9.9: Tsunamis

Although tsunamis are relatively rare and most tsunamis are small enough to cause no harm, a large tsunami can be devastating. For example, the December 2004 tsunami caused the deaths of many thousands of people around the Indian Ocean, and the March 2011 tsunami also killed thousands in Japan. Tsunamis as large as these are not frequent events but most people do not know that far bigger tsunamis have occurred in the past and will occur again. 

Tsunamis can be generated when an earthquake or volcanic eruption moves a section of seafloor or coast, when a meteorite or coastal land collapse impacts the sea surface, or when an undersea landslide takes place. Each of these events can produce an abrupt displacement of the ocean water that causes the water column to oscillate, generating waves that generally have much longer periods than wind waves. Tsunamis are not related to tides, but they are often incorrectly called “tidal waves.” Tsunami is a Japanese word that is translated as “harbor wave,” but tsunamis are definitely not restricted to harbors.

Not all undersea earthquakes, volcanic eruptions, or landslides cause tsunamis. Tsunamis are most likely to be created when such seismic events cause a section of seafloor to move vertically or to slump. The sudden vertical movement either pushes up the seafloor and overlying water to form a wave crest or lowers the seafloor and overlying water to form a wave trough. The event may cause only one, almost instantaneous, vertical movement of the seafloor. Nevertheless, a series of several waves is created as the water oscillates before returning to a level configuration. This series resembles the waves created by a stone thrown into a pond, radiating outward from the stone’s impact point. Hurricanes and extratropical cyclones can cause a different phenomenon, called a storm surge, that elevates the sea surface and may cause flooding and damage similar to that caused by tsunamis. These storm surges are not considered to be wind-driven waves, although they behave in a similar manner. They are caused by elevation of the sea surface created by the lower atmospheric pressure at the center of the storm (Chap. 7).

Tsunamis consist of a series of waves with extremely long wavelengths (typically 100 to 200 km) and periods (typically 10 to 30 min). Only a very small fraction of the ocean is deeper than 6 km (Fig. 4-3), and half of the ocean floor is less than 4 km deep. Hence, the water depth is almost always less than one-twentieth of the tsunami’s wavelength, which causes tsunamis to behave as shallow-water waves. The speed of a shallow-water wave is determined by the water depth. In water 4 km deep, tsunamis travel at approximately 200 m•s^–1 (720 km•h^–1), or nearly the speed of a jet airliner. Because tsunamis are shallow-water waves, their speed changes with depth (Fig. 9-17), and they are refracted as they pass over seafloor topography (Fig. 9-24). Tsunamis can be spread out or focused by undersea ridges or depressions, just as wind waves are as they approach a coastline.

Figure 9-24. Computer model of the paths of the tsunami wave caused by the earthquake that occurred in 2011 offshore from Sendai, Japan. The wave energy of a tsunami is refracted by seafloor topography because tsunamis are shallow water waves. Refraction focuses or spreads the tsunami wave energy in the same way that wind wave energy is focused and spread at headlands and in bays. Note that one of the darker red streaks, which indicate paths along which the tsunami wave energy is focused, travels to Crescent City, California. Crescent City is extremely vulnerable to tsunamis, as are some Pacific Islands and locations along the South American Pacific Coast, due to this focusing effect. 

When a tsunami travels over deep-ocean waters, its wave height rarely exceeds 1 or 2 m. Therefore, ships at sea are not affected by tsunamis. Indeed, it is almost impossible to detect a tsunami at sea because of its very long wavelength and limited wave height. A tsunami raises and lowers a ship only a meter or two, and each rise and fall takes several minutes. Tsunamis become dangerous only when they enter shallow water.

As a tsunami enters shallower water, it slows, and its wavelength is reduced, but its period is unchanged, as is true for any other wave (Fig. 9-17). As a wave slows, the wave height increases. Because water depth is very small in comparison to a tsunami’s wavelength, wave height builds very rapidly (Fig. 9-19). The tsunami does not break because its wavelength is so long that even a large increase in wave height does not produce steep, unstable waves. Nevertheless, the leading edge of the tsunami wave can produce tremendous surf as it flows turbulently across the shore and coast.

The tsunami reaches the shore as a wave that can be tens of meters high and can take 5 to 10 min to pass from trough to crest. As the tsunami moves inshore, sea level rises several meters above normal, and enormous quantities of water are transported onshore and into any estuaries or rivers. The water keeps pouring onshore for several minutes as the wave crest approaches. The enormous energy stored in the wave is released as the water in the wave flows turbulently onto land and past any structures it encounters. Very strong currents and the equivalent of large breaking waves are generated as the water is concentrated in flows through harbors and channels and between structures. Buildings and trees can be destroyed, and boats and debris can be carried far inshore to be left stranded when the wave recedes (Fig. 9-1). If the ocean is not calm when the tsunami arrives, wind waves will add to the tsunami wave and may contribute to the destruction as they break far inland from the normal surf zone.

The impact that creates a tsunami may initially form either a trough or a crest. Accordingly, the first indication of the tsunami’s arrival at the coast may be a rise or recession in the normal sea surface level that lasts several minutes and exposes large areas of seafloor that are not normally exposed. Regardless of whether a trough or crest arrives at the coast first, the tsunami will consist of several waves following each other. After the first wave, successive waves may be larger, but they will eventually decrease in height. Waves can continue to arrive for 12 h or more. Many drownings occur when curious sightseers or beachcombers walk out onto a beach exposed by a tsunami trough and are caught by the following crest. Despite its 10- to 20-min period, the crest of a tsunami moves onshore much too fast for someone on the beach to escape after the water begins to rise.

In 1883, the Indonesian island volcano Krakatau erupted with an extremely violent explosion that almost instantly blew a large fraction of the island’s mass into the air. The explosion and the subsequent collapse of the remaining sides of the volcano into the underwater caldera (crater) created by the explosion caused a tsunami with waves of unusually long periods (estimated to be as much as 1 to 2 h). On the island of Java, about 60 km away from the eruption, the tsunami hit with waves about 30 m high. The waves destroyed many structures and carried a ship more than 3 km inland, where it was stranded almost 10 m above sea level. Krakatau’s tsunami killed an estimated 35,000 people and was observed by water-level recorders as far as 18,000 km away in Panama.

In 1946, an earthquake in the Aleutian Trench off Alaska caused a tsunami at Scotch Cap, Alaska, that destroyed a concrete lighthouse 10 m above sea level, killed the lighthouse operators, and tore down a nearby radio mast mounted 33 m above sea level. About 5 hours later, the tsunami heavily damaged the Hawaiian Islands and swept away 150 people to their deaths. In response to this type of disaster, a tsunami warning system was developed for the Pacific Ocean. As a result, when a tsunami hit Hawaii in 1957 with waves larger than those in 1946, no lives were lost.

On 26 December 2004, a great earthquake struck in the subduction zone just offshore from the Indian Ocean coastline of the northern part of the island of Sumatra in Indonesia. The earthquake generated a tsunami that was estimated to be 30 m high when it hit the coast of Sumatra. The tsunami spread across the Indian Ocean, smashing into Thailand to the east and India and Sri Lanka to the west. Continuing across the Indian Ocean to the west, the tsunami then impacted the western coastline of Africa. More than 220,000 people were killed or disappeared, with the majority of deaths occurring in Indonesia. Many deaths also occurred in Thailand, Sri Lanka, and India; and several deaths were even reported in Somalia, more than 5000 km from where the earthquake occurred. In addition to the deaths, hundreds of thousands were rendered homeless, and the damage to property is estimated to have been many billions of dollars. In March 2011, a tsunami approximately the same size as the 2004 Indonesian tsunami hit Japan. This tsunami caused massive destruction and took many lives, but not nearly as many died in 2011 as in 2004. Some of the credit for the smaller death toll is due to the tsunami detection and warning system built after the 2004 event, and some also undoubtedly belongs to the Japanese preparedness. Earthquake and tsunami awareness are a high priority in Japan, and frequent emergency drills to respond to such events are a routine part of Japanese schoolchildren’s lives.

No doubt most of you who are reading this will have seen videos of the December 2004 or the March 2011 tsunami. Although some of the videos are horrific, they are also instructive. Many of the video records show that a tsunami does not arrive as a single short-lived wave. They also show that the often turbulent front edge of the advancing wave is followed by the mass of water in the wave continuing to pour ashore in a relentless current for several minutes before finally receding. Eyewitness accounts in both tsunamis also attest to the fact that several waves came ashore, 10 to 20 min apart, and that, in some places, the crest of the first wave arrived without warning, whereas in others, the trough arrived first, providing some warning as the ocean receded rapidly and much farther out than normal. The videos also show that it is not possible to outrun a tsunami wave unless one is lucky enough to be very close to the highest point the wave will reach. However, there were many survivors in each of these tsunamis. Some survivors saved themselves by running to higher ground at the first sign that a tsunami might hit, which was usually the shaking caused by the earthquake that generated the waves.

We should all learn from these events. It is imperative to seek higher ground immediately if we are on or near the shoreline and we either feel an earthquake or see the ocean rapidly recede in an abnormal fashion. We must also leave immediately if we see a massive wave moving toward shore, but unfortunately, fleeing might not guarantee safety. Finally, if a tsunami has occurred, even a small one, we must stay well away from the shore for at least 12 h.

Although the 2004 Indian Ocean and 2011 Japan tsunamis were events of epic human scale, they are both dwarfed by giant tsunamis that occurred before recorded history, and will probably occur again. For example, Chapter 6 describes the monstrous tsunami that is thought to have been created when a meteorite hit the Earth at Chicxulub, Mexico. In addition, studies of the Hawaiian Islands have shown that huge tsunamis may be created when large segments of volcanic islands break loose and slump to the ocean floor (Chap. 11). A slump of 350 km3 of the island of Hawaii’s coastline about 120,000 years ago led to the deposit of marine fossils in a location on the island that was at that time at least 5 km inland and at least 400 m above sea level. Blocks of coral have been found at a height of 325 m on the island of Lanai, perhaps swept upward by this same tsunami. The earliest historical record of a tsunami appears to be almost 4000 years old, when the Minoan city of Ugarit was documented on clay tablets as having been destroyed by giant waves, possibly created by a massive volcanic eruption far more powerful even than the eruption of Krakatau. 

In 2022, the Hunga Tonga volcano eruption (Chap. 4) caused a tsunami 20 m high. Only 6 people died, a testament to the success of tsunami awareness education programs around the Pacific Ocean. However, this event was notable because it caused two different tsunamis, a traditional one caused by displacement of the water by the explosion, plus a type never observed before, a meteotsunami. This meteotsunami was caused by the intense atmospheric pressure (shock) wave from the volcanic explosion, which apparently pushed a tsunami ahead of it as it passed over the oceans. This meteotsunami travelled even faster than the ocean wave tsunami and arrived at Japan’s coastline about two hours before the ocean wave tsunami.

Tsunamis are most common in the Pacific Ocean because it has many subduction zones, where earthquakes are likely to cause vertical seafloor movements. However, tsunamis may occur wherever vertical movements of the land or seafloor occur. Tsunamis are most likely to be damaging on island or continental coasts with narrow continental shelves because much of the wave energy can be dissipated as a tsunami moves over the shallow waters of a wide continental shelf. A tsunami may cause no damage at all on atolls or other islands that have no continental shelf, because the tsunami has no opportunity to build in height before it reaches the coast. Table 9-2 lists a number of destructive tsunamis that have occurred around the Pacific Ocean since 1990.

TABLE 9-2. Selected Destructive Tsunamis since 1990

Date	Location	Estimated Maximum Wave Height (m)	Number of Fatalities
2 September 1992	Nicaragua	10	170
12 December 1992	Flores Island, Indonesia	26	>1000
12 July1993	Okushiri, Japan	31	239
2 June 1994	East Java, Indonesia	14	239
14 November 1994	Mindoro Island, Philippines	7	49
9 October 1995	Jalisco, Mexico	11	1
1 January 1996	Sulawesi Island, Indonesia	3.4	9
17 February 1996	West Papua, Indonesia	7.7	161
21 February 1996	Northern coast of Peru	5	12
17 July 1998	Papua New Guinea	15	>2200
26 December 2004	Sumatra, Indonesia	30	>220,000
27 February 2010	Maule, Chile	8	ca. 500
11 March 2011	Sendai, Honshu, Japan	47	>25,000
15 January 2022	Hunga Tonga Ha’apai, Tonga	20	6

The tsunami warning system, headquartered in Hawaii, provides warnings of possible tsunamis whenever an earthquake occurs in the Pacific that might cause such waves. Often, tsunami warnings are not followed by a dangerous series of tsunami waves because the wave refraction patterns are complex and different for each tsunami. Therefore, even when a tsunami is actually observed near its source, predicting whether the tsunami will significantly affect any section of coast at a more remote location is very difficult. Even though some warnings may prove false, heeding such warnings is always wise.

In Hawaii and Oregon, tsunami warnings are broadcast through sirens at points along the coast and through the emergency radio and television broadcasting system. In the U.S. and some other nations, tsunami warnings are now broadcast on an emergency warning network. The US system is the Wireless Emergency System (WEA) that is active on all cell phones (but may be turned off, so make sure WEA is activated in your phone setting). If the tsunami is generated by a nearby earthquake the warning time will be very short so you should always leave the beach for higher ground immediately after you feel an earthquake. What feels like a “mild” earthquake may actually be a major earthquake centered some kilometers offshore, and a tsunami warning may be issued too late.