Earthquakes: Glossary

Active fault = an active fault is one that has moved once or more times in the past 10,000 years and therefore likely to move sometime in the future.

Aftershocks = smaller earthquakes that occur after the main earthquake in the same place as the mainshock. Aftershocks are linked to the size of the mainshock and if this is large can continue for many weeks, months or years.

Amplitude = the size of the wiggles on an earthquake recording.

Asthenosphere = the asthenosphere is the ductile part of the Earth just below the lithosphere, which includes the lower mantle and is about 180km thick.

Body waves = seismic vibrations that move through the Earth's interior. They are classified into:

a. Primary waves or P waves: push-pull waves, waves move back and forth in the direction in which the wave is travelling. Change both volume and shape of material in which they pass. Affect and can pass through solids, liquids, and gasses (as they all exhibit resistance to change in volume). Also called Longitudinal Waves= wave motion (amplitude) is parallel to direction of travel, e.g. accordion motion, to and from.

b. Secondary waves or S waves = vibrations occurring at right angles to direction of wave propagation. They are shake-waves (shear waves), vibrating side-to-side. They are not as fast as P-waves and result in changing only shape of material they travel through. Affect and can pass through only solid materials (only solids offer resistance to change in shape). Also called Transverse Waves = wave motion (amplitude) is perpendicular to direction of travel, e.g. spring bobbing, up and down.

Convection = as a fluid is warmed it expands and rises as the density decreases. The area of fluid which is colder and denser sinks,occupying the area of the warmed fluid. This is called convection.

Deformation = deformation is a change to the original shape of material and in earthquakes this is due to stress and strain.

Displacement = displacement is a change in the initial reference point or later position. This is defined by the amount a reference point is affected by an earthquake or moved from it’s original position before the earthquake.

Earthquake = shaking or slipping movement of the earth’s crust, followed by seismic waves and vibrations.

Earthquake Fault = earthquake faults are fractures where displacement is on either side relative to one another and parallel to the fracture.

Epicentre = the point on the Earth's surface vertically above the focus of an earthquake, i.e. directly above the true centre of the seismic disturbance from which the shock waves of an earthquake seem to radiate. The epicentre usually registers the strongest shaking.

Fault = a crack in the Earth’s crust along which the rocks slide. Faults are found at the edges of the plates where the crust is moving in different directions.

Fault plane = the planar (flat) surface along which two blocks of the earth's crust suddenly slip past one another during an earthquake

Focus = the point inside the Earth where the rock breaks off and pressure is released, The focus point generally occurs 45 miles below the ground.

Focal depth of an earthquake = the depth of the hypocentre below the Earth's surface.

Foreshocks = these are smaller earthquakes in the same area as the following larger earthquake. Until the larger earthquake hits scientists are unable to predict if they are foreshocks.

Hypocentre = the location below the Earth’s surface where an earthquake rupture begins.

Magnitude = a number that categorizes the amount of energy released during an earthquake.

Mainshock = the largest, main earthquake.

Plate tectonics = plate tectonics are the science of the process where rigid plates move across hot molten material. It helps explain the formation of mountains and the distribution of earthquakes and volcanoes.

Richter Scale = the magnitude of seismic energy released during an earthquake is measured by the Richter scale. A quake magnitude is determined by measuring the amplitude of the largest wave recorded on the seismogram. The larger the amplitude, the greater is the displacement of the recording pen and the greater the earthquake. (sensitivity of seismograph and distance of station from quake must be adjusted for in the calculation).

Seismicity = the intensity, frequency, and distribution of earthquakes in a specific area.

Seismic Waves = elastic energy that travels as waves through the Earth's crust at speeds of several miles per second and is propagated or transmitted outward radially from the focus of an earthquake.

Seismograph = a very sensitive instrument that can detect, measure and record ground vibrations and their intensities during an earthquake.

Seismograms = plot of time vs. intensity of motion, First waves to arrive at seismograph are P waves, 2nd are S waves, and 3 rd are Surface waves. Arrival of waves relates to velocity with which they are transmitted through the earth's crust. Surface waves ride immediately behind S waves which are moving in layer directly beneath them.

Seismology = the branch of geology that studies earthquakes and seismic waves that move through and around the earth as well as their effects. The people who study earthquakes and seismic waves are called seismologists.

Surface Waves = seismic vibrations that move along the outer layer of the Earth's surface. They have both vertical and horizontal components of motion, horizontal components cause the most damage. They are:

a. Love Waves = horizontal vibration, perpendicular to travel

b. Rayleigh Waves = rolling / orbital ground vibration like ocean waves.