Glossary


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ACCELEROMETER

A seismic transducer that produces an electric signal proportional to ground acceleration.

AFTERSHOCK

A seismic event that follows soon after a larger seismic event and originates at or near the focus of the larger event. Aftershocks are generally at least 1,5 magnitude units smaller than the main shock.

AMPLIFIER

An electronic device to increase the level of an analogue signal.

AMPLITUDE (wave)

The maximum height of a wave, or depth of a trough, as measured from the equilibrium particle position.

ANALOGUE TO DIGITAL CONVERTER

(A / D)

An electronic device which converts an electrical signal to numbers suitable for use in a digital computer.

APPARENT STRESS

A model independent measure of the stress change at the seismic source. It is defined as

σ A = GE / Mo

where G = modulus of rigidity, E = seismic energy, and Mo = moment.

ATTENUATION

Reduction in the amplitude of a wave as the distance to the source increases.

BEDDING PLANE

Visible planar discontinuity in sedimentary or metamorphic rock which may be sufficiently weak to form a separation.

BLOCKY

Roughly equi-dimensional fragments of rock formed between steeply intersecting joints, induced fractures, bedding planes or other discontinuities.

BODY WAVES

P waves and S waves as opposed to surface waves. S waves can not propagate in liquids.

BRUNE'S MODEL

One of the most widely used models of the seismic source. It allows the displacement (D) and average static stress drop(s S) over the area of rupture to be calculated according to

D = Mo / (6.28GRs2)

where Rs = source radius.

BURST FRACTURE

Steep-dipping shear fracture sub-parallel to the line of the face, shattered within a zone of up to several centimeters wide. Small secondary or "feather" fractures usually form at an acute angle pointing in the direction of relative displacement.

CHOKED

Descriptive of a cavity which is almost completely filled with fragmented rock that has fallen or been thrust into the opening.

CLEAVAGE SURFACE

Can split through grains or crystals. Sometimes it has curved or fan-like traces usually starting at a single point which may indicate rapid exclusion of a slab by spilling. This surface results from indirect tensile failure or extension fracture.

CLOSURE

The amount by which the vertical dimension is actually reduced by the combined effect of convergence and additional in-elastic movements such as bed-separation, dilation, bulking, footwall heave etc.

COEFFICIENT OF FRICTION

Constant of proportionality relating normal stress and corresponding critical shear stress at which sliding starts between surfaces.

CODA

That part or a seismogram that follows the source pulse, either P waves or S waves. Coda waves originate from scattering of body waves by inhomogeneities and from dispersion.

COHESION

Shear resistance at zero normal stress.

COMMINUTION

White, finely-ground rock material (rock-flour) characteristic of burst fractures. May be indicative of secondary movement on induced fractures.

COMPRESSIVE STRESS

Normal stress tending to shorten a body in the direction in which it acts.

CONVERGENCE

The amount by which the vertical dimension of an excavation (usually the stope height) is reduced as a result of elastic relaxation in the surrounding rock continuum.

CORNER FREQUENCY

The frequency at which the spectrum of ground velocity of P- or S-waves reaches a maximum. The corner frequency is generally assumed to be inversely proportional to the source dimension.

CREEP

Time-dependant deformation without seismic energy release.

CRUSHED

Descriptive of rock which has been reduced to small whitened fragments (sometimes elongated and "splintery") where cohesion has been reduced to the extent that the rock can be removed by hand.

DIP

Angle at which a stratum or planar feature is inclined from the horizontal (positive down).

DISPERSION

The distortion of a wave form due to a change of velocity with frequency.

DYNAMIC STRESS DROP

Difference between initial stress and the kinetic friction level on a fault.

EARTHQUAKE

A sudden motion in the earth’s crust caused by the abrupt release of accumulated strain of tectonic origin. When associated with mining, earthquakes are labelled "seismic events".

ENERGY INDEX

Ratio of seismic energy of a given event to the average energy of events of the same seismic moment. EI is an indicator of the relative stress level in the rock in which the event took place.

EPICENTRE

The point on the earth's surface directly above the focus (or hypo-centre) of a seismic event.

FAST FOURIER TRANSFORM (FFT)

A rapid numerical technique used to determine the frequency content of a wave.

FIRST MOTION

The initial swing in a wave radiated from a seismic event as recorded at a given station.

FLUCTUATION

Change over time, variation.

FOCUS

The initial rupture point within a seismic source at which strain energy is first converted to elastic energy.

FORESHOCK

A seismic event which precedes a larger seismic event and which originates at or near the hypo-centre of the main shock.

FRACTURE

Break in the continuity of a body of rock due to mechanical failure as a result of excessive stress. Fractures may be open or closed but not "bonded" together.

GEOPHONE

A sensor converting ground velocity into an electric signal.

HEAD WAVES

Waves critically refracted that travel along the contact surface between a high and a low velocity layer.

HYPOCENTRE

The idealised point source of a seismic event, defined in the 3D space.

JOINT

A usually smooth-walled discontinuity with no displacement. Often shows slight alteration, sometimes bonded. It is occasionally difficult to distinguish from an induced fracture.

JOINT-LAMINATIONS

Closely-spaced jointing that cuts rock into slices.

LAMINATION

Fine layering of rock due to closely-spaced bedding.

LOCAL MAGNITUDE

Measure of the strength of a seismic event; based on energy release, source size, and forces acting at the source.

MAGNITUDE

An arbitrarily defined quantity characteristic of the total energy released by a seismic event. The Richter magnitude scale is related to the logarithm of an observed displacement on a calibrated instrument and to its distance from the epicenter.

The Gutenberg-Richter formula is an empirically determined relationship between energy and magnitude :

E (In MJ) = 101.5M-1.2

Local magnitude used in mine seismology is calculated from seismic energy and/or seismic moment.

MECHANISM

(Source Mechanism)

The physical process of ground disturbance at the source of a seismic event.

MICRO-SEISMIC EVENT

A very small seismic event which is normally detectable only by means of sensitive instruments and which is unlikely to cause damage but whose effects may extend into the audible range of 'rock noise' underground.

MINE SEISMOLOGY

Application and extension of seismological techniques to seismic events associated with mining. It includes the study of the relationship between seismicity and mining-related parameters.

ORIGIN TIME

The initiation time of a seismic event. The origin time is usually determined simultaneously with the focus when an event is located.

PARTICLE VELOCITY (Ground Velocity)

The ground velocity with which a specific particle is displaced as a result of a passing seismic wave. Needs to be distinguished from the wave velocity, the speed at which a seismic wave travels in rock.

PRECAUTIONS

Measures taken to reduce the possible consequences of seismic events.

PRECURSOR

A phenomenon which precedes the occurrence of a seismic event.

PREDICTION

Anticipation of the approximate time, magnitude and position of a seismic event.

P-WAVE

The first seismic body wave emanating from a seismic source. Particle displacement is in the direction of wave propagation.

QUICK MAGNITUDE

Quick estimate of the local magnitude of a seismic event; available from the seismic system within seconds after the event.

RAYLEIGH WAVE

A type of surface wave propagated along a free surface of a semi-infinite medium. The particle motion takes place in a vertical plane parallel to the direction of propagation.

REFLECTION

A type of ray-bouncing in which part of the energy is reflected from the velocity contrast.

REFRACTION

The change in direction of wave propagation when waves pass obliquely from one rock mass layer into another.

ROCKBURST

A sudden and violent disruption of rock or disturbance of excavation walls in mines which is caused by, or accompanied by a 'shock' or tremor.

Injury to persons (or damage to equipment) resulting from an isolated fall of rock or from flying fragments does not necessarily indicate the occurrence of a rockburst, although a tremor from a distant event can cause the fall of an unstable piece of rock. Such an occurrence might be denoted as "fall of ground associated with a seismic event".

SAMPLING RATE

The rate at which an analogue to digital converter samples the analogue signal. In seismology a constant sampling rate is used.

SCALING

The non-violent formation of slabs which have become completely separated from the excavation surface.

SEISMIC ENERGY

That part of total strain energy that is converted to elastic waves and radiated away from a seismic source.

SE released by a tremor can be calculated directly from a seismogram. It is usually assumed to have spread out in a spherical wave front from the source. The energy of the passing seismic waves is calculated at each seismometer site and is used to estimate the total seismic energy radiated from the source..

SEISMIC EVENT

Sudden failure due to stresses exceeding the strength of the rock mass or a discontinuity. The resulting emission and radiation of kinetic energy in the form of ground vibrations causes a noticeable 'shock' or tremor.

SEISMIC HAZARD

Hazard (potential for loss) associated with seismicity.

SHA = SEISMIC HAZARD ASSESSMENT

Seismic hazard is, in its basic form, expressed as magnitude of the largest event expected. More complex assessments can include event frequency, expected PPV, probability of occurrene of large events etc.

SEISMIC MOMENT

Measure of the earthquake size and the leverage of the forces (couples) across the area of slip. Unlike magnitude, the seismic moment is not a scalar but a tensor (i.e. its value is different depending on the direction). This characteristic can be used to determine other source parameters, like slip direction and source radius.

Seismic moment is the product of the modulus of rigidity (G), the rupture area (A), and the displacement or "ride" (D):

Mo = GAD (in Nm)

SEISMIC NETWORK

System for data collection and analysis of seismic data utilising a number of sensors, data transmission, and storage facilities.

SEISMIC SOURCE

A volume of rock that fails suddenly, deforms and releases energy of various types.

SEISMIC PARAMETERS

A set of variables that are used to describe seismic events or the seismicity of a confined area.

SEISMOLOGY

A geophysical science which is concerned with the study of earthquakes and measurements of the elastic properties of rock.

SEISMOMETER

Contains A/D converter, data storage, and communication devices. Forms part of a seismic network.

SHATTERED

  1. Of rock - disruption into fragments so small that it is not easy to determine whether these are basically slabs, slices or blocks.
  2. Of support packs - timber component severely compressed and splintered and concrete bricks fragmented.

SLABBING

The non-violent formation of slabs which are not detached from the free surface.

SLABS

Tabular pieces of rock formed by stress-induced cleavage essentially parallel to the face surface.

SLICES

Tabular pieces of rock disposed more or less perpendicularly to the existing surface of the excavation. Typically formed in the stope H/W of tabular ore-bodies by the fracture planes induced parallel to a stope face.

SLICKEN-SIDING

Polished, linear features or grooves on discontinuities of geological age such as faults.

SLIP

Used to denote a minor fault or joint with very slight displacement.

SOURCE DIMENSION

Measure of the linear dimension of seismic source, inversely proportional to the corner frequency.

SOURCE MECHANISM

The 3-dimensional distribution of forces and resulting movements within the seismic source.

SOURCE PULSE

The part of a seismogram resulting from waves which radiate directly from the source.

SPALLING

The violent formation of slabs which separate from a strained surface. If the slab is ejected from the surface this would constitute one form of strain burst.

SPITTING

Small strain burst which causes splinters of rock to "explode" from a strained surface.

SPLINTERED

Timber so severely damaged that sticks are split into many long, slender fragments.

STRAIN BURST

A commonly used term denoting sudden rock failure at the lower end of the magnitude spectrum of violent events. The failure is associated with the surface of the excavation and involves only a few hundred kilograms of rock.

STRESS DROP

The decrease in shear stress on that part of a fault which slips during a seismic event.

STRESS-INDUCED FRACTURE

Generally clean, "cleavage type" fracture parallel to the stope face with no primary displacement.

STRIATIONS

Linear markings in a comminuted surface or recent source region indicating direction or movement.

SURFACE WAVE

Wave which propagates along a free surface (e.g. the Earth's surfac). Types of surface waves are Rayleigh and Love waves.

S-WAVE

Distortional, equi-voluminar, secondary or shear wave. Particle motion is normal to direction of wave propagation.

TALKING

The noise associated with small failures of rock occurring beyond the exposed surface and not causing any fall of rock fragments.

TIME HISTORY

The plotting of seismic parameters over time; used to detect trends in seismic activity.

WAVE VELOCITY (Phase Velocity)

The speed with which an individual wave of a certain frequency advances.