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.
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.
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.
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.
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.
Focal Depth: Guides and Explainers
These explainers connect this definition to real-world forecasting and decision-making.
Weather Education
Earthquake Depth Explained: Why Shallow Quakes Do More Damage
A magnitude 6.7 killed 57 people. A magnitude 6.8 three times deeper killed no one directly. Here is how earthquake depth controls shaking, why deep quakes are felt farther but weaker, and when depth stops mattering.
Weather Education
Why Earthquake Magnitudes Change After the First Report
An earthquake posts as M6.9, then becomes M7.1 an hour later. Here is why USGS magnitude, depth, and location estimates get revised, how much change is normal, and which number to trust.
Helpful Tools and Hubs
Use these pages to apply the concept, compare conditions, or follow related weather workflows.
Live Tracker
Live Earthquake Map
Browse recent USGS earthquake activity worldwide with magnitude and depth filters.
Feature
Earthquake Alerts
Get notified about nearby and global earthquakes with customizable thresholds.
Guide
Foreshocks, Mainshocks, and Aftershocks
What the labels really mean, how long sequences last, and why aftershocks get rarer over time but not smaller.
Guide
Richter vs Moment Magnitude
How moment magnitude replaced the Richter scale, and why each whole number means about 32 times more energy.
Explore Other Terms
Black Ice
A thin, transparent layer of ice on road surfaces that is nearly invisible, making it extremely dangerous for drivers. Black ice forms when temperatures drop rapidly after rain or from refreezing meltwater, often at night or in shaded areas.
King Tide
King tide is a popular, non-scientific name for an exceptionally high tide. King tides happen when the higher tides around a new or full moon coincide with the Moon's closest approach to Earth, called perigee. They can flood low-lying coastal areas on a clear day, and they raise the water level any storm surge is added to.
AQI
The Air Quality Index (AQI) is the U.S. Environmental Protection Agency's color-coded scale for reporting air pollution. It has six categories, from Good (0 to 50) to Hazardous (301 and higher). Values of 101 and above are unhealthy, first for sensitive groups and then, as the number rises, for everyone.
Jet Stream
A jet stream is a relatively narrow band of strong wind in the upper atmosphere, typically around 30,000 feet, that blows from west to east. The polar jet sits between 50° and 60° latitude and the subtropical jet near 30°. Jet streams follow the boundaries between cold and warm air and steer the storm systems below them.
Kp Index
The Kp index is a 0 to 9 scale of global geomagnetic activity, reported in 3-hour intervals from ground-based magnetometers. A Kp of 5 or higher indicates a geomagnetic storm, and the values map onto NOAA's G-scale from G1 (Kp 5) to G5 (Kp 9). The higher the Kp, the farther from the poles the aurora can be seen.
Tornado
A tornado is a violently rotating column of air in contact with the ground, extending from a thunderstorm. Tornadoes are rated from EF0 to EF5 on the Enhanced Fujita scale by the damage they cause. Most last less than 10 minutes, but the strongest can stay on the ground for more than an hour.
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