Earthquake
An earthquake of magnitude 8.6 releases the same amount of energy as 10,000 atomic bombs of the size used in World War II. That figure hints at the violence locked inside the shaking of the Earth's surface, a phenomenon also called a quake, a tremor, or a temblor. Some earthquakes are so weak they cannot be felt at all. Others are violent enough to propel objects and people into the air and wreck whole cities.
The shaking comes from a sudden release of energy in the lithosphere, the outer shell of the planet, which sends out seismic waves. That energy can build for decades before it lets go in seconds. Why does the ground store so much force, and what finally makes it break? Where does the rupture begin, and how do scientists measure something they cannot see? Why do some quakes kill hundreds of thousands while others pass unnoticed? And how did people explain this terror long before they understood the rock beneath their feet?
More than 100,000 people died in the 1556 Shaanxi earthquake, which struck on the 23rd of January 1556 in Shaanxi, China. The region lost up to 730,000 people afterward through emigration, plague, and famine. Most local houses were yaodongs, dwellings carved out of loess hillsides, and many victims were killed when these collapsed. It remains one of the most devastating earthquakes in recorded history.
The 1976 Tangshan earthquake killed between 240,000 and 655,000 people, making it the deadliest of the 20th century. The greatest losses of life share a grim logic: the quake struck close to a heavily populated area or close to the ocean. Where earthquakes are rare but powerful, and where seismic building codes are lax, unenforced, or nonexistent, the death tolls climb. Megacities such as Mexico City, Tokyo, and Tehran now sit in high-risk zones, and some seismologists warn a single earthquake could claim up to three million lives.
Tectonic earthquakes happen anywhere there is enough stored elastic strain energy to drive a fracture along a fault plane. The sides of a fault slide past each other smoothly only when no irregularities, called asperities, raise the friction. Most fault surfaces do have asperities, producing a stick-slip pattern. The fault locks, plate motion keeps adding stress, and strain energy builds until the stress breaks through the asperity and the fault suddenly slides.
This cycle of slow build-up punctuated by sudden failure is the elastic-rebound theory. Only about 10 percent or less of an earthquake's total energy radiates as seismic waves. Most of it powers the growth of the fracture or turns into frictional heat. So earthquakes lower the Earth's available elastic potential energy and slightly raise its temperature, though that warming is negligible next to the heat flowing out from the deep interior. The brittle upper crust and the cool descending slabs of tectonic plates are the only parts of the planet that can store and release this energy in fault ruptures.
Normal, reverse, and strike-slip are the three main fault types, and each can drive an interplate earthquake. Normal faults form where the crust is being pulled apart, as at a divergent boundary, and their quakes are generally below magnitude 7. Along spreading centers like Iceland, the brittle layer is only about 6 km thick, which limits them further. Reverse faults, by contrast, form where the crust is being shortened at convergent boundaries.
Reverse faults along convergent boundaries produce the most powerful earthquakes, the megathrust events, which account for almost all quakes of magnitude 8 or more. Megathrust earthquakes are responsible for about 90 percent of the total seismic moment released worldwide. Strike-slip faults are steep structures where the two sides slide horizontally past each other, and transform boundaries are one kind. Oriented nearly vertically, they span a width of roughly 10 km in the brittle crust, capping their quakes near magnitude 8. The greatest principal stress sets a hierarchy: thrust faults are generated by the highest stress, strike-slip by intermediate, and normal faults by the lowest.
For every unit increase in seismic magnitude, the energy released jumps roughly thirty-fold. A magnitude 6.0 event releases about 32 times the energy of a magnitude 5.0, and a 7.0 releases about 1,000 times as much as a 5.0. The energy is proportional to the area of fault that ruptures and the stress drop, so longer and wider ruptured areas mean larger magnitudes. The most important control on maximum magnitude is not length but available width, which varies by a factor of 20.
The majority of tectonic earthquakes originate in the Ring of Fire at depths not exceeding tens of kilometers. Quakes shallower than 70 km are shallow-focus; those between 70 and 300 km are mid-focus or intermediate-depth. In subduction zones, where older, colder oceanic crust descends beneath another plate, deep-focus earthquakes can occur between 300 and 700 km down. These zones are called Wadati-Benioff zones. At such depths the slab should no longer be brittle, so one proposed mechanism is faulting caused by olivine undergoing a phase transition into a spinel structure.
A tectonic earthquake begins as a small area of slip at the focus, then propagates outward along the fault surface. The rupture continues until it hits a barrier, such as the end of a fault segment, or a region where the stress is too low to keep going. The mechanics are poorly understood because such rapid motion is hard to recreate in a laboratory and hard to record close to a nucleation zone. Usually the rupture speed approaches but does not exceed the shear wave velocity of the surrounding rock.
Supershear earthquakes break that rule, propagating faster than the S wave velocity, and so far all have been large strike-slip events. The unusually wide damage zone of the 2001 Kunlun earthquake has been blamed on the sonic boom such speeds create. At the other extreme, slow earthquakes rupture at unusually low velocities. The most dangerous variant is the tsunami earthquake, where weak felt shaking from a slow great quake fails to warn the neighboring coast, as in the 1896 Sanriku earthquake. During rupture, high temperatures at the fault plane raise pore pressure and vaporize groundwater, which can lubricate the fault, while pressure spreading afterward can reactivate adjacent faults and trigger aftershocks.
Around 500,000 earthquakes occur each year that current instruments can detect, and about 100,000 of these can be felt. Minor quakes are frequent in places like California, Alaska, El Salvador, Mexico, Chile, Indonesia, Japan, and many others. Larger quakes grow exponentially rarer; roughly ten times as many quakes above magnitude 4 occur as above magnitude 5. In the low-seismicity United Kingdom, the recurrence works out to one quake of 3.7-4.6 every year and one of 5.6 or larger every 100 years, an example of the Gutenberg-Richter law.
Since 1900, the United States Geological Survey estimates an average of 18 major earthquakes of magnitude 7.0-7.9 and one great earthquake of 8.0 or greater per year, a rate that has stayed relatively stable. The apparent rise in reported quakes traces to instrumentation: seismic stations grew from about 350 in 1931 to many thousands today. Most quakes, 90 percent and 81 percent of the largest, occur in the 40,000 km horseshoe of the circum-Pacific seismic belt, the Pacific Ring of Fire. Humans add their own share through reservoirs, resource extraction, and fluid injection; the magnitude 5.7 2011 Oklahoma earthquake is thought to have come from disposing oil-production wastewater into injection wells.
Charles Francis Richter developed the first scale for measuring earthquake magnitudes in 1935, where each unit represents a ten-fold difference in shaking amplitude and a 32-fold difference in energy. The Richter scale's use has become minimal in the 21st century. Most seismological authorities now express strength on the moment magnitude scale, which accounts for the seismic moment: total rupture area, average slip, and the rigidity of the rock. Intensity scales like the Mercalli scale measure observed effects instead, and vary from place to place.
Seismometers record the different waves each quake produces. Longitudinal P waves and transverse S waves are body waves; Rayleigh and Love waves travel along the surface. In the Earth's interior, P waves travel much faster than S waves, in a ratio of about 1.7 to 1, and the gap in their arrival times reveals the distance. By such analysis Beno Gutenberg located the Earth's core in 1913. S waves and surface waves do most of the damage, because S waves shake the ground up and down and back and forth.
The damage takes many forms beyond shaking. Soil liquefaction turns water-saturated sand briefly into liquid; in the 1964 Alaska earthquake it caused buildings to sink and collapse. Fire killed more people in the 1906 San Francisco earthquake than the quake itself. Offshore quakes can displace the seabed and launch tsunamis that cross thousands of kilometers, and most destructive tsunamis come from quakes of magnitude 7.5 or more. Against all this, earthquake engineering, seismic retrofitting, insurance, and warning systems try to blunt the impact before the next rupture begins.
From the lifetime of the Greek philosopher Anaxagoras in the 5th century BCE to the 14th century CE, earthquakes were usually attributed to air, or vapors, in the cavities of the Earth. Pliny the Elder called earthquakes underground thunderstorms. Thales of Miletus, who lived from 625 to 547 BCE, was the only documented person who believed earthquakes came from tension between the earth and water. Each culture reached for a story to tame the terror underfoot.
In Norse mythology, earthquakes were the violent struggle of the god Loki, punished for the murder of Baldr, god of beauty and light. In Greek mythology, Poseidon was both cause and god of earthquakes. Japanese mythology blamed Namazu, a giant catfish, while Taiwanese folklore named the Te-gu, a giant earth buffalo. Matthew's Gospel even records earthquakes after the death of Jesus and at his resurrection. Modern fiction keeps the dread alive, from Heinrich von Kleist's novella The Earthquake in Chile, set in the destruction of Santiago in 1647, to the hypothetical Big One long expected of California's San Andreas Fault. And the shaking is not Earth's alone: scientists have observed marsquakes on Mars and moonquakes on the Moon.
Common questions
What was the deadliest earthquake in recorded history?
The 1556 Shaanxi earthquake, which struck on the 23rd of January 1556 in Shaanxi, China, killed more than 100,000 people, and the region lost up to 730,000 people afterward through emigration, plague, and famine. The 1976 Tangshan earthquake, which killed between 240,000 and 655,000 people, was the deadliest of the 20th century.
What is the largest earthquake ever recorded?
The 1960 Chilean earthquake is the largest measured on a seismograph, reaching 9.5 magnitude on the 22nd of May 1960 with its epicenter near Cañete, Chile. It released roughly twice the energy of the next most powerful quake, the Good Friday earthquake of the 27th of March 1964 in Prince William Sound, Alaska.
What causes earthquakes?
Earthquakes are caused mostly by the rupture of geological faults, as the elastic-rebound theory describes, when stored strain energy releases suddenly along a fault. They can also be caused by volcanic activity, landslides, and human activities such as mining, fracking, and nuclear weapons testing.
How is the size of an earthquake measured?
Charles Francis Richter developed the first magnitude scale in 1935, in which each unit represents a ten-fold difference in shaking amplitude and a 32-fold difference in energy. Most seismological authorities now use the moment magnitude scale, which accounts for total rupture area, average slip, and the rigidity of the rock.
How many earthquakes happen each year?
Around 500,000 earthquakes occur each year that current instruments can detect, and about 100,000 of these can be felt. Since 1900, the United States Geological Survey estimates an average of 18 major earthquakes of magnitude 7.0-7.9 and one great earthquake of 8.0 or greater per year.
Where do most earthquakes occur?
Most of the world's earthquakes, 90 percent and 81 percent of the largest, occur in the circum-Pacific seismic belt, a 40,000 km horseshoe-shaped zone known as the Pacific Ring of Fire. Many quakes also occur along other plate boundaries, such as along the Himalayan Mountains.
How were earthquakes explained in mythology and ancient belief?
From Anaxagoras in the 5th century BCE to the 14th century CE, earthquakes were usually attributed to air or vapors in the cavities of the Earth, and Pliny the Elder called them underground thunderstorms. In Norse mythology they were the struggle of Loki, in Greek mythology Poseidon caused them, and Japanese mythology blamed the giant catfish Namazu.