One of the most devastating consequences of subduction is the generation of the massive ocean waves known as tsunamis (from the Japanese for “harbor wave”). Large subduction earthquakes, like the magnitude 9.1 earthquake that occurred off the island of Sumatra’s northwest coast in December 2004, and the magnitude 9.0 Tōhoku earthquake that struck eastern Japan in March 2011, release vast amounts of energy and cause significant vertical movement of the seabed. When this occurs, a portion of the energy is transferred to the ocean water above the earthquake and generates powerful seismic sea waves capable of crossing an entire ocean basin. When these waves reach the coastline they unleash their energy and cause widespread devastation.
Tsunamis are commonly generated along locked (jammed together) segments of a subduction zone when stresses built up over a long period are suddenly released. If the two plates at a subduction zone lock together while the stress from plate motion continues, the front of the overriding plate is dragged downward, slowly storing energy like a stretched spring. Finally, it recoils upward, generating a powerful earthquake that releases the stored energy, lifting the seabed and the ocean water above (Fig 9B)
So unlike a normal wave, which concentrates its energy at the surface of the sea, a tsunami carries its energy from the ocean bottom (where the earthquake occurs) to the surface. Ships at sea scarcely notice a tsunami passing, for in the deep ocean a tsunami raises only a broad, gentle swell no more than a few feet high. But the shallower seafloor close to land compresses the wave and focuses its power into a series of major waves that can reach heights of up to 40 meters (130 feet).
Figure 9B: Generation of Tsunami. Tsunamis are formed above subduction zones as a result of the drag and release of the upper plate (red arrows) by the lower one. The overlying ocean water is first pushed away from the shore, only to return as a tsunami. In the case of the Sumatra earthquake, the tsunami was generated about 10 minutes after the earthquake, slamming into the coast of Sumatra just 15 minutes later.
Following the Sumatra earthquake, a killer tsunami sped across the Indian Ocean like a ripple across a pond but at speeds of about 800 kilometers (500 miles) per hour (Fig. 9C). Slamming first into Sumatra (Fig. 9D) and then Thailand, Sri Lanka and India just two hours later, the waves killed over 230,000 people.
Figure 9C: Tsunami Following Sumatra Earthquake. This is a computer simulation of the tsunami generated by the 2004 Sumatra earthquake: (a) 30 minutes after earthquake, (b) after 2 hours, (c) after 3.5 hours, (d) after 5.5 hours. The tsunami crossed the entire Indian Ocean to reach South Africa 11 hours later.
Figure 9D: Tsunami damage in Sumatra. A coastal village in Sumatra lies obliterated by the tsunami launched by the 2004 Sumatra earthquake. Traveling inland, the wave reached a height of 30 meters (100 feet)
Only five years later, another massive subduction earthquake off the coast of Japan, the strongest ever recorded in that country, launched a devastating tsunami that slammed into Japan’s east coast a half an hour later (Fig. 9E). Locally reaching a height of 40 meters (130 feet), the waves obliterated coastal towns, caused meltdowns and radiation release at the Fukushima Daiichi nuclear power plant, and cost the lives of almost 20,000 people.
Crossing the entire Pacific Ocean, the tsunami reached California and Oregon as a surge up to 2.4 meters (8 feet) high some ten hours later, damaging docks and harbors and causing over $10 million in damage. When it eventually reached the ice shelf of Antarctica, almost 18 hours after the earthquake, the tsunami broke off an iceberg the size of Manhattan Island.
Figure 9E: Tsunami comes ashore. This aerial view shows a tsunami wave coming ashore at Iwanuma on the east coast of Japan on March 11, 2011. The 4-meter (13-foot) tsunami swept boats, cars, buildings, and tons of debris miles inland.
Because it is almost entirely surrounded by subduction zones, it is the Pacific Ocean that is most at risk for tsunamis, and here an oceanwide tsunami warning system exists. Based in Hawaii, this detects tsunami-generating earthquakes and predicts the time of the tsunami’s arrival. In this way, those areas at risk can be alerted before the tsunami waves arrive and the potential loss of life is greatly reduced. In 2011, this was the case for the Japanese tsunami for those living on the Pacific rim, but in Japan itself there was little time to warn the affected population because of its proximity to the earthquake site. In the Indian Ocean, where tsunamis are less frequent, no such early warning system exists. As a result, the death toll of the 2004 Sumatra tsunami was far higher.