Glossary

when the structure of a loose, saturated sand breaks down due to some rapidly applied loading. As the structure breaks down, the loosely-packed individual soil particles attempt to move into a denser configuration. In an earthquake, however, there is not enough time for the water in the pores of the soil to be squeezed out. Instead, the water is "trapped" and prevents the soil particles from moving closer together. This is accompanied by an increase in water pressure which reduces the contact forces between the individual soil particles, thereby softening and weakening the soil deposit. (Jörgen Johansson, 2000) Seismic waves are waves of sound that travel through the core of the earth or other elastic bodies, for example as a result of an earthquake, explosion, or some other process that imparts light. Seismic waves are studied by seismologists, and measured by a seismograph, which records the output of a [[mmeter], or geophone. Earthquakes create various types of waves with different velocities; when reaching seismic observatories, their different travel time enables the scientists to locate the epicenter. (Jörgen Johansson, 2000)
 * LIQUEFACTION**
 * SEISMIC WAVES**

The **epicenter** or **epicentre** is the point on the Earth's surface that is directly above the hypocenter or focus, the point where an earthquake or underground explosion originates. The word derives from the Neolatin noun epicentrum[1] from the Greek adjective ἐπίκεντρος (epikentros) "central", from ἐπί (epi) "on, upon, at" and κέντρον (kentron) "centre". In the case of earthquakes, the epicenter is directly above the point where the fault begins to rupture, and in most cases, it is the area of greatest damage. However, in larger events, the length of the fault rupture is much longer, and damage can be spread across the rupture zone. (Jörgen Johansson, 2000)
 * EPICENTER**

An earthquake's **hypocenter** or **focus** is the position where the strain energy stored in the rock is first released, and is the point where the fault begins to rupture. This occurs at the focal depth below the epicentre. (Jörgen Johansson, 2000)
 * HYPOCENTER/ FOCUS**

In geology, **subduction** is the process that takes place at convergent boundaries by which one tectonic plate moves under another tectonic plate, sinking into the Earth's crust, as the plates converge. A **subduction zone** is an area on Earth where two tectonic plates move towards one another and subduction occurs. Rates of subduction are typically measured in centimeters per year, with the average rate of convergence being approximately 2 to 8 centimeters per year (about the rate a fingernail grows) Subduction zones involve an oceanic plate sliding beneath either a continental plate or another oceanic plate. Subduction zones are often noted for their high rates of volcanism, earthquakes, and mountain building. This is because subduction processes result in melt of the mantle that produces a volcanic arc as relatively lighter rock is forcibly submerged. (Jörgen Johansson, 2000)
 * SUBDUCTION**

The crust of the Earth is composed of a great variety of igneous, metamorphic, and sedimentary rocks. The crust is underlain by the mantle. The upper part of the mantle is composed mostly of peridotite, a rock denser than rocks common in the overlying crust. The boundary between the crust and mantle is conventionally placed at the Mohorovičić discontinuity, a boundary defined by a contrast in seismic velocity. Earth's crust occupies less than 1% of Earth's volume. The oceanic crust of the sheet is different from its continental crust.The oceanic crust is 5 km (3 mi) to 10 km (6 mi) thick[1] and is composed primarily of basalt, diabase, and gabbro. The continental crust is typically from 30 km (20 mi) to 50 km (30 mi) thick, and is mostly composed of slightly less dense rocks than those of the oceanic crust. Some of these less dense rocks, such as granite, are common in the continental crust but rare to absent in the oceanic crust. Both the continental and oceanic crust "float" on the mantle.
 * CRUST**

In geology, a **fault** is a planar fracture or discontinuity in a volume of rock, across which there has been significant displacement. Large faults within the Earth's crust result from the action of tectonic forces. Energy release associated with rapid movement on active faults is the cause of most earthquakes. (Jörgen Johansson, 2000)
 * FAULT**