GS ENVS 102 CH 8 Tsunami Versus Wind-Caused Waves

HEBGEN LAKE, MONTANA, 17 AUGUST 1959 Shortly before midnight on August 17, two faults running beneath the northern end of Hebgen Lake moved in magnitude 6.3 and 7.5 earthquakes. These two normal faults had their southwestern sides drop 7 and 7.8 m (23 and 26 ft) down fault surfaces inclined 45° to 50° to the southwest, also dropping the northern end of Hebgen Lake (see figures 5.26 and 5.27). We thought the dam had broken. . . . Then we went up to the dam. When we got there we couldn't see much, but I walked over to the edge of the dam and all we could see was blackness. There was no water. And then here came the water. It had all been up at the other end of the lake. . . . We rushed back when we heard the water coming. We could hear it before we could see it. When it came over the dam, it was a wall of water about three to four feet high completely across that dam, and it flowed like that for what seemed to me to be 20 minutes, but possibly it could have been 5 or 10. Then it all cleared away, and no water again. The lake was completely dry as far as we could see. All we could see down the dam was darkness again. It seemed like a period of maybe 10 to 15 minutes, and the water came back, and then it repeated the same thing over again.

Hungerford was eyewitness to a spectacular seiche event in which lake water sloshed back and forth for 11.5 hours. A seiche is analogous to what happens to your bathwater when you stand up quickly from a full tub—it sloshes back and forth. More than 130 km2 (50 mi2) of land on the northern side of Hebgen Lake dropped down 3.3 m (11 ft). The warping of the lake floor set off a huge series of seiches.

During early November 2000, the global positioning system (GPS) measured this large block moving at an accelerated rate that peaked at 6 cm (2.3 in) per day. The days-long movement was equivalent to an earthquake of moment magnitude 5.7.

It is possible that this large hunk of Hawaii, with its more than 10,000 residents, could be plunged into the sea following a large earthquake or during a major movement of magma.

LAND SUBSIDENCE Over the centuries, as the Pacific plate subducted downward beneath Japan, it caused the upper plate carrying Japan to warp upward. When the plates came unstuck, starting the earthquake, the seafloor was thrust upward at the same time as coastal land was moved downward along a 400 km (250 mi) length of coastline.

Land subsidence was up to 1.2 m (4 ft) on the Oshika Peninsula in Miyagi Prefecture. The land subsidence is permanent. High tides and storm waves will now reach farther across the coastline and increase coastal flooding.

Rogue Waves (cont) Rogue waves have been measured at 34 m (112 ft) in height. The problems they present also include the steepness of the wave front descending into the wave trough. A small, short boat is maneuverable and in good position to ride over the rogue wave, as long as it does not get hit sideways and rolled, or tossed from the front of one wave onto the back of the next wave.

Large, long ships face either being uplifted at their midpoint, leaving both ends suspended in air, or having both ends uplifted with no support in their middle. Either case creates severe structural strains that break some ships apart.

Japanese Tsunami, 11 March 2011 (cont) The Earth's speed of rotation jolted, and this caused one of the largest earthquakes in recorded history to hit off the east coast of Japan. Most people assume that the earthquake caused a recorded polar shift, but this planet is far too massive for a solitary earthquake to move the planet's axis - the Earth shifted on its axis before the earthquake.

NASA scientists determined that the Earth's axis shifted approximately 6 1/2 inches (17 centimeters). This may not seem like a considerable movement, but it was significant enough to shift the planet and cause massive crustal movement responding to a redistribution in the planet's mass. This slip of the axis has caused the Earth to rotate faster, shortening the length of the day by about 1.8 microseconds[i]. According to Kenneth Hudnut of the U.S. Geological Survey, Japan's main island moved east about 8 feet. The largest crustal movement was off the east coast of Japan, breaking offshore about 231 miles (373 kilometers) northeast of Tokyo and 80 miles (130 km) east of Sendai. The earthquake created a massive tsunami that devastated Japan's northeastern coastal areas, and at least 20 continuous earthquakes, registering a 6.0 magnitude or higher, followed the main quake.

CHILE, 22 MAY 1960: THIRD WAVE BIGGEST (cont) Since 1960, Hawaiians have been given warnings before tsunami arrive. The Pacific Tsunami Warning Center evaluates large earthquakes in the Pacific Ocean and then, using maps like the one in figure 8.18, provides people with hours of advance warning, including shrieking sirens.

At 6:47 p.m. Hawaii time, the Chilean tsunami was predicted to arrive in Hilo, Hawaii, about midnight. The first wave arrived just after midnight and was 1.2 m (4 ft) high. At 12:46 a.m., the second wave came in about 2.7 m (9 ft) high. Many people thought the danger had passed and returned home. Then the sea level dropped 2 m (7 ft) below the low-tide level before the third wave, the largest wave, came in 6 m (20 ft) higher (15 m total?). The third wave killed 61 people in Hilo and seriously injured another 282. The tsunami raced on to Japan, where it killed another 185 people, 22.5 hours after the earthquake. The energy in this set of tsunami was so great that it was recorded on Pacific Ocean tide gauges for a week as the energy pulses bounced back and forth across the entire ocean basin.

Landslide-Caused Tsunami Gravity pulls a variety of rock and sediment masses into and beneath the seas and lakes.

Energy from these mass movements is transferred to the water, locally causing higher and larger run-ups of water than are caused by earthquake-generated tsunami.

The vastly different wavelengths and periods of windblown versus tsunami waves can be further appreciated by looking at islands.

Huge wind-blown waves such as at Waimea, Hawaii, hammer the north shore of Oahu, the windward shore. The relatively short wavelengths and periods of the Waimea waves prevent them from affecting the other shores, the leeward or protected shores.

Other clues can alert us to the possibility of powerful tsunami. Before the first big wave of a tsunami, the sea may either withdraw significantly far from shore or suddenly rise. Sometimes the ocean water changes character or makes different sounds, or something else out of the ordinary may happen. Notice these changes in ocean behavior.

If you think tsunami might be coming—take action (table 8.5). If you wait until you see the tsunami, you have waited too long. No matter how strong a swimmer you are, it is not just the water that can hurt you, but also the debris it carries (figure 8.29).

Earthquake-Caused Tsunami Fault movements of the seafloor that generate large earthquakes may also cause powerful tsunami.

In general, to create tsunami, the fault movements need to have a vertical component, either uplifting or down dropping the seafloor, and have an earthquake magnitude of at least 7.5MW (table 8.4).

Puerto Rico, 11 October 1918 Even the Atlantic Ocean has a few subduction zones along its Caribbean boundary and to the far south (see figure 2.13).

On 11 October 1918, an earthquake of magnitude 7.3Ms shook loose a 22 km (14 mi) wide submarine landslide. Tsunami up to 6 m (20 ft) high hit the northwestern Puerto Rico coastline, killing 116 people.

Papua New Guinea, 17 July 1998 At 6:49 p.m. on Friday evening, the north shore of Papua New Guinea was rocked by a 7.1 magnitude earthquake occurring about 20 km (12 mi) offshore. As the shaking ended, witnesses saw the sea rise above the horizon and shoot spray 30 m (100 ft) high. About 4 to 5 minutes later, the people again heard a rumbling sound and saw a tsunami about 4 m (13 ft) high approaching. But if you can see the wave coming, it is too late to escape. Many people living on the barrier beach were washed into the lagoon (figure 8.26).

Several minutes later, a second wave approached, but this one was about 14 m (45 ft) tall. A tsunami does not have the shape of a typical wave; it is more like a pancake of water. This tsunami averaged about 10 m (33 ft) high and measured 4 to 5 km (2.5 to 3 mi) across. This tsunami event was a three-wave sequence that washed thousands of people and their homes into the lagoon. A barrier beach that hosted four villages was swept clean. The estimated 2,200 fatalities were mostly those least able to swim—the children. The Papua New Guinea tsunami apparently was not caused directly by the earthquake but by a submarine landslide triggered by the shaking.

When tsunami approach

run fast and gain elevation—up a hill, upstairs in a strong building, or up a tree.

EARTHQUAKE-TRIGGERED MASS MOVEMENTS Earthquakes not only generate tsunami directly, but their energy can also

trigger the movement of large masses of rock or sand whose kinetic energy causes tsunami.

How tall a wave becomes depends on

(1) the velocity of the wind, (2) the duration of 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 thousands of kilometers away from the winds that created them.

It is the vertical-fault movements at subduction zones that most commonly cause tsunami (table 8.2). In the 20th century,

141 damaging tsunami combined to kill more than 70,000 people. Early in the 21st century, tsunami have killed more than 265,000 people.

NOAA has set in place 39 DART II stations close to subduction zones that can generate killer tsunami. Other countries combined have placed

19 stations on the sea floor. Data are handled and tsunami warnings issued for the Pacific Ocean by the Pacific Tsunami Warning Center in Hawaii and for the west coast of North America by the West Coast and Alaska Tsunami Warning Center in Alaska. The Japan Meteorological Agency also has a sophisticated tsunami warning system in place. Tsunami warning systems are being developed for the Indian Ocean, the Caribbean Sea, the Mediterranean Sea, and the northeast Atlantic Ocean.

Canary Islands in the Atlantic Ocean At least three of the Canary Islands have had mega-collapses: Tenerife, La Palma, and Hierro. The last known major event happened on Hierro just 15,000 years ago. When the next collapse occurs, powerful tsunami could hit coastal cities along the east coasts of North and South America and along the west coasts of Europe and Africa. Although these events are rare, they are real and can be destructive.

A computer simulation was made by Steven Ward and Simon Day assuming a 500 km3 (120 mi3) flank collapse from Cumbre Vieja Volcano on La Palma in the Canary Islands. The model forecast that tsunami with heights of 10 to 20 m (30 to 65 ft) could travel across the Atlantic Ocean and strike the east coasts of the Americas (figure 8.24).

Tsunami and You If you are near the coast and feel an earthquake, think about the possibility of tsunami. A sharp jolt and shaking that lasts a few seconds suggest that the epicenter was nearby, but the earthquake was too small to create powerful tsunami.

A relatively mild shaking that lasts for 25 or more seconds suggests that the epicenter was far away, but the energy released during that long shaking could create dangerous tsunami if the fault moved the seafloor. Remember that the most powerful earthquakes can create tsunami that kill many thousands of people who are too far away to feel the earthquake.

Tsunami at shoreline No. A tsunami arriving at the shoreline does not look like a gigantic version of the breaking waves we see every day. A typical tsunami hits the coastline like a very rapidly rising tide or whitewater wave, but it does not stop on the beach; it keeps rushing inland (figure 8.11d).

A tsunami event may begin with a drawdown or retreat of sea level if the trough of the wave reaches shore first. The drawdown can cause strong currents that pull seawater, boats, and swimmers long distances out to sea. Or the tsunami wave crest or front may reach shore first. Then a strong surge of seawater, resembling a faster and stronger rising tide, rushes across the beach and pushes far inland.

ALASKA, 1 APRIL 1946: FIRST WAVE BIGGEST As 1 April 1946 began in the Aleutian Islands, two large subduction movements occurred and shook the area severely. The five workers in the Scotch Gap lighthouse were shaken awake and wondered what lay ahead during the dark night. The lighthouse was built of steel-reinforced concrete, and its base sat 14 m (46 ft) above mean low-water level (figure 8.17a).

About 20 minutes after the second earthquake, a tsunami approximately 30 m (100 ft) high swept the lighthouse away (figure 8.17b). This time, the first wave was the biggest; it killed all five men. Tsunami are not just local events. The waves race across the entire Pacific Ocean. The April Fool's Day tsunami traveled about 485 mph in the deep ocean, slowing to about 35 mph as it neared shore in Hilo, Hawaii. Humans are no match for these massive waves. The long wavelengths allow the wave front to rush on land for long distances; there is no trough immediately behind, waiting to pull the water back to the ocean. This tsunami killed 159 people in Hilo, Hawaii. One of the ironies of this tragedy is that some of those killed were people who were warned. After being told a tsunami was coming, some folks laughed, said they knew it was April Fool's Day, and ignored the warning.

NICARAGUA, 1 SEPTEMBER 1992 Can a major earthquake occur nearby and offshore without you feeling it—and yet send killer tsunami? Yes, on a Tuesday evening at 6:16 p.m., an unusual earthquake occurred that was large (magnitude 7.6) but barely felt.

About 45 minutes later, a 10 m (33 ft) high tsunami ravaged a 300 km (185 mi) long section of the Nicaraguan coastline. What happened? Why did the ground shake very little, yet the ocean water still become agitated into large tsunami? The fault moved so slowly that the high-velocity, short-period seismic waves were relatively weak and shook the ground only slightly, about 1/100 of what is expected. However, the slow-moving fault released a lot of long-period energy into the water, thus creating powerful tsunami. The earthquake was a subduction event in which a 100 km (62 mi) long segment of oceanic plate moved 1 m in 2 minutes. The slow-motion fault movement was especially efficient at pumping energy into water. The seawater absorbed the energy, sending tsunami onto the beach. The coastal residents were caught without warning; 13,000 homes were destroyed, and 170 people were killed, mostly sleeping children.

CHILE, 22 MAY 1960: THIRD WAVE BIGGEST The most powerful earthquake ever measured occurred in Chile on 22 May 1960. Tsunami generated by this magnitude 9.5 subduction movement killed people throughout the Pacific Ocean basin. In Chile, the main seism broke loose at 3:11 p.m. on Sunday. Chileans are familiar with earthquakes, so many people headed for high ground in anticipation of tsunami. About 15 minutes after the seism, the sea rose like a rapidly rising tide, reaching 4.5 m (15 ft) above sea level. Then the sea retreated with speed and an incredible hissing and gurgling noise, dragging broken houses and boats out into the ocean. Some people took the "smooth wave" as a sign that these tsunami could be ridden out at sea, thus saving their boats.

About 4:20 p.m., the second tsunami arrived as an 8 m (26 ft) high wave traveling at 125 mph. The wave crushed boats and their terrified passengers, as well as wrecking coastal buildings. But the third wave was the largest; it rose 11 m (35 ft) high, but it traveled at only half the speed of the second wave. More than 1,000 Chileans died in these tsunami.

Fukushima Daiichi Nuclear Disaster A triple disaster occurred in Japan on 11 March 2011. First, a magnitude 9 earthquake broke loose. Second, tsunami were sent racing toward the coastline. Third, the three operating nuclear reactors at Fukushima Daiichi power plant automatically shut down soon after the earthquake. But 41 minutes later, tsunami burst through the plant's defenses and flooded the nuclear reactor buildings. The tsunami broke the connections to the power system and knocked out the emergency generators, leaving the plant without power to run the cooling systems.

As bad as these failures are, they could have been worse. The skilled workforce risked their lives and stayed inside the plant, despite blackout conditions. Japanese officials initially assessed the disaster as Level 4 on the International Nuclear Event Scale, but as the days went by, the level was successively raised until it reached Level 7, the maximum scale value. A year after the tsunami, the Japanese government estimated it will take 40 years to clean up and decommission the three reactors.

BRITISH COLUMBIA, WASHINGTON, OREGON, 26 JANUARY 1700 The ghost forests of gray, dead tree trunks still standing in tidal marshes today were described in the 1850s. The annual growth rings of the trees were thick and healthy to the end of their lives, which ended rather quickly. Sea level rising did not kill the trees, the culprit was the land subsiding rapidly below sea level during a great earthquake (see chapter 4 and figures 4.16 and 4.17).

As much as an 800 km (500 mi) length of coastal land—from Vancouver Island, Canada, through Washington, Oregon, and into northernmost California—dropped 1 to 2 m (6.5 ft) during an earthquake of about magnitude 9.

INDIAN OCEAN 26 DECEMBER 2004 On 26 December 2004 (Boxing Day), a killer tsunami swept through the Indian Ocean and crossed Asian and African shorelines, causing death and destruction in 14 countries (see figure 8.1). The estimated death total was 245,000, but the true number is almost certainly higher and will never be known.

Countries hit especially hard were Indonesia (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 tsunami threat when you are vacationing at a beach resort (figure 8.15).

The long wavelengths and periods of tsunami allow them to bend around many islands and hit all shores with high waves. Tsunami wavelengths typically are longer than the dimensions of an island.

During the 26 December 2004 Indian Ocean tsunami, the island nation of Sri Lanka was hit by 4 to 7 m (13 to 23 ft) high tsunami at sites on all shores (figure 8.13). The tsunami were directed at the east shore, 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.

Tsunami Versus Wind-Caused Waves Although the periods and wavelengths of wind-blown waves vary by storm and season, they are distinctly different from those of tsunami (table 8.3). Wind-blown waves rise up as they near the beach, roll forward, run up the beach for several seconds, and then withdraw (figure 8.11a). Wind-blown waves not only come and go quickly, but the water run-up and retreat is confined to the beach (figure 8.11b).

Even huge wind-blown waves are different from tsunami. For example, at Waimea on the north shore of Oahu Island in Hawaii, the world-famous surfing waves may reach 15 m (50 ft) in height, but each wave is a solitary unit. These huge waves have short wavelengths and brief periods, meaning that each wave is an entity unto itself; there is no additional water mass behind the wave front. These waves are spectacular to view or ride, but what you see is what you get; the wave is the entire water mass.

Seiches Seiches are oscillating waves that slosh back and forth within an enclosed body of water, such as a sea, bay, lake, or swimming pool. The word seiche (pronounced "SAY-sh") comes from a Swiss-French word that means to sway back and forth. The energy to cause a seiche can come from a variety of sources. Winds blowing across a lake can cause the water body to oscillate at some natural period. Seiches are common in the Great Lakes of Canada and the United States, where they may be called sloshes.

For example, Lake Erie with its elongate shape and relatively shallow water can experience seiches when strong winds blow. The oscillating water mass can form seiches up to 5 m (16 ft) high, alternating from one end of the lake to the other.

When the wave height-to-wavelength ratio (H:L) reaches about 1:7, the wave front has grown too steep, and it topples forward as a breaker (figures 8.7 and 8.8).

Note that the 1:7 ratio is reached by changes in both wave height and wavelength; wave height is increasing at the same time that wavelength is decreasing. The depth of water beneath a breaker is roughly 1.3 times the wave height as measured from the still-water level. At this depth, the velocity of water-particle motion in the wave crest is greater than the wave velocity, thus the faster-moving wave crest outraces its bottom and falls forward as a turbulent mass.

Seismic Signals and Pressure Sensors After the mega-killer Indian Ocean tsunami in 2004, the interest in and support for tsunami warning systems increased significantly.

Now, rapid analyses of seismic waves for their tsunami-generating potential begin with the arrival of the first P waves. Quick initial determinations are made of epicenter location, depth to hypocenter, earthquake magnitude, and vertical versus horizontal components of fault movement.

Rogue Waves (deaths) 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 recreational fishing boat Fish-n-Fool sank beneath a sudden "wall of water" in the Pacific Ocean near a Baja California island.

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 seas became rough. Then a thunderous disruption shocked people as a freak 22 m (70 ft) high wave slammed into the ship, breaking windows, sending furniture flying, flooding more than 60 cabins, and injuring four passengers. The wave even ripped out whirlpools on deck 10. On occasion, rogue waves strike the shoreline and carry people away from the beach. On 4 July 1992, a rogue wave 5.5 m (18 ft) high rose out of a calm sea at Daytona Beach, Florida, crashed ashore, and smashed hundreds of cars parked on the beach, causing injuries to 75 of the fleeing people.

KRAKATAU, INDONESIA, 26-27 AUGUST 1883 - Explosions Krakatau sits in the sea between the major Indonesian islands of Sumatra and Java (figure 8.20). One of the most famous eruption sequences in history occurred here in 1883, including the killing events of 26-27 August. In the evening, some of the eruptions blasted large volumes of gas-charged rocky debris rapidly down slope, across the shoreline, and into the sea, putting energy into the water that radiated outward as tsunami that ravaged villages on distant shorelines. The highly irregular coastline in this region affects tsunami height and run-up in the various harbors, inlets, and peninsulas (figure 8.20).

On Monday morning, gigantic explosions occurred around 5:30, 6:45, and 8:20. Each explosion sent tsunami with their maximum energy focused in different directions, wiping out different villages. These explosions may have been due to seawater coming in contact with the magma body and rapidly converting the thermal energy of the magma into the mechanical energy of tsunami. The eruption sequence culminated about 10 a.m. with an overwhelming explosion that is commonly attributed to the volcano mountain collapsing into the void formed by its partly emptied magma chamber. The resulting blast was heard for thousands of miles. Tsunami pushed into harbors and ran up and over some coastal hills up to 40 m (135 ft) high. The tsunami during this 20-hour period destroyed 165 villages and killed more than 36,000 people. This tsunami death total was not exceeded until 26 December 2004, again in Indonesia.

Difference between Tsunami and wind-caused waves

Period Length Speed The contrasts in velocities of wind waves versus tsunami are also great. A wind-blown wave moving through water deeper than half its wavelength (L) has its velocity (v) determined by v = 1.25 √L Tsunami velocity requires a different calculation. Tsunami wavelengths are so long and so much greater than the depth of the deepest ocean that their velocity is calculated by: v = √gD A tsunami of 1 m height in the deep ocean may be moving nearly 500 mph. As tsunami enter shallower water, the increasing friction with the seafloor and internal turbulence of the water slow their rush, but they still may be moving at freeway speeds.

The National Oceanic and Atmospheric Administration (NOAA) has pre-computed scenarios for tsunami-generating earthquakes that could occur along the subduction zones all around the Pacific Ocean. Early tsunami watches and warnings now can be issued within several minutes.

The NOAA earthquake/tsunami scenario closest to the epicenter becomes improved by measurements made by pressure sensors that are anchored to the seafloor. The pressure sensors are DART II (Deep-ocean Assessment and Reporting of Tsunami) stations (figure 8.32). As tsunami pass by, changes in water pressure are recorded by the bottom pressure sensors.

TSUNAMI WARNINGS Sitting in the middle of the earthquake-prone Pacific Ocean basin, Hawaii is hit by numerous tsunami. The heights and run-ups of tsunami at the shoreline vary due to differences in local topography, both onshore and offshore.

The broad-scale threats to homeowners and businesses are presented in a tsunami-hazard map for the Big Island of Hawaii (figure 8.30).

HUMANS CAN INCREASE THE HAZARD Mapping of coastlines in Indonesia, India, and Sri Lanka following the killer tsunami of 2004 showed how human activities increased the damages and life loss in some areas.

The coastal areas where forests had been removed suffered more extensive damage than neighboring areas with the natural vegetation intact. Trees and shrubs reduce the amplitude and energy of incoming waves. With the forest gone, houses, bridges, and other human-built structures were left to absorb the tsunami energy. In Sri Lanka, many of the hardest-hit coastlines were ones where coral reefs had been removed. Coral reefs there are mined for souvenirs to sell, removed to open beaches for tourists to use, and blown up to stun and catch the fish inside them. With coral barrier reefs removed, tsunami charge farther inland and with greater energy.

Newfoundland, Canada, 18 November 1929 At 5:02 p.m. on 18 November 1929, an earthquake of magnitude 7.2Mw occurred offshore of eastern Canada (figure 8.25).

The earthquake triggered the submarine movement of a sediment mass with an estimated volume of 200 km3 (50 mi3). Especially hard hit was the Burin Peninsula of Newfoundland, where about 40 villages were damaged and 28 people died. The tsunami began arriving about 2.5 hours after the earthquake; the waves came in three major pulses during a 30-minute interval. The long narrow bays caused 1 m high tsunami to build to 3 m (10 ft) in many inlets and to 7 m (23 ft) in Taylor's Bay. When tsunami reached the heads of inlets and bays, they had so much energy and momentum that their run-up onto land reached 13 m (43 ft) elevation in some areas, causing significant damage. The tsunami dealt a crippling blow to the local fishing industry and almost drove Newfoundland into bankruptcy.

SIMEULUE ISLAND, INDONESIA, 26 DECEMBER 2004 Simeulue Island is the inhabited land closest to the epicenter of the magnitude 9.1 earthquake in 2004. Within 30 minutes of the earthquake, tsunami 10 m (33 ft) high ravaged their earthquake-damaged coastal villages and destroyed much of what remained. After the earthquake and tsunami, a count of the residents found that only 7 out of 75,000 inhabitants had died. Why were so few people killed?

The islanders remembered the stories passed down as oral history from their ancestors: when the ground shakes, run to the hills before the giant waves arrive. Remembering the lessons of their history paid off big for the Simeulue islanders.

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 (figure 8.16).

The last major tsunami event in the Indian Ocean occurred in 1883 when the volcano Krakatau collapsed into the sea, also offshore from Sumatra. The 1883 tsunami killed about 36,000 people. The dramatic growth of the human population during those 121 years helped increase the death total to 245,000 people in the 2004 tsunami.

Lituya Bay, Alaska, 9 July 1958 The largest historic wave run-up known occurred on 9 July 1958, when a massive rockfall dropped into Lituya Bay in Glacier Bay National Park, Alaska (figure 8.27). It was after 10 p.m. when the Fairweather fault moved in a 7.7M earthquake, causing about 90 million tons of rock and ice to drop more than 900 m (3,000 ft) into the water. Three boats were anchored in the bay. One was a 40 ft fishing boat operated by a father and son who later reported an earsplitting crash that caused them to look up the bay, only to see a huge wall of water about 30 m (100 ft) high roaring toward them faster than 100 mph. They only had enough time to turn the bow (front) of the boat toward the wave.

The onrushing tsunami swept over 54 m (176 ft) high Cenotaph Island, and then it hit them. Their anchor chain snapped, and the boat soared near vertically upward like a high-speed elevator in a tall building. Reaching the crest of the wave, they dropped down the back side and survived. The second boat was carried across the sandbar beach into the ocean, and the crew survived. The third boat fired up its engine and tried to outrun the tsunami; this was a bad decision. The wave hit that boat on the stern (backside), flipped it, destroyed it, and killed the crew. Looking at beautiful Lituya Bay after the traumatic evening showed that the rock-fall impact had sent a surge of water up the opposite slope, stripping away mature trees up to 525 m (1,720 ft) above sea level. The tsunami destroyed and stripped away mature trees along both walls of the bay, 35 m (110 ft) above sea level, all the way to the open ocean.

Deep-Water Wave Velocity, Length, Period, and Energy Waves moving through water deeper than one-half their wavelength are essentially unaffected by friction with the bottom. The waves move as low, broad, evenly spaced, rounded swells with velocities related to wavelength by: VW = 1.25 √L where Vw equals wave velocity and L equals wavelength. ex: A swell with 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 equation is telling us that wave velocity in deep water depends on the wave's length—as wavelength increases, so does velocity.

The period (T) is the amount of time it takes for two successive wave crests to pass a given point. Since the distance between successive wave crests is the wavelength, there must be a relationship between period (T) in seconds and wavelength (L) in meters. This relationship may be defined by: V = distance traveled/time = L/T which may be simplified to: L = 1.56T^2 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 example, waves with a period of 10 seconds are moving about 35 mph.

INDIAN OCEAN 26 DECEMBER 2004 (cont) The seafloor west of northern Sumatra in Indonesia overcame frictional resistance, triggering faulting that ruptured northward for 1,200 km (740 mi) during almost 7 minutes; this 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 earth rupture occurred due to compression caused by the subducting Indian-Australian plate (see figure 4.10) that raised the seafloor tens of feet, thus charging the seawater with energy that rapidly moved outward as tsunami racing throughout the Indian Ocean.

Subduction zones generate more killer tsunami than any other source.

The subduction zone along the coastline reaching from British Columbia, Washington, Oregon, and northernmost California is active (figure 8.4). Subduction is still occurring, volcanoes are still erupting, and moderate size earthquakes are shaking—as we build to the upcoming 9M earthquake and tsunami.

The Chile Tsunami of 1868 Several ships were moored in the harbor at Arica (then part of Bolivia), including the USS Wateree, a two-masted side wheeler with a broad, flat bottom. About 4 p.m., the ship began vibrating and chains rattled as a huge earthquake shook down houses in Arica.

The sun rose upon such a spectacle of desolation as can rarely have been seen. We were high and dry, three miles from our anchorage and two miles inland. The wave had carried us at an unbelievable speed over the sand dunes which line the shore, across a valley, . . . leaving us at the foot of the coastal range of the Andes. Note several items in this thrilling account: (1) After the first earthquake, the initial seawater reaction was a deadly onrushing mass of water that killed people on the jetty. (2) After the second earthquake, the initial seawater reaction was a huge withdrawal of the sea, leaving ships sitting on the seafloor. (3) The biggest tsunami occurred hours after the earthquakes. (4) The phosphorescent glow of the huge incoming tsunami is a commonly reported phenomenon due to bioluminescence of small sea life caught up in the monster wave. (5) A large U.S. warship was carried 2 mi inland.

Lake Tahoe, California and Nevada The lake is 35 km (22 mi) long, 19 km (12 mi) wide, and over 500 m (1,600 ft) deep; it is the 10th deepest lake in the world. This broad and deep lake was created by subparallel normal faults dropping the land between them (figure 8.28). The faults are active.

The underwater faults have a 4% probability of causing a magnitude 7 earthquake in the next 50 years. The likely fault movement could drop the lake bottom about 4 m (13 ft) and could generate 10 m (33 ft) high waves that rush over the populated shoreline.

Japanese Tsunami, 11 March 2011 (cont) Recapping the event At 2:46 p.m. on a Friday, a powerful earthquake broke loose 70 km (43 mi) offshore from northeastern Japan, with the most violent earth movements lasting 6 minutes. The seafloor thrust upward 5 to 8 m (16 to 26 ft), loading the seawater with energy that drove tsunami ashore less than 30 minutes later.

The unstoppable tsunami inflicted severe destruction and death along a 670 km (415 mi) length of coastline, pushed inland as far as 10 km (6 mi) in Sendai, ran up as high as 40.5 m (133 ft) above sea level in Miyako, inundated 560 km2 (215 mi2) of land, damaged or destroyed 330,000 buildings, ruined roads and railways, caused one dam to fail, triggered numerous fires, and caused nuclear-power plants to fail. The deaths were caused by tsunami drowning or blunt force (93%), earthquake-collapsed buildings (4%), burns (1%), and unknown (2%).

Where might an event like this happen next? Quite likely on the active volcano Kilauea on the southeast side of the Big Island of Hawaii (figure 8.22).

The zones of normal faults associated with magma injection in the active volcano Kilauea also appear to be head scarps of giant mass movements (figure 8.23; see figure 5.44). One moving mass on southeastern Kilauea is more than 5,000 km3 (1,200 mi3) in volume and is sliding at rates up to 25 cm/yr (10 in/yr). The slide mass extends offshore about 60 km (37 mi) to depths of about 5 km (3 mi) (figure 8.23). The area on the moving mass includes the 80 km (50 mi) long coastal area southeast of Kilauea.

VOLCANO COLLAPSES Hawaii in the Pacific Ocean The largest submarine mass movements were recognized first on the seafloor along the Hawaiian Islands volcanic chain. Slump and debris-avalanche deposits there cover more than five times the land area of the islands (figure 8.21). Some individual debris avalanches are more than 200 km (125 mi) long with volumes greater than 5,000 km3 (1,120 mi3), making them some of the largest on Earth.

These events are not just loose debris sliding down the side of the volcano; they are catastrophic flank collapses where the whole side of an oceanic volcano breaks off and falls into the sea. There have been at least 70 flank collapses from the Hawaiian Islands in the past 20 million years. Pause and think about this a moment: each Hawaiian Island has major structural weaknesses that lead to massive failures. The not-so-solid Earth here betrays us; it can fail rapidly and massively. For example, the island of Molokai has no volcano. Where did it go? Apparently, the northern part of the island fell into the ocean (figure 8.21), leaving steep cliffs behind.

Tsunami Versus Wind-Caused Waves (cont) Tsunami are different. Tsunami arrive as the leading edge of an elevated mass of water that rapidly runs up and over the beach and then floods inland for many minutes (figures 8.11c, d). Tsunami arrive as a series of several waves separated by periods usually in the 10- to 60-minute range. The waves are typically a meter high in the open ocean and 6 to 15 m (20 to 50 ft) high on reaching shallow water, except where topography, such as bays and harbors, focuses the energy to create much taller waves.

They may be no taller than the wind-blown waves we see at the beach every day, but they are much more powerful. Even a knee-high tsunami can kill you. The power of the fast-moving water can knock you down, then beat your body and head with debris, and then drown you. Tsunami wavelengths can be as great as 780 km (485 mi) (table 8.3), meaning that ocean water would be disturbed to depths of 390 km (240 mi). But the ocean's average depth is only 3.7 km (2.3 mi), and the deepest trenches just exceed 11 km (6.9 mi). Therefore, the energy pulse of a tsunami moves the entire water column it passes through. Tsunami have such long wavelengths that they are always dragging across the ocean bottom, no matter how deep the water. The ocean basin has enough topography on its bottom to slow most tsunami down to the 420 to 480 mph range.

In a subduction zone, the plates may become stuck together (figure 8.14a). Because the overriding plate is stuck to the subducting plate, its seaward (leading) edge is dragged downward while the area behind (landward) bulges upward (figure 8.14b).

This movement goes on for centuries, building up elastic strain all the while. When the stuck area ruptures, causing an earthquake, the leading edge of the overriding plate breaks free, springs seaward and upward, causing a tsunami (figure 8.14c). At the same time, the landward bulge warps downward, lowering coastal land below sea level. Tsunami race through the ocean for hours, but the subsided land will remain down for generations (figure 8.14d).

Earthquakes frequently cause seiches. For example, a large earthquake near Lake Tahoe may rock the land and water enough that the water body continues oscillating in seiches.

This seiche is different from the tsunami caused by a large landslide moving into Lake Tahoe. People living in seismically active areas commonly get to watch seiches during an earthquake when the water in swimming pools sloshes back and forth and overflows the sides.

ALASKA, 27 MARCH 1964: FIFTH WAVE BIGGEST The Good Friday earthquake in Alaska was a magnitude 9.2 monster whose tsunami ravaged the sparsely populated Alaska coastline, killing 125 people. Alaska sits on the upper plate and was shifted horizontally (seaward) up to 19.5 m (64 ft) and uplifted as much as 11.5 m (38 ft). Another landward block was down-dropped as much as 2.3 m (7.5 ft). Some of the seafloor offsets were even greater. More than 285,000 km2 (110,000 mi2) of land and sea bottom were involved in these massive movements. More than 25,000 km3 (6,000 mi3) of seawater were jolted and moved. The sudden uplift of this huge volume of water resulted in tsunami racing through the entire Pacific Ocean.

Three hours after the earthquake (see figure 8.18), Port Alberni on Vancouver Island was hit by a 6.4 m (21 ft) high tsunami that destroyed 58 buildings and damaged 320 others. Thanks to advance warning, the Crescent City, California, waterfront area was evacuated, and residents waited upslope while tsunami arrived and did their damage. After watching four tsunami and seeing their sizes, many people could no longer stand the suspense of not knowing the condition of their properties. Some people went down to check them out—a big mistake. In this tsunami series, the fifth wave was the biggest; it was 6.3 m (21 ft) high, and it killed 12 of the curious people. All the fatalities of this event at Crescent City were caused by the fifth tsunami.

A deadly example hit Japan on 15 June 1896. An offshore earthquake swayed the seafloor; then, about 20 minutes later, the sea withdrew, only to return in 45 minutes with a sound like a powerful rainstorm.

Tsunami hit all the beaches hard but reached their greatest heights of 29 m (95 ft) where they crowded into narrow inlets. The tsunami destroyed more than 10,000 homes and killed more than 27,000 people. The fishermen on the 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.

WHY A WIND-BLOWN WAVE BREAKS Waves undergo changes when they move into shallow water—water with depths less than one-half their wavelength.

Wave friction on the floor of the shallow ocean interferes with the orbital motions of water particles, so waves begin slowing (figure 8.6). As waves slow down, their wavelengths decrease, thus concentrating water and energy into shorter lengths and causing the waves to grow higher.

Tsunami The country with the most detailed history of these killer waves is Japan, and the waves are known by the Japanese word tsunami (tsu = harbor; nami = waves). The reference to harbor waves emphasizes the greater heights waves reach in inlets and harbors because the narrowed topography focuses the waves into smaller spaces. For example, an 8 m (26 ft) high wave on the open coast may be forced to heights of 30 m (100 ft) as it crowds into a narrow harbor.

a long high sea wave caused by an earthquake, submarine landslide, or other disturbance.

If a small one-kilometer asteroid (0.621371 miles in diameter) hit a landmass, it would create a dust cloud that would block out sunlight for

at least a year. These sizes seem too big to be real, but in comparison to the vastness of the Universe, these dimensions are actually very small.

As many as 32,000 underwater mountains have been identified around the world, and the majority are believed to be volcanic. Several thousand of these may be active, but very few have

been studied because of their depth beneath the ocean and their remoteness.

As awareness of tsunami hazards increases, signs are

being posted in coastal areas of many states to provide warning (figure 8.31).

The Indian plate subducts northward beneath the Burma plate (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,

causing adjoining areas to move downward (figure 8.10). The uplifts and down warps 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 tsunami water surface had shapes similar to those of the newly deformed seafloor topography that generated them.

When a violent exchange of rock and seawater is this pronounced, it can

destabilize the volcanic cone and cause a landslide and tsunami.

The Newfoundland tsunami is not an isolated event. Beneath the Atlantic Ocean off the

east coast of North America, new images of the seafloor show significant scars where big submarine landslides have occurred. Similar landslides in the future will generate tsunami.

When a large volume of ocean water is suddenly moved, gravity causes waves to be

generated that spread out from the disturbance. Imagine the size of the waves formed when a really big rock drops in the water, such as the caldera collapse of the volcano Krakatau in 1883.

Volcanoes can also cause a tsunami. In May 2012, scientists were shocked to discover a dangerous problem that had gone unnoticed. The Monowai volcano near Tonga, 300 miles SSE of the Fiji Islands, had

grown to huge heights in just two weeks, and the revelation was "a wake-up call" that the sea-floor is now more dynamic than they ever imagined. Even scientists studying the seabed have fallen prey to the Normalcy Bias, and assumed that the ocean depths would always remain stable.

Japanese Tsunami, 11 March 2011 Japan is the nation most prepared for tsunami. The Japanese have built

high walls to stop tsunami along much of their coastline, constructed huge metal gates at the entrances to some harbors, and created warning systems to alert the people.

The biggest tsunami are caused by the rarest events, the impact of

high-velocity asteroids and comets. Consider the amount of energy injected into the ocean when a 10 km (6 mi) diameter asteroid hits at 30,000 mph.

Subsidence of coastal land is common during great earthquakes caused by subduction, as occurred in Japan in 2011,

in Indonesia in 2004, in Alaska in 1964, and in Chile in 1960. Land subsidence was a starting point for recognizing a ∼9M earthquake in prehistoric time in the Pacific Northwest.

ROGUE WAVES An ocean is such an extensive body of water that different storms are likely to be operating in different areas. Each storm creates its own wave sets. As waves from different storms collide, they

interfere with each other and usually produce a sea swell 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 rogue wave (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 disaster may occur.

Most human populations are largely concentrated on coastlines, so if a 200 km (124.274 miles in diameter) object hit the Pacific Ocean,

it would create a tsunami of catastrophic destruction.

Wavelength and Period Versus Height People tend to attribute the destructive power of tsunami mostly to the great height of their waves, but the height of tsunami commonly is

not as important as the momentum of their large masses separated by ultra-long wavelengths and periods (table 8.3). Visualize a flat or gently sloping coast hit by tsunami with a 60-minute period. The tsunami can rush inland, causing destruction for about 30 minutes before the water is pulled back to help form the next wave. A view of the aftermath of the 1960 Chilean tsunami in Hilo, Hawaii, shows the effects of the long wavelengths and long periods between tsunami wave sets (figure 8.12). The powerful tsunami was able to charge upslope, through the city, for a long distance and a long time before receding to help form the next tsunami wave set.

Earthquakes in the Tonga region are

on the rise, and a quake can suddenly break apart the seafloor.

Seismic detectors on the Cook Islands discovered violent activity around the Tonga volcano over a mere five-day period. Overnight,

part of the volcano's summit had collapsed by as much as 18.8 meters (62 feet), while new lava flows had raised another area by 79 meters (259 feet). Most shocking was the rapid creation of an entirely new volcanic cone. These changes are bigger than most volcanoes in modern history. Only Vesuvius and Mount St Helens have recorded larger growth rates.

What happens to the ocean when a gigantic chunk of island drops into it and flows rapidly underwater? Huge tsunami are created. For example,

prehistoric giant waves washed coral, marine shells, and volcanic rocks inland, where they are found today as gravel layers on Lanai lying 365 m (1,120 ft) above sea level and on Molokai more than 2 km inland and more than 60 m (200 ft) above sea level. Tsunami of this size would not only ravage Hawaii but would cause death and destruction throughout the Pacific Ocean basin.

In the United States, tsunami have been called "tidal waves," but this is

rather silly because tsunami have nothing to do with the tides. Nor do tsunami have anything to do with winds or storms; they are created by huge injections of energy or "splashes" in deep ocean water by fault movements, volcanic eruptions or caldera collapses, underwater landslides, meteorite impacts, and such.

No longer can we assume that big tsunami are only caused by giant earthquakes in distant places. Tsunami can be created by

smaller local faults that cause unstable sand and rock masses to slip and slide under water. For example, Southern California is protected from Pacific Ocean basin tsunami by its offshore system of sub-parallel island ridges. But these ridges have been created by active faults, and their movements could trigger undersea landslides that send tsunami across the densely populated Southern California coastline.

IN BAYS AND LAKES The constricted topography of bays and lakes allows

some landslides to create huge tsunami of local extent.

Tsunami are most commonly created during earthquakes, more specifically

subsea fault movements with pronounced vertical offsets of the seafloor that disturb the deep ocean 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.

"Tilly's story is a simple reminder that education can make a difference between life and death. All children should be

taught disaster reduction so they know what to do when natural hazards strike." In 2004, nations throughout the Indian Ocean region were unprepared for tsunami, and 245,000 people were killed in 14 countries (figure 8.1).

Wind-Caused Waves Waves transfer energy away from some disturbance. Waves moving through a water mass cause water particles to rotate in place, similar to the passage of seismic waves (figure 8.5; see figure 3.18). You can feel the orbital motion within waves by standing chest-deep in the ocean. An incoming wave will pick you up and carry you shoreward and then drop you downward and back as it passes. At the water surface, the diameter of the water-particle orbit is the same as

the wave height. The diameters of water orbits decrease rapidly as water deepens; wave orbital motion ceases at a depth of about one-half of the wavelength. Most waves are created by the frictional drag of wind blowing across the water surface. A wave begins as a tiny ripple. Once formed, the side of a ripple increases the surface area of water, allowing the wind to push the ripple into a higher and higher wave. As a wave gets bigger, more wind energy is transferred to the wave.

Japanese Tsunami, 11 March 2011 (cont) Despite careful preparation, their defenses were overrun in many areas by powerful tsunami on 11 March 2011 (figure 8.2). The tsunami caused

the world's most expensive natural disaster and killed 19,184 people. The earthquake 70 km (43 mi) off Japan's northeast shore was a 9.0 Mw; it was felt by everyone in the region (see chapter 4). The Japanese know tsunami. After a huge earthquake they know what's coming. About nine minutes after the earthquake, warnings of a major tsunami were issued by the Japan Meteorological Agency at the highest end of the warning scale.

Which wave in the series will be the biggest? As the preceding events show, it is

unpredictable. In 1946, 1960, and 1964, the biggest wave was the first, third, and fifth, respectively.

As we build to the upcoming 9M earthquake and tsunami Major cities such as Victoria, Vancouver, Seattle, Tacoma, and Portland are built along

water inlets; they will probably be most affected by the great earthquake and less by the tsunami. The small cities and towns along the Pacific Ocean coastline will be hammered by the tsunami, as seen in Japan in 2011.

Deep-Water Wave Velocity, Length, Period, and Energy (cont) Higher-velocity waves carry more energy, but how much more? Wave energies per unit length can be computed with the following relationship: Ew = 0.125ρ gH^2L

where Ew equals wave energy, ρ (rho) equals density of water, g equals gravitational acceleration, H equals wave height in meters, and L equals wavelength. Some representative values computed from this equation are listed in table 8.1. Notice that doubling the wavelength doubles the wave energy, but that doubling the wave height quadruples the wave energy.

Volcano-Caused Tsunami Volcanic action can create killer tsunami (see table 8.2). Volcanoes can put jolts of energy into a water body in several ways.

• They can explode, • they can collapse, and • they can send avalanches of debris into the water. It seems likely that tsunami were generated by all three mechanisms during the eruption of Krakatau in 1883.

LEARNING OUTCOMES Tsunami and everyday waves have different origins and characteristics. After studying this chapter you should

• know how wind creates the everyday waves in water, how water moves within a wave, and how waves break on the shoreline. • be able to explain four different origins for tsunami. • understand the differences in wavelength and period between wind-caused waves and tsunami. • comprehend how elastic strain builds up at subduction zones and finally is released creating killer tsunami. • be able to explain, in order, the actions to take to survive a tsunami. • be familiar with tsunami-warning systems.

TSUNAMI TRAVEL THROUGH THE PACIFIC OCEAN After the earthquake, a computer-generated map was issued predicting tsunami heights throughout the Pacific Ocean basin (figure 8.3). Actual tsunami heights and experiences include: ∙ Canada, Vancouver Island: 1 m (3.3 ft) high surges. ∙ Oregon, Curry County: 2.4 m (8 ft) high surges caused $7 million in harbor damages. ∙ California, Crescent City: 2.4 m (8 ft) high tsunami damaged docks and 35 boats, and killed one person.

∙ California, Santa Cruz: 2.4 m (8 ft) high surges did $17 million in damage to the harbor, sank 18 boats, and damaged others for another $4 million in losses. ∙ Hawaii: 3 m (10 ft) high waves did tens of millions of dollars in damage to boats, houses, and hotels, especially in Kona. ∙ Midway Atoll: 1.5 m (5 ft) high tsunami swept across Spit Island, killing more than 110,000 nesting sea birds. ∙ Chile: 18,000 km (11,000 mi) away, 3 m (10 ft) high tsunami damaged more than 200 houses. ∙ Antarctica: tsunami broke up a 125 km2 (48 mi2) area of Sulzberger Ice Shelf into 80 m (260 ft) thick icebergs.