Dating earthquakes.Dating earthquakes In figuring out the history of the earth, geologists generallyuse pickaxes, hammers and mechanical equipment. Gordon Jacoby of the Lamont-Doherty Tree Ring Laboratory in Palisades Palisades, cliffs along the west bank of the Hudson River, NE N.J. and SE N.Y., extending from N of Jersey City, N.J., to the vicinity of Piermont, N.Y., with a general altitude of from 350 ft to 550 ft (107–168 m). , N.Y., uses trees. Several years ago, he and a Lamont colleague looked at thegrowth rings of trees from a seismically active part of Alaska and suggested that a strong earthquake in 1899 had moved the land on which the trees stood to a more protected environment where they grew more quickly (SN: 2/5/83, p.90). Now Jacoby, Lamont colleague Paul R. Sheppard and Kerry Sieh of 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 suggest, based on tree ring evidence, that an 1812 earthquake thought not to have been along the San Andreas fault San Andreas fault, great fracture (see fault) of the earth's crust in California. It is the principal fault of an intricate network of faults extending more than 600 mi (965 km) from NW California to the Gulf of California. actually was. If this is the case, they note, it brings into question the current understanding of how the fault works. The width of tree rings depends on their growth rate--inparticularly bad years, they may lay down no rings at all. Jacoby and his colleagues looked at cores taken from 65 trees along the fault in Wrightwood, Calif. They compared the rings to 30 trees in the area but not on the fault, and found "dramatic and extended' growth suppression in trees along the fault beginning in 1813, indicating that something had happened between the 1812 and 1813 growth seasons. The area had not been settled in 1812, and there are no records of the quake in Wrightwood. Two quakes were felt 60 miles away at San Juan Capistrano San Juan Capistrano (săn wän kăpĭsträ`nō), city (1990 pop. 26,183), Orange co., S Calif.; inc. 1961. San Juan Capistrano has some manufactures, including aircraft parts, medical apparatus, and boats, but the economy is inDecember of 1812. One knocked down a mission building, killing several dozen worshippers inside. Because San Juan Capistrano is about 50 miles from the San Andreas San Andreas is an Anglicisation of the Spanish language San Andrés (Saint Andrew, the Apostle). It may refer to:
But a quake that affects the fault in Wrightwood is a SanAndreas quake, and an 1812 San Andreas quake skews the suspected periodicity periodicity /pe·ri·o·dic·i·ty/ (per?e-ah-dis´i-te) recurrence at regular intervals of time. pe·ri·o·dic·i·ty n. 1. of the fault. Geological evidence based on changes in streambed streambed or stream channel Any long, narrow, sloping depression on land that had been shaped by flowing water. Streambeds can range in width from a few feet for a brook to several thousand feet for the largest rivers. paths indicates that the Wrightwood area experienced an earthquake in 1550, plus or minus 50 years, and people living in the area in 1857 reported a quake. An 1812 Wrightwood quake, as the tree ring evidence indicates, suggests that ruptures more frequent than every few hundred years are possible on that part of the fault. Alternatively, Caltech's Sieh suggests, Wrightwood may be atan overlap of two fault segments, and can rupture along either one. |
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