P-Wave
The primary or compressional wave, the fastest seismic wave and the first to reach any recording station. P-waves push and pull rock along their direction of travel and can pass through both solids and liquids. Their head start over more damaging waves is what makes earthquake early warning physically possible.
More Earthquakes & Seismology Terms
Magnitude
A single number estimating the size of an earthquake at its source, derived from seismograms. Older scales read it from the peak amplitude of the recorded waves; the modern standard, moment magnitude, is computed from the physical size of the fault rupture instead. The scale is logarithmic: each whole number represents about 10 times more ground motion and roughly 32 times more energy released. Magnitude describes the earthquake itself, not how strongly it was felt at any particular place.
Moment Magnitude
The modern standard magnitude scale (Mw), based on seismic moment: the rupture area multiplied by the average slip and the rigidity of the rock. Unlike older scales it does not saturate, so it stays accurate for the largest earthquakes. USGS reports it under labels such as Mww, derived from W-phase moment tensor solutions.
Richter Scale
The original local magnitude scale (ML) developed by Charles Richter in 1935 for Southern California. It saturates above roughly magnitude 7, meaning it underestimates the largest earthquakes, so seismologists replaced it with moment magnitude for significant events. News coverage still refers to the Richter scale out of habit even when the figure quoted is actually moment magnitude.
Epicenter
The point on Earth's surface directly above where a fault rupture begins. It is a single map location, not the whole earthquake: a large event can break hundreds of kilometers of fault, so severe shaking often extends far beyond the epicenter.
Hypocenter
The actual point underground where fault rupture begins, also called the focus. It is described by latitude, longitude, and depth. The epicenter is simply the hypocenter projected straight up to the surface.
Focal Depth
How far below the surface an earthquake began, reported in kilometers. Shallow earthquakes (roughly 0 to 70 km) shake the surface hardest because the energy travels a shorter distance; very deep earthquakes can be felt across huge areas but often cause less damage. When depth cannot be resolved from the available data, USGS may fix it at a default value such as 10 km.
Fault
A fracture in Earth's crust where blocks of rock have moved past one another. Earthquakes occur when accumulated stress overcomes friction on a fault and the rock slips suddenly. The main types are strike-slip (horizontal motion, as on the San Andreas), normal (pulling apart), and reverse or thrust (compression).
Aftershock
A smaller earthquake following a mainshock in the same area, caused by the crust adjusting to redistributed stress. Aftershocks are most frequent in the first hours and days and taper off over time, but can continue for months or years after a large event. If one turns out to be larger than the original earthquake, it is reclassified as the mainshock.
Foreshock
An earthquake that precedes a larger event on or near the same fault. Foreshocks can only be identified after the fact, which is one reason earthquakes cannot be reliably predicted: at the time it happens, a foreshock looks like any other small earthquake.
Earthquake Swarm
A cluster of earthquakes in a small area over days to months with no single dominant mainshock. Swarms are common in volcanic and geothermal regions, where moving magma or underground fluids change stress in the surrounding crust.
S-Wave
The secondary or shear wave, which arrives after the P-wave and moves rock perpendicular to its direction of travel. S-waves shake far more strongly than P-waves and cause much of the damage in an earthquake. They cannot travel through liquids, which is how scientists established that Earth's outer core is molten.
Seismometer
An instrument that detects and records ground motion; the recording it produces is called a seismogram. Networks of seismometers locate an earthquake by comparing wave arrival times across many stations, which is why reported location, depth, and magnitude get more accurate as additional stations report in.
Modified Mercalli Intensity
A scale from I to XII describing how strongly shaking was felt and what damage occurred at a specific location, rather than the size of the earthquake at its source. One earthquake has a single magnitude but many intensity values, which decrease with distance and vary sharply with local soil conditions.
ShakeMap
A USGS product that maps estimated ground shaking and intensity across an affected region within minutes of an earthquake, combining instrument recordings, public felt reports, and models. Emergency managers use it to identify where damage is most likely before reports arrive from the ground.
Liquefaction
A process in which water-saturated, loosely packed soil temporarily behaves like a liquid during strong shaking and loses its ability to bear weight. It can tilt or sink buildings, float buried tanks and pipes upward, and cause the ground to crack and spread sideways. Reclaimed land, river deltas, and sandy fill are especially vulnerable.
Subduction Zone
A plate boundary where one tectonic plate is forced beneath another. Subduction zones produce the largest earthquakes on Earth, including every recorded magnitude 9 event, along with deep earthquakes, chains of volcanoes, and most destructive tsunamis.
Ring of Fire
The horseshoe-shaped belt of subduction zones, faults, and volcanic arcs surrounding the Pacific Ocean, running from New Zealand up through Asia, across Alaska, and down the Americas. About 90 percent of the world's earthquakes occur along it.
Tsunami Warning
The highest alert level issued by a tsunami warning center, meaning a dangerous tsunami is expected or already underway and people in the threatened zone should move inland or to high ground immediately. The lower levels are an advisory (dangerous currents near the water), a watch (a distant event may pose a threat, stay alert), and an information statement (no threat expected).
Earthquake Early Warning
A system that detects the fast P-wave near an earthquake's origin and issues an alert before the slower, more damaging shaking arrives, providing seconds to tens of seconds of notice. It is detection, not prediction: the earthquake has already begun. Public systems operate in regions including the U.S. West Coast (ShakeAlert), Japan, and Mexico.
Explore Other Terms
Tornado
A violently rotating column of air extending from a thunderstorm to the ground. Tornadoes can produce winds exceeding 300 mph and cause catastrophic damage along their path. They are rated on the Enhanced Fujita (EF) scale from EF0 to EF5.
Supercell
A severe thunderstorm characterized by a deep, persistently rotating updraft called a mesocyclone. Supercells are the most dangerous type of thunderstorm and can produce tornadoes, large hail, damaging winds, and flash flooding.
Freezing Rain
Rain that falls as liquid but freezes on contact with surfaces at or below 32°F (0°C), creating a coating of ice called glaze. Freezing rain is extremely dangerous as it can make roads impassable and cause widespread power outages from ice-coated power lines.
Sleet
Frozen raindrops that bounce when hitting the ground. Sleet forms when snowflakes partially melt falling through a warm layer, then refreeze in a cold layer near the surface. Unlike freezing rain, sleet is frozen before it hits the ground.
Barometric Pressure
The weight of the atmosphere pressing down on a given point, measured in inches of mercury (inHg) or millibars (mb). Rising pressure typically indicates improving weather, while falling pressure suggests approaching storms or precipitation.
Dew Point
The temperature at which air becomes saturated and water vapor begins to condense into liquid. A higher dew point indicates more moisture in the air. Dew points above 65°F feel humid, while above 70°F is considered oppressive.
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Earthquake Depth Explained
Why a shallow quake shakes far harder than a deeper one of the same magnitude, and when depth stops mattering.
Glossary
Earthquake Early Warning
A system that detects the fast P-wave near an earthquake's origin and issues an alert before the slower, more damaging shaking arrives, providing seconds to tens of seconds of notice.
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