Coral clues: rise and fall of reefs record quakes' effects.Shallow coral reefs coral reefs, limestone formations produced by living organisms, found in shallow, tropical marine waters. In most reefs, the predominant organisms are stony corals, colonial cnidarians that secrete an exoskeleton of calcium carbonate (limestone). around islands west of Sumatra chronicled the uplift and subsidence that resulted from massive quakes that struck that region recently, a new study shows. From data recorded in this biological database, scientists may learn why two undersea ruptures stopped where they did. On Dec. 26, 2004, a magnitude 9.3 temblor beneath the Indian Ocean Indian Ocean, third largest ocean, c.28,350,000 sq mi (73,427,000 sq km), extending from S Asia to Antarctica and from E Africa to SE Australia; it is c.4,000 mi (6,400 km) wide at the equator. It constitutes about 20% of the world's total ocean area. spawned killer tsunamis (SN: 1/8/05, p. 19) and a flood of scientific interest (SN: 8/27/05, p. 136). Researchers rushed to the affected region to install sophisticated instruments, many of them in time to record the effects of a magnitude 8.7 quake in March 2005 (SN: 4/2/05, p. 211). But some of the best sensors--the reefs surrounding the region's islands--had been in place all along, says Richard W. Briggs, a geologist at the California Institute of Technology California Institute of Technology, at Pasadena, Calif.; originally for men, became coeducational in 1970; founded 1891 as Throop Polytechnic Institute; called Throop College of Technology, 1913–20. in Pasadena. Many corals, especially those in the genus Porites, naturally record sea level. They grow to the water's surface but can't tolerate dry conditions for more than a few minutes. Once a mass of corals including those species reaches the sea's surface, it stops growing taller but continues to grow laterally, says Briggs. Thus, a flat top on a Porites colony marks the previous level of a site's lowest fide. Briggs and his colleagues surveyed reefs around islands west of Sumatra to estimate quake-induced rises and falls Rise and Fall redirects here. For the Belgian hardcore band, click here. Rises and falls is a category of the ballroom dance technique that refers to rises and falls of the body of a dancer achieved through actions of knees and feet (ankles). . They report their results in the March 31 Science. The two quakes approached the 100-kilometer-long island of Simeulue from different directions. The island's northwestern tip rose 1.45 meters during the December 2004 temblor, says Briggs. However, the southeastern tip of the island, farther from the quake's epicenter, didn't rise or fall noticeably. In contrast, after the March 2005 quake, the island's northwestern tip held a steady elevation, while the southeastern tip was lifted about 1.65 m. Reefs along the midsection mid·sec·tion n. A middle section, especially the midriff of the body. of Simeulue rose only 50 centimeters or so in response to the 2004 and 2005 temblors, says Briggs. That pattern suggests that in central Simeulue, stress on the fault had been relieved by previous small quakes, including one in 2002, or during periods when the fault had slipped without causing a temblor. That stress relief might have prevented the quakes of 2004, and 2005 from progressing through this region. Alternatively, the geometry of the fault beneath the island may have arrested the progress of the quakes' ruptures, says Briggs. Understanding why the Indonesian ruptures of 2004 and 2005 stopped where they did will be important for evaluating seismic risk Seismic risk takes the results of seismic hazard analysis, and calculates the 'follies of man'. Your safety depends on what you build. You can locate in a region of high seismic hazard, but still sleep fairly soundly at night if you have built to sound engineering principles. throughout Southeast Asia Southeast Asia, region of Asia (1990 est. pop. 442,500,000), c.1,740,000 sq mi (4,506,600 sq km), bounded roughly by the Indian subcontinent on the west, China on the north, and the Pacific Ocean on the east. , says Roger Bilham, a seismologist seis·mol·o·gy n. The geophysical science of earthquakes and the mechanical properties of the earth. seis at the University of Colorado University of Colorado may refer to:
THE RISEN DEAD The top of the coral in the foreground indicates the level of low tide at this site in Indonesia before March 28, 20065. That day, a magnitude 8.7 earthquake permanently lifted the spot almost 2 meters out of the water, killing the reef. [ILLUSTRATION OMITTED] |
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