Earthquakes, Seismology, and Earth's Interior

Earthquake

A sudden shaking of the ground caused by the release of energy stored in rocks along faults or other parts of the Earth's crust.

Fault

A fracture in Earth’s crust where blocks of rock have moved past each other.

Hypocenter (focus)

The point inside Earth where an earthquake originates and seismic energy is first released.

Epicenter

The point on Earth’s surface directly above the earthquake’s focus.

Seismic wave

Vibrations that travel through Earth, carrying the energy released by an earthquake.

Elastic rebound

The sudden release of stored energy when rocks break and snap back to their original shape, causing an earthquake.

Aftershock

A smaller earthquake that follows the main shock, occurring as the crust adjusts to the new stress distribution.

Foreshock

A smaller earthquake that occurs before the main seismic event, often near the same fault.

Megathrust fault

A large, low-angle fault at subduction zones where one tectonic plate is forced beneath another; capable of producing the most powerful earthquakes.

Fault creep

Slow, gradual movement along a fault without causing significant earthquakes.

Seismology

The scientific study of earthquakes and the propagation of seismic waves through Earth.

Seismograph (seismometer)

An instrument that detects and records ground motion caused by seismic waves.

Inertia

The tendency of an object to resist changes in motion; this principle allows seismographs to measure ground movement.

Seismogram

The recorded trace or graph of seismic waves produced by a seismograph.

Body waves

Seismic waves that travel through Earth’s interior, including P and S waves.

Surface waves

Seismic waves that move along Earth’s surface, causing the most damage during earthquakes.

Primary (P) waves

The fastest seismic waves, compressional in nature, that travel through solids, liquids, and gases.

Secondary (S) waves

Slower seismic waves that move with a shearing motion and can travel only through solids.

Intensity

A measure of an earthquake’s effects or the amount of ground shaking at a particular location.

Magnitude

A measure of the total energy released by an earthquake at its source.

Modified Mercalli Intensity scale

A scale that rates earthquake effects based on observed damage and human perception, from I (not felt) to XII (total destruction).

Richter scale

A logarithmic scale that measures earthquake magnitude based on the amplitude of seismic waves.

Moment magnitude

A scale that measures the total energy released by an earthquake, calculated from fault size, slip, and rock properties; now the most widely used scale.

Liquefaction

The process by which saturated soil loses strength and acts like a liquid during intense shaking.

Tsunami

A series of large ocean waves usually caused by underwater earthquakes, volcanic eruptions, or landslides.

Circum-Pacific belt

The seismically active region surrounding the Pacific Ocean, also known as the “Ring of Fire.”

Precursor

A smaller event or change in natural conditions that may signal an upcoming earthquake.

Seismic gap

A segment of an active fault known to produce earthquakes but which has not slipped recently, suggesting potential for a future quake.

Paleoseismology

The study of ancient earthquakes using geological evidence to understand long-term seismic activity.

Crust

Earth’s outermost solid layer, composed of continental and oceanic rocks.

Mantle

The thick, solid layer between Earth’s crust and core, composed mainly of silicate minerals rich in iron and magnesium.

Lithosphere

The rigid outer layer of Earth, including the crust and the uppermost mantle; it is broken into tectonic plates.

Asthenosphere

The partially molten, plastic-like layer of the mantle beneath the lithosphere on which tectonic plates move.

Core

Earth’s innermost layer, composed mainly of iron and nickel, divided into a liquid outer core and solid inner core.

Outer core

The liquid layer of the core whose movement generates Earth’s magnetic field.

Inner core

The dense, solid center of Earth, composed primarily of iron and nickel.