7.3.4: Tsunamis

How Earthquakes Generate Tsunamis

A tsunami is a fast-moving, long-wavelength water wave generated by a sudden displacement of water by undersea landslides, earthquakes, or volcanic activity. They are commonly called "tidal waves", but are by no means created by the tides. The sudden slippage of the ocean floor near the source of an earthquake can send a train of seismic waves across the ocean. When an overriding plate along a subduction zone suddenly breaks free, it moves upward, raising the sea floor and the water above. The waves move outward in ever-expanding circles, nearly imperceptible in deep ocean water. As they approach land, water recedes from the shore. This dramatic action often entices the curious to investigate. But very shortly, a water wave rises rapidly from the ocean surface, crashing onto shore and rushing landward.

Figure \(\PageIndex{1}\): Formation of a tsunami (Courtesy USGS; Source: Surviving a Tsunami—Lessons from Chile, Hawaii, and Japan, USGS Circular 1187.(Public Domain; via USGS).

Figure \(\PageIndex{1}\) illustrates how an underwater earthquake at a subduction zone can generate a tsunami.

A cross-sectional view of the Earth shows an oceanic plate descending beneath a continental plate along a sloping plate boundary. The two plates are locked together at part of the boundary, creating a stuck area where stress accumulates over time.

Near the plate boundary, a starburst symbol marks the location of an earthquake. A label indicates that the stuck area ruptures, suddenly releasing stored energy. When the rupture occurs, the overriding plate rebounds upward while adjacent areas move downward.

This abrupt vertical displacement of the seafloor pushes the overlying ocean water upward, creating a broad wave at the ocean surface. Dashed curved lines show the movement of the water as the disturbance spreads outward across the ocean.

An upward-pointing arrow beneath the wave indicates the uplift of the seafloor that initiates the tsunami.

The figure demonstrates the process of tsunami generation by a subduction-zone earthquake. As tectonic plates converge, stress builds where the plates are locked together. When the fault suddenly ruptures, the seafloor is displaced vertically, transferring energy to the water above. This displacement creates long-wavelength tsunami waves that travel across the ocean and can grow in height when they reach shallow coastal waters.

On December 26^th, 2004 subduction between the Indian and Eurasian (more specifically the Burma) plates off the coast off the coast of Indonesia resulted in a magnitude 9 earthquake and large tsunami that devastated South Asia.The earthquake resulted from the release of stress as the Indian plate slid beneath the Burma plate. The resulting vertical movement of the ocean floor displaced hundreds of cubic meters of water and large waves propagated outward from the focus of the quake. The massive tsunami devastated coastal regions of South Asia as it crashed ashore.

One of the largest earthquakes to occur in recent memory occurred March 11, 2011 off the coast of Japan. The huge tsunami it created devastated the northeast coast of Japan, destroying towns and infrastructure. Learn more about this magnitude 9 earthquake and its aftermath by "Digging Deeper: The Great East Japan Earthquake of 2011", or skip and continue reading.

Digging Deeper: The Great East Japan Earthquake of 2011

On March 11, 2011 the Island of Japan was rocked by a violent earthquake. The magnitude 9 Tōhoku earthquake, officially named the Great East Japan Earthquake, originated 129 km (80 miles) offshore east of Sendai, Honshu, Japan, at a depth of 30 km. It was the fourth-largest earthquake in the world since 1900. Honshu lies near the junction of the North American, Eurasian, Pacific, and Philippine Sea plates. The massive earthquake radiated from plates slipping along the subduction zone, actually moving the entire country. Areas near Ishinomaki moved southeast by 5.3 meters (17 feet), while parts of Tokyo moved 9 inches.

Figure \(\PageIndex{2}\): Location of the Tōhoku earthquake. (Public Domain; Courtesy USGS).

Figure \(\PageIndex{2}\) shows the location of the 2011 Tōhoku earthquake off the northeastern coast of Japan.

The main map focuses on the island of Honshu, Japan's largest island. Several cities are labeled, including Sendai, Natori, Minamisanriku, and Tokyo. Offshore to the east, a prominent line marks the Japan Trench, a deep ocean trench associated with a subduction zone.

A large red circle surrounded by concentric rings is located offshore, east of Honshu and west of the Japan Trench. The concentric rings represent the earthquake's epicenter and the outward propagation of seismic energy from the rupture area.

The earthquake occurred near the boundary where the Pacific Plate is subducting beneath Japan. The epicenter is positioned relatively close to the Japan Trench and offshore from the heavily populated Sendai region.

An inset globe in the upper left corner shows the global location of Japan, marked by a star, providing geographic context within the western Pacific Ocean.

Latitude and longitude coordinates frame the map, and a scale bar indicates distances of approximately 100 kilometers (60 miles).

The map illustrates the geographic location of a major subduction-zone earthquake along the Japan Trench. It shows the relationship between the earthquake epicenter and the tectonic plate boundary responsible for the event. The figure emphasizes that large earthquakes commonly occur near convergent plate boundaries where one tectonic plate is forced beneath another. Such offshore earthquakes can generate significant tsunamis that impact nearby coastal communities.

As the North American plate slid upward over the Pacific plate, a 16 foot bulge of water rose and swept across the Pacific at an estimated 500 mph. As the water approached land, the wave increased to incredible heights. A huge tsunami washed across the northern coast of Japan within moments. Sendai airport was inundated by a wall of water 12 m(39 ft) high. An estimated tsunami height of 37.9 m (124 ft) swept across the slope of a mountain 200 m (656 ft) from the coast. Flooded fields several kilometers inland remained so after more than a week due in part to coastal subsidence. Land subsidence at Oshika Peninsula Miyagi was as much as 1.2 m (3.93 ft). Those coastal locations suffering from subsidence are now more susceptible to flooding during high tides.

The effects of the tsunami were far more deadly than the earthquake itself. Nearly 15,000 people were killed and 10,000 missing. Hundreds of thousands were left homeless as whole towns were devastated. The earthquake and subsequent tsunami severely damaged the Fukushima nuclear reactor station in the towns of Okuma and Futaba, leaking radioactivity into the air and water. Large evacuations of the surrounding area and contamination of food and water supplies resulted from the leakage.

Figure \(\PageIndex{3}\): The flooded Port of Sendai, Japan. (Public Domain; Courtesy US Navy via Wikimedia Commons).

Damage from the tsunami was not confined to Japan. Tens of millions of dollars in damage to public infrastructure and private property occurred in Hawaii. Along the west coast of North America docks and harbor facilities were damaged as a tsunami surge of up to 2.4 m (8 ft) struck portions of California and Oregon. An California man attempting to photograph the rising water was swept out to sea and died.

Figure \(\PageIndex{4}\): Achi, Japan. A large ferry boat washed ashore sits amid the ruins of houses. (Public Domain; U.S. Marine Corps photo by Lance Cpl. Garry Welch via Wikimedia Commons).