Seismology Glossary
Key terms and definitions used in seismology and earthquake science.
A
Accelerometer
A sensor that measures acceleration of ground motion, critical for earthquake engineering. Modern strong-motion acceler…
Aftershock
A smaller earthquake that follows the mainshock in the same fault region. Aftershock sequences can last weeks to years,…
Asthenosphere
The partially molten, ductile layer of Earth's upper mantle beneath the lithosphere, extending from about 100-700 km de…
B
Base Isolation
An earthquake engineering technique that decouples a building from ground motion using flexible bearings at the foundat…
Blind Thrust Fault
A thrust fault that does not reach the surface, making it invisible at ground level and harder to detect. The 1994 Nort…
Body Wave
Seismic waves that travel through the Earth's interior, including P-waves and S-waves. They travel faster than surface …
Body-Wave Magnitude (mb)
A magnitude scale based on the amplitude of P-waves recorded at teleseismic distances. Useful for measuring deep earthq…
Broadband Seismometer
A seismometer capable of recording seismic waves across a wide frequency range (0.001-50 Hz). The primary instrument in…
Building Code (Seismic)
A set of legal requirements governing the design and construction of buildings to ensure minimum levels of earthquake s…
b-value
The slope of the Gutenberg-Richter frequency-magnitude relationship. A b-value near 1.0 is typical; higher values indic…
C
Cascading Failures
A chain reaction of failures triggered by an earthquake where the failure of one system causes others to fail — such as…
Catastrophe Bond (CAT Bond)
A financial instrument that transfers earthquake risk from insurers to capital market investors. If a qualifying earthq…
CERT (Community Emergency Response Team)
A volunteer program that trains community members in basic disaster response skills including fire suppression, search …
Coda Wave
The tail portion of a seismogram after the main seismic wave arrivals, caused by scattering of waves in the heterogeneo…
Continental Drift
The theory proposed by Alfred Wegener in 1912 that continents move across Earth's surface over geological time. Later e…
Convergent Boundary
A plate boundary where two plates move toward each other. Can produce subduction zones (ocean-continent), mountain buil…
Coulomb Stress Transfer
The process by which an earthquake changes stress on nearby faults, potentially triggering or delaying future earthquak…
D
Did You Feel It? (DYFI)
A USGS program that collects intensity reports from the public after earthquakes to create community-derived intensity …
Divergent Boundary
A plate boundary where two plates move apart from each other, creating new crust as magma rises from the mantle. Mid-oc…
Drop, Cover, and Hold On
The internationally recommended protective action during earthquake shaking. Drop to your hands and knees, take cover u…
E
Earthquake Clustering
The tendency for earthquakes to occur in clusters (mainshock-aftershock sequences or swarms) rather than randomly in ti…
Earthquake Deductible
The percentage of a property's insured value that the policyholder must pay before insurance coverage begins. Earthquak…
Earthquake Early Warning (EEW)
A system that detects an earthquake and sends alerts to people and systems before strong shaking arrives. Can provide s…
Earthquake Emergency Kit
A pre-assembled collection of supplies for surviving the aftermath of an earthquake, typically including water (1 gallo…
Earthquake Energy
The total seismic energy radiated by an earthquake, measured in joules. A magnitude 9 earthquake releases the energy eq…
Earthquake Insurance
A specialized insurance policy covering damage caused by earthquakes, typically purchased as a separate policy from sta…
Earthquake Prediction vs Forecasting
Prediction claims to specify exact time, place, and magnitude of a future earthquake — currently impossible. Forecastin…
Earthquake Preparedness
The ongoing process of planning and preparation to minimize earthquake impact, including securing furniture, creating c…
Earthquake Recurrence Interval
The average time between major earthquakes on a particular fault. Estimated from paleoseismology and historical records…
Earthquake Swarm
A sequence of earthquakes occurring in a localized area over days to months with no clearly dominant mainshock. Often a…
Earthquake-Triggered Landslide
The downslope movement of soil and rock triggered by earthquake shaking. Landslides can bury entire communities and may…
Emergency Communication Plan
A pre-arranged plan for family members to contact each other after an earthquake, including out-of-area contacts, meeti…
Epicenter
The point on the Earth's surface directly above the hypocenter (focus) where an earthquake originates underground. Ofte…
Exposure Analysis
The identification and quantification of assets (buildings, infrastructure, population) at risk from earthquake shaking…
F
Fault Creep
The slow, continuous movement along a fault without generating significant earthquakes. Some sections of the San Andrea…
Fault (Geology)
A fracture in rock along which movement has occurred. Faults range from millimeters to thousands of kilometers long. Ma…
Fault Line
The trace of a fault on the Earth's surface, visible as a line or zone of broken rock. Active fault lines are mapped by…
Fault Rupture
The breakage of rock along a fault during an earthquake, releasing stored elastic energy as seismic waves. Rupture leng…
Fault Scarp
A cliff or steep slope formed by vertical displacement along a fault during an earthquake. Fault scarps can be meters h…
Fault Segment
A distinct section of a larger fault system with characteristic slip behavior. Different segments may rupture independe…
Foreshock
An earthquake that occurs before the mainshock in the same region. Foreshocks can only be identified in retrospect — th…
G
Global Seismographic Network (GSN)
A worldwide network of 150+ broadband seismograph stations that provides comprehensive monitoring of global earthquake …
GPS Geodesy
The use of Global Positioning System receivers to measure tectonic plate motion and crustal deformation with millimeter…
Ground Rupture (Surface Faulting)
Visible displacement of the ground surface along a fault during an earthquake. Structures built across a surface ruptur…
Gutenberg-Richter Law
A statistical law describing the relationship between earthquake frequency and magnitude: for each unit increase in mag…
H
Hotspot (Geology)
A location in the mantle where hot rock rises as a plume, creating volcanic activity independent of plate boundaries. H…
Hypocenter (Focus)
The actual point within the Earth where an earthquake rupture initiates. Also called the focus. Depth of the hypocenter…
I
Induced Seismicity
Earthquakes triggered by human activities such as hydraulic fracturing (fracking), wastewater injection, mining, or res…
InSAR (Interferometric SAR)
A satellite radar technique that measures ground deformation with centimeter accuracy by comparing radar images taken b…
L
Lateral Spreading
The horizontal movement of soil blocks toward a free face (cliff or stream bank) during liquefaction. Can cause extensi…
Liquefaction
A phenomenon where saturated, loose soil temporarily loses strength and behaves like a liquid during strong shaking. Ca…
Lithosphere
The rigid outer layer of Earth, comprising the crust and upper mantle, broken into tectonic plates. The lithosphere ave…
Local Magnitude (ML)
The original Richter magnitude, calculated from the maximum amplitude recorded on a Wood-Anderson seismograph within 60…
Locked Fault
A section of a fault where friction prevents movement, causing stress to accumulate. When a locked fault finally ruptur…
Loss Estimation
The process of predicting economic losses and casualties from a potential earthquake scenario. FEMA's HAZUS software is…
Love Wave
A type of surface wave that causes horizontal shearing of the ground. Named after mathematician A.E.H. Love, these wave…
M
Magnitude
A single number that quantifies the total energy released by an earthquake. Each whole number increase represents rough…
Mainshock
The largest earthquake in a sequence, which defines the overall magnitude of the event. Preceded by foreshocks (sometim…
Mantle Convection
The slow circulation of rock within Earth's mantle driven by heat from the core. This process provides the driving forc…
Maximum Credible Earthquake (MCE)
The largest earthquake that could reasonably occur on a particular fault or in a specific area, based on geological evi…
Mid-Ocean Ridge
An underwater mountain range formed by divergent plate boundaries where magma rises to create new oceanic crust. The gl…
Modified Mercalli Intensity
A 12-point scale (I-XII) that measures the observed effects of an earthquake at a specific location, from imperceptible…
Moment Magnitude Scale
The modern standard for measuring earthquake size (Mw), based on the seismic moment — the product of fault area, averag…
Moment-Resisting Frame
A structural system where beams and columns are rigidly connected to resist lateral earthquake forces through bending. …
P
Paleoseismology
The study of prehistoric earthquakes through geological evidence such as fault trenches, uplifted terraces, and tsunami…
Peak Ground Acceleration (PGA)
The maximum acceleration of the ground during an earthquake, measured in g (gravitational acceleration). A key paramete…
Performance-Based Seismic Design
An advanced design approach that targets specific performance levels (operational, life-safe, collapse prevention) for …
Plate Boundary
The edge where two tectonic plates meet. Most earthquakes, volcanic eruptions, and mountain building occur at plate bou…
Plate Collision
The process of two continental plates converging, creating massive mountain ranges like the Himalayas. Continental coll…
Probabilistic Seismic Hazard Analysis (PSHA)
A method for quantifying earthquake hazard that considers all possible earthquake sources, magnitudes, and ground motio…
Probable Maximum Loss (PML)
An estimate of the maximum loss an insurance portfolio or property is likely to experience from a single earthquake eve…
P-Wave (Primary Wave)
The fastest seismic wave, traveling through both solid rock and liquid at 5-8 km/s. P-waves compress and expand materia…
R
Rayleigh Wave
A surface wave that causes the ground to move in an elliptical motion, similar to ocean waves. Named after Lord Rayleig…
Reverse (Thrust) Fault
A fault where the hanging wall moves upward relative to the footwall, caused by compressional forces. Thrust faults at …
Richter Scale
The original logarithmic magnitude scale developed by Charles Richter in 1935 to measure local earthquake magnitude. La…
Rift Zone
An area where the Earth's crust is being pulled apart, often associated with divergent boundaries. The East African Rif…
Ring of Fire
A horseshoe-shaped zone around the Pacific Ocean where about 90% of the world's earthquakes occur. It spans 40,000 km a…
S
Search and Rescue (SAR)
Organized efforts to locate and extract survivors trapped in collapsed structures after an earthquake. The first 72 hou…
Secondary Earthquake Hazards
Hazards triggered by earthquake shaking rather than the shaking itself — including tsunamis, landslides, liquefaction, …
Seiche
A standing wave oscillation in an enclosed or semi-enclosed body of water caused by earthquake shaking. Seiches can occ…
Seismic Alert System
Mexico's SASMEX, one of the world's first public earthquake early warning systems, operational since 1991. Provides up …
Seismic Damper
A device installed in buildings to absorb and dissipate earthquake energy, reducing structural movement. Types include …
Seismic Design
The practice of designing structures to withstand earthquake forces. Modern seismic design aims to prevent collapse and…
Seismic Gap
A section of an active fault that has not produced an earthquake for a long time compared to neighboring sections. Seis…
Seismic Hazard Map
A map showing the probability of earthquake shaking exceeding specified levels over a given time period. Used by engine…
Seismic Intensity
A measure of the strength of shaking at a particular location, determined by observed effects on people, structures, an…
Seismic Moment
A measure of the total energy released by an earthquake, calculated as the product of the fault area, average displacem…
Seismic Network
A coordinated group of seismograph stations that continuously monitor earthquake activity. The Global Seismographic Net…
Seismic Retrofit
Strengthening an existing building to improve its earthquake resistance. Common methods include adding steel bracing, r…
Seismic Risk Assessment
The process of evaluating earthquake hazard, building vulnerability, and potential losses for a specific area or struct…
Seismic Tomography
A technique that uses seismic wave travel times to create 3D images of Earth's interior structure, similar to a medical…
Seismic Wave
An elastic wave generated by an earthquake or explosion that propagates through the Earth. Seismic waves carry the ener…
Seismogram
The recorded output of a seismograph, showing ground motion as a function of time. Seismologists analyze seismograms to…
Seismograph
An instrument that detects and records ground motion caused by seismic waves. Modern digital seismographs can detect mo…
ShakeAlert
The US earthquake early warning system operated by USGS and university partners. Covers the West Coast (California, Ore…
ShakeMap
A USGS product that displays the distribution of ground shaking intensity after an earthquake. Combines seismograph dat…
Shear Wall
A structural wall designed to resist lateral forces from earthquake shaking. Shear walls are the primary lateral force-…
Shelter-in-Place
The practice of staying in your current location during or after an earthquake rather than evacuating. Appropriate when…
Slip Rate
The average rate of displacement along a fault, typically measured in millimeters per year. Higher slip rates generally…
Soft Story
A building story (usually ground floor) that is significantly weaker than the floors above, often due to large openings…
Soil Amplification (Site Effect)
The increase in shaking intensity caused by soft soil or sediment layers amplifying seismic waves. Structures built on …
Strike-Slip Fault
A fault where blocks of rock move horizontally past each other. The San Andreas Fault and North Anatolian Fault are maj…
Strong-Motion Sensor
An instrument designed to record the intense ground shaking near large earthquakes without going off-scale. Essential f…
Structural Resonance
The amplification of building motion when earthquake wave frequency matches the building's natural frequency. Low-rise …
Subduction Zone
A region where one tectonic plate dives beneath another into the mantle. Subduction zones produce the world's largest e…
Surface Wave
Seismic waves that travel along the Earth's surface rather than through its interior. Slower than body waves but typica…
Surface-Wave Magnitude (Ms)
A magnitude scale based on Rayleigh wave amplitude at a period of about 20 seconds. Works well for shallow earthquakes …
S-Wave (Secondary Wave)
Seismic waves that move rock perpendicular to the direction of travel, arriving after P-waves. S-waves cannot travel th…
T
Tectonic Plate
A massive segment of Earth's lithosphere that moves, floats, and sometimes fractures. There are 7 major and about 8 min…
Transform Boundary
A plate boundary where two plates slide horizontally past each other. The San Andreas Fault in California is the most f…
Tsunami
A series of ocean waves generated by sudden displacement of the seafloor during an underwater earthquake. Tsunamis can …
Tsunami Evacuation Zone
A designated area at risk of tsunami inundation with marked evacuation routes to higher ground. Evacuation should begin…
U
Unreinforced Masonry (URM)
Brick or block construction without steel reinforcement, which is extremely vulnerable to earthquake shaking. URM build…
USGS (United States Geological Survey)
The primary US government agency responsible for monitoring earthquakes, operating the National Earthquake Information …
V
Volcanic Earthquake
An earthquake associated with volcanic activity, caused by magma movement, gas pressure, or rock fracturing near a volc…
Vulnerability (Fragility) Function
A mathematical function describing the probability of various damage states for a specific building type given a level …