Earthquake
An earthquake is the shaking of the ground caused by the sudden release of stored energy when rock breaks and slips along a fault.
Definition
An earthquake is a sudden shaking of the ground that happens when rocks deep underground that have been pushed and squeezed for a long time finally break and slip. It is the release of elastic energy stored in rock when stress along a fault exceeds friction; the point of first rupture underground is the focus (hypocenter), the point directly above it on the surface is the epicenter, and energy travels outward as seismic waves. Magnitude measures the energy released, intensity measures the shaking felt at a location, and most earthquakes occur along plate boundaries. Earthquakes result from frictional instability on faults described by elastic rebound theory. Moment magnitude $M_w$ is derived from seismic moment $M_0 = \mu A D$, the product of rock rigidity, rupture area, and average slip, with $M_w = \frac{2}{3}\log_{10} M_0 - 6.07$ (in newton-meters). Rupture propagates at about $2$ to $3\ \text{km/s}$, a magnitude $9$ rupture can span $1{,}000\ \text{km}$ and last several minutes, aftershocks decay by Omori's law, and the Gutenberg-Richter relation gives roughly ten times more earthquakes for each unit decrease in magnitude. Intraplate earthquakes in the eastern U.S. occur on ancient faults reactivated by far-field stress.
Example
Bend a wooden pencil slowly: it bends a little, then snaps all at once, and rocks along a fault do the same, with the snap being the earthquake. The 2020 Sparta, North Carolina, earthquake (magnitude $5.1$) was the strongest in the state in over a century and was felt in Charlotte, and the 1886 Charleston earthquake (about magnitude $7$) damaged buildings across the Carolinas. The 1960 Valdivia, Chile, earthquake ($M_w$ $9.5$) is the largest ever recorded. Eastern U.S. earthquakes are felt over areas up to ten times larger than western ones of the same magnitude because the old, cold crust transmits seismic energy efficiently.
Key Insight
Small earthquakes happen every day, thousands of them, and most are too weak for anyone to feel. Each step up in magnitude means about $32$ times more energy, so a magnitude $7$ releases about a thousand times the energy of a magnitude $5$. Earthquakes cannot be predicted in the sense of a date and place, but they can be forecast statistically. Early warning systems now detect the first P-waves and send alerts seconds before destructive S-waves arrive.