Quake Vocabulary

crust : The Earth is composed of three main layers of rock: the core, the mantle and the crust. The core is the center of the Earth and is separated into a solid, metallic inner core and a molten outer core with an overall thickness of about 2161 km. The mantle is a solid layer about 1789 km thick and the crust is the brittle surface layer that ranges from about 5 km thick in the ocean to about 70 km thick in continental mountains. The Earth can be further divided into the asthenosphere and the lithosphere. The asthenosphere is a part of the mantle about 250 km thick that is so hot and weak it is ductile. The lithosphere is the cool, brittle layer above it consisting of the crust and the upper mantle and is about 100 km thick.

plate tectonics : The theory of plate tectonics was proposed in 1968 to explain why similar fossils and rock structures are found in landmasses now seperated by vast oceans. It says that the lithosphere is divided into about 20 rigid plates that are in continuous motion in relation to one another. It is the movement of these plates that creates stresses in the rock that result in earthquakes, most of which occur along plate boundaries. This theory displaced the earlier notion of continental drift, an earlier explanation of how similar animal species could be found on continents separated by vast oceans and how similar rock types and structures could be found along the coasts of now separated continents.

elastic energy : energy created when a rock is deformed elastically, like a stretched rubber band. Rocks can also be deformed viscously, like silly putty, or in a combination of the two.

triangulation : A mathematical method for locating the epicenter of an earthquake. Using recorded data, the distance of the epicenter from three different seismic stations can be calculated. The distance becomes the radius of an imaginary circle drawn around each station. Where the three circles intersect is the exact location of the epicenter.

Earth's interior : Because S waves decrease in velocity within the asthenosphere, it can be reasoned that this layer of rock is hot and weak with about 10 percent of it being molten. Also, a shadow zone, or an area of absent data, for P waves has been recorded at about 105-140 degrees latitude. This suggests that there is an abrupt change in the physical state of the rock at the mantle-core boundary, most likely from solid to liquid, since the P waves would seem to bend along that boundary.

Earthquake Basics