Why the Eastern US has Fewer Earthquakes than the Western US

By Vickie Frantz, AccuWeather.com Staff Writer

October 21,2012; 4:13PM,EDT

 

 

 

 

When you hear about an earthquake in Maine, you are more shocked than when you hear about an earthquake in San Francisco. There is no doubt that earthquakes in the western U.S. are more frequent than those to the east of the Rocky Mountains, according to the United States Geological Survey (USGS).
Just how more frequent are earthquakes in California than in Maine? For the years of 2010 through present, 13 earthquakes of a magnitude of 2.0 or greater were reported in Maine. In the state of California, 57 earthquakes of a magnitude 2.0 or greater were reported during the week of Oct. 13, 2012 through Oct. 20, 2012.
The reason for the disparity of earthquakes between the two states is explainable. However, before the explanation can be given, the term earthquake needs to be defined.
An earthquake is the movement of huge masses of rock, along breaks or weakness known as faults, according to AccuWeather Senior Meteorologist Jim Andrews.
"Earthquakes are the result of the release of pent-up energy that stresses the rocks at the Earth's outer layers or deeper," Andrews said.
What causes the stress on the rocks? There are vast slabs of rock that are part of the Earth's rigid outer layer. These rock slabs are called tectonic plates. The plates shift and move against each other. The boundaries between the tectonic plates where the shifting takes place is where the most energy builds up.
Now, let's look at how the tectonic plates effect areas of the U.S.
Western North America
"A complex plate boundary runs between the floor of the Pacific Ocean and the North American continent. This boundary runs roughly parallel to the coast, but doesn't follow it exactly," said Andrews.
Parts of the tectonic plate are on land and parts of it are offshore. The most famous fault caused by the location of this plate is the San Andreas fault, located in California.
 
This image of the San Andreas fault and the movement of the tectonic plates is courtesy of USGS.

"From the Sea of Cortes, through the Mexican mainland, into Baja, California, western Canada and onshore through southern Alaska lies another plate boundary," Andrews stated.
"Every place along that boundary experiences earthquakes, with some places being more prone to earthquakes than other."
Central and Eastern U.S
Much of North America lies far from any tectonic plate boundary. The nearest plate boundary to Eastern North America is in the Atlantic Ocean, according to Andrews.
"For much of the interior and Eastern areas of North America, earthquakes are relatively unknown," said Andrews.
There are some exceptions; the recent earthquake in Maine, an earthquake in Quebec on Oct. 10, 2012 and Virginia's earthquake in 2011.
Some earthquakes in this area have been large and have caused a lot of damage.
1755 Massachusetts
An intensity VIII earthquake was the largest to ever impact Massachusetts. It happened in 1755. The earthquake caused extensive damage to homes around Cape Ann and Boston.
1811-1812 New Madrid
The New Madrid Seismic Zone is located in the area of Missouri, Arkansas, Kentucky and Tennessee. Several large earthquakes and aftershocks occurred in the New Madrid Seismic Zone in 1811-1812.
 
This map of the New Madrid Seismic Zone is courtesy of USGS.

Three earthquakes of 7.5 or greater impacted northeast Arkansas and Missouri between the dates of Dec. 16, 1811 and Feb. 7, 1812, according to USGS.
The 1811 earthquake was felt over a large area of the East Coast. Reports of shaking came in from New York City, Washington, D.C., and Charleston, S.C.
In Missouri, the earthquake of February 7. destroyed the town of New Madrid.
1886 South Carolina
A magnitude 7.3 earthquake that occurred on Sept. 1, 1886, was the largest to ever impact South Carolina.
 
Brick House at 157 Tradd Street, wrecked by Charleston earthquake of August 31, 1886. Photo from the Earth Science Photographs from the U.S. Geological Survey Library, by Joseph K. McGregor and Carl Abston.

The earthquake damage or destroyed several buildings in the city of Charleston and killed 60 people.
This earthquake was the factor that caused South Carolina to develop early building standards to help buildings withstand the damage caused by earthquakes.
An interesting fact about the earthquakes that occur east of the Rocky Mountains is that they can be felt over broader regions than those in the western U.S., according to USGS.
A magnitude 4.0 earthquake in the eastern U.S. can be felt as far as 60 miles away from the epicenter. People can feel a magnitude 5.5 quake as far as 300 miles from where it occurred.
Andrews said, "Earth is constantly changing at a very slow rate and every now and then, there is a hiccup and an earthquake happens."