What Causes Earthquakes?

Why do faults suddenly slip?

An earthquake is a sudden slip of two blocks of the earth past each other, along a surface called a fault.^[1] The crust and the top of the mantle form a thin outer skin on the planet, broken into many pieces called tectonic plates that keep slowly moving, sliding past and bumping into each other.^[1] Plate boundaries contain many faults, and these faults are where most of the world's earthquakes happen.^[1]

Because the plate edges are rough, they get stuck while the rest of the plate keeps moving, and the energy that would make the blocks slide is stored up instead.^[1] When the force finally overcomes the friction of the fault's jagged edges, the fault comes unstuck and all of that stored energy is released.^[1]

This slow build-up and sudden release of stress is known as the elastic rebound theory.^[2] It comes from H. F. Reid, a geology professor at Johns Hopkins University, who studied how the ground shifted in the California earthquake of April 18, 1906.^[2] The U.S. Geological Survey (USGS) compares it to a stretched rubber band that is cut: the Earth's crust slowly stores elastic stress and then releases it suddenly.^[2] The USGS also illustrates the idea with a fence built straight across the San Andreas Fault: as the Pacific Plate moves northwest, the fence slowly bends, and when the earthquake strikes, the two parts become straight again but offset.^[2] According to the USGS, most earthquakes are the result of this sudden elastic rebound.^[2]

What are the main types of faults?

Scientists classify faults by their angle to the surface, called the dip, and by the direction in which the rock slips.^[3]

Faults that combine dip-slip (up or down) and strike-slip (sideways) motion are called oblique-slip faults.^[3] Not every fault moves in jumps: some move slowly, in a process called creep.^[3]

Where does an earthquake start, and how do the waves travel?

The point below the surface where an earthquake starts is the hypocenter, and the point on the surface directly above it is the epicenter.^[1] The released energy spreads out from the fault in all directions as seismic waves, like ripples on a pond.^[1] The main earthquake is always followed by smaller aftershocks in the same place, which can continue for weeks, months or even years.^[1]

P waves, or compressional waves, make the rock vibrate in the direction the wave is moving; they are the fastest and arrive first.^[4] S waves, or shear waves, shake the rock at right angles to the direction of travel, generally move at about 60% of the speed of P waves in rock, and always arrive after them.^[4] The slowest are surface waves, which are usually the largest recorded, because their energy spreads out only along the surface rather than through the whole planet.^[4]

What is the difference between magnitude and intensity?

Magnitude is the size of an earthquake, and each earthquake has only one.^[5] Intensity is the strength of the shaking at a particular place, which varies mostly with distance from the fault; in the United States it is measured on the Modified Mercalli Intensity Scale, in Roman numerals.^[5]

The USGS says the Richter scale is no longer commonly used except for small, locally recorded earthquakes, and that the moment magnitude (Mw) scale is more accurate for all others.^[5]

Magnitude is measured on a logarithmic scale: each whole-number step up means a tenfold increase in the wave amplitude recorded on a seismogram and about 32 times more energy released.^[5] Two whole steps therefore mean roughly 32 × 32, or about 1,000 times, more energy (a calculation based on the USGS figure).^[5]

Depth also matters: the magnitude 6.7 Northridge earthquake in California shook harder than the slightly larger magnitude 6.8 Nisqually earthquake in Washington because it started closer to the surface, 3 to 11 miles deep instead of 30 to 36 miles.^[5]

Where and how often do earthquakes happen?

Earthquakes can strike anywhere, but they follow the same general pattern year after year, mostly in three large zones.^[6] The circum-Pacific seismic belt, nicknamed the "Ring of Fire", runs around the rim of the Pacific Ocean and accounts for about 81 percent of the planet's largest earthquakes.^[6] The Alpide belt, from Java to Sumatra through the Himalayas and the Mediterranean into the Atlantic, accounts for about 17 percent of the largest earthquakes, and a third belt follows the underwater Mid-Atlantic Ridge.^[6]

The USGS National Earthquake Information Center locates about 20,000 earthquakes around the world each year, or roughly 55 a day.^[7] Based on records since about 1900, the USGS expects about 16 major earthquakes a year: 15 in the magnitude 7 range and one of magnitude 8.0 or greater.^[7] Smaller quakes are far more common: in 2021, the most recent year in the worldwide table on the USGS statistics page as of October 2026, it counted 2,047 earthquakes of magnitude 5 to 5.9 but only three of magnitude 8 or more.^[8]

• Magnitude 5 to 5.92,047
• Magnitude 6 to 6.9140
• Magnitude 7 to 7.916
• Magnitude 8 or more3

Source: U.S. Geological Survey (Earthquake Hazards Program) [8] · Counts change from year to year.

Can scientists predict earthquakes?

The USGS says they cannot: scientists have tried many ways of predicting earthquakes, none has worked, and it is unlikely they ever will.^[1] On any particular fault, scientists know there will be another earthquake at some point, but they have no way of telling when.^[1] A temporary rise or fall in the number of earthquakes is part of normal variation and is not a sign that a large earthquake is about to happen.^[7]