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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, 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 in Pasadena suggest, based on tree ring evidence, that an 1812 earthquake thought not to have been along the San Andreas fault 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 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, seismologists have thought that these were not San Andreas earthquakes.

But a quake that affects the fault in Wrightwood is a SanAndreas quake, and an 1812 San Andreas quake skews the suspected periodicity of the fault. Geological evidence based on changes in streambed 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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Title Annotation:using tree rings to date earthquakes
Author:Siberner, Joanane
Publication:Science News
Date:Apr 18, 1987
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