HOW THE EARTH SHAKES

How the center is found

Earthquakes make several kinds of seismic wave. The fast P-waves (primary, compressional) arrive first, followed by the slower, more damaging S-waves (secondary, shear), and then the surface waves (Love and Rayleigh), which cause most damage.

The longer the gap between the P and S waves at a seismometer, the farther away the quake. Distances from three or more stations give three circles that intersect at the epicenter, a method called triangulation. Depth is calculated from the wave arrival times at nearby stations.

The National Earthquake Information Center

In the United States the job of locating earthquakes worldwide belongs to the National Earthquake Information Center (NEIC) of the US Geological Survey, based in Golden, Colorado, which monitors the globe around the clock. It combines data from hundreds of seismic stations, including the Global Seismographic Network, and publishes the location, depth and magnitude, usually within minutes for a large quake. The data are free, and they feed the live maps on this site.

Other earthquake centers

Many nations and groups run their own centers: the Japan Meteorological Agency, the European-Mediterranean Seismological Centre (EMSC), GFZ Potsdam's GEOFON, and the International Seismological Centre, which compiles the definitive worldwide bulletin months after the fact. The Pacific and Alaska Tsunami Warning Centers use the same data to decide quickly whether a tsunami warning is needed.

Numbers can differ slightly between centers, and an early location is often revised as more stations report.

Magnitude and intensity

• Magnitude measures the energy released at the source. Modern reports use the moment magnitude (Mw), which replaced the older Richter scale for large quakes.
• Each whole step is about 32 times more energy, and about ten times more ground motion. A M8 releases roughly a thousand times the energy of a M6.
• Intensity (the Modified Mercalli scale, I to XII) describes the shaking and damage at a particular place. One earthquake has one magnitude but many intensities.

Normal faults

The crust is pulled apart and one block drops down relative to the other. They are common where plates are diverging, such as in rifts and the Basin and Range of the western US. Quakes are usually moderate, up to about M7.5.

Reverse and thrust faults

The crust is squeezed and one block is pushed up and over the other. A low-angle thrust is a megathrust when it forms the plate boundary at a subduction zone. All of the M9 earthquakes ever recorded were megathrust quakes. These are the largest and the ones most likely to cause tsunamis.

Strike-slip faults

Blocks slide horizontally past each other. The San Andreas, North Anatolian and Alpine faults are examples. They can produce major earthquakes of M7–8, and the surface may be offset by several metres, but they typically do not trigger large tsunamis.

Tectonic

The great majority. Strain builds as plates move, and is released suddenly in slip on a fault. All of the biggest and deadliest earthquakes are tectonic.

Volcanic

Caused by magma and hot fluids moving beneath volcanoes, which may fracture rock or make the ground swell and collapse. They are usually small, but they are a key warning sign. See Earthquakes and Volcanoes.

Induced

Caused by human activity. Examples include filling reservoirs behind big dams, injecting wastewater from oil and gas production deep underground (notably in Oklahoma, Texas and Kansas), geothermal energy projects, and mining. Most are small, but some have exceeded M5.

Collapse and explosion

Roof collapses in mines and caves, and underground explosions such as nuclear tests, generate seismic waves. Seismologists can tell them apart from natural quakes because explosions send out mostly compressional (P) waves.

Shallow (0–70 km)

About three quarters of the energy released worldwide. They are the most damaging because the focus is close to the surface.

Intermediate (70–300 km)

Occur inside sinking slabs of oceanic plate. Strong ones can cause damage over wide areas, but the shaking is spread out.

Deep (300–700 km)

Occur only in the coldest, oldest sinking slabs, such as beneath the Sea of Okhotsk, the Tonga Trench and Bolivia. The 2013 M8.3 Okhotsk earthquake was 600 km deep and was felt as far away as Moscow, but caused little damage.

Foreshock, mainshock, aftershock

The biggest quake in a sequence is the mainshock. Smaller ones before it are foreshocks, and those after are aftershocks. Aftershocks fall off in number over days to years, following a rule called Omori's law, and may include damaging events. Foreshocks cannot be recognised as such until the mainshock has happened.

Doublets and sequences

Sometimes two or more earthquakes of similar size occur close together, such as the 2023 Turkey–Syria quakes (M7.8 and M7.5), and the 2019 Ridgecrest, California pair (M6.4 and M7.1). See also earthquake swarms.

Slow-slip events

In some subduction zones the plates slip slowly over days or months, releasing energy with hardly any shaking at all, in "silent" earthquakes. They have been observed in Cascadia, Japan and New Zealand, and scientists are studying whether they play a role in triggering great quakes.

Intraplate quakes

Rare but real earthquakes far from plate boundaries, such as New Madrid (1811–12), Charleston, South Carolina (1886), Bhuj, India (2001) and Bulnay, Mongolia (1905). Old faults and rifts are reactivated by stress carried across the plate.

The big idea

• Alfred Wegener proposed continental drift in 1912, but could not explain how continents could move. His idea was rejected for decades.
• In the 1950s and 60s, mapping of the sea floor found mid-ocean ridges, symmetrical magnetic stripes, and deep trenches, which showed that sea floor is created at ridges and consumed at trenches.
• By the late 1960s the theory of plate tectonics was established. Earthquakes and volcanoes, when mapped, outlined the plates.

What drives the plates

• Slab pull. Old, dense oceanic plate sinks at subduction zones and pulls the rest of the plate along. This is thought to be the biggest force.
• Ridge push. New crust at ridges is high and slides down off the ridge.
• Mantle convection. Slow flow in the hot mantle interacts with the plates from below.
• Plates move about 2 to 10 cm a year, roughly as fast as fingernails grow.

Three kinds of boundary

• Convergent: plates collide. Where one dives beneath the other (subduction), it produces the largest earthquakes, deep trenches, and volcanic arcs. Where two continents meet, as in the Himalaya, thrust faults build mountains.
• Divergent: plates separate, as at the Mid-Atlantic Ridge and East African Rift, producing moderate quakes and volcanism.
• Transform: plates slide past each other, as at the San Andreas Fault.

The Ring of Fire

A horseshoe of subduction zones and volcanoes surrounds the Pacific Ocean, from New Zealand through Japan, Alaska and the Americas. It accounts for about 90% of the world's earthquakes and about 80% of the largest ones, including most of the largest earthquakes on this page's list.

Away from the plate edges, intraplate quakes are rare, and are usually caused by old weaknesses in the crust.

Prediction vs forecast vs warning

• Prediction: a specific time, place and size. Not currently possible.
• Forecast: a probability over years or decades. For example, the US Geological Survey's model for California puts the chance of at least one M6.7 or larger earthquake in the state over 30 years at roughly nine in ten.
• Early warning: an alert after the quake starts. Fast P-waves are detected before the slower, damaging S-waves and surface waves arrive, giving seconds to tens of seconds of notice.

Why it is so hard

• The faults are miles underground, where we cannot measure stress directly.
• A fault behaves like a complex system. Tiny changes in the stress or friction can decide whether a small slip stays small or grows into a large one.
• Earthquakes are rare, so there are few big events from which to learn patterns. Recurrence intervals are not regular: they vary a lot.

What is known

• Gutenberg–Richter law: for every M7 earthquake, there are about ten M6, a hundred M5, and so on.
• Seismic hazard maps use fault slip rates, past earthquakes and GPS strain to show where strong shaking is most likely over decades, and are used in building codes.
• Seismic gaps: sections of a major fault that have not ruptured for a long time may be accumulating strain. The idea is useful but has a mixed record.
• Aftershock forecasts: after a large quake, the likelihood of further strong shaking in coming days can be estimated, and these are published by the USGS.

Haicheng, China (1975)

An evacuation was ordered the day before a M7.3 earthquake after a surge of foreshocks, saving many lives. It was hailed as the first successful prediction. But the following year the Tangshan earthquake (1976) struck without warning and killed about 240,000 people, showing that the method was not reliable.

Parkfield, California

In the 1980s the USGS forecast that a M6 earthquake would hit Parkfield on the San Andreas Fault by about 1993 based on a run of similar quakes at roughly 22-year intervals. It arrived in 2004, over a decade later, and an extensive instrument network recorded it in detail, but it showed that quakes are not clockwork.

L'Aquila, Italy (2009)

After a swarm of small tremors, scientists advising the civil protection authority were criticised after a M6.3 quake killed more than 300 people. Six scientists and an official were convicted of manslaughter in 2012, but the scientists were acquitted on appeal in 2014. The case showed how difficult it is to communicate uncertainty, and that swarms are very rarely followed by a big quake.

Animals and "earthquake weather"

Stories of animals behaving strangely before earthquakes are common, but no study has shown a reliable signal. There is no link between weather and earthquakes, since quakes begin kilometres underground, far below the weather.

Radon, electrical and other signals

Changes in radon gas, groundwater, electromagnetic signals and ionospheric activity have been reported before some earthquakes. None has been shown to work consistently, and they often show up without any quake at all. The methods are still being researched.

Planetary alignments and tides

Claims that the Moon, planets or "supermoons" cause earthquakes do not hold up. Tides can slightly nudge fault stress and tremor, but there is no evidence of a predictive link to large quakes.

Early warning systems

Japan's national system, Mexico's SASMEX, and the US West Coast's ShakeAlert detect the P-wave and send alerts to phones, trains, factories and hospitals. They can slow trains and open fire-station doors, giving seconds that can save lives. The farther from the quake, the more time you have.

Machine learning and lab quakes

Researchers use artificial intelligence to detect tiny quakes in continuous recordings and to find patterns in laboratory-scale earthquakes. Results so far show promise in labs, but real faults are more complex.

Fibre optics, GPS and satellites

Telecommunications cables are being used as dense seismic sensors, GPS stations show millimetres of strain, and satellite radar maps ground deformation. Together they may eventually reveal where faults are locked, and where they are creeping or slipping slowly.

Swarm or aftershock sequence?

In a normal sequence, a big mainshock is followed by smaller aftershocks that die away. In a swarm, the biggest event can come at any point, and there may be several of similar size. Swarms often begin and end gradually and may last for weeks.

What causes them

• Magma and hot fluids forcing their way through rock in volcanic areas.
• Water and gases moving through fractures and changing the pressure on faults, such as in geothermal areas.
• Slow-slip events on faults, which can trigger small quakes.
• Human activity, such as the injection of wastewater.

Do swarms lead to big earthquakes?

Usually not. Most swarms fade away. But in rare cases, a swarm is followed by a bigger quake, so scientists monitor them. In volcanic regions, a swarm may be a sign that magma is on the move and that an eruption could follow, as in Iceland, but in many places, such as Yellowstone, it is routine.

Earthquakes caused by volcanoes

• Volcano-tectonic (VT) quakes are caused by rock fracturing as magma pushes in. They are the usual early sign of unrest.
• Long-period (LP) events and tremor are caused by the resonance of fluids in cracks and conduits. A steady tremor can mean that magma is moving.
• Explosion quakes come from eruptions, and collapse quakes when a caldera floor subsides, as at Bárðarbunga in 2014 and at Kīlauea in 2018.

Earthquakes that trigger eruptions

A large earthquake can change stress in the crust and shake bubbles out of the magma. The eruption of Puyehue–Cordón Caulle in Chile about 38 hours after the 1960 M9.5 earthquake is a famous example. Statistical studies suggest that eruptions are a little more likely in the months after a great quake, within a few hundred kilometres of the rupture, though this is a modest effect.

Volcano-linked quakes that are large

Most volcanic earthquakes are small, but some are not. In 1975 a M7.7 quake struck Kīlauea's flank, in 1980 a M5.1 shock triggered the Mount St. Helens landslide and eruption, and in 2018 a M6.9 shock accompanied the eruption at Kīlauea. Large caldera collapses can generate earthquakes of M5 or more.

Reading the signs

Volcano observatories combine earthquake counts, depth patterns and migration (where swarms rise toward the surface), with GPS and satellite measurements of swelling, gas emissions and heat. Rising numbers of shallow quakes and continuous tremor are among the most important warning signs. Not every swarm ends in an eruption, and not every eruption has a long warning.