Earthquakes and tsunamis Tsunamis are commonly caused by earthquakes. The hard rock surface of the earth, or the crust, is made up of plates that move on top of a liquid rock layer underneath, or the mantle. This is the idea behind the theory of plate tectonics. When two plates come in contact with each other, they may rub against each other laterally or side by side. This site of contact may be apparent on the earth’s crust as a fault line, like the San Andreas Fault that runs along the length of California. Sometimes the earth’s tectonic plates spread apart, like the mid-Atlantic Ridge. Or, one plate may sink beneath the other. This is called subduction, and an area where this occurs is known as a subduction zone. As one plate sinks, the other plate is lifted up. Mountain ranges, islands and deep ocean trenches can form in subduction zones. For example, the Mariana Islands lie on the edge of the zone where the Pacific Plate sinks beneath the Philippine Plate, forming the Marianas Trench, the lowest elevation on the earth’s surface. Around 43 million years ago, the release of magma from volcanic activity in this area resulted in the formation of Guam, the oldest of the Mariana Islands chain. Though not always detected by humans, the earth’s plates are continuously moving, and they experience a considerable strain at these points of contact. When the strain becomes too great, they move in a way that can be detected at the surface as an earthquake. Scientists measure the strength of an earthquake using an instrument called a seismograph. The strength or magnitude of an earthquake is designated with a numeric ranking on the Richter Scale, which goes from 0-10. Typically, stronger earthquakes that can be felt and cause damage measure between 5.5 and 8.9 on the scale. The earthquake that caused the March 2011 Japan tsunami measured 9.0 on the scale. The memorable earthquake that struck Guam in August 1993 measured 8.1, although some sources recorded it at 7.9. The earthquake in American Samoa that caused a tsunami in September 2009 also measured 8.1. American Samoa, Japan and Guam experience frequent earthquakes. This is because they are located in a region geologists commonly refer to as the Ring of Fire. The Ring of Fire is an area in the Pacific Ocean where the movement of the earth’s plates causes frequent earthquakes and volcanic activity. The ring extends along the edges of the large Pacific plate, from New Zealand, through Indonesia, past the Mariana Islands and Japan, and along the Aleutian Islands in Alaska. The ring continues further along the whole west coast of the North American continent, Central America and South America to the southernmost point in Chile. These coastal areas are particularly vulnerable to the effects of tsunamis even if the epicenter of an earthquake is located thousands of miles away. When an earthquake occurs in the ocean, the energy released from the movement of the plates on the earth’s crust can generate a series of waves on the ocean surface. The waves radiate outward from the center of the quake (or epicenter). The effect resembles the rippling waves caused by throwing a stone in a still pond. Offshore, tsunami waves have a short height (or amplitude), but a very long wavelength (or the distance between waves). Under normal conditions, oceanic wavelengths are about 30 to 40 meters, but a tsunami may have wavelengths several hundred kilometers apart. In deep water, the effect of a tsunami is hardly detectable because of this huge wavelength. However, as a tsunami approaches a shoreline where the waters are shallow, the waves slow down. This causes the wavelength to shorten and the wave height to grow enormously. The destructive force of a tsunami, though, occurs not only when the wave crashes into the shoreline at high speed, but also by the receding of the water as it goes back towards the ocean, dragging buildings, vehicles, trees and other debris out to sea. According to geologists, a tsunami may feature multiple waves, and sometimes it may take several hours for the waves to travel thousands of miles across open water. In fact, not all earthquakes will produce tsunamis that actually make it to a shoreline. Whether a destructive tsunami is generated after an underwater earthquake is difficult to detect. Sometimes, an indicator of an impending tsunami is the drawback of water on the shore, exposing fish, coral and other features that would normally be under water. When such a drawback of the ocean occurs, it is a good idea for people to head inland or move to higher ground. Today, sophisticated instruments are used to monitor changes in sea level, especially after an earthquake, and to alert coastal zones of impending danger from destructive tsunamis. Guam receives its information from the Pacific Tsunami Warning Center, which has detectors on buoys located throughout the Pacific Ocean. The PTWC is based in Honolulu, Hawai’i, and operated by the National Oceanic and Atmospheric Administration, an agency of the United States government. By virtue of the island’s location along the Ring of Fire, it is not surprising that Guam is the site of regular earthquake activity. In fact, earthquakes occur every day on Guam, but the majority of them are imperceptible without a seismograph. Occasional destructive force earthquakes occur every few years, as evidenced in the landscape and geological record, as well as in historic accounts. The volcanic activity in the region causes much of the earthquake activity on Guam. In addition, there are thousands of fault lines that zigzag across the island, resulting from the collision (and subduction) of the Pacific plate and the Marianas plate. A major fault line divides the island geologically between the limestone plateau of the north, and the mountainous volcanic region in the south. Named the Adelup-Pago Point fault, the fault line runs roughly between Pago Bay on the east coast of the island, to Asan on the west—near the location of the Governor’s Complex in Adelup. The Tamuning-Yigo fault moves south-southwest from Mt. Santa Rosa in Yigo, along Latte Heights, past the airport and to the boundary between Tamuning and East Agana. Further south, the Cabras Fault runs from Facpi Point (which lies about five kilometers north of Umatac) and moves northeast along the west coast toward Piti Bay. In addition, a number of smaller fault lines run along the valleys of southern Guam. It is no wonder, then, that Guam should experience so many earthquakes during the year.
Posted on: Sat, 31 May 2014 09:18:55 +0000
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