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A map of epicenters.

An earthquake is the result of a sudden release of energy in the Earth's top 700 km that creates seismic waves. These waves are detected with seismometers and amplified electronically so they can be displayed as a function of time by a seismograph as a seismogram. The size of an earthquake is given by its open ended logarithmic scale of magnitude, often referred to as the Richter scale. Shocks smaller than magnitude 2.5 are usually not felt and those with magnitude 7 cause serious damage over large areas. Intensity of shaking is measured on the modified Mercalli scale, ranging from 1 far from the epicenter to a maximum near it, which can reach 12 in the strongest earthquakes..

At the Earth's surface, earthquakes manifest themselves by shaking and sometimes displacement of the ground. When the epicenter of a large earthquake is located offshore, the seabed sometimes is uplifted enough to cause a tsunami. The shaking in earthquakes can also trigger landslides and occasionally volcanic activity. (read more...)

A propagation of seismic waves.

Seismology (from the Greek seismos (σεισμός) = earthquake and λόγος (logos) = knowledge) is the scientific study of earthquakes and the propagation of elastic waves through the Earth. The field also includes studies of earthquake effects, such as tsunamis as well as diverse seismic sources such as volcanic, tectonic, and artificial processes (explosions). A related field that uses geology to infer information regarding past earthquakes is paleoseismology.

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Eruptions in the Lassen volcanic area in the last 70,000 years. Circle shows base of Mt Tehama. See media:Eruptions in the Lassen area in the last 50,000 years.png
The geology of the Lassen volcanic area presents a record of sedimentation and volcanic activity in the area in and around Lassen Volcanic National Park in Northern California, U.S.. The park is located in the southernmost part of the Cascade Mountain Range in the Pacific Northwest region of the United States. Oceanic tectonic plates have plunged below the North American Plate in this part of North America for hundreds of millions of years. Heat from these subducting plates have fed scores of volcanoes in California, Oregon, Washington and British Columbia over at least the past 30 million years (see Geology of the Pacific Northwest) and is also responsible for activities in the Lassen volcanic area.

Between 3 and 4 million years ago, volcanic-derived mud flows called lahars streamed down several major mountains that included nearby but now extinct Mount Yana and Mount Maidu to become the Tuscan Formation. Basaltic and later andesitic to dacitic flows of lava covered increasingly larger areas of this formation to eventually form the lava plateau that the park sits on. About 600,000 years ago, Mount Tehama started to rise as a stratovolcano in the southwestern corner of the park, eventually reaching an estimated 11,000 feet (3,350 m) in height. Following a series of eruptions approximately 350,000 years ago, its cone collapsed into itself to form a two-mile (3 km) wide caldera.

Did you know...

  • ...that the 1946 Nankaido earthquake caused a 5-6 meter (16-20 feet) tsunami that took out another 2,100 homes after the initial destruction of the earthquake?
  • ...that the 2003 Hokkaido earthquake was not given a cost in US dollars because it occurred offshore and did not cause as much damage as it would have on the mainland of Japan?


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WikiProject Earthquakes is the central point of coordination for Wikipedia's coverage of earthquakes, seismology, tsunamis, and related subjects. Please feel free to join and help!

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