Earhquake glossary (copy)

Active fault

One that has moved once or more times in the past 10,000 years and therefore likely to move sometime in the future.

Aftershock

an earthquake that follows a larger earthquake, or main shock, and originates along the same fault as the main shock.

Amplitude

a measurement of the energy of a wave. Is the displacement of the medium from zero or the height of a wave crest or trough from a zero point.

Body waves

Seismic vibrations that move through the Earth's interior

Primary 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)

Longitudinal Waves 

wave motion (amplitude) is parallel to direction of travel, e.g. accordion motion, to and from.

Secondary waves

vibrations occurring at right angles to direction of wave propagation. They are shake-waves (shear waves), vibrating side-to-side

Transverse Waves

wave motion (amplitude) is perpendicular to direction of travel, e.g. spring bobbing, up and down

Braces or Bracing 

structural elements built into a wall to add strength. These may be made of various materials and connected to the building and each other in various ways.

Compression

squeezing, being made to occupy less space.

Continental drift

the theory, first advanced by Alfred Wegener, that Earth's continents were originally one land mass, pieces of which split off and gradually migrated to form the continents we know.

Diagonal braces

structural elements that connect diagonal joints. These braces may be made of solid materials or flexible materials. How they function depends on what they are made of and how they are connected.

Duration

the length of time that ground motion at a given site shows certain characteristics. Most earthquakes have a duration of less than one minute, in terms of human perceptions, but waves from a large earthquake can travel around the world for hours.

Earthquake

a sudden shaking of the Earth caused by the passage of seismic waves. These waves are caused in turn by the release of energy stored in the Earth's crust.

Earthquake hazard

any geological or structural response to an earthquake that poses a threat to human beings and their environments.

Epicenter

the point on Earth's surface directly above the location (focus) of the earthquake below the surface.

Fault

a break or fracture in Earth's crust along which movement has taken place

Focus (plural foci)

the point within the Earth that is the origin of an earthquake, where strain energy is first released as wave energy.

Foreshock

an earthquake that precedes a larger earthquake, or main shock, and originates along the same fault as the main shock.

Friction

any force or combination of forces that acts in opposition to motion.

Frequency

the rate at which a motion repeats, or oscillates. Is the number of oscillations in an earthquake wave that occur each second.

Hazard

a risk; an object or situation that holds the possibility of injury or damage.

Hertz

the unit of measurement for frequency, as recorded in cycles per second.

Horizontal Load

the sum of horizontal forces (shear forces) acting on the elements of a structure.

Intensity

 a subjective measure of the amount of ground shaking an earthquake produces at a particular site, based on human observations of the effect on human structures and geologic features.

Isoseismal line

a line on a map that encloses areas of equal earthquake intensity. It is usually a closed curve around the epicenter.

Joint

a break or fracture in the Earth's crust along which movement has not taken place

Lag time

the difference between the arrival time of P waves (Tp) and S waves (Ts)

Landslide

an abrupt movement' of soil and bedrock downhill in response to gravity. Can be triggered by an earthquake or other natural causes.

Liquefaction

the process in which a solid (soil) takes on the characteristics of a liquid as a result of an increase in pore pressure and a reduction in stress.

Load

the sum of vertical force (gravity) and horizontal forces (shear forces) acting on the mass of a structure.

Load path

the path a load or force takes through the structural elements of a building

Longitudinal waves

This term is used to emphasize that p-waves move particles back and forth in the same line as the direction of the wave.

Love waves

surface waves that move in a back and forth horizontal motion.

Magnitude

a number that characterizes the size of an earthquake by recording ground shaking on a seismograph and correcting the distance to the epicenter of the earthquake.

Mass

the quantity of matter in a body as measured by its inertia.

Mass movement

the movement of surface material caused by gravity.

Modified Mercalli scale of 1931

a qualitative scale of earthquake effects that assigns an intensity number to the ground shaking for any specific location on the basis of observed effects.

Natural hazard

any of the range of natural Earth processes that can cause injury or loss of life to human beings and damage or destroy human-made structures.

Oscillation or vibration

the repeating motion of a wave or a material.

P waves

They travel faster than S waves, or secondary waves and arrive at the station first. These waves carry energy through the Earth as longitudinal waves, moving particles in the same line as the direction of the wave

Paleomagnetism

the natural magnetic traces that reveal the intensity and direction of Earth's magnetic field in the geologic past. .

Paleoseismology

the study of ancient earthquakes.

Period

the time between two successive wave crests

Plate tectonics

the theory that Earth's crust and upper mantle (the lithosphere) are broken into a number of more or less rigid, but constantly moving, segments, or plates

Rayleigh waves

surface waves that carry energy along Earth's surface by elliptical particle motion, which appears on the surface as a ripple effect.

Recurrence interval

the approximate length of time between earthquakes in a specific seismically active area.

Resonance

an increase in the amplitude (a measurement of the wave crest) in a physical system (such as a building) that occurs when the frequency of an applied oscillatory force (such as earthquake shaking) is close to the natural frequency of the system.

Retrofitting

making changes to a completed building to meet needs that were not considered at the time it was built; in this case, to make it better able to withstand an earthquake.

Richter magnitude

the number that expresses the amount of energy released during an earthquake, as measured on a seismograph or a network of seismographs, using the scale developed by Charles Richter in 1935.

Richter scale

the magnitude scale developed by Charles Richter in 1935 as a means of categorizing local earthquakes.

Rigid connections

connections that do not permit any rotation of the structural elements they connect.

Seismic zone

a region in which earthquakes are known to occur.

Seismogram

the record of earthquake ground motion recorded by a seismograph

Seismograph

an instrument that records vibrations of the Earth, especially earthquakes.

Shear force

force that acts horizontally (laterally) on a wall. These forces can be caused by earthquakes and by wind, among other things.

Shear walls

walls added to a structure to carry horizontal (shear) forces. These are usually solid elements, and are not necessarily designed to carry the structure's vertical load.