Earthquake reports combine measurements, geological concepts and descriptions of observed effects. This shortened glossary explains commonly encountered terms in plain language.
Reading note: Magnitude describes an earthquake’s size, while intensity describes shaking and effects at a particular place. Early event values may be revised as more seismic stations report.
A
- Acceleration
- The rate at which velocity changes. Ground acceleration is commonly expressed in metres per second squared or as a fraction of standard gravity, g.
- Active fault
- A fault considered capable of renewed movement within the period relevant to a hazard assessment. The time criteria vary by region and application.
- Aftershock
- An earthquake occurring after the largest event in a sequence and within the same general source area. Aftershocks can continue for weeks, months or longer.
- Asthenosphere
- A mechanically weak and slowly deforming region of the upper mantle beneath the lithosphere. Tectonic plates move above it.
B
- Blind fault
- A fault that does not reach the ground surface. Movement on a blind fault can still produce strong shaking and folding above the rupture.
- Body waves
- Seismic waves that travel through Earth’s interior. P waves and S waves are the principal types.
- Building code
- Rules establishing minimum requirements for the design, construction and occupancy of structures. Seismic provisions seek to reduce collapse and protect life during shaking.
C
- Continental crust
- The outer solid layer forming the continents. It is generally thicker and less dense than oceanic crust.
- Critical structures
- Facilities whose continued operation is important during an emergency, or whose failure could have severe consequences, such as hospitals, major utilities, bridges and emergency-response facilities.
- Crust
- Earth’s outermost major solid layer. Its thickness varies beneath continents and oceans, and its brittle portions host many earthquakes.
D
- Deep earthquake
- An earthquake with a hypocentre deeper than 300 kilometres under a widely used scientific classification. Deep events occur mainly within descending tectonic plates.
- Depth
- The distance below Earth’s surface to an earthquake’s hypocentre. Depth is often less certain than horizontal location in an initial report.
- Duration
- The interval during which measurable or strong ground motion persists. Perceived duration varies with distance, geology and building response.
E
- Earthquake
- Ground motion and radiated seismic energy produced most commonly by sudden slip on a fault, and sometimes by volcanic activity or other abrupt stress changes.
- Earthquake risk
- The potential for harm or loss when earthquake hazards interact with exposed people, buildings, infrastructure and their vulnerability.
- Epicentre
- The point on Earth’s surface directly above the hypocentre where rupture begins. It is not necessarily the location of the strongest shaking or greatest effects.
F
- Fault
- A fracture or zone of fractures across which blocks of rock have moved relative to one another.
- Fault plane
- The surface along which fault movement occurs.
- Fault scarp
- A step-like landform produced where displacement on a fault reaches or deforms the surface.
- Fault trace
- The line where a fault intersects the ground surface, or its representation on a geological map.
- Focal depth
- Another term for the depth of an earthquake’s hypocentre.
- Focus
- Another name for the hypocentre: the point within Earth where rupture starts.
G
- Ground motion
- Movement of the ground caused by seismic waves. It may be described using displacement, velocity, acceleration, frequency and duration.
- Gutenberg–Richter relationship
- An empirical relationship showing that smaller earthquakes generally occur much more often than larger ones in a defined region and period.
H
- Hazard
- A potentially damaging earthquake-related phenomenon, including shaking, surface rupture, liquefaction, landslides, deformation and tsunami. Hazard is not the same as risk.
- Hypocentre
- The point within Earth where an earthquake rupture initiates. It is also called the focus.
I
- Intensity
- A description of shaking severity and observed effects at a particular location. Intensity can vary widely across the area affected by one earthquake.
- Intermediate-depth earthquake
- An earthquake whose hypocentre is approximately 70 to 300 kilometres below the surface.
J
- JMA seismic intensity
- A scale used by the Japan Meteorological Agency to describe local shaking intensity. It is distinct from earthquake magnitude.
L
- Landslide
- Downslope movement of rock, soil or debris under gravity. Earthquake shaking can trigger landslides on susceptible slopes.
- Latitude
- Angular distance north or south of the equator, expressed in degrees.
- Liquefaction
- A loss of strength in water-saturated granular soil when shaking raises pore-water pressure. The affected ground can deform and lose its ability to support structures.
- Lithosphere
- Earth’s rigid outer shell, comprising the crust and uppermost mantle and divided into tectonic plates.
- Local site conditions
- The soil, rock, groundwater and topographic characteristics that influence shaking at a location.
- Locked fault
- A fault segment where friction prevents steady slip while tectonic strain may continue to accumulate.
- Longitude
- Angular distance east or west of the prime meridian, expressed in degrees.
M
- Magnitude
- A logarithmic measure of an earthquake’s size derived from seismic recordings. A one-unit increase represents ten times the recorded wave amplitude and roughly 32 times the energy release, although the precise relationship depends on the measure used.
- Mainshock
- The largest earthquake in a recognised sequence. An event can be reclassified if a larger earthquake follows.
- Mantle
- The major rocky layer between Earth’s crust and core, extending to a depth of about 2,900 kilometres.
- Modified Mercalli intensity
- A twelve-level scale, written with Roman numerals, describing observed shaking and effects from not felt to extreme.
- Moment magnitude (Mw)
- A magnitude scale based on seismic moment, which reflects fault area, slip and rock rigidity. It performs well across a broad range of earthquake sizes.
O
- Oceanic crust
- The relatively thin, dense crust beneath the oceans, composed mainly of basaltic rock.
- Oceanic trench
- A long, deep depression in the seafloor commonly associated with a subduction zone.
- Oscillator
- A system that moves back and forth around an equilibrium position. Simplified oscillators are used to model how structures respond to shaking.
P
- P wave
- The fastest seismic body wave. It alternately compresses and expands material in the direction the wave travels and can pass through solids and fluids.
- Peak ground acceleration (PGA)
- The largest ground acceleration measured or estimated during an earthquake. PGA is one useful engineering measure but does not describe every aspect of damaging motion.
- Plate
- A large, comparatively rigid section of the lithosphere that moves relative to neighbouring plates.
- Plate tectonics
- The framework explaining the motion and interaction of lithospheric plates. Many earthquakes occur at subduction zones, spreading ridges and transform boundaries.
- Preparedness
- Planning, knowledge, supplies, training and coordination developed before an emergency to support an effective response and recovery.
R
- Resilience
- The capacity of people, communities and systems to withstand disruption, maintain essential functions and recover.
- Reverse fault
- An inclined fault on which the block above the fault plane moves upward relative to the block below.
- Risk
- The probability and consequences of loss arising from the interaction of a hazard with exposure and vulnerability.
- Risk assessment
- A structured analysis of hazards, exposed assets, vulnerability and capacity used to understand potential consequences and support decisions.
- Rupture
- The process by which slip spreads across a fault during an earthquake. Rupture may propagate mainly in one direction or in several directions.
S
- S wave
- A shear body wave whose particle motion is perpendicular to its direction of travel. S waves pass through solids but not fluids.
- Seismic hazard
- The probability that specified earthquake effects, such as a level of ground motion, will occur at a location during a stated period.
- Seismic station
- A site where instruments record ground motion or other geophysical observations.
- Seismic waves
- Elastic waves generated by an earthquake or another sudden energy source. Body waves travel through Earth, while surface waves travel near its exterior.
- Seismicity
- The distribution of earthquakes in space, time and magnitude within a region.
- Seismogram
- The recorded signal produced by a seismic instrument.
- Seismometer
- An instrument that detects and measures ground motion. A recording system paired with the sensor is often called a seismograph.
- ShakeMap
- A map estimating the distribution of ground motion and shaking intensity after an earthquake using instrument readings, observations and models.
- Shallow earthquake
- An earthquake with a hypocentre less than about 70 kilometres deep. Shallow events can produce strong local shaking, though damage cannot be inferred from depth alone.
- Strike-slip fault
- A usually steep fault on which blocks move primarily horizontally past one another.
- Subduction
- The process in which one tectonic plate descends beneath another into the mantle.
- Surface faulting
- Permanent displacement or deformation that reaches the ground surface during fault movement.
- Surface wave
- A seismic wave that travels along or near Earth’s surface. Love and Rayleigh waves are common types.
T
- Thrust fault
- A low-angle reverse fault on which the upper block moves up and over the lower block.
- Tsunami
- A series of long ocean waves caused by substantial displacement of water, most commonly from seafloor movement during a large earthquake and sometimes from landslides or volcanic activity.
V
- Velocity
- The rate and direction of ground displacement over time. Ground velocity is often reported in centimetres per second.