Science / Medicine : Calculating Exact Magnitude Can Take Months Or Years

No single agency in the world has the final authority to say what the magnitude of an earthquake is. But two of the most important assessments are made by the National Earthquake Information Center of the U.S. Geological Survey in Golden, Colo., and the International Seismological Center in Edinburgh, Scotland.

It takes as long as two years before the international center makes its estimate, and it is usually five or six months after the earthquake before the Colorado center does.

Preliminary estimates of magnitude, which frequently differ, are often made by various agencies within minutes of a sizable earthquake. The most common preliminary magnitude assessments of the Oct. 1 Whittier Narrows earthquake were 6.0 or 6.1.

Several weeks later, after Caltech seismologists received data from all their seismological stations scattered through Southern California, they revised their estimate down to 5.9, and this was accepted at the time by the National Earthquake Information Center.

However, according to Waverly Person, a director at the center, since then mean readings of other data from seismological stations throughout the world have led the center to revise downward its estimate to 5.8. This is not final. No final mean will be arrived at, Person said, for another month or two.

That figure will be forwarded to Scotland, but it will probably not make its estimate, based on other data forwarded to it from elsewhere in the world, until 1989.