RR166 An appraisal of existing seismic hazard estimates for the UK Continental Shelf

115 Pages·2003·1.51 MB·English

HSE
Health & Safety
Executive
An appraisal of existing seismic hazard

estimates for the UK Continental Shelf

Prepared by The Mallard Partnership for the
Health and Safety Executive 2003
RESEARCH REPORT 166
HSE
Health & Safety
Executive
An appraisal of existing seismic hazard

estimates for the UK Continental Shelf

D.J. Mallard B.Sc.(Eng) M.Sc. C.Eng. M.I.C.E. FGS
B.O. Skipp B.Sc. Ph.D. C.Eng. F.I.C.E. C.Geol FGS
& W.P. Aspinall B.Sc. Ph.D. C.Geol FGS
The Mallard Partnership
26 Gloucester Street
Winchcombe
GLS4 5LX
The first seismic hazard estimates for the UKCS were made for HSE in 1986, with later assessments
being made in 1993 and 2002.Since the work on the most recent of these studies was completed, a
draft ISO standard and a NORSOK standard have been published. Also, a medium-sized earthquake
has occurred in the central North Sea, in an area previously considered to have low seismic activity. In
this report, the most recent estimates (given in report OTO 2002/005) are synthesized with those from
the earlier studies, using a specially-designed rating scheme, to generate hybrid contour maps of the
peak ground acceleration (pga) hazard. The reliability of these hybrid results is assessed by
comparison with provisional, newly-calculated, de-minimis hazard figures based on catalogue
completeness considerations. Until such time as a uniformly-derived hazard map is developed it is
recommended that the 10-4 p.a.probability of exceedance pga hazard figure at any point in the
offshore UKCS east of longitude 10°W should be taken to be highest of the figures given by: the new
map of allowable minimal hazard levels, the new synthesized hazard map, and the estimate given by
report OTO 2002/005. As there is no current information on hazard exposure at the lower
frequencies that are of concern when designing offshore installations, recommendations are made as
to how such estimates could be calculated. The characterization of ground motion is discussed, noting
the significance of the differences between hard and soft sites and current thinking on vertical motion.
Attention is drawn to policy and technical issues that remain unresolved and suggestions made as to
how those issues could be addressed.
This report and the work it describes were funded by the Health and Safety Executive (HSE). Its
contents, including any opinions and/or conclusions expressed, are those of the authors alone and do
not necessarily reflect HSE policy.
HSE BOOKS
© Crown copyright 2003
First published 2003
ISBN 0 7176 2778 0
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or by e-mail to [email protected]

ii
CONTENTS

Executive summar y v

1 INTRODUCTION 1

1.1 Background and objectives 1

1.2 Scope 3

1.3 Approach adopted for conducting this appraisal 4

1.4 Structure of this report 6

2 GENERIC ISSUES RAISED BY THE REPORTS CONSIDERED IN THIS APPRAISAL 8

3 SOME FUNDAMENTAL ASPECTS OF THE PRACTICE OF SEISMIC HAZARD

ASSESSMENT 11

3.1 General matters 11

3.2 Modelling seismicity 13

3.3 The value of hazard maps at low probabilities 17

3.4 The role of geological information 20

3.5 The effects on hazard levels of varying model parameter values 21

3.6 Concluding remarks 22

4 SYNTHESIS OF PGA HAZARD MAPS 23

4.1 The existing hazard maps 23

4.2 Making a synthesis of the existing hazard maps 27

4.3 Discussion 33

5 CHARACTERISING EARTHQUAKE GROUND MOTION FOR THE UKCS 42

5.1 Introduction 42

5.2 The ground motion characterisations proposed in the existing reports 44

5.3 The adequacy of the ground motion characterisations proposed in the existing

reports 45

5.4 Time histories 52

6 VERTICAL MOTION 54

7 TOWARDS ESTIMATING THE 1HZ HAZARD ACROSS THE UKCS 56

8 CONCLUDING REMARKS AND RECOMMENDATIONS 58

8.1 The ground motion hazard in terms of pga 58

9 REFERENCES 66

9.1 The reports forming the input to this appraisal 66

9.2 References quoted in main text 66

9.3 References quoted in appendices 68

9.4 A brief selected bibliography 68

iii
10 Appendix 1: Notes on the individual Reports 71
10.1 Report 1 – Principia Mechanica Ltd. (1986) 71
10.2 Report 2 – BGS & Ove Arup and Partners (1997) 72
10.3 Report 3 – NORSAR & NGI (1998) 74
10.4 Report 4 – EQE International Ltd. (2002) 77
11 Appendix 2: Attenuation Relationships 80
11.1 Report 1 – Principia Mechanica Ltd. (1986) 80
11.2 Report 2 – BGS and Ove Arup & Partners (1997) 80
11.3 Report 3 – NORSAR & NGI (1988) 81
11.4 Report 4 – EQE International Ltd. (2002) 83
Figures
Following page 83:
Figure 1 Report 1: replicated 10-2 p.a. pga contours

Figure 2 Report 1: replicated 10-4 p.a. pga contours

Figure 3a Report 2: replicated 10-2 p.a. pga contours (east of 10ºW)

Figure 3b Report 2: replicated 10-2 p.a. pga contours (west of 1ºW)

Figure 4a Report 2: replicated 10-4 p.a. pga contours (east of 10ºW)

Figure 4b Report 2: replicated 10-4 p.a. pga contours (west of 1ºW)

Figure 5 Report 4: replicated 10-2 p.a. pga contours (east of 10ºW)

Figure 6 Report 4: replicated 10-4 p.a. pga contours (east of 10ºW)

Figure 7 UK Continental Shelf Designations [2001] showing areas covered by existing reports

Figure 8 Hybrid 10-2 p.a. pga hazard contour map (east of 10ºW)

Figure 9 Hybrid 10-4 p.a. pga hazard contour map (east of 10ºW)

Figure 10 Hybrid 10-4 p.a. pga hazard contour map compared with onland site-specific

hazard estimates
Figure 11 Macroseismic earthquake magnitude completeness thresholds in and around
the British Isles
Figure 12 Minimal 10-4 p.a. pga hazard contour map determined from catalogue
completeness considerations
Figure 13 Difference contours for hazards indicated by Figure 9 and Figure 12
Figure 14 Combination 10-4 p.a. pga hazard contour map (the hazard levels plotted are
the higher of those indicated at each point by Figure 9, or by Figure 12)
Figure 15 Comparison of PML [1981] spectral shapes for hard and soft sites
Figure 16 Comparison of various spectral shapes for bedrock/hard sites
Figure 17 Comparison of various spectral shapes for soft sites
Figure 18 Difference contours for hazards indicated by Figure 14 and Figure 6 (i.e.
Figure 14 minus Figure 6)
Figure 19 Recommended 10-4 p.a. pga hazard contour map (the hazard levels plotted
are the higher of those indicated at each point by Figure 14 or by Figure 6)
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Executive summary
The first seismic hazard estimates for the offshore UK Continental Shelf (UKCS) were made
for HSE in 1986, with later subsequent studies being published in 1993 and 2002. Since the
work on the most recent of these studies was completed, a number of developments have
taken place. Two design standards for offshore structures have been published (a draft ISO
standard and a NORSOK standard), and a medium-sized (4.2M ) earthquake has occurred in
L
the central North Sea, in an area previously thought to be characterised by low seismic
activity.
With this background, the Offshore Safety Division of the Health and Safety Executive (HSE
OSD) commissioned The Mallard Partnership to carry out an evaluation of several existing
hazard studies (i.e. not only the most recent report, but also some of the earlier work) in the
light of recent methodological advances. HSE OSD also wished to be informed about such
future work as might be necessary to address any issues revealed by this appraisal that cannot
be resolved using available information. As part of the exercise, comparisons were to be
made with the requirements of various design standards, including the new draft ISO
standard.
The existing reports made available by HSE OSD for this appraisal are:
Principia Mechanica Ltd. (1986) 'North Sea seismicity'. UK Dept. of Energy
Offshore Technology Report No. OTH 86 219, HMSO, London.
BGS & Ove Arup and Partners (1997) 'UK continental shelf seismic hazard'. Health
and Safety Executive Offshore Technology Report OTH 93 416, HMSO,
London.
NORSAR & NGI (1998) 'Seismic zonation for Norway'. Report for the Norwegian
Council for Building Standardization.
EQE International Ltd. (2002) 'Seismic hazard œ UK continental shelf. Health and
Safety Executive Offshore Technology Report 2002/005.
The seismic hazard results provided by each of these studies, which are presented solely in the
form of peak ground acceleration contour maps for certain annual probabilities of exceedance,
and for various differing parts of the UKCS area, are taken and re-plotted to a uniform format
for direct comparison and appraisal.
The strengths and weaknesses of the individual reports are discussed in depth, and a specially-
devised scheme is used to weight their various results impartially, according to their
respective merits. The results are then combined to generate a set of hybrid seismic hazard
maps, representing an appropriate amalgamation of the information that is provided by the
four separate studies. For seismic hazard at the 10-4 p.a. probability of exceedance, the hybrid
contour map of offshore peak horizontal ground acceleration (pga) is compared with
corresponding pga hazard levels determined in site-specific studies at a number of coastal
sites around Britain.
The reliability of the 10-4 p.a. probability of exceedance pga hybrid contour map is assessed
by comparison with a provisional map of ”minimal‘ pga hazard levels, determined
independently, and specifically for this appraisal, from generic catalogue completeness
v
considerations. Difference contours between this minimal hazard map and the levels of
hazard indicated by the hybrid map are plotted to illustrate variations across the expanse of
offshore waters around the British Isles. The two maps are then combined to form a newly
derived 10-4 p.a. pga hazard contour map for areas of the offshore UKCS east of longitude
10˘W (the hazard levels plotted being the higher of those indicated at each point on either
map).
The spatial differences between the hazard levels indicated by this combination map and by
the results in the most recent of the existing reports (EQE International Ltd., 2002) are then
explored. Whilst these differences are modest, and explicable, there are indications of
systematic divergences in different parts of the UKCS area. A final composite map is
constructed, therefore, showing the pga contours that are obtained by selecting the higher of
these alternative hazard estimates at each point on the map.
Thus, this final composite map provides a transparently-derived estimate of seismic hazard
levels which takes appropriate account of the different opinions that have been expressed in
the existing hazard studies and includes precautionary adjustments in areas with no known
earthquakes.
It is recognised that the other limitations in the existing reports are likely to have had some
effects on the reliability of this reconciliation of their results, which cannot be resolved within
the scope of the present appraisal. Nevertheless, for the purpose of providing guidance on
seismic hazard levels in UK offshore areas, it is judged that the final composite pga hazard
map derived here will suffice until a more authoritative study, providing a robust, coherent
and uniform assessment across the whole UKCS area, can be undertaken.
A significant deficiency in the information currently available to HSE OSD arises from the
fact that the existing reports provide little or no information on low frequency ground motion
hazard exposure and its variability. As it is precisely these lower frequencies that are of most
concern in the design of offshore installations, recommendations are made as to how simple
provisional estimates of the 10-4 p.a. probability of exceedance ground motion at 1Hz could
be made using different amplification ratios, depending on whether the site concerned is hard
or soft. For the present, any such estimates could only be regarded as provisional as several
technical elements of the problem need more detailed consideration before improved
regulatory guidance for decision-making can be fully developed.
Consideration has also been given to the spectral shapes that should be used to characterise
the earthquake ground motion across the offshore UKCS, stressing the need in defining such
characterisations to distinguish between hazard spectra and design spectra. In comparison
with the other available alternatives, the spectral shape recommended in the NORSAR & NGI
(1998) and EQE International Ltd. (2002) reports is considered a not unreasonable
characterisation of the ground motion hazard for hard sites in areas close to the British Isles.
This shape is likely to be rather less satisfactory, however, for more distant localities in the far
west of the UKCS. Whether or not this same spectral shape should be adopted as a design
spectrum for hard sites is a matter of policy: to-date, most authorities have preferred to
employ a standard piecewise-linear spectrum for this purpose. The proposals made in these
same two reports concerning the treatment of soft sites is one that utilises an approach which
relates only to a ground motion characterisation intended as a design input (these reports,
therefore, do not touch on the ground motion hazard characterisation for such sites).
However, there are many issues that would have to be resolved before it could be
recommended that this proposal could be accepted as a standard design spectrum. One major
factor influencing policy decisions on spectral shapes is the existence of the recent draft
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document produced by ISO which deals precisely with this topic: on a purely technical level,
for much of the UKCS the shape recommended in the current draft would appear to be
somewhat over-cautious at frequencies below about 0.4Hz.
In the related matter of selecting corresponding time histories appropriate for dynamic
analysis, the existing reports are found to offer only limited assistance: several technical
questions would need to be developed and resolved before an adequate generic approach
could be formulated for representing conditions across the whole area of the UKCS.
With regard to vertical ground motions, only very limited information is provided in the four
existing reports: a cautious position, for the time being, would be to accept the
recommendations made in the NORSAR & NGI (1998) report. In fact, this is a topic which,
generally, has received insufficient attention in recent times and, whilst it seems possible that
the Norwegian proposal could be over-conservative, further work would be necessary to
resolve whether or not this is the case.
As a final general suggestion to HSE OSD, having completed the appraisal, it is concluded
that a site-specific analysis of its exposure to earthquake ground motion hazard should
probably be undertaken in the case of any particularly sensitive location or special structure.
vii
viii
1 INTRODUCTION
1.1 BACKGROUND AND OBJECTIVES
Over the last twenty years or so a large number of reports have been produced providing
probabilistic estimates of the hazard due to earthquake ground motion across and around
Britain. Even allowing for the fact that the concern here is just with the offshore areas of the
UK Continental Shelf, there have still been several such studies, emanating from sources in
the UK and elsewhere, and they have produced differing results. Because of the nature of
probabilistic seismic hazard assessments (see Section 2 below), the existence of several sets
of results makes complex the provision of definitive guidance either for the satisfactory
design of new safety-critical facilities or for the carrying out of proper safety reviews of
existing facilities.
At the request of the Offshore Safety Division of HSE [HSE OSD], an appraisal is here
presented of four of the most significant and recent hazard assessments which deal with the
offshore areas. The objective is to provide recommendations as to the position that should
currently be adopted for passing on, with confidence, to users: in short, HSE OSD wish to
know how best use can now be made of the results of the four studies in the light of current
and developing engineering practice.
Clearly, in making such recommendations, there are many issues that have to be considered:
for example, the following questions were posited in the original proposal for this project:
In terms of hazard levels
· Which report is best founded - how do the reports compare, one with another -
which results to use?
· How robust are the general hazard levels particularly against the occurrence of
new earthquakes?
· How do the hazard results compare with other seismic hazard maps, existing or in
the pipeline (e.g. CEN, ISO)?
· How robust are the low hazard levels assigned to areas of sparse earthquake
information? In such areas, are there any geological factors not fully accounted
for?
· What values of hazard should be used for sites in the locality of the 1931 North
Sea earthquake?
· How do the hazard contours compare with existing site-specific hazard results on
land?
· What is the nature of any conservatism in the hazard modelling?
· What are the features of hazard modelling and contouring software that need to
be understood in appraising the relative merits of the different reports?
1