Earthquake

An earthquake is any sudden shaking of the ground caused by the passage of seismic waves through Earth's rocks, produced when energy stored in the crust is suddenly released. The shaking originates when stress on a stuck plate edge overcomes friction and energy is released in waves that travel through the crust.

About 50,000 earthquakes large enough to be noticed without the aid of instruments occur annually over the entire Earth, and approximately 100 are of sufficient size to produce substantial damage if their centres are near areas of habitation. The National Earthquake Information Center locates about 12,000 to 14,000 earthquakes each year. Very great earthquakes occur on average about once per year, and over the centuries they have been responsible for millions of deaths and an incalculable amount of damage to property.

An earthquake occurs when two blocks of the earth slip past one another along a surface known as a fault or fault plane. The location below the earth's surface where the earthquake starts is called the hypocenter, and the location directly above it on the surface is called the epicenter. A fault slips when the force of friction along it is overcome.

A fault is a planar or gently curved fracture in crustal rocks where compressional or tensional forces displace the rocks on opposite sides. Normal dip-slip faults are produced by vertical compression as the crust lengthens, with the hanging wall sliding down relative to the footwall; they bound many mountain ranges and the rift valleys along spreading margins of tectonic plates. Reverse dip-slip faults result from horizontal compressional forces, with the hanging wall moving up and over the footwall, and thrust faults are reverse faults that dip less than 45 degrees. Strike-slip faults release energy by horizontal displacement.

The San Andreas Fault, the primary boundary between the Pacific Plate and the North American Plate, is more than 650 miles long and extends to depths of at least 10 miles. The Pacific Plate grinds northwestward past the North American Plate at a rate of about two inches per year.

The Earth's outermost layer is fragmented into a dozen or more large and small plates that move relative to one another atop hotter, more mobile material. The crust and the top of the mantle together form a thin skin broken into these moving plates.

In 1912 Alfred Wegener introduced the theory of continental drift, contending that around 200 million years ago the supercontinent Pangaea began to split apart. Wegener was intrigued by unusual geologic structures and by identical fossil species found on the matching coastlines of South America and Africa, now separated by the Atlantic Ocean, evidence that the two continents were once joined. Four major scientific developments beginning in the 1950s spurred the theory of plate tectonics, including documentation that earthquake and volcanic activity concentrates along oceanic trenches and submarine mountain ranges.

The global mid-ocean ridge system, more than 50,000 kilometers long and rising an average of about 4,500 meters above the sea floor, is the most prominent topographic feature on the surface of our planet. As seafloor spreading was accepted, scientists came to appreciate the oceanic crust as a natural tape recording of the history of the reversals in the Earth's magnetic field. In 1968 the research vessel Glomar Challenger drilled core samples criss-crossing the Mid-Atlantic Ridge, and dating of the samples provided the clinching evidence that proved the seafloor spreading hypothesis. By the late 1920s seismologists were beginning to identify inclined earthquake zones parallel to ocean trenches, later named Wadati-Benioff zones after Kiyoo Wadati of Japan and Hugo Benioff of the United States.

Major earthquakes occur mainly in belts coinciding with tectonic plate margins, with an estimated 80 percent of presently released seismic energy coming from epicentres in the Circum-Pacific Belt. Because at many places the Circum-Pacific Belt is associated with volcanic activity, it has been popularly dubbed the "Pacific Ring of Fire". A second global belt, the Alpide Belt, passes through the Mediterranean region eastward through Asia and joins the Circum-Pacific Belt in the East Indies, releasing about 15 percent of the world total energy.

A seismometer records the ground shaking caused by seismic waves, while the term seismograph refers to the combined instrument and recording device. On the resulting record, a short wiggle means a small earthquake and a long, large wiggle means a large one.

On a seismogram the P wave is the first wiggle bigger than background signals, the S waves follow and are usually bigger, and the surface Love and Rayleigh waves travel slower still and arrive just after the S waves. Because P waves travel faster than S waves, the time between their arrivals indicates distance to the earthquake but not direction; locating an earthquake requires three seismographs, with circles drawn around each station at its distance intersecting at the epicenter. In one worked example, a 24-second P–S separation indicated an epicenter 215 kilometers away.

Magnitude is a single value for the size of an earthquake, while shaking intensity varies by location.

Scientists no longer use the Richter scale; instead they use a number of different scales depending on what kind of motion they want to detect, and the one usually reported to the public is the local magnitude. Magnitudes can be based on seismic moment estimates, teleseismic body-wave amplitudes, surface waves, local recordings, or signal duration. The Richter scale, the first widely used method, was developed by Charles F. Richter in 1934 and used a formula based on the amplitude of the largest wave recorded on a specific type of seismometer. Today the moment magnitude scale, abbreviated MW, is preferred because it works over a wider range of earthquake sizes and is applicable globally.

The original intensity scale was invented by Giuseppe Mercalli in 1902 and modified by Harry Wood and Frank Neumann in 1931 to become the Modified Mercalli Intensity Scale, which uses roman numerals.

The energy released does not rise at the same rate as the recorded amplitude: for every step up in magnitude, the annual number of earthquakes falls by roughly a factor of ten ^{3How Often Do Earthquakes Occur? · NSF SAGE / EarthScope Consortium ↗}.

Under the standard magnitude classes, a great earthquake is one of magnitude 8 or greater, while a major earthquake falls in the range of magnitude 7 to 7.9 ^{12How do I interpret magnitudes and magnitude types? · NSF SAGE / EarthScope Consortium (IRIS Data Services FAQ) ↗}.

Earthquake magnitude is expressed on a logarithmic scale with a base of ten, so that each whole-number step corresponds to a tenfold increase in the amplitude of ground motion recorded by a seismograph ^{11How Do We Measure Earthquake Magnitude? · Michigan Technological University (UPSeis) ↗}. Each whole-number increase on the Richter scale corresponds to a 31.6-fold rise in released energy ^{10What does the Richter scale tell us about energy released by an earthquake? · NSF SAGE / EarthScope Consortium ↗}.

Earth is an active place, and earthquakes are always happening somewhere. That more earthquakes can be detected and located each year does not mean that the annual average number of earthquakes has increased.

Mainshocks always have aftershocks, which are smaller earthquakes occurring afterwards in the same place. These aftershocks can continue for weeks, months, and even years.

It is not earthquake prediction; rather, it is a tool indicating that shaking is imminent.

During an earthquake, if you are inside, stay there and do not run outside. If you are in bed, turn face down and cover your head and neck. If you are in a car, pull over and stop; if you are outside, move to an open area away from buildings, trees, streetlights and power lines.

Scientists cannot predict earthquakes, and it is unlikely that they ever will. On any fault, another earthquake will happen sometime, but there is no way of telling when.

NASA's geosphere research studies the processes and changes in the Earth's core, mantle, and crust, along with surface topography, composition, and geology, and the hazards they generate. NASA's Crustal Dynamics research informs the assessment, mitigation, and forecasting of natural hazards, including earthquakes, tsunamis, landslides, and volcanic eruptions.

• 1 November 1755
  Lisbon earthquake
  A series of earthquakes struck Lisbon on the morning of All Saints' Day, killing an estimated 60,000 people in the city alone, and the shock, tsunami and fires made it a seminal event in European history.^{16Lisbon earthquake of 1755 | Great Lisbon Quake, Tsunami & Fire · Encyclopaedia Britannica ↗}
• 18 April 1906
  Northern California earthquake
  The magnitude 7.9 earthquake was the most damaging in US history. The 1906 rupture on the San Andreas fault was traced 202 miles (325 km) to the northwest and 93 miles (241 km) to the southeast, and strong shaking was felt in the Bay Area for 45 to 60 seconds.^{17A Virtual Tour of the 1906 Earthquake in Google Earth · U.S. Geological Survey (USGS) ↗}
• 1 September 1923
  Tokyo-Yokohama (Great Kanto) earthquake
  A magnitude 7.9 earthquake struck the Tokyo-Yokohama metropolitan area near noon on 1 September, and the death toll was estimated to have exceeded 140,000 people as fires consumed hundreds of thousands of houses.^{18Tokyo-Yokohama earthquake of 1923 | Death Toll & Facts · Encyclopaedia Britannica ↗}
• 22 May 1960
  Chile earthquake
  Its magnitude of 9.5 makes the 1960 Chile earthquake the largest recorded in the 20th century; the rupture extended over an estimated 900 to 1,000 km of the Nazca Plate off southern Chile and sent a tsunami across the Pacific. Most casualties resulted from a tsunami that rose up to 80 feet (25 metres) high along the Chilean coastline.^{19Chile earthquake of 1960 | Causes, Effects, & Facts · Encyclopaedia Britannica ↗}
• 27 March 1964
  Alaska earthquake
  A magnitude 9.2 earthquake in Prince William Sound, the most powerful recorded in U.S. history, lasted about 4.5 minutes and later proved key to the developing theory of plate tectonics.^{20M9.2 Alaska Earthquake and Tsunami of March 27, 1964 · U.S. Geological Survey (USGS) ↗}
• 26 December 2004
  Sumatra-Andaman earthquake and Indian Ocean tsunami
  A magnitude 9.1 megathrust earthquake off Sumatra ruptured the subduction zone where the India Plate, an oceanic plate, is being subducted beneath the Burma micro-plate, and it generated a devastating ocean-wide tsunami.^{21Tsunami Generation from the 2004 M=9.1 Sumatra-Andaman Earthquake · U.S. Geological Survey, Pacific Coastal and Marine Science Center ↗}
• 11 March 2011
  Tohoku earthquake and tsunami
  A magnitude 9.0 earthquake occurred near the east coast of Honshu, Japan on 11 March 2011 as a result of thrust faulting on or near the subduction zone interface plate boundary between the Pacific and North American plates. Its focus lay at a depth of 24.4 km, about 130 km east of Sendai, and it generated a tsunami warning across the Pacific.^{22Special Event: Tohoku, Japan earthquake · NSF SAGE / EarthScope Consortium ↗}

The earliest seismoscope was invented by the Chinese philosopher Chang Heng in A.D. It took the form of a large urn on whose outside were eight dragon heads facing the eight principal directions of the compass. When an earthquake occurred, one of the dragon mouths would release a ball into the open mouth of a toad below, and the instrument is reported to have detected an earthquake 400 miles away that was not felt at its location.