Distribution
Seismologists have been monitoring the frequency and locations of earthquakes for most of the 20th century. Seismologists generally classify naturally occurring earthquakes into one of two categories: interplate and intraplate. Interplate earthquakes are the most common; they occur primarily along plate boundaries. Intraplate earthquakes occur where the crust is fracturing within a plate. Both interplate and intraplate earthquakes may be caused by tectonic or volcanic forces.
A Tectonic Earthquakes
Tectonic earthquakes are caused by the sudden release of energy stored within the rocks along a fault. The released energy is produced by the strain on the rocks due to movement within the Earth, called tectonic deformation. The effect is like the sudden breaking and snapping back of a stretched elastic band.
B Volcanic Earthquakes
Volcanic earthquakes occur near active volcanoes but have the same fault slip mechanism as tectonic earthquakes. Volcanic earthquakes are caused by the upward movement of magma under the volcano, which strains the rock locally and leads to an earthquake. As the fluid magma rises to the surface of the volcano, it moves and fractures rock masses and causes continuous tremors that can last up to several hours or days. Volcanic earthquakes occur in areas that are associated with volcanic eruptions, such as in the Cascade Mountain Range of the Pacific Northwest, Japan, Iceland, and at isolated hot spots such as Hawaii.
LOCATIONS
Seismologists use global networks of seismographic stations to accurately map the focuses of earthquakes around the world. After studying the worldwide distribution of earthquakes, the pattern of earthquake types, and the movement of the Earth’s rocky crust, scientists proposed that plate tectonics, or the shifting of the plates as they move over another weaker rocky layer, was the main underlying cause of earthquakes. The theory of plate tectonics arose from several previous geologic theories and discoveries. Scientists now use the plate tectonics theory to describe the movement of the Earth’s plates and how this movement causes earthquakes.
They also use the knowledge of plate tectonics to explain the locations of earthquakes, mountain formation, and deep ocean trenches, and to predict which areas will be damaged the most by earthquakes. It is clear that major earthquakes occur most frequently in areas with features that are found at plate boundaries: high mountain ranges and deep ocean trenches. Earthquakes within plates, or intraplate tremors, are rare compared with the thousands of earthquakes that occur at plate boundaries each year, but they can be very large and damaging.
Earthquakes that occur in the area surrounding the Pacific Ocean, at the edges of the Pacific plate, are responsible for an average of 80 percent of the energy released in earthquakes worldwide. Japan is shaken by more than 1,000 tremors greater than 3.5 in magnitude each year. The western coasts of North and South America are very also active earthquake zones, with several thousand small to moderate earthquakes each year.
Intraplate earthquakes are less frequent than plate boundary earthquakes, but they are still caused by the internal fracturing of rock masses. The New Madrid, Missouri, earthquakes of 1811 and 1812 were extreme examples of intraplate seismic events. Scientists estimate that the three main earthquakes of this series were about magnitude 8.0 and that there were at least 1,500 aftershocks.
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