Seismology

An aftershock is a smaller earthquake that follows a larger mainshock in the same region, occurring as the crust adjusts to the stress changes caused by the main event. Aftershocks can persist for months or years and pose significant risk to already-damaged structures.

An earthquake is the sudden release of accumulated elastic strain energy in the Earth's crust, typically along a fault, that generates seismic waves and causes ground shaking. Earthquakes range from imperceptible microtremors to catastrophic events that reshape cities and claim hundreds of thousands of lives.

The epicenter is the point on the Earth's surface directly above the hypocenter (focus) of an earthquake. It is typically the location reported in news media and is used as a reference point for measuring epicentral distance in seismological and engineering applications.

A fault is a fracture or zone of fractures in the Earth's crust along which relative displacement of the two sides has occurred. Faults are the primary sources of earthquakes, and their geometry, slip rate, and mechanical properties govern the size, frequency, and characteristics of seismic events.

The hypocenter, also called the focus, is the point within the Earth where an earthquake rupture initiates. Its depth below the surface is a critical parameter that strongly affects the intensity and distribution of ground shaking at the surface.

Intensity is a measure of the severity of earthquake effects at a specific location, based on observed damage to structures, ground deformation, and human perception. It varies from place to place for a single earthquake, unlike magnitude, which is a single value for the event.

Magnitude is a logarithmic measure of the size of an earthquake, quantifying the energy released at its source. Unlike intensity, which varies from place to place, magnitude is a single value assigned to each earthquake, independent of where it is observed.

Rupture in seismology refers to the propagation of a shear fracture along a fault surface during an earthquake, releasing accumulated elastic strain as seismic waves. The rupture's length, width, slip distribution, and propagation speed govern the size of the earthquake and the characteristics of the resulting ground motion.

A seismic wave is an elastic wave that propagates through the Earth, carrying the energy released by an earthquake or other seismic source. Seismic waves are classified as body waves (P and S) that travel through the Earth's interior, or surface waves (Love and Rayleigh) that travel along its surface.

A seismograph is an instrument that records the motion of the ground during an earthquake. Modern seismographs — more accurately called seismometers with digital recorders — are sensitive enough to detect motions of nanometers and form the backbone of global earthquake monitoring networks.