Related Links

http://www.geophys.washington.edu/tsunami/general/historic/historic.html

http://students.ist.psu.edu/~sos104/earth101.html

http://www.tsunamicommunity.org/TCgen2.html

http://www.drgeorgepc.com/Tsunami1964PrWilliam.html

http://www.emporia.edu/earthsci/student/geert1/trigger.htm

The advance of a tsunami on the shores of Japan on May 26, 1983.

On July 17, 1998 an earthquake measuring 7.0 occured in the Bismark Sea. Twenty minutes later a series of three tsunami waves hit a twenty mile stretch of shore from Aitape to Serai. The tsunami killed 2,202 people and injured 1,000 others.

This photo was taken on April 1, 1946 in Hilo, Hawaii.

Map of Lituya Bay

This is a picture of Lituya Bay, citing the area of the rock slide (A), and the barren point (B).

This picture shows a boat and other scattered debris after a tsunami hit the shores of Hokkaido in the Sea of Japan. The tsunami, lasting 2-5 minutes, occured on July 12, 1993 because of an earthquake with a magnitude of 7.8. There were around 200 deaths and $600 million in property damage.

An earthquake of 7.4 magnitude happened on August 17, 1999 in western Turkey on the northern Anatolian Fault. This pictures the Izmit Bay coastline flooded by water.

Tsunamis

Introduction

Tsunamis are giant water waves that have the potential to cause great destruction and devistation. Tsunami is a Japanese word meaning "harbor waves", because the waves became known for destroying harbors. For years they were called "tidal waves", which are the result of the transition between high and low tides, caused by the mon's gravitational pull. This is a common misconception.

Geologic Processes

Tsumnamis can be caused when a fault slip suddenly displaces the sea-floor surface. An earthquake below the ocean causes the water above the deformed region to be displaced from its equilibrium position. A wave forms when gravity acts on the water, as it attempts to return to equilibrium. The wave in the overlying water then flows away from the epicenter. Around the Pacific Ocean are many subduction zones where the oceanic crust slides beneath lighter continental crust. These zones create many tsunamis.

Similar waves can also be created by forces from above such as landslides that dump rock into adjacent bodies of water, and even meteorite impacts. The momentum from the falling debris is transferred to the water, creating a wave. Volcanic explosions beneath the surface of the ocean create an impulsive force that uplifts the water. Submarine slumps (when large areas along the margins of continents or islands suddenly slip downslope) also can create tsunamis. These usually occur with earthquakes, which disturb the water when sediment slumps downslope and redistributes across the ocean floor. Unlike the tsunamis that occur due to earthquakes in the Pacific, these lose power quickly, and rarely affect distant coasts.

Products of the Geologic Process

A small tsunami, created by little sea-floor displacement, could only affect nearby coasts. But tsunamis generated by earthquakes of m = 8 or higher move much larger sections of sea floor and in turn lots more water. These waves can cross the entire ocean reaching speeds of 800 km per hour, which is about the speed of a jet. In the open ocean, where there may be depths of 4 km, a large tsunami could only be 30 cm high, comparable to the vertical displacement of the sea-floor. This same wave could be 200 km across, as measured perpendicular to the crest, and can carry more than 50,000 tons of water per kilometer. Because a wave's energy loss is inversely related to its wave length, tsunamis grow at high speeds, and lose little energy while crossing an ocean. A tsunami's energy flux, which keeps almost constant, depends on the height and speed of the wave. As the waves gets closer to the shore, friction with the sea floor slows it down, but the water builds up forming a huge wave measuring up to 30 m high. If this same wave happens to flow into a narrow harbor or bay, it could reach heights of 70 m. Tsunamis are very unpredictable. It could reach a coast as a rapidly rising or falling tide, a series of breaking waves, or a bore.

Impacts

When a tsunami comes to shore, water first recedes far below the low-tide level. Then a wall of water rushes in swallowing everything along the beach. Some of the wave energy is reflected offshore, while shoreward energy is lost through bottom friction and turbulance. As the tsunami moves inland, it sweeps ships onto shore, crushes buildings, erodes the ground, and floods low-lying land. Finally the water returns to the ocean leaving ships hundreds of meters inland, while carrying smaller debris out to sea.

The largest recorded tsunami took place in Lituya Bay, Alaska on July 7, 1958. Thirteen miles from Lituya Bay, an 8.0 earthquake occured along the Fairweather Fault. This caused a rock slide to happen on the eastern side of Gilbert Inlet, creating a splash that reached 1720 feet across the point of the opposite inlet. The mass of water left bare rock on the point, and started a 100 ft. tsunami across the bay. The tsunami destroyed five square miles of land, and sent water 3,600 ft. inland. Only two boaters were killed, while the other two witnesses, who had their boats on shore, survived.

The deadliest tsunami came after the August 7, 1887 eruption of Krakatau volcano in the East Indies. Waves reaching 100 ft took the lives of 36,000 people off the coasts of Java and Sumatra. Although tsunamis can happen anywhere Japan has been hit the most. Around 250 tsunamis have killed aproximately 100,000 people in the past centuries.

During April of 1946, a tsunami in Hilo put a boxcar under the Hatada Bakery.

Literature Cited

Marshak, Stephen Essentials of Geology. W.W . Norton and Company, Inc. 2004, New York. p.232-233.

http://www.geophys.washington.edu/tsunami/general/physics/physics.html

http://www.usc.edu/dept/tsunamis/alaska/1958/webpages/lituyacloseup.html

http://www.ngdc.noaa.gov/seg/hazard/slideset/tsunamis/

http://www.soultsunami.com/whatis.html

Link to other Student Webpages for 2004 Earlham Physical Geology

Copyright © 2004 Earlham College. Revised April 20, 2004 . Send corrections or comments to Andrew Krickenbarger