Why should people in the eastern United States be concerned about earthquakes?
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.
Earthquakes of the same magnitude affect larger areas in the East than in the West
A magnitude 5.8 earthquake in 2011 in Mineral, Virginia, was felt up to 600 miles from the epicenter. Tens of millions of people in the eastern United States and southeastern Canada felt this earthquake.
For comparison, a magnitude 6.0 earthquake in 2014 in Napa, California, was felt only as far as 250 miles from the epicenter. Despite the Napa earthquake releasing about twice as much energy as the Virginia earthquake and causing much more damage near the epicenter, it wasn’t felt nearly as far away.
As another example, the magnitude 4.1 earthquake that occurred in December 2017 near Dover, Delaware, was felt approximately 200 miles from the epicenter. The region that felt this earthquake is about the same size as that of the much larger California event, which released about 700 times more energy.
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.
Seismic waves travel farther in the East
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.
Challenges of assessing earthquake hazards in the East
The geology of the eastern United States and the relatively sparse history of earthquakes to study make it difficult for scientists to assess how frequently earthquakes will occur and how large they can be.
Eastern earthquakes are more of a mystery because they do not take place at a plate boundary where most other earthquakes originate.
Scientists do not fully understand the state of stress within tectonic plates, and they are studying how stresses accumulate and evolve and how earthquakes are triggered.
Where are the most active faults in the East?
Scientists also do not know precisely where most active faults are located in the East.
Most faults have not had major earthquakes or movement in the past few million years, and the faults that are active may only have earthquakes every few thousand or tens of thousands of years.
Furthermore, any evidence of past earthquakes on the land’s surface in the eastern U.S. is often obscured by vegetation or is more subdued because of erosion.
The New Madrid Seismic Zone is about 30 years overdue for a magnitude 6.3 earthquake. A magnitude 7.6 earthquake, as serious as the 1811-12 series, may arrive by 2069. So my question is… Are you ready? [USGS]
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