At Last Count -- June 1996

Perpetual Motion

Earthquake Activity 1974-1993
by Christine Powell
Earthquake Activity: 1974-1993

Map by Matthew Sibol, using data from a U. S. Geological Survey Earthquake Catalog

COUNTING only those earthquakes that register a magnitude of 2.0 or greater on the Richter scale, there are more than two quakes a day, on average, in the lower forty-eight states. Although it takes a quake of 5.5 or greater to cause significant damage to buildings, a 2.0 quake will still give a rude jolt to people within a few miles of its epicenter.

The states along the Pacific coast, of course, have the highest concentration of quakes, because the fault lines that run through this region constitute a boundary between two giant tectonic plates: the North American plate and the Pacific plate. Four of the five largest quakes during the twenty-year period covered by this map were in California or off its coast.

But seismic activity is also common in places far away from this plate boundary. A prominent band of earthquakes extends northward from Nevada to Montana, in a zone between deformed rocks and stable rocks which has been active periodically for the past 600 million years. Another band of earthquake activity extends westward from Yellowstone National Park. The cause of these earthquakes, and of the unusual volcanic activity at Yellowstone, is the presence of a "hot spot"--a pipe of molten rock that blasts its way up to the surface. (Pressure from this molten rock was so intense from 1923 to 1984 that it gradually raised the park's elevation by three feet.)

Earthquakes that occur east of the Rocky Mountains are probably associated with ancient geological structures formed as a result of past plate-tectonic activity. For example, 600 million years ago North America attempted to split apart into two continents along the present location of the Mississippi River, and as a result the crust is cut by thousands of ancient faults. A prominent zone of activity near the border between Missouri and Tennessee produced three of the most violent earthquakes on record, in 1811 and 1812. New England, which is characterized by persistent but scattered quakes, has experienced at least two quakes of 6.0 or greater since the early 1800s. Although most earthquake-related attention focuses on the West Coast, it is entirely possible that the country's next major quake may strike somewhere in the East.

Is any part of the country safe from earthquakes? Judging by data from the past three centuries, the safest places are eastern Texas and Florida.

[Editors' Note]

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