The most common intensity scale is the Mercalli scale, which ranges from I to XII

I - Not felt - Not felt except by very few under the right conditions.

V - Moderate - Felt by most. Some dishes and windows are broken.

VIII - Severe - Considerable damage to ordinary buildings. Fall of chimneys and factory stacks. Heavy furniture overturned.

IX - Violent - Considerable damage to specially designed buildings. Liquefaction occurs.

XI - Catastrophe - Few, if any, structures remain standing.

XII - Enormous Catastrophe - Damage is total.

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When a fault ruptures, it creates seismic waves that travel through the earth and result in shaking. Loose, soft materials, like unconsolidated sediments, cause the shaking to intensify because the seismic waves must slow down which cause them to increase in amplitude. The thicker the sediment layer, the more intense the shaking. So, valleys tend to have more intense shaking than places located on hard rock. [8]

As seismic waves travel through the earth, they begin to lose energy. This means that at farther distances, intensity begins to decrease. The decrease will not be uniform in all directions though, because the waves will pass through different types of materials. Also, when the fault slips it may focus its energy more strongly in one direction than another. [8]

An earthquake produces several types of seismic waves that differ in the way they travel. There are two main categories, body waves and surface waves. Body waves travel through the inner layers of the earth and surface waves travel along the earth’s surface.

Surface Waves:

These arrive last and move side-to-side or in a rolling motion like a wave. These waves are often what cause large damages.

[2]

Body Waves:

P or primary waves are always the first to arrive. They are compressional, which means they travel how a slinky moves when you push it towards someone holding the other end.

S or secondary waves are the second to arrive. These are shear waves, so they travel how a slinky moves if you shake it side to side. The amplitude for S-waves are generally larger than the amplitude for P-waves. Because S-waves rely on friction, they do not travel through liquid.

[2]

The map to the left show seismometers operated by the University of Utah Seismograph Stations in Utah as triangles. Seismometers are the instruments that are used to record the ground movement produced by earthquakes. Color indicates the intensity calculated at each seismometer, with red being the most intense and yellow being the least intense. Intensity by zip code that was estimated using felt reports collected by the United States Geological Survey can also be added to the map. These squares are scaled by the number of reports and colored by intensity.

Aftershocks occur because the mainshock causes major stress changes in the surrounding area and it must readjust. Aftershocks are natural and expected. As time goes on, the frequency and size of aftershocks generally decrease. This is only an average though, which means large aftershock can occur several days or weeks later. [4]

A common misconception is that large earthquakes release enough stress to prevent an even larger earthquake in the future. An earthquake increases the chance of having a larger earthquake by a small amount (~5%) for the next few weeks. Besides, a one unit increase in magnitude corresponds to ~32x more energy, so there is no way for a magnitude 5 to release enough energy to prevent a magnitude 6. [11]

As of November 4, 2020, the Magna earthquake had 2496 aftershocks, including 6 that were greater than magnitude 4.

In the map to the right, the aftershocks are shown as red circle. There are two cluster of earthquakes, one closer to Magna and one closer to Salt Lake City. The earthquakes closer to Magna occur on the Salt Lake City Segment of the Wasatch Fault. The cluster closer to Salt Lake City are likely occuring on part of the West Valley Fault System. These faults are antithetic to the Wasatch Fault, meaning they dip to the east and intersect with the Wasatch Fault at depth.

To see how these aftershocks look projected onto the cross-section line A-B, see the Depth section.

Magnitude is a logarithmic scale! This means that a magnitude 3 earthquake is ten times larger than a magnitude 2 earthquake, and 100x larger than magnitude 1 earthquake. [4]

The largest earthquake ever recorded was a magnitude 9.5! This earthquake moved an area around 1000 km long and 150 km wide [10]. Since earthquake size is limited by the area available for rupture, this large of an earthquake cannot occur in Utah. The largest possible earthquake for Utah is likely between a magnitude 7 and 7.5 [1] .

When an earthquake occurs it releases energy in the form of seismic waves. Energy is more directly related to the expected damages than earthquake size. For every unit increase in magnitude, ~32x more energy is released! So, magnitude 6 may not seem that much greater than a magnitude 5, but it would certainly feel much stronger! [7]

The Magna earthquake had an approximate slip of 16 square kilometers, with a displacement of just 28 centimeters. Try playing around with the sliders below to see how that compares to other sized earthquakes! [7]

Depth affects how an earthquake will feel at the surface. A good analogy is a flashlight. The closer a flashlight is to a surface, the smaller and brighter the circle of light. As you move the flashlight farther away, the circle grows but decrease in brightness. Therefore, shallow earthquakes are more intense but are more focused whereas deeper earthquakes are less intense but felt by a wider area. [6]

The scatter plot to the right shows the Magna earthquakes projected onto the cross-section line A-B from the Aftershocks section to show where the earthquakes occur at depth.