Earthquake & volcano glossary
Clear definitions of the key terms in seismology and volcanology — magnitude, epicenter, faults, subduction and more.
Magnitude
Magnitude is a number that measures the size of an earthquake by the amount of energy released at its source. Because the scale is logarithmic, each whole step up represents roughly 32 times more energy. Unlike intensity, magnitude is a single value for an earthquake regardless of where you feel it.
Epicenter
The epicenter is the point on the Earth's surface directly above where an earthquake begins underground. It is usually where shaking is felt most strongly and damage is greatest. Seismologists locate it by comparing the arrival times of seismic waves at different stations.
Hypocenter (Focus)
The hypocenter, also called the focus, is the actual point underground where an earthquake's rupture starts. It lies directly below the epicenter, at a depth that can range from just a few kilometers to hundreds of kilometers. Shallow hypocenters tend to cause more surface damage than deep ones.
Fault
A fault is a fracture or zone of fractures in the Earth's crust where blocks of rock have moved past each other. When stress builds up and the rocks suddenly slip, the released energy produces an earthquake. Faults can range from tiny cracks to boundaries thousands of kilometers long.
Subduction
Subduction is the process by which one tectonic plate slides beneath another and sinks into the mantle. It happens where a denser oceanic plate meets a lighter continental plate, and it drives some of the planet's largest earthquakes and volcanoes. The world's biggest recorded quakes occur in subduction zones.
Aftershock
An aftershock is a smaller earthquake that follows a larger one, called the mainshock, in the same area. They occur as the crust readjusts to the stress changes caused by the main rupture. Aftershocks can continue for days, months, or even years, and occasionally cause additional damage to weakened structures.
Foreshock
A foreshock is a smaller earthquake that occurs before a larger mainshock in the same region. It can sometimes signal that a bigger event is coming, but it is only identified as a foreshock after the larger quake happens. Not every earthquake has foreshocks, which makes them unreliable for prediction.
Richter Scale
The Richter scale is a logarithmic measure of earthquake magnitude developed by Charles Richter in 1935. Each whole number increase represents about ten times greater ground motion and roughly 32 times more energy. Although widely known to the public, scientists now prefer the moment magnitude scale for large earthquakes.
Moment Magnitude Scale
The moment magnitude scale (Mw) is the modern standard for measuring the size of large earthquakes. It is based on the seismic moment, which accounts for the area of the fault that ruptured, how far it slipped, and the rigidity of the rock. Unlike the Richter scale, it does not saturate for very large quakes, giving more accurate values.
Mercalli Scale
The Mercalli scale measures the intensity of an earthquake by its effects on people, buildings, and the landscape. It runs from I (barely felt) to XII (total destruction) and is based on observations rather than instruments. Unlike magnitude, intensity varies from place to place depending on distance and local conditions.
Tsunami
A tsunami is a series of powerful ocean waves usually triggered by an underwater earthquake, landslide, or volcanic eruption. In the open sea the waves are barely noticeable, but they grow into towering walls of water as they reach shallow coastlines. Tsunamis can travel across entire oceans and cause devastating flooding far from their origin.
Liquefaction
Liquefaction happens when strong shaking causes water-saturated, sandy soil to lose its strength and behave like a liquid. During an earthquake the ground can no longer support buildings, which may sink, tilt, or float upward. It is a major cause of damage in coastal and reclaimed land areas.
Tectonic Plate
A tectonic plate is one of the large slabs of rock that make up the Earth's outer shell, or lithosphere. These plates slowly drift over the softer mantle beneath them, at speeds of a few centimeters per year. Where their edges meet, they collide, pull apart, or grind past each other, creating earthquakes and volcanoes.
Ring of Fire
The Ring of Fire is a horseshoe-shaped zone around the edges of the Pacific Ocean where most of the world's earthquakes and volcanic eruptions occur. It marks the boundaries of several tectonic plates and their subduction zones. About 75% of the planet's active volcanoes and 90% of its earthquakes happen along this belt.
Seismograph
A seismograph is an instrument that detects and records the ground motion caused by earthquakes and other vibrations. It produces a trace called a seismogram, which scientists use to determine an earthquake's location, depth, and magnitude. Modern digital seismographs can pick up tremors from the other side of the planet.
P-Waves
P-waves, or primary waves, are the fastest seismic waves and the first to arrive at a recording station. They travel by compressing and expanding the material they pass through, and they can move through solids, liquids, and gases. Because they arrive first, they are used by early-warning systems to give a few seconds of alert before stronger shaking.
S-Waves
S-waves, or secondary waves, are slower than P-waves and arrive at a station after them. They move the ground side to side, perpendicular to their direction of travel, and can only pass through solids, not liquids. This inability to travel through liquid is how scientists discovered that the Earth's outer core is molten.
Surface Waves
Surface waves travel along the Earth's surface rather than through its interior, and they arrive after the P- and S-waves. They move more slowly but often have the largest amplitude, making them the most destructive during an earthquake. The two main types are Love waves, which shake side to side, and Rayleigh waves, which roll like ocean swells.
Earthquake Swarm
An earthquake swarm is a sequence of many earthquakes in a small area over a short period, without a single clearly dominant mainshock. Unlike a mainshock-aftershock sequence, the events are similar in size. Swarms are common in volcanic regions and can be a sign of magma moving underground.
Seismic Gap
A seismic gap is a segment of an active fault that has not produced a large earthquake for an unusually long time, while neighboring segments have. Because stress keeps accumulating there, such gaps are considered likely locations for future major quakes. Scientists monitor them closely to assess earthquake hazard.
Strike-Slip Fault
A strike-slip fault is one where two blocks of rock slide horizontally past each other, with little vertical movement. The motion is driven by shearing forces in the crust, and the famous San Andreas Fault is a classic example. These faults can produce powerful earthquakes when the built-up strain is suddenly released.
Normal Fault
A normal fault forms where the crust is being pulled apart, causing one block to slide down relative to the other. It is typical of regions where tectonic plates are moving away from each other, such as rift valleys. Normal faults lengthen and thin the crust over time.
Reverse Fault
A reverse fault occurs where the crust is being compressed, pushing one block of rock up and over another. It is common where tectonic plates collide, and it helps build mountain ranges. When the dip of the fault is very shallow, it is called a thrust fault.
Magma
Magma is molten rock that lies beneath the Earth's surface, along with dissolved gases and suspended crystals. It forms when high temperatures partially melt rock in the crust or upper mantle. When magma rises and erupts onto the surface, it becomes lava.
Lava
Lava is magma that has reached the Earth's surface through a volcanic eruption. It can flow as a fluid stream or ooze slowly, depending on its temperature and chemical composition. As it cools, lava solidifies into volcanic rock such as basalt.
Eruption
A volcanic eruption is the release of magma, gases, and ash from a volcano onto the surface. Eruptions can be gentle, with slow lava flows, or violently explosive, blasting ash and rock high into the sky. Their style depends largely on how thick the magma is and how much gas it contains.
Caldera
A caldera is a large, bowl-shaped depression that forms when a volcano collapses into the emptied magma chamber beneath it after a major eruption. Calderas can be several kilometers wide and often fill with water to become lakes. Some, like Yellowstone, sit above enormous volcanic systems capable of huge eruptions.
Stratovolcano
A stratovolcano is a tall, steep-sided volcano built up from alternating layers of hardened lava, ash, and volcanic rock. They tend to have explosive eruptions because their magma is thick and traps gas. Famous examples include Mount Fuji, Vesuvius, and Mount St. Helens.
Shield Volcano
A shield volcano is a broad, gently sloping volcano built almost entirely from fluid lava flows. Its runny lava spreads far before cooling, creating a wide, low profile that resembles a warrior's shield. Hawaii's Mauna Loa is one of the largest shield volcanoes on Earth.
Pyroclastic Flow
A pyroclastic flow is a fast-moving avalanche of hot gas, ash, and rock fragments that rushes down the slopes of a volcano during an explosive eruption. It can reach speeds of hundreds of kilometers per hour and temperatures of several hundred degrees. Pyroclastic flows are among the deadliest volcanic hazards and destroyed the Roman city of Pompeii.