Earthquakes

Focus

The place within Earth where earthquake waves originate; also referred to as the hypocenter.

Epicenter

The point on the surface directly above the focus or hypocenter of an earthquake.

Seismic waves

Acoustic waves (vibrations) caused by frictional slip along the fault plane, transmitted through surrounding rock.

Seismographs

Machines (seismometers) that measure and record the seismic waves generated by earthquakes.

Body waves

Seismic waves that travel through earth’s interior.

Surface waves

Seismic waves that travel along earth’s surface; they have the slowest velocity and include Rayleigh and Love waves.

Rayleigh waves

A type of surface wave that rolls counterclockwise from the direction of wave propagation.

Love waves

A type of surface wave that sways side to side with no vertical motion.

P waves

Also known as primary waves; they have a push-pull (compressional) motion and travel through solids, liquids, and gases.

S waves

Also known as shear waves; they travel only through solids and have a slower velocity than P waves.

Intensity

A measure of the earthquake shaking at a given location based on the amount of damage; measured with the modified Mercalli intensity scale.

Magnitude

A measure of seismic wave amplitude (energy release) and is a function of distance from the focus.

Moment magnitude

The standard used by seismologists to measure very small to very large earthquakes.

Liquefaction

A process where spaces between soil grains filled with water destabilize, causing soil and buildings to sink during shaking.

Earthquake hazards

Potential dangers associated with earthquakes, including ground shaking, liquefaction, tsunamis, landslides, and ground subsidence.

Earthquake forecasting

Estimates the probability that an earthquake of a certain magnitude will occur in a specific area during a specific time.