1) Severe earthquakes have occurred in the eastern U.S.:
In November of 1755, an earthquake with an estimated magnitude of 6.0 and a maximum intensity of VIII occurred about 50 miles northeast of Boston, Massachusetts. Boston was heavily damaged. Other strong earthquakes recorded in the continental US were centered in southeastern Missouri near the border with Arkansas, Kentucky and Tennessee. In the winter of 1811-1812, a series of three powerful earthquakes of magnitudes about 7.0 to 7.8 and hundreds of aftershocks 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, created a vast area of ground deformation and liquefaction features; and they were felt widely along the east coast of the U.S. 800 to 1000 miles away. 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 6.8 to 7.2. Much 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.
2) 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.
Eastern North America has older rocks, some of which formed hundreds of millions of years before those in the West. These older formations have been exposed to extreme pressures and temperatures, making them harder and often denser. Faults in these older rocks have also had more time to heal, which allows seismic waves to cross them more effectively when an earthquake occurs. In contrast, rocks in the West are younger and broken up by faults that are often younger and have had less time to heal. So when an earthquake occurs, more of the seismic wave energy is absorbed by the faults and the energy doesn’t spread as efficiently.