave direction

Figure 8.5 Waves are energy fronts passing through water, causing water particles to rotate in place. Rotational Figure 8.6 As a wave moves into shallower water, it rises movement becomes insignificant at depth about one-half of the higher. Circular rotating water touches bottom, flattening into wavelength. a back-and-forth motion. Deep water __..,..r-- Shallow water •._ __ T_r_a_ns_i_tio_n __ ~•~r••B_re_a_k_e~~~ii~•t----s_u_rt __...... ;:n~sh~ r zone zone zone 1 I -

Figure 8.7 Schematic cross-section of deep-water waves entering shallow water. Wavelengths decrease and wave heights increase, causing the water to pitch forward as breakers.

204 Chapter 8 Tsunami Versus Wind-Caused Waves produce a that is the result of the constructive and destructive interference of multiple sets of ocean waves (figure 8.9a). However, every once in a while, the various waves become briefly synchronized, with their energies united to form a spectacular tall wave, the so-called (figure 8.9b). The moving waves quickly disunite, and the short-lived rogue wave is but a memory. But if a ship is present at the wrong time, a may occur. During World War II, the Queen Elizabeth was operating as a troop transport passing Greenland when a rogue wave hit, causing numerous deaths and injuries. On 3 June 1984, the three-masted Marques was sailing 120 km (75 mi) north of Bermuda when two rogue waves quickly sent the ship under, drowning 19 of the 28 people on board. In 1987, the recre­ Figure 8.8 A . Circular rotating water is ational fishing boat Fish-n-Fool sank beneath a sudden "wall slowed at the base but rolls forward on top. of water" in the near a Baja island. © Royalty-Free/Corbis. On 10 April 2005 in New York, 2,300 eager passengers boarded the 295 m (965 ft) long Norwegian Dawn for a one­ week vacation cruise to the Bahamas. On the return trip, the changes in both wave height and wavelength; wave height became rough. Then a thunderous disruption shocked is increasing at the same time that wavelength is decreasing. people as a freak 22 m (70 ft) high wave slammed into the The depth of water beneath a breaker is roughly 1.3 times ship, breaking windows, sending furniture flying, ­ the wave height as measured from the still-water level. At this ing more than 60 cabins, and injuring four passengers. The depth, the velocity of water-particle motion in the wave crest wave even ripped out on deck 10. Damage to the is greater than the wave velocity, thus the faster-moving wave hull forced an emergency stop for inspection and repairs in crest outraces its bottom and falls forward as a turbulent mass. Charleston, South Carolina. Spring vacation was interrupted by a rogue wave. ROGUE WAVES On occasion, rogue waves strike the shoreline and carry An ocean is such an extensive body of water that different people away from the beach. On 4 July 1992, a rogue wave are likely to be operating in different areas. Each 5.5 m (18 ft) high rose out of a calm sea at Daytona Beach, creates its own wave sets. As waves from different Florida, crashed ashore, and smashed hundreds of cars parked storms collide, they interfere with each other and usually on the beach, causing injuries to 75 of the fleeing people.

Figure 8.9 Waves on the sea surface. (a) At ~\\\\\\1111111111////////1, #'''''1111111111111111//1111,,,,!. ~,,,,~ ~ ... -.... # ~--.... ~/ any time, there usually are several different -~~·.--... ' · ··· ~~ ··· L_; · ··· ' .•··. ~~ - · ~ ... ' ~~ storms, each producing its own waves of e-- ...... \. . # .. . . , .. .. · . . ' ·. . characteristic wavelength. Different wave sets ·...... ' ·.. ~ ,, ...... \ ~~... ,, .... •... , ' ~ , # , \ usually int erfere with each other. (b) On rare .... _... ~ ~~ --; ' occasions, the various wave sets combine to ~ -#' ~4't. ~,,,,~ produce an unexpected giant- a rogue wave. W/1111111111111111\\~ (a) and (b) are separate drawings; they do not (a) Multiple wave sets directly correlate.

(b) Rogue wave

Wind-Caused Waves 205 In Greater Depth

Deep-Water Wave Velocity, Length, Higher-velocity waves carry more energy, but how much more? Wave energies per unit length can be computed wit h the Period, and Energy following relationship:

Waves moving through water deeper than one-half their wave­ Ew = 0.125p gH2L length are essentially unaffected by friction with the bottom. The waves move as low, broad, evenly spaced, rounded swells with where Ew equals wave energy, p (rho) equals density of water, g velocities related to wavelength by: equals gravitational acceleration, H equals wave height in meters, and L equals wavelength. Some representative values computed Vw = 1.25VL from this equation are listed in table 8.1. Notice that doubling the wavelength doubles the wave energy, but that doubling the wave where Vw equals wave velocity and L equals wavelength. A swell height quadruples the wave energy. w ith a wavelength of 64 m would have a velocity of the square root of 64 (i.e., 8) times 1.25, or 10 m/sec (22.4 mph). The equa­ tion is telling us that wave ve locity in deep water depends on the TABLE 8.1 wave's length- as wavelength increases, so does velocity. The period (T) is the amount of time it takes for two successive Some Ocean-Wave Energies wave crests to pass a given point Since the distance between suc­ Wave Wave- Wave Energy cessive wave crests is the w avelength, there must be a relationship Period (T) length (L) Height (H) in Joules between period (T) in seconds and wavelength (L) in meters. This relationship may be defined by: 10 seconds 156m lm 2.39 x 105

V = distance traveled/time = UT 2m 9.57 X II which may be simplified to: 3m 21.54 X II 4.79 x 105 L = 1.56T2 14.l seconds 312m Im

2m 19.15 x II As a rule of thumb, the velocity of waves in miles per hour may be estimated as 3.5 times the wave period in seconds. For exam­ 3m 43.08 X II ple, waves with a period of 10 seconds are moving about 35 mph.

Rogue waves have been measured at 34 m (112 ft) in A deadly example hit on 15 June 1896, a sum­ height. The problems they present also include the steepness mer day when fishermen were out to sea and beaches were of the wave front descending into the wave trough. A small, crowded with vacationers. An offshore swayed short boat is maneuverable and in good position to ride over the seafloor; then, about 20 minutes later, the sea withdrew, the rogue wave, as long as it does not get hit sideways and only to return in 45 minutes with a sound like a powerful rolled, or tossed from the front of one wave onto the back of rainstorm. Tsunami hit all the beaches hard but reached their the next wave. Large, long ships face either being uplifted at greatest heights of29 m (95 ft) where they crowded into nar­ their midpoint, leaving both ends suspended in air, or having row inlets. The tsunami destroyed more than I 0,000 homes both ends uplifted with no support in their middle. Either case and killed more than 27 ,000 people. The fishermen on the creates severe structural strains that break some ships apart. open ocean did not feel the earthquake or the tsunami; they learned of it when they sailed back into a bay littered with the wreckage of their houses and the bodies of their families. Tsunami In the United States, tsunami have been called "tidal The biggest, most feared waves of all pass mostly unno­ waves," but this is rather silly because tsunami have noth­ ticed across the open sea and then rear up and strike the ing to do with the . Nor do tsunami have anything to shoreline with devastating blows. The country with the do with winds or storms; they are created by huge injections most detailed history of these killer waves is Japan, and of energy or "splashes" in by move­ the waves are known by the Japanese word tsunami ments, volcanic eruptions or caldera collapses, underwater (tsu = harbor; nami = waves). Being a Japanese word, it , impacts, and such. You can approxi­ is the same in both singular and plural; there are no "tsuna­ mate a tsunami by throwing a rock into a still body of1water, mis," just as there are no "sheeps." The reference to harbor sending off trains of concentric waves heading away from waves emphasizes the greater heights waves reach in inlets the impact point. When a large volume of ocean water is and harbors because the narrowed focuses the suddenly moved, gravity causes waves to be generated that waves into smaller spaces. For example, an 8 m (26 ft) high spread out from the disturbance. Imagine the size of the wave on the open coast may be forced to heights of 30 m waves formed when a really big rock drops in the water, such (100 ft) as it crowds into a narrow harbor. as the caldera collapse of the volcano Krakatau in 1883.

206 Chapter 8 Tsunami Versus Wind-Caused Waves The biggest tsunami are caused by the rarest events, the Before a trough comes ashore, the sea retreats, but there is impact of high-velocity asteroids and comets. Consider the no retreating sea before a crest comes ashore. amount of energy injected into the ocean when a 10 km It is the vertical-fault movements at zones (6 mi) diameter asteroid hits at 30,000 mph. that most commonly cause tsunami (table 8.2). In the Tsunami are most commonly created during , 20th century, 141 damaging tsunami combined to kill more more specifically subsea fault movements with pronounced than 70,000 people. Early in the 21st century, tsunami have vertical offsets of the seafloor that disturb the deep ocean­ killed more than 265,000 people. water mass. Water is not compressible; it cannot easily absorb the fault-movement energy. Therefore, the water transmits the energy throughout the ocean in the waves we ~ call tsunami. West East- The Indian plate subducts northward beneath the (see figure 4.10). When a breaking point was ------reached on 26 December 2004, seafloor along a 1,200 km (740 mi) length snapped upward several meters, caus­ ing adjoining areas to move downward (figure 8.10). The uplifts and downwarps of the seafloor along a north- south trend set powerful long-wavelength tsunami in motion, with the greatest energy directed to the west and east. The tsu­ 200 km (125 mi) nami water surface had shapes similar to those of the newly deformed seafloor topography that generated them. Notice Figure 8.10 Schematic cross-section showing movements of that both troughs (downdrops) and crests (uplifts) formed. the seafloor and the tsunami they generated, 26 December 2004.

TABLE 8.2 Notable Tsunami in Recent Times

Date Cause Height Site Deaths 1 November 1755 Earthquakes !Om , Portugal 30,000 21 May 1792 Volcano !Om Japan (Unzen) >14,000 11 April 1815 Volcano eruption !Om Indonesia (Tambora) >10,000 8 August 1868 Earthquake 15 m >25,000 27 August 1883 Volcano eruption 35 m Indonesia (Krakatau) 36,000 15 June 1896 Earthquake 29m Japan 27,000 11 October 1918 Subsea 6m 116 2March 1933 Earthquake 20m Japan 3,000 1 Earthquake 15 m 175 22May 1960 Earthquake lOm Chile > 1,250 27 March 1964 Earthquake 6m Alaska 125 l September 1992 Earthquake !Om Nicaragua 170 12 December 1992 Earthquake 26m Indonesia >1,000 12 July 1993 Earthquake 31 m Japan 239 2 June 1994 Earthquake 14m Indonesia 238 17 July 1998 Subsea landslide 15 m >2,200

26 December 2004 Earthquake !Om Indonesia, Sri Lanka, India ~245 , 000 17 July 2006 Earthquake 7m Indonesia >600 29 September 2009 Earthquake 14m Samoa 190 27 February 2010 Earthquake 8m Chile 200 11 March 2011 Earthquake 13 m Japan 19,184

Tsunami 207 Tsunami velocity requires a different calculation. Tsu­ Tsunami Versus nami wavelengths are so long and so much greater than the Wind-Caused Waves depth of the deepest ocean that their velocity is calculated by: The typical ocean waves created by winds vary in size dur­ v =ygD ing the course of a year. Although the periods and wave­ lengths of wind-blown waves vary by storm and season, they where v equals wave velocity, g equals acceleration due to are distinctly different from those of tsunami (table 8.3). gravity (9 .81 m/sec2 or 32 ft/sec2), and D equals depth of Wind-blown waves rise up as they near the beach, roll ocean water. The Pacific Ocean has an average depth of forward, run up the beach for several seconds, and then 5,500 m (18,000 ft). Calculating the square root of g times D withdraw (figure 8.1 la). Wind-blown waves not only come yields a tsunami velocity of 232 m/sec (518 mph). and go quickly, but the water run-up and retreat is confined The calculated velocities of tsunami are faster than are to the beach (figure 8.llb). typically measured. The energy pulse that makes the wave Even huge wind-blown waves are different from tsunami. also puts the water into a rotating motion to depths of about For example, at Waimea on the north shore of Oahu Island in one-half the wavelength. Tsunami wavelengths can be as , the world-famous surfing waves may reach 15 m (50 ft) great as 780 km (485 mi) (table 8.3), meaning that ocean in height, but each wave is a solitary unit. These huge waves have water would be disturbed to depths of 390 km (240 mi). But short wavelengths and brief periods, meaning that each wave is the ocean's average depth is only 3.7 km (2.3 mi), and the an entity unto itself; there is no additional water mass behind deepest trenches just exceed 11 km (6.9 mi). Therefore, the the wave front. These waves are spectacular to view or ride, but energy pulse of a tsunami moves the entire it what you see is what you get; the wave is the entire water mass. passes through. Tsunami have such long wavelengths that Tsunami are different. Tsunami arrive as the leading they are always dragging across the ocean bottom, no matter edge of an elevated mass of water that rapidly runs up and how deep the water. The ocean basin has enough topogra­ over the beach and then inland for many minutes phy on its bottom to slow most tsunami down to the 420 to (figures 8.1 lc, d). Tsunami are dangerous because their tre­ 480 mph range. mendous momentum carries water and far inland. A tsunami of 1 m height in the deep ocean may be mov­ They may be no taller than the wind-blown waves we see ing nearly 500 mph. As tsunami enter shallower water, the at the beach every day, but they are much more powerful. increasing friction with the seafloor and internal turbulence Even a knee-high tsunami can kill you. The power of the of the water slow their rush, but they still may be moving fast-moving water can knock you down, then beat your body at freeway speeds. For example, when a tsunami is in water and head with debris, and then drown you. 50 m (165 ft) deep, using v = y'gD yields a velocity of The contrasts in velocities of wind waves versus tsunami 22 m/sec = 50 mph. are also great. A wind-blown wave moving through water As the velocity of the tsunami front slows and the wave­ deeper than half its wavelength (L) has its velocity (v) deter­ length decreases, the water behind it begins to build up and mined by increase in amplitude. If it reaches a height of 15 m (50 ft), v = 1.25VL it will not be like the Waimea, Hawaii, solitary wave with (see In Greater Depth, page 206). From table 8.3, take a nothing behind it. The visible tsunami wave is only the lead­ wave with 156 m wavelength and calculate its velocity as ing edge of a tabular sheet of water that will flow on land for 15.6 m/sec = 35 mph. minutes (figure 8.1 ld).

TABLE 8.3 Representative Wave Periods and Lengths Periods Lengths Wind-Blown Short: 5 seconds 39m (130 ft) Ocean Medium: 10 seconds 156m (510 ft) Waves Long: 20 seconds 624m (2,050 ft) Tsunami Maximum: 3,600 seconds 780,000 m (2,560,000 ft) (deep ocean) (60 minutes) (485 mi)

Common: 900 seconds 20,000 ID (65,000 ft) (nearer shore) (15 minutes) (12 mi)

208 Chapter 8 Tsunami Versus Wind-Caused Waves Figure 8.11 Ocean waves. (a) A wind-blown wave rolls onto the beach in Natal, South Africa. (b) Daily wind-blown waves break on the beach and do not flood higher areas. (c) Tsunami pour across the beach and flood inland for many minutes. Even small tsunami can knock you down, batter your body with debris, and kill you. (d) Tsunami overruns shoreline and a 3 m (10 ft) high tsunami barrier wall; then flows through M iya ko, . 11 March 2011. (a)© Digital Vi sion/Getty Images RF (d) ©JU I Press/AFP/Getty Images' .

(b)

(c)

(d)

Tsunami Versus Wind-Caused Waves 209 A Classic Disaster

The Chile Tsunami of 1868 into the night, we first made out a thin phosphorescent line which, like a strange kind of mirage, seemed to be rising Several ships were moored in the harbor at (then part of higher and higher in the air: its crest, topped by the baleful Bolivia), including the USS Wateree, a two-masted sidewheeler light of that phosphorescent glitter, showed frightful masses with a broad, flat bottom. About 4 p.m., the ship began vibrat­ of black water below. Heralded by the thunder of thousands ing and chains rattled as a huge earthquake shook down houses of breakers all crashing together, the wave that we had in Arica. Despite worries about tsunami, the desire to help people dreaded for hours was at last upon us. ashore caused the Wateree captain to drop extra anchors, close Of all the horrors, this seemed the worst. We were the hatches, lash the guns, rig lifelines, and then send a yawl with chained to the bed of the sea, powerless to escape .... We 13 men to the jetty to help. Here are some words written by Lieu­ could only hold on to the rails and wait for the catastrophe. tenant L. G. Billings, who remained aboard the Wateree: With a terrifying din, our ship was engulfed, buried under a half-liquid, half-solid mass of sand and water. We stayed . survivors were coming down the beach and crowding under for a suffocating eternity; then, groaning in all her on the little jetty, calling to the crews ... to carry them to timbers, our solid old Wateree pushed her way to the sur­ the apparent safety of the anchored vessels ... all at once a face, with her gasping crew still hanging on to the rails . hoarse murmuring noise made us look up; looking towards Our survival was certainly due to the construction of the the land we saw, to our horror, that where a moment before ship ... which allowed the water to pour off the deck almost there had been the jetty, all black with human beings, there as quickly as if she had been a raft. The ship had been car­ was nothing: everything had been swallowed in a moment ried along at a very great speed, but all at once she became by the sudden rising of the sea, which the Wateree, floating motionless ... we lowered a lantern over the side and discov­ upon it, had not noticed. At the same time we saw the yawl ered that we had run aground. . The rose upon such a carried away by the irresistible wave towards the lofty, verti­ spectacle of desolation as can rarely have been seen. We were cal cliff of the Morro, where it disappeared in the foam as high and dry, three miles from our anchorage and two miles the wave broke against the rock. inland. The wave had carried us at an unbelievable speed over ... there was another earthquake shock. Once more we the sand dunes which line the shore, across a valley, . leav­ saw the ground move in waves. This time the sea drew back ing us at the foot of the coastal range of the Andes. from the land until we were stranded and the bottom of the sea was exposed, so that we saw what had never been seen Note several items in this thrilling account: (1) After the first before, fish struggling on the and the monsters of the earthquake, the initial reaction was a deadly onrushing deep aground. The round-hulled ships rolled over on their sides, mass of water that killed people on the jetty. (2) After the second while our Wateree sat down upon her flat bottom; and when earthquake, the initial seawater reaction was a huge withdrawal the sea came back, returning not as a wave, but rather like a of the sea, leaving ships sitting on the seafloor. (3) The biggest huge , it made our unhappy companion [ships] turn turtle, tsunami occurred hours after the earthquakes. (4) The phosphores­ whereas the Wateree rose unhurt on the churning water. cent glow of the huge incoming tsunami is a commonly reported It had been dark for some time when the lookout hailed phenomenon due to bioluminescence of small sea life caught up in the deck and said that a breaking wave was coming. Staring the monster wave. (5) A large U.S. warship was carried 2 mi inland.

Tsunami arrive as a series of several waves separated by A tsunami event may begin with a drawdown or retreat periods usually in the 10- to 60-minute range. The waves of if the trough of the wave reaches shore first. are typically a meter high in the open ocean and 6 to 15 m The drawdown can cause strong currents that pull seawater, (20 to 50 ft) high on reaching shallow water, except where boats, and swimmers long distances out to sea. Or the tsu­ topography, such as bays and harbors, focuses the energy to nami wave crest or front may reach shore first. Then a strong create much taller waves. surge of seawater, resembling a faster and stronger rising tide, rushes across the beach and pushes far inland.

TSUNAMI AT THE SHORELINE Wavelength and Period Versus Height What does a tsunami look like when it comes onshore? People tend to attribute the destructive power of tsunami Does it resemble the animation in the Hollywood movie mostly to the great height of their waves, but the height of Deep Impact, where a beautiful, symmetrical wave curves tsunami commonly is not as important as the momentum high above the buildings of New York City? No. A tsunami of their large masses separated by ultra-long wavelengths arriving at the shoreline does not look like a gigantic version and periods (table 8.3). Visualize a flat or gently sloping of the breaking waves we see every day. A typical tsunami coast hit by tsunami with a 60-minute period. The tsunami hits the coastline like a very rapidly rising tide or whitewa­ can rush inland, causing destruction for about 30 minutes ter wave, but it does not stop on the beach; it keeps rushing before the water is pulled back to help form the next wave. A inland (figure 8.1 ld). view of the aftermath of the 1960 Chilean tsunami in Hilo,

210 Chapter 8 Tsunami Versus Wind-Caused Waves 50 km

-N-~ ~ 8°N SRI LANKA

33 ft

6°N Tsunami damage to Hilo, Hawaii, following Figure 8.12 33 ft 0 the magnitude 9.5 Ch ilean earthquake, 22 . Notice ~ 10 ...... ~'+--~-~~ 33 ft the Pacific Ocean in the background and how far inland the .s tsunami traveled, thanks to the long wavelengths and periods. c 0 NOAA/NGDC, U.S. Navy. ~ > Q) [iJ Hawaii, shows the effects of the long wavelengths and long 80° E 82° E periods between tsunami wave sets (figure 8.12). The pow­ erful tsunami was able to charge upslope, through the city, Figure 8.13 Tsunami from 4 to 7 m (13 to 23 ft) high were for a long distance and a long time before receding to help common all around Sri Lanka. The long wavelengths of the form the next tsunami wave set. tsunami allowed them to encircle the island. The vastly different wavelengths and periods of wind­ Sou rce: Science, v. 308 (2005). blown versus tsunami waves can be further appreciated In a subduction zone, the plates may become stuck by looking at islands. Huge wind-blown waves such as at together (figure 8.14a). Because the overriding plate is Waimea, Hawaii, hammer the north shore of Oahu, the stuck to the subducting plate, its seaward (leading) edge is windward shore. The relatively short wavelengths and peri­ dragged downward while the area behind (landward) bulges ods of the Waimea waves prevent them from affecting the upward (figure 8.14b). This movement goes on for centu­ other shores, the leeward or protected shores. ries, building up elastic strain all the while. When the stuck The long wavelengths and periods of tsunami allow them area ruptures, causing an earthquake, the leading edge of the to bend around many islands and hit all shores with high overriding plate breaks free, springs seaward and upward, waves. Tsunami wavelengths typically are longer than the causing a tsunami (figure 8. l 4c ). At the same time, the dimensions of an island. During the 26 December 2004 landward bulge warps downward, lowering coastal land tsunami, the island nation of Sri Lanka was hit below sea level. Tsunami race through the ocean for hours, by 4 to 7 m (13 to 23 ft) high tsunami at sites on all shores but the subsided land will remain down for generations (figure 8.13). The tsunami were directed at the east shore, (figure 8.14d). where more than 14,000 people died, but more than 10,000 were killed on the south shore and more than 6,000 on the north shore. Nearly 100 people were killed in the capital city of Colombo on the "protected" west shore.

Vertical Movement Earthquake-Caused Tsunami Duration of Seafloor (sec) (m) Fault movements of the seafloor that generate large ­ quakes may also cause powerful tsunami. In general, to create 7 0.6 23 0.2 (0.7 ft) tsunami, the fault movements need to have a vertical com­ 8 2.7 70 0.7 (2.3 ft) ponent, either uplifting or downdropping the seafloor, and ' 9 9 200 2.3 (7.5 ft) have an earthquake magnitude of at least 7.5Mw (table 8.4). 9.5 7 (23 ft) Reverse (thrust) and normal fault movements of the seafloor 27 330 can inject lots of energy into the overlying seawater. Source: Science, v. 78 (1997).

Earthquake-Caused Tsunami 211 Overrltllng plate

(a)

Figure 8.15 The leading edge of a tsunami slams into a resort, 26 December 2004. © Reuters/Corbis. (b)

Tsunami starts during earthquake (about 198,000 dead), Sri Lanka (about 30,000 killed), India (about 11,000 dead), and Thailand (about 6,000 killed). More than 3,000 of the deaths were European and North American tourists enjoying warm ocean water and sunny coastlines during the winter. Remember the tsu­ nami threat when you are vacationing at a beach resort (figure 8.15). What did Earth do to cause so many deaths? The sea­ floor west of northern in Indonesia overcame (c) frictional resistance, triggering faulting that ruptured north­ ward for 1,200 km (740 mi) during almost 7 minutes; this Tsunami waves spread created the third largest earthquake in the world in more than 100 years (see table 4.2). The rupture began 30 km (19 mi) below the seafloor and caused movements of up to 20 m (65 ft) that shifted the positions of some Indonesian islands and tilted other ones. The huge earthquake must have collapsed many nearby buildings that fell and killed many thousands of people, but the evidence of earthquake damage was largely erased by the powerful tsunami that swept across the land minutes later. The earthquake-causing (d) earth rupture occurred due to compression caused by the subducting Indian-Australian plate (see figure 4.10) that Figure 8.1 4 Subduction zones and tsunami. (a) Plates raised the seafloor tens of feet, thus charging the seawater may stick together. (b) Seaward edge of the overriding plate with energy that rapidly moved outward as tsunami racing is dragged downward. (c) When stuck area ruptures, the overriding plate springs upward starting a tsunami. (d) Tsunami throughout the Indian Ocean. The most powerful tsunami race landward and seward. moved east to kill people in Indonesia and Thailand, and Source.· USGS Circular 1187. west to slaughter people in Sri Lanka and India. A second great earthquake occurred in Indonesia on 28 March 2005 as another subduction event created an 8.6Mw seism just southeast of the 2004 rupture; it generated another tsunami INDIAN OCEAN 26 DECEMBER 2004 (figure 8.16). On 26 December 2004 (Boxing Day), a killer tsunami swept The last major tsunami event in the Indian Ocean occurred through the Indian Ocean and crossed Asian and African in 1883 when the volcano Krakatau collapsed into the sea, shorelines, causing death and destruction in 14 countries also offshore from Sumatra. The 1883 tsunami killed about (see figure 8.1). The estimated death total was 245,000, but 36,000 people. The world population of humans in 1883 was the true number is almost certainly higher and will never about 1.6 billion, but it had grown to 6.4 billion people in be known. Countries hit especially hard were Indonesia 2004. The dramatic growth of the human population during

212 Chapter 8 Tsunami Versus Wind-Caused Waves Figure 8.16 Devastation caused by tsunami in Indonesia, 28 March 2005. U.S. Air Force photo by Technical Sgt. Scott Reed.

those 121 years helped increase the death total to 245,000 people in the 2004 tsunami. An overwhelming event such as this far-reaching tsu­ nami results in many dramatic stories. In Sri Lanka, the train known as the Queen of the Sea left Colombo at 7:30 a.m. with upwards of 1,000 passengers. The train chugged slowly up the palm-fringed coast until about 9:30 a.m., when the tsunami struck. It knocked the railroad cars off the track and rolled them into a thick marsh, killing more than 800 pas­ sengers. The force of the tsunami was so great that wheels were torn off and tracks were twisted into odd shapes. In India, it was Full Day, and many Hindus were at the ocean's edge doing ritual bathing when they were pulled out to sea by the tsunami. In Thailand, at the peak of the tourist season, the tsunami pushed snorkelers across sharp coral reefs only to pull them back out to sea, along with sunbathers. Meanwhile, farther offshore, scuba divers enjoyed the sights in deeper water unaware of the tsunami that raced past them. Immediately following this great , people and countries of the world mobilized to bring aid to the sur­ vivors. People needed shelter, food, clean water, and sani­ tary conditions. One of the big concerns was an outbreak of disease. With outside help, diseases such as cholera, typhoid, hepatitis A, and dysentery were prevented from causing another disaster.