Frequently Asked Questions

question3How do you measure earthquake magnitude?

answer3Today, geologists use two different scales to measure how strong an earthquake is. The Richter scale measures the actual size (or amplitude) of the wave generated by a particular earthquake on a seismograph. [Source: Newton's Apple (ericir.syr.edu)]

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This is an indirect measure of the amount of energy released by the earthquake. A one-point increase on the Richter scale equals a tenfold amplitude of wave increase, which equals approximately 31 times more earthquake energy. A second type of scale, the modified Mercalli intensity scale, measures the amount and type of damage that earthquakes do to buildings and other structures, and their effects on humans. Because of these differences in measurements, an earthquake with a low Richter magnitude reading that occurs in a densely populated area like Los Angeles can actually have a higher Mercalli value or intensity than a high-magnitude quake in a desolate region like Antarctica. [Source: Newton's Apple (ericir.syr.edu)]

Answers to other Frequently Asked Questions regarding earthquakes can be found at USGS Earthquake FAQs.

question3

What is Richter Magnitude?

answer3The Richter magnitude scale was developed in 1935 by Charles F. Richter of the California Institute of Technology as a mathematical device to compare the size of earthquakes. The magnitude of an earthquake is determined from the logarithm of the amplitude of waves recorded by seismographs. [Source: US Geological Survey]

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Adjustments are included for the variation in the distance between the various seismographs and the epicenter of the earthquakes. On the Richter Scale, magnitude is expressed in whole numbers and decimal fractions. For example, a magnitude 5.3 might be computed for a moderate earthquake, and a strong earthquake might be rated as magnitude 6.3. Because of the logarithmic basis of the scale, each whole number increase in magnitude represents a tenfold increase in measured amplitude; as an estimate of energy, each whole number step in the magnitude scale corresponds to the release of about 31 times more energy than the amount associated with the preceding whole number value. Although the Richter Scale has no upper limit, the largest known shocks have had magnitudes in the 8.8 to 8.9 range. [Source: US Geological Survey]

question3

What is Local Magnitude?

answer3The SA National Seismograph Network operated by the Council for Geoscience in Pretoria calculates magnitude from recorded ground motion (Hutton & Boore, 1987):

CGSmag

where A is the the recorded max. amplitude in nano metres and D ist the hypo-central distance in km.

question3

What is South Africa’s mine magnitude?

answer3The magnitude scale most commonly used on SA mines is based on seismic energy (En) and seismic moment (Mo). Both are estimated from waveforms:

StankMag

This scale ensures that the largest mine tremors receive similar magnitude values, whether they are recorded by mine networks or by the national seismic network (This email address is being protected from spambots. You need JavaScript enabled to view it., 2005).

question3

What is the purpose of seismic monitoring?

answer3A number of standard monitoring objectives have been formulated that make use of modern, state-of-the-art seismic systems with increased location accuracy, sensitivity, and data throughput. The accurate LOCATION of seismic events and the immediate reporting to management of event coordinates can assist in rescue operations after intermediate to large seismic events.
[After: A handbook for rock engineering practice for tabular hard rock mines, SIMRAC 1999]

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A critical assessment of design parameters in mine planning as well as rock mass modelling can lead to the correction of layouts, mining rate and sequence etc. and thereby result in the PREVENTION of seismicity related damage.

A correlation between spatio-temporal changes in seismicity and the stability of seismic deformation in an area of interest can, if successful, facilitate the implementation of CONTROL measures to reduce exposure to seismicity or determine the desired location and timing of triggered events.

A knowledge of strong changes and characteristic pattens in seismicity leading to instabilities could result in WARNINGS and improved management of exposure to potential rockbursts.

The creation and maintenance of a continuous seismic data base allows the evaluation of historic data in BACK ANALYSIS in terms of optimum mining strategy and potentially hazardous geological features.

[After: A handbook for rock engineering practice for tabular hard rock mines, SIMRAC 1999]

question3

Can earthquakes be predicted?

answer3The issue of prediction has always been one of the establishment of the probability that an earthquake will occur within a specified time interval, a specified space interval, and a specified magnitude range.
[Leon Knopoff, Dep. of Physics and Inst. of Geophysics and Planetary Physics, Univ. of California, CA, in Proc.Natl.Aca.Sci., USA 1995]

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Contraction of these intervals remains an elusive goal. As recently as 20 years ago, the problems of earhquake prediciton were approached through a compilation of a succession of isolated case histories of presumed precursors to subsequent large and small earthquakes. The hope was that these precursory phenomena would appear before many, if not all, subsequent events. Alas, some of these hopes have either evaporated or have proved extremely difficult to document.

[Leon Knopoff, Dep. of Physics and Inst. of Geophysics and Planetary Physics, Univ. of California, CA, in Proc.Natl.Aca.Sci., USA 1995]

question3

Are there legal requirements in terms of seismic hazard monitoring?

answer3At every mine where a significant risk of rockbursts or rock falls exist, the employer must engage the services of competent persons who...

  • Proactively identify dangerous situations and take remedial action before persons are injured or workings are damaged.

  • Participate in any relevant planning activities that need to take account of, could influence or be influenced by, the risk of rockfalls or rockbursts.

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  • Evaluate all face layout positions to determine any significant risks relating to rockfalls or rockbursts created by or likely to be created by mining operations.

  • Identify, review and make recommendations to the employer regarding systems, procedures, and techniques used or to be used by the employer to eliminate, control, or minimise rockfall or rockburst hazards.

  • Establish monitoring, recording and reporting systems and procedures, that ensure that relevant information related to rockfalls and rockbursts is timeously provided to those persons involved in the planning and operating of mining activities.

  • Approve plans for mining sequences to ensure that the probability of seismic events/rockbursts are minimised; the factors affecting the stability of off-reef excavations are taken into account; support systems accommodate current and anticipated rock conditions; and all precautions necessary for remnant mining are stipulated.

  • In seismically active mines, advise the employer in developing a strategy for their mine's COP to reduce the incidence and the effects of rockbursts.

  • On mines or sections of mines where the bulk of ore reserves are located in remnants or pillars, make a periodic detailed analysis of the whole mine with emphasis on sequencing or phasing the extrcation of pillars or remnants in such a way as to ensure that they are mined out as safely as possible.

  • Monitor, interpret and analyse seismicity, where necessary assisted by seismic specialists using appropriate monitoring equipment.

  • Regularly monitor pillar performance to ensure that they conform to design requirements.

  • Inspect all major rockbursts and large or serious falls of grounds and submit a report making recommendations to the employer.

  • Give input on risk assessment matters pertaining to rock related issues.

For more details on the contents of the mandatory Code of Practice to combat rock falls and rockburst acccidents go to CoP.

[Source: Department of Minerals and Energy, DME 7/4/118 AB1 of 2001]