What Areas Of The Us Are Prone To Earthquakes?

The Pacific Coast of the United States is the region most susceptible to earthquakes. However, any state can experience earthquakes depending on what's happening in the Earth's crust.

Earthquakes are a product of the constantly moving forces in the Earth's crust. The location of particular faults and plate boundaries are heavy determinants in predicting why certain areas are more prone earthquakes than others. In the United States, almost every state has experienced earthquakes, but the states along the Pacific Coast experience more earthquakes than any other region in the United States.

The geology of Earth's crust explains why and where earthquakes occur. The Earth is comprised of several layers. The outer layer is the source of all seismic activity. Plates in the outer layer move and slide over, past, and under one another. Any displacement of neighboring plate boundaries or within a plate can trigger an earthquake.

Movement between neighboring plates produces the majority of earthquakes in the United States. There are three different types of plate boundaries. One type of plate boundary is called the spreading zone. The spreading zone is where molten rock is raised, separating the plates. Spreading zones are found in the mid-Atlantic oceans. The second type of plate boundary is the transform fault. Transform faults occur where the plates slide over one another. An example of a transform fault is the San Andreas Fault in California. Finally, subduction zone plate boundaries occur along the Northwestern coast of the United States and in Southern Alaska. These types of boundaries are also found in mountain ranges that have active volcanoes. Plate boundary quakes created many mountain ranges in the United States, including the Wasatch Range in Utah. Earthquakes continue to occur every 350 years along the Wasatch fault.



A movement within the plates can also trigger an earthquake. These movements amass stress deep within the plate and at the edges of the plate, creating weak zones that trigger the occurrence of a trembler. Earthquakes of this type are typical for intercontinental areas. Movement within the North American Plate was responsible for a massive quake in 1812 that leveled the city of New Madrid, Missouri and affected six states in the Mississippi River Valley, including Ohio, Kentucky, Arkansas, Illinois, and Tennessee. The effects of the New Madrid quake were felt in states as far as Virginia and Massachusetts. The Charleston quake of 1886 is another example. Sixty people died in the one-minute trembler that affected a 120-mile radius.

Earthquakes are sure to occur where there is a fault. Faults result in the displacement of rocks in the Earth's crust. Three types of faults exist. Normal faults result in the displacement of young rocks over older rocks. Examples include the Death Valley Fault in California, the Sevier Fault in Utah, and the Conjugate Normal Fault in Canyonlands National Park in Southern Utah. Reverse thrust faults occur when older rocks displace younger rocks. Examples of reverse thrust faults include the Lewis and Fold Thrusts in Montana. A third fault type, called the strike-slip fault, occurs in areas where there is side-by-side displacement of the rocks. Las Vegas, Nevada and Coos Bay in Oregon experience strike-slip faults. The San Andreas Fault in California also exhibits strike-slip motion, and quakes may occur every 50 to 200 years along segments of this particular fault.

By far, the greatest earthquake activity occurs along the Pacific Belt. The Pacific Coast states are part of an area known as the Pacific Ring of Fire, which is a zone of high volcanic activity and where many earthquakes occur. This is due to the movement of the Pacific Plate with other plates. The Pacific is also an area rich in subduction zones. Approximately 80% of all earthquakes in the United States occur in Alaska and California. In fact, according to the United States Geologic Society, the top ten states that experienced the most quakes lie in the Western United States. Included in that list are Alaska, California, Hawaii (7.3%), Nevada (3.7%), Washington (2.0%), Idaho (1.9%), Wyoming (1.0%), Montana (0.9%), Utah (0.7%), and Oregon (0.3%). In fact, Alaska experiences a magnitude 7 earthquake every year.

Because the majority of the quakes occur in the West, this is not to say that the rest of the United States is immune. Ohio residents near the Grenville Mountains may experience seismic activity every so often because of a collision in the plate boundaries between the North American plate with an eastern plate. When earthquakes occur on the Eastern seaboard, the damage is much more pronounced because the areas are heavily populated and the rocks in the Earth's crust in these areas are older and more rigid than the rocks on the West Coast. The physical composition of the region is such that seismic waves can propagate to greater distances, affecting a greater number of states as was the case of the New Madrid earthquake.

The states that have the smallest number of earthquakes include Florida and North Dakota, followed by Iowa and Wisconsin. The United States Geologic Society recorded no earthquakes in these states in 1995.

Earthquakes are due to stressed regions in the Earth's crust resulting from movement among and within the plates. The more famous quakes of California and Alaska create media frenzy, while those in outlying regions receive little mention. It is important to realize no matter where you live, the constant movement of the plates in the Earth's crust means that no single state in the United States is completely immune from seismic activity.

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