An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in intensity, from those that are so weak that they cannot be felt, to those violent enough to propel objects and people into the air, damage critical infrastructure, and wreak destruction across entire cities. The seismic activity of an area is the frequency, type, and size of earthquakes experienced over a particular time period. The seismicity at a particular location in the Earth is the average rate of seismic energy release per unit volume. The word tremor is also used for non-earthquake seismic rumbling.

At the Earth's surface, earthquakes manifest themselves by shaking and displacing or disrupting the ground. When the epicenter of a large earthquake is located offshore, the seabed may be displaced sufficiently to cause a tsunami. Earthquakes can also trigger landslides.

In its most general sense, the word earthquake is used to describe any seismic eventwhether natural or caused by humansthat generates seismic waves. Earthquakes are caused mostly by rupture of geological faults but also by other events such as volcanic activity, landslides, mine blasts, and nuclear tests. An earthquake's point of initial rupture is called its hypocenter or focus. The epicenter is the point at ground level directly above the hypocenter.

The 1811–1812 New Madrid earthquakes () were a series of intense intraplate earthquakes beginning with an initial earthquake of moment magnitude 7.2–8.2 on December 16, 1811, followed by a moment magnitude 7.4 aftershock on the same day. Two additional earthquakes of similar magnitude followed in January and February 1812. They remain the most powerful earthquakes to hit the contiguous United States east of the Rocky Mountains in recorded history. The earthquakes, as well as the seismic zone of their occurrence, were named for the Mississippi River town of New Madrid, then part of the Louisiana Territory and now within the U.S. state of Missouri.

The epicenters of the earthquakes were located in an area that at the time was at the distant western edge of the American frontier, only sparsely settled by European pioneers. Contemporary accounts have led seismologists to estimate that these stable continental region earthquakes were felt strongly throughout much of the central and eastern United States, across an area of roughly 50,000 square miles (130,000 km2), and moderately across nearly 3 million km2 (1 million sq mi). The 1906 San Francisco earthquake, by comparison, was felt moderately over roughly 6,200 sq mi (16,000 km2). The New Madrid earthquakes were interpreted variously by American Indian tribes, but one consensus was universally accepted; the powerful earthquake had to have meant something. For many tribes in Tecumseh's pan-Indian alliance, it meant that Tecumseh and his brother the Prophet must be supported.