Addressing misconceptions regarding seismic hazard assessment in mines: b-value, Mmax, and space-time normalization J. Wesseloo1 Affiliation: 1 Australian Centre for Geomechanics, The University of Western Australia, Australia.
Synopsis Seismic hazard assessment, in some form or another, has formed part of seismic risk management in seismically active hard-rock mines for decades. Some misconceptions, however, exist in the mining industry which may lead to errors in interpretation and poor risk management decisions. This paper addresses some misconceptions the author has encountered in the mining industry. This is done by exploring the meaning and implications of the frequency–magnitude distribution. The meaning of Mmax, the methods of assessing it, and the topic of space and time normalization necessary for the evaluation of seismic hazard, are also addressed. The scope of this paper does not include the evaluation of strong ground motion exceedance which also forms part of the evaluation of seismic hazard at mine sites.
Correspondence to: J. Wesseloo
Email:
johan.wesseloo@uwa.edu.au
Dates:
Received: 29 Jun. 2019 Revised: 5 Dec. 2019 Accepted: 11 Dec. 2019 Published: January 2020
Keywords Seismic risk, frequency–magnitute distribution, normalization probability.
How to cite:
Wesseloo, J. Addressing misconceptions regarding seismic hazard assessment in mines: b-value, Mmax, and space-time normalization. The Southern African Insitute of Mining and Metallurgy DOI ID: http://dx.doi.org/10.17159/24119717/855/2020
This paper was first presented at the Deep Mining 2019 Conference, 24–25 June 2019 Misty Hills Conference Centre, Muldersdrift, Johannesburg, South Africa.
Introduction At any seismically active mine, considerable effort is invested into the effective management of seismic risk (see Potvin et al., 2019), of which seismic hazard assessment is, of course, a fundamental component. Over many years I have come across several misconceptions regarding the assessment of the seismic hazard which adversely affect the standard of seismic risk management in our industry. Some of these misconceptions are widespread and deeply rooted, while others crop up from time to time and seem to migrate through the industry. The aim of this paper is to address some of those misconceptions. The frequency–magnitude (FM) distribution is foundational to understanding seismic hazard. It appears, however, that many of the misunderstandings regarding seismic hazard arise from inadequate understanding of the meaning and implications of the FM distribution. For this reason, a large proportion of this paper will be devoted to the FM distribution and its implications. The fact that ‘Mmax’ is used for several different concepts, further creates confusion and misunderstanding. Another topic that does not seem to be well understood is that of normalization of hazard, with respect to space and time, and the related issue of separating sources of seismicity with different behaviour. The assessment of seismic hazard in mines also includes the evaluation of strong ground motions at specific locations. This topic is, however, excluded from the scope of this paper.
What is the Gutenberg–Richter relationship really? It appears that many misconceptions in the mining industry stem from an inadequate understanding of the FM distribution and its implications. Many rock engineers use the FM chart without realizing that it is simply a reverse cumulative distribution1 of magnitude, with the vertical axis plotted on a log scale. The straight line fit, or any other curve fitted to the data, is therefore simply a statistical best-fit model and is conceptually the same as, for example, a normal distribution fitted to UCS data (see Figure 1).
1
lso referred to as Complementary Cumulative Distribution or Inverse Cumulative Distribution. The term ‘Inverse Cumulative Distribution’, A however, is also used to refer to the quantile function.
The Journal of the Southern African Institute of Mining and Metallurgy
VOLUME 120
JANUARY 2020
67 ◀
