What Is a Tsunami?

Major undersea earthquakes occasionally set in motion a series of large ocean waves that are known by the Japanese name tsunami (“harbor wave”). Most tsunamis are generated by displacement along a megathrust fault that suddenly lifts a large slab of seafloor (Figure 1- Tsunami generated by displacement of the ocean floor)

Figure 1 – Tsunami generated by displacement of the ocean floor The speed of a wave correlates with ocean depth. Waves moving in deep water advance at speeds exceeding 800 kilometers (500 miles) per hour. Speed gradually slows to 50 kilometers (30 miles) per hour at depths of 20 meters (65 feet). As waves slow in shallow water, they grow in height until they rush onto shore with tremendous force. The size and spacing of the swells shown here are not to scale.

Once generated, a tsunami resembles a series of ripples formed when a pebble is dropped into a pond. In contrast to ripples, tsunamis advance across the ocean at amazing speeds, about 800 kilometers (500 miles) per hour— equivalent to the speed of a commercial airliner. Despite this striking characteristic, a tsunami in the open ocean can pass undetected because its height (amplitude) is usually less than 1 meter (3 feet), and the distance separating wave crests ranges from 100 to 700 kilometers (60 to 450 miles). However, upon entering shallow coastal waters, these destructive waves “feel bottom” and slow, causing the water to pile up (see Figure 1). A few exceptional tsunamis have exceeded 30 meters (100 feet) in height. As the crest of a tsunami approaches the shore, it appears as a rapid rise in sea level with a turbulent and chaotic surface; it does not resemble a breaking wave (Figure 2).

Figure 2 – Tsunami generated off the coast of Sumatra, 2004
(AFP/Getty Images, Inc.)

The first warning of an approaching tsunami is often the rapid withdrawal of water from beaches, the result of the trough of the first large wave preceding the crest. Some inhabitants of the Pacific basin have learned to heed this warning and quickly move to higher ground. Approximately 5 to 30 minutes after the retreat of water, a surge capable of extending several
kilometers inland occurs. In a successive fashion, each surge is followed by a rapid oceanward retreat of the sea. Therefore, people experiencing a tsunami should not return to the shore when the first surge of water retreats.

Tsunami Damage from the 2004 Indonesia Earthquake

A massive undersea earthquake of MW 9.1 occurred near the island of Sumatra on December 26, 2004, sending waves of water racing across the Indian Ocean and Bay of Bengal. It was one of the deadliest natural disasters of any kind in modern times, claiming more than 230,000 lives. As water surged several kilometers inland, cars and trucks were flung around like toys in a bathtub, and fishing boats were rammed into homes.
In some locations, the backwash of water dragged bodies and huge amounts of debris out to sea. The destruction was indiscriminate, destroying luxury resorts as well as poor fishing hamlets along the Indian Ocean. Damages were reported as far away as the coast of Somalia in Africa, 4100 kilometers (2500 miles) west of the earthquake epicenter.

Japan Tsunami

Because of Japan’s location along the circum-Pacific belt and its expansive coastline, it is especially vulnerable to tsunami destruction. The most powerful earthquake to strike Japan in the age of modern seismology was the 2011 Tohoku earthquake (MW 9.0). This historic earthquake and devastating tsunami resulted in at least 15,890 deaths, more than 3000 people missing, and 6107 injured. Nearly 400,000 buildings, 56 bridges, and 26 railways were destroyed or damaged.
The majority of human casualties and damage were caused by a Pacifi-wide tsunami that reached a maximum height of about 40 meters (130 feet) and travelled inland 10 kilometers (6 miles) in the region of Sendai, Japan (Figure 3).

Figure 3 – Aftermath of the Japan tsunami, March 2011 This tsunami devastated parts
of Natori City. (Photo by Mike Clark/AFP/Getty Images)

In addition, meltdowns occurred at three nuclear reactors in Japan’s Fukushima Daiichi Nuclear Complex. Across the Pacific in California, Oregon, Peru, and Chile, some loss of life occurred, and several houses, boats, and docks were destroyed. The tsunami was generated when a slab of seafloor located 60 kilometers (37 miles) off the east coast of Japan was suddenly “thrust up” an estimated 5 to 8 meters (16 to 26 feet).

Tsunami Warning System

In 1946, a large tsunami struck the Hawaiian Islands without warning. A wave more than 15 meters (50 feet) high left several coastal villages in shambles. This destruction motivated theU. S. Coast and Geodetic Survey to establish a tsunami warning system for coastal areas of the Pacific that today includes 26 countries. Seismic observatories throughout the region report large earthquakes to the Tsunami Warning Center in Honolulu. Scientists at the center use deep-sea buoys equipped with pressure sensors to detect energy released by an earthquake.
In addition, tidal gauges measure the rise and fall in sea level that accompany tsunamis, and warnings are issued within an hour. Although tsunamis travel very rapidly, there is sufficient time to warn all except those in the areas nearest the epicenter. For example, a tsunami generated near the Aleutian Islands would take 5 hours to reach Hawaii, and one generated near the coast of Chile would travel 15 hours before reaching the shores of Hawaii (Figure 4).

Figure 4 – Tsunami travel times Travel times to Honolulu, Hawaii, from
selected locations throughout the Pacific. (Data from NOAA)

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