Eastern US can identify its faults, but has trouble pinpointing quakes

New York State's recent shocks show that earthquakes happen east of the Rocky Mountains, too. The difference is that geologists understand these Eastern quakes far less than they do those around the Pacific ``Ring of Fire.'' Among other things, Eastern quakes could suddenly erupt in unsuspected areas, as has happened with some moderate quakes in recent years.

New York's shocks were fairly mild. The main quake on Oct. 19 -- magnitude 4.0 -- did little damage. However, it came less than 3 years after a moderate -- magnitude 5.7 -- earthquake centered in New Brunswick, Canada, took geologists by surprise on Jan. 9, 1982.

Canada's quake was felt well down into New England, as was the magnitude 5.4 major aftershock Jan. 11. In fact, that aftershock gently rocked the Massachusetts Institute of Technology lecture hall where, coincidentally, the Boston earthquake of 1755 was under discussion. Like the magnitude 5.2 quake near Sharpsburg, Ky., on July 27, 1980, and the swarm of earthquake tremors that erupted in January 1982 in northern Arkansas and is only now dying out, the New Brunswick shock was in a region not identified

as earthquake-prone. It was followed by an unrelated magnitude 4.5 earthquake near Franklin, N.H., Jan. 18.

There is little doubt that the Northeastern United States and neighboring parts of Canada -- along with many other Eastern US regions -- are earthquake-prone. Moreover, some recent quakes have bordered on the magnitude range where shaking begins to be quite destructive. MIT geophysicist M. Nafi Toksoz observed after the New Brunswick earthquake that either it or the 1775 Boston quake (magnitude 5.9) could ``do extensive damage in a populated area.''

But while geologists can warn of Eastern quake hazards, they can't be as specific as they can be in the West. Earth's crust is broken into 10 or so great plates. Some 95 percent of the world's earthquakes occur along the plate edges where they rub, jostle, and override each other. California's San Andreas fault marks such a plate boundary. Scientists can explain earthquakes in terms of plate interaction fairly well. It is the relatively rare quake that occurs in the middle of a plate, as in the Eastern US, that stumps them.

A few places that have had spectacular earthquakes are beginning to yield their secrets. Thanks partly to work of the US Geological Survey and research sponsored by the Nuclear Regulatory Commission, which is concerned about nuclear power plant siting, geologists have identified fault structures responsible for some Eastern quakes. These often seem to be ancient features, formed hundreds of millions of years ago, and now reactivated. Just what reactivates faults or triggers specific quakes often remains

obscure.

Three of the greatest earthquakes in North American history occurred near New Madrid, Mo., on Dec. 16, 1811, and on Jan. 23 and Feb. 7, 1812. Their estimated magnitudes were 8.6, 8.4, and 8.7 respectively. That's considerably stronger than the recent Mexican earthquake of magnitude 8.1. Geologists have mapped fault structures responsible for this still seismically active region, as well as the faults that may be related to the magnitude 6.6 quake that shook Charleston, S.C., in 1886. While causes remain

uncertain, geological evidence of previous earthquakes suggests a recurrence period of some hundreds of years. This further suggests, but does not guarantee, that there will be no recurrence of major shocks in these areas anytime soon.

On the other hand, causes of such shocks as the New Brunswick quakes or the Arkansas tremors have not been established at all. Arch C. Johnston of Memphis State University, who has led much of the tremor research, says the cause of this swarm may never be known. He doubts large quakes can happen in a zone without a history of small quakes. But, he notes, as in the case of New Brunswick, ``there have been surprises.''

Under the circumstances, earthquake wariness should be part of land-use planning and building codes in Eastern states, as well as of power plant siting. Otto W. Nuttli of St. Louis University, who has studied the New Madrid area, has urged seismic safety building codes for the entire Mississippi Valley. Mr. Toksoz has stressed a need for them in New England. Such pleas parallel those of hurricane and tornado meteorologists who also believe that sound community planning and proper construction can minimi ze potential disaster.

Building codes and land use should be considered in light of all the hazards a region faces. There may be a common set of requirements that can meet these multiple safety needs. This should be easier to justify than a code designed solely to deal with rare earthquakes.

A Tuesday column. Robert C. Cowen is the Monitor's natural science editor.