Why should people in the Eastern United States be concerned about earthquakes?

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Welcome to CoreFacts, where we're always short on time and big on science. I'm Brian Campbell. Today's question is …

Why should people in the eastern US be concerned about earthquakes?

There are a number of reasons.

First, severe earthquakes have occurred in the East-

In November of 1755, an earthquake with an estimated magnitude of 6.0 and a maximum intensity of VIII occurred 200 miles off the coast of Cape Ann, Massachusetts. Boston was heavily damaged. The strongest earthquakes recorded in the continental US were not in the West; they were centered in eastern Missouri near the border with Kentucky and Tennessee. In the winter of 1811-1812, a series of three earthquakes of magnitudes 8.4 to 8.7 and maximum intensities of XI occurred near New Madrid, Missouri. These shocks were so strong that observers reported that the land distorted into visible rolling waves. They changed the course of the Mississippi River; they made church bells ring in Boston and Washington, D.C. Because the surrounding area was mostly undeveloped at the time, few deaths were reported and these events stirred relatively little attention then. In August of 1886, a strong earthquake occurred in Charleston, South Carolina. Magnitude is estimated at 6.6 and maximum intensity was X. Most of the city of Charleston was damaged or destroyed. Earthquakes in the East are not confined to these areas; they have been recorded in every State east of the Mississippi. Damaging earthquakes have occurred historically in nearly every eastern State.

Another reason why people in the eastern US should be concerned about earthquakes is because earthquakes of the same magnitude affect larger areas in the East than in the West-

The size of the geographic area affected by ground shaking depends on the magnitude of the earthquake and the rate at which the amplitudes of body and surface seismic waves decrease as distance from the causative fault increases. Comparison of the areas affected by the same Modified Mercalli intensity of ground shaking in the 1906 San Francisco, California, the 1971 San Fernando, California, the 1811-12 New Madrid, Missouri, and the 1886 Charleston, South Carolina, earthquakes shows that a given intensity of ground shaking extends over a much larger area in the Eastern United States. Ground shaking affects a larger area because amplitudes of seismic waves decrease more slowly in the east than in the west as distance from the causative fault increases.

A third reason why people in the eastern US should be concerned-

Eastern states’ building codes are not as stringent as those in the West. Modern building codes in Eastern states are not as strict as those in California and much of the West. Not only that, but older buildings, which predate modern building codes completely, are more prevalent in the East than in the West.

Finally, the causes of earthquakes in the East are not well-understood-

Hundreds of millions of years ago, the East coast was this continent's active plate tectonic boundary, as the West coast is today. If the East is not now in an active plate margin, why do we have earthquakes here and why do we have them in the center of the continent? One possible explanation is that ancient faults or rifts are stressed. If this is true, what is the cause of the stress? In many areas of the East where earthquakes have occurred historically, specific faults causing the quakes have not been mapped or even identified. Another problem we encounter when evaluating earthquake risk is that we only have earthquake records for the last couple of hundred years. Establishing geologic patterns over human time scales is difficult at best.

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