The deepest earthquakes typically occur at plate boundaries where the Earth's crust is being subducted into the Earth's mantle. These occur as deep as 750 km (400 miles) below the surface.
An estimated 500,000 detectable earthquakes occur in the world each year. About 100,000 of those can be felt and 100 of them cause damage.
Like all mountain ranges, the Wasatch Range, which runs north to south through Utah, was created by a series of earthquakes. The 240-mile Wasatch Fault is made up of several segments, each capable of producing a magnitude 7.5 earthquake. During the past 6,000 years, there has been a magnitude 6.5 or higher earthquake about once every 350 years, and it has been 150 years since the last powerful earthquake.
Florida and North Dakota have the fewest earthquakes in the United States.
Alaska is the most earthquake-prone state and one of the most seismically active regions in the world. Alaska experiences a magnitude 7 earthquake almost every year, and a magnitude 8 or greater one on average every 14 years.
When the Chilean earthquake occurred in 1960, the largest earthquake recorded since 1900 at 9.5 magnitude, seismographs recorded seismic waves that traveled all around the Earth for days after the event. This phenomenon is called the free oscillation of the Earth.
Source: U.S. Geological Survey
The Richter Scale
In 1935, Charles Richter of the California Institute of Technology developed the Richter Scale as a mathematical device to compare the size of earthquakes. The scale is based on the logarithm of the amplitude of waves recorded by seismographs. Adjustments are included for the variation 
in the distance between the various seismographs and the epicenter of the earthquakes.
Earthquakes with magnitude of about 2.0 or less are usually called microearthquakes; they are not commonly felt by people and are generally recorded only on local seismographs. Events with magnitudes of about 4.5 or greater -- there are several thousand such shocks annually -- are strong enough to be recorded by sensitive seismographs all over the world. Great earthquakes, such as the 1964 Good Friday earthquake in Alaska, have magnitudes of 8.0 or higher. On the average, one earthquake of such size occurs somewhere in the world each year.
The Richter Scale has no upper limit and is not used to express damage.
Source: U.S. Geological Survey
The Mercalli Scale
The Modified Mercalli Intensity Scale, developed in 1931 by American seismologists Harry Wood and Frank Neumann, is not based on math but on observation. It takes into account factors such as people awakening, movement of furniture, up to total destruction.
This scale, composed of 12 increasing levels of intensity that range from almost imperceptible shaking to catastrophic destruction, is designated by Roman numerals ranging from I to XII.
The Mercalli intensity value assigned to a specific site after an earthquake has a more meaningful measure of severity to the nonscientist than the magnitude because intensity refers to the effects actually experienced at that place. After the occurrence of widely felt earthquakes, the Geological Survey mails questionnaires to postmasters in the disturbed area requesting the information so that intensity values can be assigned. The results of this postal canvass and information furnished by other sources are used to assign an intensity within the felt area. The maximum observed intensity generally occurs near the epicenter.
Source: U.S. Geological Survey
Kinds of Faults
A fault is a fracture along which the blocks of the Earth's crust on either side have moved relative to one another parallel to the fracture.
Strike-slip faults are vertical (or nearly vertical) fractures where the blocks have mostly moved horizontally. If the block opposite an observer looking across the fault moves to the right, the slip style is termed right lateral; if the block moves to the left, the motion is termed left lateral.
Dip-slip faults are inclined fractures where the blocks have mostly shifted vertically. If the rock mass above an inclined fault moves down, the fault is termed normal, whereas if the rock above the fault moves up, the fault is termed reverse. A thrust fault is a reverse fault with a dip of 45° or less. Oblique-slip faults have significant components of different slip styles.
Source: U.S. Geological Survey
Ten Largest Earthquakes Since 1900
Source: Earthquake Hazards Program, U.S. Geological Survey
Most Destructive Known Earthquakes on Record 
Source: U.S. Geological Survey
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