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 Greekseismos(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.
"The earthquake occurred as a result of thrust-faulting on the boundary between the Australia plate and the Sunda plate. On this part of their mutual boundary, the Australia plate moves north-northeast with respect to the Sunda plate at about 59 mm/year. The Australia plate thrusts beneath the Sunda plate at the Java trench, south of Java, and is subducted to progressively greater depths beneath Java and north of Java. The earthquake occurred on the shallow part of the plate boundary, about 50 km north of the Java trench."