S wave
A seismic body wave that shakes the ground back and forth perpendicular to the direction the wave is moving. This wave moves more slowly than the P wave but in an earthquake is usually bigger. Also called the secondary, shear, transverse or shake wave.

Sea floor spreading
Happens at the mid-oceanic ridge where a divergent boundary is causing two plates to move away from one another resulting in spreading of the sea floor. As the plate moves apart from each other, a line of ridges form as the magma reaches the ocean bottom and solidifies.

A highly vesicular, usually light weight, volcanic rock of basaltic or andesitic composition

A standing wave oscillating in a partially or fully enclosed body of water. It may be initiated by an earthquake, wind, water waves or a tsunami.

Refers to the geographic and historical distribution of earthquakes.

The study of earthquakes and seismic waves that move through and around the earth.

A scientist who studies earthquake sand seismic waves.

Means capable of generating earthquakes.

Seismic gap
A section of a fault that has produced earthquakes in the past but is now quiet.

A record written by o show the ground motions produced by vibrations on the Earth’s surface produced by an earthquake, explosion or other ground motion sources.

An instrument used to detect and record earthquakes. Generally, it consists of a mass attached to a fixed base. During an earthquake, the base moves and the mass does not. The motion of the base with respect to the mass is commonly transformed into an electrical voltage. This used to be recorded on paper (seismograph) or magnetic tape. Now the voltage is expressed as waveforms on computers and stored on servers for processing.

Seismic wave
A seismic wave is an elastic wave generated by an impulse such as an earthquake or an explosion. Seismic waves may travel either along or near the earth’s surface (Rayleigh or Love waves) or through the earth’s interior (P and S waves). Seismic waves are vibrations generated by sudden movements of rock. After earthquakes occur, the seismic waves propagate from the hypocentre to the surface of the Earth.

Seismic zone
An area of seismicity probably sharing a common cause.

A number of earthquakes that occurred in a specific area of a certain time period that maybe characterized by foreshocks, a main shock and aftershocks.

Shadow zone
An area of the earth from angular distances of 104 to 140 degrees from a given earthquake that does not receive any direct P waves. The shadow results from S waves being stopped entirely by the liquid core and P waves being refracted by the liquid core.

Shield Volcano
A gently sloping volcano in the shape of a flattened dome and built most of lava flows.

A chemical combination of silicon and oxygen

A tabular body of intrusive igneous rock, parallel to the layering of the rocks into which it intrudes.

Soufrière, the name of several Caribbean volcanoes, and the name commonly used in the Caribbean for fumarole or fumarolic area, means “source of sulphur”.

Vertical or nearly vertical fractures where the blocks have mostly moved horizontally. If the block opposite an observer looking across the fault moves to the right, the slip style is termed right lateral; if the block moves to the left, the motion is termed left lateral.

The study of rock strata, especially of their distribution, deposition, and age.

 A large volcanic edifice constructed of layers of lava flows interbedded with pyroclastic deposits. Stratovolcanoes tend to have a steeply conical form, and may display very persistent activity (e.g. Etna and Stromboli in Italy); erupt at fairly regular intervals (e.g. Kick ‘em Jenny) or alternate periods of activity with long periods of inactivity (e.g. Mt. Pelée).

Strombolian eruption
Strombolian eruptions are named after Stromboli volcano off the west coast of Italy, where a typical eruption consists of the rhythmic ejection of incandescent cinder, lapilli, and bombs to heights of a few tens or hundreds of feet. Lava flows may also occur, and ash may be present in relatively minor amounts.

Strong motion
Is ground motion of sufficient amplitude and duration to be potentially damaging to a building or other structure.

St. Vincent style eruption
Explosive eruption of vesiculated basaltic andesite from an open crater, resulting in scoria and ash flows as well as surges, lapilli and ash fall.

Subduction/subduction zone
The process by which the oceanic plates collides with and descends below the less dense continental plate. Most volcanoes on land and volcanic arcs occur parallel to and inland from the boundary between the two plates.

Surface wave
Is a seismic wave that is trapped near the surface of the earth.

A ring-shaped cloud of gas and suspended solid debris that moves radially outward at high velocity as a density flow from the base of a vertical eruption column.

A series of minor earthquakes, none of which may be identified as the main shock, occurring in a limited area and time.