Wednesday, May 19, 2010

Short Essay on Tsunami


Here is a short essay on Tsunami and Earthquake... On Good Friday, March 1964, at 5:36 pm local time, the strain of the warping crust at the plate boundaries of the Pacific and North American plates under Northern Prince William Sound, Alaska, gave way to cause the second largest earthquake in recorded history. The earthquake was recorded as having a moment magnitude of 8.4, but later revised to the astonishing 9.2. The epicenter was located 20 – 30 kilometers below the surface of the earth, at latitude 61.04 N, and longitude 147.73 W near Prince William Sound. The rupture in the rocks and the ground deformation extended 800 kilometers, parallel to the Aleutian trench. Along the shoreline of southern Alaska was a belt of uplift up to 10 meters on the coast and 15 offshore, while inland there was subsidence. (The 1964 Alaskan Earthquake)

The 1964 Alaskan earthquake was caused by a thrust fault subduction zone that is occurring in southern Alaska and forming the Aleutian Island Arc. Running parallel to the volcanic island arcs is the Aleutian trench which marks the meeting of the two plates (Wood). This is where the Pacific plate is being thrust underneath the North American plate at a rate of 5 to 7 centimeters per year. The pressure and the bending of the rocks and the sudden release of energy by the sudden southeastward motion of portions of coastal Alaska as they move back over the subducting Pacific plate is what caused the Alaskan earthquake (Wood). Once the rocks met their elastic limit, the massive amounts of stored energy were released in the short 4 minute rupture time of the earthquake. However, the energy was continually released throughout the following months. On the first day, 11 more earthquakes were recorded as having a magnitude greater that 6 on the Richter scale. In the next three week 9 more were recorded. Smaller aftershocks continued on the remainder of the year (Wood). These aftershocks occurred in an area that was about 250 kilometers wide, and it extended 800 kilometers from Prince William Sound to the southwest end of Kodiak Island. (Sokolowski)

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The earthquake caused some amazing surface ruptures with vertical displacements ranging from 11.5 meters of uplift to 2.3 meters of subsidence (Earth). Ocean floors were uplifted to the point where they are now dry ground. Off the southwest end of Montague Island, the land had a vertical uplift of 15 meters. The earthquake caused the greatest vertical uplift ever recorded. Buildings were completely torn in half, topography was completely altered, and areas of Alaska were completely changed. A zone of subsidence covered about 285,000 square kilometers, including the north and west parts of Prince William Sound, the west part of the Chugach Mountains, most of Kenai Peninsula, and almost all the Kodiak Island group (Earth). The earthquake was also accompanied by vertical uplift over an area of 170,000-200,000 square miles. The major area of uplift trended northeast from southern Kodiak Island to Prince William Sound, and east-west to the east of the sound. (Earth)

The earthquake generated a maximum intensity of XI on the intensity scale. It was recorded as having an IX-X throughout most of the area. The maximum intensity of IX-X was felt over approximately 7,000,000 square miles of Alaska, and portions of western Yukon Territory and B.C., Canada. This was one of the most violent earthquakes ever recorded. Only the sparse population and time of occurrence when schools were closed, business areas were not crowded, and the tides were low prevented the death toll from surpassing 131. (1964 Earthquake)

The earthquake took 131 lives, 115 in Alaska, and 16 in Oregon and California. The tsunami created by the earthquake took 122 of the lives, whereas the earthquake itself, only took 9. The tsunami was the second largest ever recorded (Sokolowski). Significant damage from the earthquake was seen over 130,000 square kilometers. Liquefaction occurred over most of this area and it added to the damage reports and cost (Vallejo). As stated previously, the death toll was extremely small for a quake of this size, due to low population density, the time of day (holiday) and type of material used to construct many buildings (wood). The overall damage was reported as being between 300,000,000 and 400,000,000 dollars (1964 dollars) (1964 Earthquake).

With such an intensive earthquake, there were numerous landslides, avalanches, and reports of liquefaction. Liquefaction is where the ground shakes so much that it looses all of its density and its ability to support mass (Vallejo). The buildings and structures on top of these areas just sink into the ground and fall apart. The ground turns into a thin mud, almost liquid. The landslides and avalanches were created when the shaking loosened their hold on the surrounding ground. The result was massive amounts of rock, mud, and snow sliding down the mountains and hills. These were some of the most destructive aspects of the earthquake. Homes, schools, roads, sewer, and gas lines were all destroyed because of these landslides and avalanches (Barnhardt).

The tsunami that this earthquake created was recorded as being the second largest one in history. When a sudden vertical uplift occurred in the Alaskan seafloor, the result was the tsunami that took 122 lives. The tsunami traveled at a speed of around 500 miles per hour and was recorded as being as high as 220 feet. Pictures of splintered, leveled, trees that were between 88 and 101 feet above sea level, and 24 inches in diameter showed how powerful and deadly the tsunami was. The tsunami affected the entire world. Boats in harbors as far away as Louisiana were sunk due to seiches, a sort of sloshing of water back and forth in a small body of water like a boat harbor or swimming pool. Oscillations in the height of water in wells as far away as South Africa were also reported. (The 1964 Alaskan Earthquake)

A local tsunami was also reported when at least 5 local landslides hit the ocean. The tsunami, though much smaller than the one created by the vertical uplift of the sea floor, still caused damage throughout the region. This local tsunami is what caused the damage and spreading of oil, which was on fire and floating on the water, throughout the homes and buildings that were in the water and in the nearby area. (The Great Alaska Earthquake of 1964)

The seismic waves that the earthquake created traveled around the world for several weeks. Because of their magnitude and intensity, the waves basically rang the whole earth like a bell (1964 Earthquake). States as far away as Texas and Florida reported vertical displacements from 5 to 10 centimeters in height. No substantial damage was reported from these waves, but it is necessary to note just how phenomenal of an act that this is (Moulton). The earthquake had the energy to ring the whole earth like a bell for several weeks. That energy was great enough to move the ground in areas thousands of miles away (Moulton). This was one of the most substantial and influential events the earth has ever seen.

Many things have been learned from the study of this earthquake. It has been found that the south central seismic zone that produced the 1964 quake extends as part of a system of interior faults. Alaska has a large number of these faults although not many are unidentifiable since they are often buried under sediment and sometimes do not reach the surface. Faults are also often obscured by rivers that erode their natural weakness in the earth's crust. The Tanana River is thought to hide a fault from near Delta Junction to Fairbanks, creating a highly seismic area. (Sokolowski)

If anything good can be said about this earthquake, it is that luckily this earthquake occurred in such a remote, unpopulated area. If this earthquake would have happened in San Francisco or Japan, the death toll would have been phenomenal. Complete destruction would have occurred, skyscrapers would have fallen, millions and millions of people would have been affected, deaths would be very high, and relief would be hard to get for such a large number of people. The cost of the damage could be in the tens of billions of dollars. The numerous aftershocks that were felt would be just an added danger for a populated area. With rescuers and relief efforts, the aftershocks would be very deadly. The magnitude of this earthquake in an area around the San Andreas Fault could have triggered other violent earthquakes from the same fault or nearby ones. Luckily, this earthquake happened in the remote area of Alaska.

In conclusion, the 1964 Alaskan Earthquake was one of the most phenomenal events the earth has ever seen. The amount of energy that was released could never be matched with any man made device. The earthquake devastated half of the earth, and affected the other half. One can only hope that another earthquake of this magnitude never happens again, but it is only a matter of time. Hopefully, the next one will also be in a remote area where losses would be minimal, and lives could be saved.

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