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Visible waves are viable.

Visible waves are viable

A number of people who have experienced earthquakes firsthand have reported seeing waves or bulges in the solid earth --from centimeters to tens of meters tall--plow past them like leisurely ocean waves shortly after the rapid shaking of the quake had ceased. But over the years, such claims of "visible waves' have been eyed with skepticism by many seismologists.

However, one seismologist, Rene Rodriguez at the University of Kentucky in Lexington, says he has been convinced of the waves' existence after seeing one follow an earthquake in Japan 18 years ago. Since then he has collected reports made by other seismologists who saw ground waves in connection with 26 quakes around the world. What's more, he and a co-worker have recently improved and expanded calculations, first done in 1967 by another researcher, to test the viability of large surface waves and have found them to be theoretically possible.

In Rodriguez's model, which he says is more realistic than its predecessor, the earth's surface is represented as two layers. The topmost layer is more elastic than its underlying companion. The researchers showed that surface waves of the type reported could be generated if the ratio of the two layers' elasticities fell within a certain range. This criterion, it turns out, is met if, for example, the top layer is made of clay and the bottom of limestone or granite. And this is very much like the makeup of sedimentary basins, in which visible waves were reported for all 26 earthquakes.

Rodriguez argues that surface waves would explain a number of phenomena that cannot be explained by the rapid ground shaking of quakes alone. For instance, a large-amplitude wave could have applied stresses to some floors of a hospital in San Ferando, Calif., in 1971, causing them to collapse into others that had remained unperturbed. The most dramatic example, he says, are adobe walls built by Mayas in Guatemala; a wall parallel to the direction of seismic wave travel has been imprinted with a wavelike pattern on the top, where adobe crumbled, while walls lying perpendicular to wave motion remained unscathed.
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Title Annotation:ground waves following earthquake
Author:Weisburd, Stefi
Publication:Science News
Date:May 4, 1985
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