English: This is a seismogram for the Norris Junction seismic station in Yellowstone, Wyoming. The prominent noise about one-third up from the bottom of the graph (click on the image once or twice to zoom in) was caused by shock waves from a magnitude 6.5 earthquake that hit western Nevada at 4:03 AM, local time, on 15 May 2020. The shock waves first arrived at the seismic station at about 5:05 AM, Yellowstone time, which is ~2 minutes after the quake hit. The United States Geological Survey reports that the earthquake's hypocenter was at 2.8 kilometers depth. The epicenter is at 38° 09' 32" North latitude, 117° 52' 30" West longitude. This is at an alluvial fan on the western flanks of the Monte Cristo Range in Esmeralda County, Nevada. Mountain ranges in Nevada are "horsts" - crustal blocks that have moved upward by normal faulting. Adjacent valleys are "grabens" - crustal blocks that have moved downward. Each mountain range and valley in Nevada is bordered by a major normal fault. Nevada is at the heart of a large region of crustal extension called the Basin and Range Physiographic Province. Crustal stretching here has occurred throughout the Cenozoic Era.

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An earthquake is a natural shaking or vibrating of the Earth caused by sudden fault movement and a rapid release of energy. Earthquake activity is called "seismicity". The study of earthquakes is called "seismology". The actual underground location of an earthquake is the hypocenter, or focus. The site at the Earth's surface, directly above the hypocenter, is the epicenter. Minor earthquakes may occur before a major event - such small quakes are called foreshocks. Minor to major quakes after a major event are aftershocks.

Most earthquakes occur at or near tectonic plate boundaries, such as subduction zones, mid-ocean ridges, collision zones, and transform plate boundaries. They also occur at hotspots - large subsurface mantle plumes (Examples: Hawaii, Yellowstone, Iceland, Afar).

Earthquakes generate four types of shock waves: P-waves, S-waves, Love waves, and Rayleigh waves. P-waves and S-waves are body waves - they travel through solid rocks. Love waves and Rayleigh waves travel only at the surface - they are surface waves. P-waves are push-pull waves that travel quickly and cause little damage. S-waves are up-and-down waves (like flicking a rope) that travel slowly and cause significant damage. Love waves are side-to-side surface waves, like a slithering snake. Rayleigh waves are rotational surface waves, somewhat like ripples from tossing a pebble into a pond.

Earthquakes are associated with many specific hazards, such as ground shaking, ground rupturing, subsidence (sinking), uplift (rising), tsunamis, landslides, fires, and liquefaction.

Some famous major earthquakes in history include: Shensi, China in 1556; Lisbon, Portugal in 1755; New Madrid, Missouri in 1811-1812; San Francisco, California in 1906; Anchorage, Alaska in 1964; and Loma Prieta, California in 1989.