NUMO-TR-12-05

TOPAZ ProjectLong-term Tectonic Hazard to
Geological Repositories
an extension of the ITM probabilistic hazard assessment methodology to 1 Myr

Neil Chapman1, Mick Apted2, Willy Aspinall3, Kelvin Berryman4,
Mark Cloos5, Charles Connor6, Laura Connor6, Olivier Jaquet7,
Koji Kiyosugi6, Ellie Scourse1, Steve Sparks3, Mark Stirling4,
Laura Wallace4, Junichi Goto8
1

MCM Consulting, Switzerland
INTERA, USA
3
University of Bristol, UK
4
GNS Sciences, New Zealand5
University of Texas, USA
6
University of South Florida, USA
7
In2Earth Modelling Ltd, Switzerland
8
NUMO, Japan
2

October 2012
Nuclear Waste Management Organization of Japan (NUMO)

2012 年 10 月

初版発行

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NUMO-TR-12-05

TOPAZ ProjectLong-term Tectonic Hazard to
Geological Repositories
an extension of the ITM probabilistic hazard assessment methodology to 1 Myr

Neil Chapman1, Mick Apted2, Willy Aspinall3, Kelvin Berryman4,
Mark Cloos5, Charles Connor6, Laura Connor6, Olivier Jaquet7,
Koji Kiyosugi6, Ellie Scourse1, Steve Sparks3, Mark Stirling4,
Laura Wallace4, Junichi Goto8
1

MCM Consulting, Switzerland
INTERA, USA
3
University of Bristol, UK
4
GNS Sciences, New Zealand5
University of Texas, USA
6
University of South Florida, USA
7
In2Earth Modelling Ltd, Switzerland
8
NUMO, Japan
2

October 2012
Nuclear Waste Management Organization of Japan (NUMO)

各章の和文要約
1 序論
NUMOは，地震・断層活動や火山・火成活動などの自然現象の地層処分システムに対する影響を評価する手法の一つとして，ITM 1の海外専門家とともに，確率論的な評価手法（ITM手法）の構築を進めてきた。ITM手法は，将来 10 万年程度の期間と広域的な領域の評価を対象とし，これまで東北地方や九州地方のケーススタディを通じて日本の地質環境に対する基本的な適用性を確認してきた（Chapman, et al., 2009 など）。
地層処分における安全評価では，数 10 万～100 万年といった超長期の自然現象の影響評価が求められる可能性がある。このため，将来 10 万年を超える期間と処分場規模の数km四方の領域における自然現象の影響の確率論的評価手法を構築するために，
TOPAZ 2プロジェクトを設立した。本手法（TOPAZ手法）では，まず将来の時間枠毎に，プレート運動の変遷に伴い生じうる広域およびサイトスケールの現象を記述する広域
変遷シナリオ（RES 3）とサイト変遷シナリオ（SES 4）を設定する。次に，それらの現象の地層処分システムに対する影響を記述する影響シナリオ（IS 5）を設定する。それらをロジックツリーに統合し，専門家の意見集約（EE 6）やITM手法に基づき，各シナ
リオの確信度（起こりやすさ）を設定し，確率論的な評価を行う。以上の方法論に基づ
き，ITM手法による検討実績のある東北地方を対象にケーススタディを実施した。
2 広域変遷シナリオ（RES）とサイト変遷シナリオ（SES）の設定
日本列島周辺の過去数千万年前からの情報を分析し，現在～１万年後，１万年後～10
万年後，10 万年後～100 万年後の三つの時間枠に対して，四つの RES（RES1~4）を設定した。RES1 は現在から将来 100 万年にかけてプレート収束速度が現在のまま継続する，RES2 はプレート収束速度が 2 倍になる，RES3 はプレートの運動方向が変化す
る，RES4 は太平洋プレートの沈み込み角度が急傾斜になるというシナリオである。次に，地質条件の異なる四つの仮想サイト（A, B, C, D）を設定し，上記四つの RES のもとに各サイトにおいて生じうる自然現象を概念的に記述した SES を作成した。
3 ロジックツリーの構築
上記のシナリオを統合し，ロジックツリーを構築する方法について検討した。地震・断層活動については，四つの RES の後に岩盤の変形速度の変化に対する三つの SES を設定した（変形速度が現在と同様，2 倍に増大，1/2 に減少）。さらに，それぞれの SESについて，それらの発生確率を求めるための ITM 手法の三つのモデル（GPS，地震，
1 ITM：International Tectonics Meeting の略。わが国のようなテクトニクス活動が盛んな環境での地層処分事業におけるサイト選定のための調査・評価技術の妥当性について，国内外のテクトニクスの専門家の合意形成・情報発信の場として NUMO が主催した会議体。
2 TOPAZ：Tectonics Of Preliminary Assessment Zones の略。
3 RES：Regional Evolution Scenario の略。
4 SES：Site Evolution Scenario の略。
5 IS：Impact Scenario の略。
6 EE：Expert Elicitation の略。

i地表変位に基づく歪速度モデル）の選択肢を示す分岐を設定した。火成活動については，
四つの RES の後に，まず火山の活動頻度の変化に関する SES の分岐を設定し，さらに火山の発生確率を求めるための ITM 手法（カーネル法，コックスプロセス法）の選択
肢を示す分岐を設定した。
4 専門家の意見集約（EE）によるシナリオの確信度の設定
ロジックツリーの各分岐の確信度（起こりやすさ）は，透明性・正当性・中立性・公平性を確保しながら専門家の意見を集約する科学的な手法（Cooke, 1991）を用いて設定する。この手法では，専門家に対して事前に課題に関連する専門分野のテストを行い，正しい値を答える確からしさと回答の振れ幅（不確実性）の与え方の傾向を示す二つの指標を得て，専門家ごとに重み付け係数を設定する。続いて専門家の議論において各専門家の意見（回答）に係数を適用した上で集約し，専門家の意見の総意となる回答と不確実性（回答の取りうる範囲）を得る。今回，国内の専門家の協力を得て，一部のロジ
ックツリーのシナリオについて確信度を設定する演習を行い，基本的な手順を確認した。
5 影響シナリオ（IS）における事象の発生確率の設定
地層処分システムへの影響評価では，対象とする事象を定義し，発生確率を設定する必要がある。ここでは，M6.5~7.5 の地震とM5 の火山噴火 7を例に，発生確率の設定方
法を提示した。地震については，ITM手法の三つのモデル（GPS，地震，地表変位）による歪速度を地震モーメントに変換し，さらにグーテンベルク－リヒター分布から
M6.5~7.5 の地震の発生頻度（確率）を求めた。火山噴火については，ITM手法の二つの手法（カーネル法，コックスプロセス法）を用いて発生確率を求めた。
6 影響シナリオ（IS）の設定
まず既存情報に基づき，影響シナリオの前提となる廃棄体や人工バリアの状態の時間的な変遷について整理した。続いて，地震・断層活動，火山・火成活動，隆起・侵食に起因する様々な事象の影響のシナリオを設定するための考え方を提示した。さらに，断層変位と地表接近の有無，そしてマグマの直撃と間接的影響の有無のパターンの組合せ表を作成し，多数のシナリオを統合して効率的に評価を行う方法を考案した。
7 結論
規制機関を含む様々なステークホルダーと自然現象の影響や放射性物質による健康被害について議論する際には，それらの発生可能性や不確実性を決定論的あるいは確率論的に明確に示す必要がある。TOPAZ 手法は，安全性の評価に向けて幅広い地球科学的な情報や専門家の意見を集約するための有効なツールとなりうる。これまで，ケーススタディの限られた範囲の中で方法論の適用性を検討してきた。今後は，実際への適用に向けて，手法のさらなる検証・改良を進める必要がある。
7 M5 の火山噴火：ここでの M（マグニチュード）は，噴出物の体積や噴煙の高さに基づき噴火の強さを表すための指標(M0~M8)。M5 は噴出量 1km3，噴煙高 25km 程度。

ii

Contents
1

Introduction and Context ..................................................................................................... 1
1.1 The ITM Methodology: probability – the likelihood of future tectonic impacts on arepository ............................................................................................................................... 1
1.2 Timescales and tectonic impacts on a geological repository ......................................... 3
1.3 Going Beyond 100,000 years: the TOPAZ project ......................................................... 5
1.4 This report ..................................................................................................................... 10
1.5 TOPAZ International Project Team Members .............................................................. 10

2

Regional and Site Evolution Scenarios ............................................................................. 12
2.1 Tectonic Setting of Tohoku ........................................................................................... 12
2.1.1

Basement of Tohoku ............................................................................................ 12

2.1.2

Forearc Region .................................................................................................... 13

2.1.3

Volcanic Arc Region ............................................................................................. 13

2.1.4

Backarc Region .................................................................................................... 13

2.1.5

Active Tectonics ................................................................................................... 14

2.2 Overview of Regional Evolution Scenarios for Tohoku ................................................ 15
2.2.1

RES 1: Tectonic boundary conditions are the same from the present to 1 Myr .. 15

2.2.2 RES 2: The amount of relative plate convergence accommodated in the upper
plate doubles from present to 1 Myr in the future............................................................. 18
2.2.3 RES 3: Pacific-Eurasia relative plate motion becomes more oblique from present
to 1 Myr in the future. ....................................................................................................... 20
2.2.4 RES 4: The amount of plate convergence accommodated within the upper platedecreases linearly to a neutral state at 1 Myr .................................................................. 23
2.2.5 Examples of additional aspects of tectonic evolution that could be integrated into
the RES. ........................................................................................................................... 25
2.3 Site Evolution Scenarios ............................................................................................... 25

3

2.3.1

Description of sites used in the SES demonstration ............................................ 26

2.3.2

Site Evolution Scenarios for Site A ...................................................................... 32

2.3.3

Site Evolution Scenarios for Site B ...................................................................... 32

2.3.4

Site Evolution Scenarios for Site C ...................................................................... 33

2.3.5

Site Evolution Scenarios for Site D ...................................................................... 33

Estimating Scenario Probabilities Using Scenario Logic Trees ........................................ 35
3.1 The Rock Deformation and Uplift/Exhumation Logic Trees ......................................... 35
3.2 The Volcanism Logic Tree ............................................................................................ 37

4

Using Expert Elicitation to Derive Scenario Probabilities .................................................. 41
4.1 Background to Expert Elicitation ................................................................................... 41
4.2 Questions Posed in the Expert Elicitation ..................................................................... 43
4.3 Evaluation of RESs ....................................................................................................... 44
4.4 Weights for the SESs .................................................................................................... 50
4.5 Scenario tree weights leading to SES probabilities ...................................................... 50

iii

4.6 Discussion of the Expert Elicitation .............................................................................. 54
4.7 Output of the Expert Elicitation ..................................................................................... 55
5

Estimation of IS probabilities for Location ‘C’ .................................................................... 56
5.1 Demonstration of the Quantitative Estimation of the Probability of a Tectonic
Deformation Scenario for a M6.5-7.5 Future Earthquake .................................................... 56
5.1.1

Methodology......................................................................................................... 57

5.1.2

Observations ........................................................................................................ 60

5.2 Demonstration of the Quantitative Estimation of the Probability of a M5 Volcanic
Scenario ............................................................................................................................... 61

6

5.2.1

Estimation of SES probability at Location C ........................................................ 61

5.2.2

Estimation of IS probability of a VOGRIPA M5 event at Location C ................... 62

5.2.3

Implications for potential repository sites ............................................................. 66

Repository Relevance of the Selected Impact Scenarios ................................................. 68
6.1 Evolution of a HLW Repository ..................................................................................... 68
6.2 Illustrative Impact Scenarios ......................................................................................... 71
6.2.1

Rock Deformation Impact Scenario ..................................................................... 72

6.2.2

Volcanism Impact Scenario ................................................................................. 73

6.2.3

Uplift, Erosion and Exhumation Impact Scenarios............................................... 76

6.3 Aggregation of Impact Scenarios ................................................................................. 77
6.4 Summary....................................................................................................................... 78
7

Conclusions ....................................................................................................................... 80

References .............................................................................................................................. 81
Appendix: Concealed Active Faults Workshop ....................................................................... 85
1

Context.......................................................................................................................... 85

2What do we mean by CAF? .......................................................................................... 85

3

When is a CAF a threat to a repository? ...................................................................... 86

4

How can we identify CAFs? .......................................................................................... 86

5Conclusions .................................................................................................................. 87iv

1

Introduction and Context

NUMO (the Nuclear Waste Management Organisation of Japan) is responsible for the siting,development and operation of a deep geological repository for high-level waste (HLW) andTRU in Japan. The process is expected to take at least 15 years to reach the point of
repository construction. During the period before this, NUMO will need to evaluate sites thatemerge from the ‘volunteer process’ (whereby local communities have been invited tovolunteer to be considered as potential hosts for the repository) and select a preferred site.
This evaluation will involve initially surface-based and then underground site-characterisation
work. Underground characterisation work will only take place at the preferred site.
Prior to the surface-based investigations, volunteer sites will have first had to pass a test ofgeneral suitability (NUMO, 2004) and NUMO will then have carried out a detailed, literaturebased preliminary evaluation of suitability, prior to accepting them as ‘Preliminary
Investigation Areas’ (PIAs). Because Japan lies in such a tectonically active region of theworld on the Pacific rim (the so called ‘ring of fire’), a key aspect of all these steps isconsideration of the susceptibility of a site to future tectonic activity and tectonically driven
processes and events. For repository safety evaluation, the focus is predominantly on longterm (thousands of years) post-closure tectonic processes leading to progressive
perturbations of the repository environment. These may possibly lead to the initiation of
disruptive tectonic events at repository depth, although potential impacts of other tectonic
events during the multi-decade, operational period also have to be taken into account in many
locations. The importance of this requirement was dramatically reinforced by the impacts of
the March 2011 Tohoku post-earthquake tsunami on the coastal Fukushima nuclear power
plant and the ‘permanent’ co-seismic tectonic deformation and crustal changes in the region. .
The present study is concerned with repository susceptibility to post-closure tectonic impacts
over many thousands of years. In this respect, the potential for long-term volcanic and rockdeformation impacts on a repository site needs to be considered in particular, at each stageof NUMO’s siting programme. Whilst the nationwide evaluation factors for qualification(EFQs) for PIA acceptance are designed to remove clearly unsuitable sites fromconsideration, they cannot guarantee that, over the next tens of thousands of years, the risksof tectonic hazard for a chosen PIA will be acceptable. This is because large parts of Japanthat are potentially geologically suitable for siting are directly affected to varying extents byrock deformation, the peripheral impacts of volcanic activity or the possibility of new magma
intrusion or volcanic activity. The EFQs were only intended as preliminary screening
guidelines to prevent obviously unsuitable candidates entering the siting process.

1.1

The ITM Methodology: probability – the likelihood of future tectonic
impacts on a repository

NUMO recognised that an integration of additional and more refined techniques would be
required to evaluate sites that pass the EFQ test, so that they could have a clear, quantitative
indication of the likelihood and potential impacts of tectonic events and processes at each PIA.
NUMO’s ITM project (2005 – 2009) developed such a methodology (the ‘ITM Methodology’),based upon state-of-the-art approaches used internationally, developed and extended for the
specific purposes of NUMO and the specific conditions of Japan.
The ITM methodology is essentially probabilistic in nature. A probabilistic approach was seenby the ITM expert group (Chapman et al., 2008) as the only realistic means of addressing theuncertainties in predicting possible hazards when there is marked variability in the spatial
distribution, the timing, the intensity and style of the volcanic and deformational events, andprocesses being evaluated (for convenience, in this report, we frequently group thesetogether within the general term ‘tectonic events and processes’). The consequences ofignoring very low probability events with significant impacts were highlighted by the
Fukushima disaster.
During the course of the ITM project, both NUMO and the Japanese regulatory agencies wereconsidering how best to handle the evaluation of low probability, disruptive events (e.g.
volcanic intrusion, fault rupture) and deformation processes that are discontinuous in time andmagnitude in response to continuous regional strain, when carrying out safety assessments1

of geological repositories for radioactive wastes. Essentially, two approaches have been
adopted internationally to address this situation:1•

To calculate the health risk to people in the future by combining the probability of a
disruptive event occurring with its radiological consequences in terms of releases from a
repository: simply, risk = probability x consequence. With this approach, regulatory
standards or targets can be defined in terms of risk to an individual.

•

To consider the impacts of a disruptive event and calculate the radiological doses topeople in the future and then, separately, to discuss the likelihood that this might happen(the so-called ‘disaggregated’ approach). With this approach, separate regulatory targets
for radiation doses might be set for events (or scenarios) with different degrees oflikelihood (often expressed qualitatively; e.g. ‘likely’, ‘less likely’, ‘highly unlikely’).

2

In either approach, an evaluation of probability is essential: in the first ‘risk approach’ a soundquantitative estimate will provide more confident estimation of risk; in the second, some formof quantification of ‘likelihood’ is needed to decide which category to place an event or
scenario into.
The probabilistic approach developed by ITM is based upon and strongly supported bydeterministic models of the underlying tectonic processes that lead to magma intrusion,
volcanism and rock deformation.
The ITM methodology will be used at three important stages of NUMO’s repository siting
programme:•

SITING STAGE 1: during the literature survey (LS) stage when potential PIAs arebeing assessed. The ITM methodology will use currently available information toallow comparison of sites in terms of confidence that they are likely to prove
acceptable with respect to tectonic impacts.

•

SITING STAGE 2: during the planning of the PIA site investigations, to identify
geoscientific information requirements that will be needed to refine the Stage 1
analysis.

•

SITING STAGE 3: at the point where PIAs are being evaluated and compared inorder to select a preferred site (or sites) for detailed investigation (as DIAs).

The ITM project was mainly concerned with Stages 1 and 2 and focussed on evaluating
2
comparative hazards of small (25 km ) areas within a regional or sub-regional context of
2
100,000 to 10,000 km . This is partly because the project originally developed to compare
several possible alternative volunteer sites that might arise within a region. However, it isclear that regional to sub-regional scale assessment of tectonic hazard will also be required
even for single sites.
Application of the methodology in Siting Stage 3 is several years into the future and it isexpected that it will be most efficient to carry out any necessary updates/refinements on aregion-specific basis during the PIA investigations, when NUMO has narrowed down to agroup of sites. The overall structure of the ITM methodology is described in Chapman et al.
(2008) and consists of:1•

assembling nationally available data and alternative models of the nature, causes
and locations of tectonic processes and events;•using probabilistic techniques to evaluate the likelihood and scale of future tectonic
processes and events, shown as a function of their type and geographicaldistribution;

Health risk is normally defined as the risk of death or serious genetic effects.

2

Of course, a radiological dose can also be expressed in terms of health risk, by applying accepted
dose-to-risk conversion factors.

2•feeding information on these potential likelihoods and impacts to NUMO’s
performance assessment team so that feedback can be provided on repository
performance under tectonic stress;•

providing clearly justified and traceable input to decision-making on consequent site
suitability.

For convenience, the methodology for rock deformation and volcanic hazards assessment
has been applied as two parallel tasks. This recognises the fact that, although the concept ofeach approach as shown above is similar, in some parts of the methodology they differ
significantly in detail. Consequently, it was found that two teams with different specialities(structural, geophysics and tectonics specialists; volcanologists) worked efficiently in parallel.
At the time of the ITM Methodology development it was noted that the two ‘discipline’ teams
would need to integrate their work efficiently, as there are clear overlaps in the processes
being evaluated (e.g. magma intrusion has an impact on rock stress regimes and vice versa).
NUMO recognised it is important that such integration be carried out effectively when themethodology is applied to ‘real’ sites.
The methodology was first tested during its development by means of a Case Study of the
Tohoku region of northern Honshu (Chapman et al., 2008) and then further developed andtested by application to a second Case Study region covering the whole of Kyushu (Chapman
et al, 2009). The complete methodology is described in the latter report and is not presented
again here in detail. The Tohoku Case Study looked into the varied strain response of thecrustal plate to subduction of the Pacific Ocean plate (the key current tectonic driver for muchof Japan) and the mechanisms that underlie the apparent clustering of Quaternary volcanoes
in much of Honshu. The Kyushu Case Study region is among the more dynamic and rapidlychanging plate boundaries in the world, with the tectonic situation being intrinsically more
complex than in Tohoku. The observed modes of strain accommodation in Kyushu are varied
but not yet fully understood, and the style of volcanism and geochemistry of the magmas
varies considerably across Kyushu, compared to the reasonably simple arc volcanism in
Tohoku. Forecasting future volcanism and faulting in Kyushu is less certain because thegeological setting is evolving more rapidly. The fundamental assumption in Tohoku, that theplate boundary configurations and plate motions influencing rock deformation and volcanism
are relatively stable over periods up to a million years, are less appropriate in Kyushu.
Consequently, these two Case Studies allowed development and testing of the ITM
Methodology under significantly different tectonic conditions that span a significant part of therange of tectonic environments across the Japanese archipelago.

1.2

Timescales and tectonic impacts on a geological repository

The impact of tectonic events and processes on the long-term safety of a geologicalrepository is highly dependent on the time at which they might occur. Figure 1.1 shows thedeclining hazard of HLW (and spent fuel) as a function of time after production. Hazard isrepresented here as the ingestion radiotoxicity of all the radionuclides in the waste relative tothe radiotoxicity of the amount of uranium ore required to manufacture the fuel from which the
HLW was produced. It can be seen that the hazard declines rapidly over the first hundreds ofyears and, after about 3000 to 4000 years, reaches the same level as the uranium ore. At thistime after disposal, a HLW repository would have a similar hazard potential (in terms ofmobilisation and migration of radionuclides in groundwater to cause exposure to people andthe environment) as a rich uranium ore body buried at a similar depth. It can be argued thatthe isolation and containment function of the geological repository for HLW has been largely
fulfilled within the first ten thousand years.
Nevertheless, quantitative safety evaluations of repositories for HLW are required in mostcountries out to times of at least 100,000 years – the period addressed in the ITM project. In
several countries, even those dealing with disposal of spent fuel, which can be seen from
Figure 1.1 not to reach the natural cross-over point with uranium ore until about 100,000
years after disposal, only qualitative statements of safety are required after 100,000 years.
Such statements are expected to discuss the longevity of safety functions into the distantfuture, to compare hazard to natural radiotoxic hazards and to discuss the long-term fate of3the wastes as the repository degrades. The engineered barriers and the waste itself are
expected to be substantially degraded in the period 100,000 to 1 million years and beyond.

Figure 1.1: Declining radiotoxicity of HLW and spent fuel as a function of time after production: see text
for description (after NUMO, 2004 and Chapman & Hooper, 2012).

One of the main reasons that only qualitative descriptions are expected in many countries isthat many uncertainties associated with making quantitative estimates of actual radiologicalhealth impacts increase significantly and such forecasts loose credibility as actual indicatorsof safety. The other main reason is that, as can be seen, the hazard potential of the waste bymobilisation into the deep groundwater system will, over these protracted timescales,
eventually start to approach levels similar to naturally occurring sources of radioactivity in theenvironment. The concepts discussed above are shown schematically in Figure 1.2.

Figure 1.2: Schematic representation of the progressive degradation of a geological repository with time,the diminishing ability to make quantitative forecasts as uncertainty increases, and the decreasing
importance of tectonic impacts to safety as the hazard potential also decreases. The ITM and TOPAZ
project timescales are shown on the right hand side of the diagram.

Returning to tectonic impacts, it can be seen that events and processes that might disrupt a
repository over the first 10,000 years are critical and need to be avoided, so far as possible,
by correct siting in regions of low tectonic susceptibility. Thus, the probability of occurrence of4events and processes is an extremely important factor. The ITM project develops the tools toassess such probabilities out to 100,000 years – not only the period for which quantitativeevaluations of doses or risk are typically required, but also the limit of confident forecasting of
the tectonic framework of the Japanese islands, before major restructuring might be expected.
However, as the regulatory framework for geological disposal is still under development inJapan, NUMO wished to prepare a measure for providing information out to longer periods –up to 1 million years – again in the form of probabilities (or ‘likelihoods’) that could be used toexpress degrees of belief to alternative forecasts of the evolution and fate of a repository.

1.3

Going Beyond 100,000 years: the TOPAZ project

Consequently, following the completion of the ITM project, NUMO proposed that the expert
team involved look at the possibilities and constraints of making forecasts of the likelihoodand nature of tectonic impacts for longer periods into the future, from 100,000 years out toone million years.
The ‘TOPAZ’ project (Tectonics Of Potential Assessment Zones) was developed to see how
the ITM methodology could be extended for this purpose. The approach developed is anextension of the concepts developed in the preceding ITM project. Although intended forlooking at very long times into the future, it is important to recognise that the approach is not
limited to this period and can be deployed for the whole period over which hazard
assessment is required. The main steps of the TOPAZ methodology (see Figure 1.3) involvethe development of alternative conceptual models characterising how the tectonic situation ina region might develop in the future and attaching expert degrees of belief to these alternative‘Regional Evolution Scenarios’ (RES), using a formal expert elicitation methodology. These,in turn, are used to develop ‘Site Evolution Scenarios’ (SES), which describe how an RES
might ‘play out’ at a specific location within the region being evaluated.
The ITM Kyushu Case Study (KCS) illustrated the importance of considering alternative
framework models of evolution, especially in complex regions. In the KCS, the time period
being assessed was 100,000 years but, owing to the complexity of the tectonic regime(compared to Tohoku, for example), regional evolution scenarios were considered, even forthis timeframe. Thus, the KCS initiated thinking on the RES approach for TOPAZ and also led
the expert team to consider that the TOPAZ methodology could be especially useful forregions with complex tectonics when considering any period of time into the future.

Figure 1.3: The main steps of the TOPAZ methodology

5

Stage 1 of the TOPAZ methodology thus involves the identification of credible alternative
tectonic evolution scenarios at a regional level that incorporate different views on how major
plate driving forces or geometries may change over a period of 1 million to a few million years,
or on how crustal and upper mantle units may respond to such changes. These RES are2developed at a scale typically of the order of 100,000 km (the same area as the ITM Case
Study regions). The procedure is as follows:
1. Identify region to be evaluated (typically of the order of 100 x 100 km around apotential repository siting area) – for the purposes of developing and demonstrating
the TOPAZ methodology, the ITM Tohoku Case Study area was used as an example;
2. A set of RESs is proposed by one or more experts, for review and analysis by a wider
group. The set should encompass know alternative views on tectonic evolution,
including any ‘outlier’ opinions;
3. A group of experts reviews the set and adds any alternative RESs that it considers
might be plausible, based upon current scientific knowledge;
4. The group is asked to provide relative weights to each RES, such “weights” being
quantitative expressions of their belief that any particular RES is the most likely, or
the relative likelihoods of alternative RESs for the next 1 to a few million years: this isdone by a formal process of expert elicitation.
The development of a set of RESs for the Case Study area used in the TOPAZ project is
described in Section 2.
A key feature of TOPAZ methodology is to incorporate the range of opinions of a wide groupof experts in an objective manner to derive quantitative estimates of relative probabilities(‘weights’) of each RES (and, subsequently in Stages 2 and 3, of other key scenarios). Thisis done by a formal process of Expert Elicitation. This is a critical step as it allows NUMO tosolicit expert opinion from across the Japanese and international geoscience communities, aswell as to ensure that the full range of opinion is incorporated into its site-suitability decisionmaking procedures. The establishment and testing of the expert elicitation approach and its
applications to the set of RESs for the Case Study area used in the TOPAZ project is
described in Section 4.
Stage 2 of the TOPAZ Methodology looks at the evolution of a specific site of interest(potential repository site) in response to the RESs. As shown in Figures 1.3 and 1.4, in the
TOPAZ project, an example site was chosen from the set used in the ITM-TCS.

Figure 1.4: The example location used in TOPAZ to demonstrate the methodology: location ‘C’ from the
ITM Tohoku Case Study.

6

As with ITM, sites are defined as being 5 km x 5 km in size, which allows the hazard mappingdeveloped in ITM for this Case Study to be used for TOPAZ. As discussed below, however, it
may also be necessary to consider applying the ITM-style hazard analyses at a greater scale
to account for peripheral impacts on the repository. The procedure for Stage 2 is similar tothat for Stage 1 and is as follows:
1. Apply each of the most highly weighted RESs to the site and develop a descriptive
Site Evolution Scenario (SES) that postulates how the site will respond to theconditions imposed by the RES – this description will be in terms of whether specifictectonic events and processes become more or less likely or dominant in theevolution of the site. Each RES might generate one or more SESs;
2. Define Impact Scenarios (ISs) for each SES, identifying how specific impacts ofrelevance to the safety case might evolve or occur in response to each SES. This isfacilitated if the safety assessment analysts are able to specify events that may be ofconcern or interest for them to analyse (i.e. omitting those of little significance to theperformance of the repository). For instance, an IS might be development of a new
volcano at the site, leading to occurrence of a volcanic event greater than a certain
magnitude, or evolution of a fracture system to an active fault generating
displacement greater than a certain magnitude.
3. Use the expert elicitation approach described for Stage 1 to assign quantitative
degrees of belief (weights) to each SES and IS.
Stage 3 of the methodology integrates RES, SES and IS weights from Stage 2 with the ITMderived probabilities of events (from the regional hazard maps developed using the ITM
methodology) to produce an overall probability for each IS. The approach can be represented
by a series of branches in a logic tree, as shown in Figure 1.5 below:

Figure 1.5: Estimating the probability of an Impact Scenario for a specific site (‘Site A’) by means ofweighted degrees of belief in alternative scenarios on a logic tree.

In a comprehensive analysis, such as shown in Figure 1.5, a wide range of ISs might be
developed, although with numerous redundancies of ISs expected along the many RES-SES
branches. As many ISs will be very similar, or their consequences for the repository will bebroadly equivalent, they can be amalgamated into ‘bins’ in a complete assessment (withindividual probabilities of common ISs being added together).
For the purposes of developing and demonstrating the TOPAZ methodology, not all ISs wereidentified for the expert elicitation process, and a reduced set of three was identified for7

consideration (one each for volcanic intrusion, fault displacement and uplift/exhumation), with
only two of these being propagated through to a full probabilistic analysis for the Case Study
Example Location. For the three ISs, four scenario logic trees were developed (see Section3), as follows:•

Logic Tree 1: volcanic activity of any type and magnitude occurring within a 5 km x 5
km area at the site;•

Logic Tree 1: an explosive volcanic event > VOGRIPA M5 occurring within a 5 km x
5 km area at the site;•

Logic Tree 2: rock deformation occurring that leads to a Mw 6.5-7.5 earthquake at<10 km depth within the 5 km x 5 km area of the site;•

Logic Tree 3: rock deformation or tectonic uplift occurring that leads to point
estimates of partial exhumation of the repository exceeding 200 m.

The two that were selected for propagation through the full TOPAZ methodology concerned
new volcanic intrusion and fault displacement, and are defined in more detail as:•

New volcano formation occurring within a 5 km x 5 km area at the site:•from this, using the VOGRIPA frequency v. magnitude database, theprobability of an explosive volcanic event > VOGRIPA M5 occurring within a
5 x 5 km area at the site is estimated.

A shallow (<5 km) M 6.5-7.5 earthquake occurring on a concealed, undetected fault
within the 5 km x 5 km area of the site that could be expected to propagate into therepository rock volume (approx. equivalent to a 0.3 m displacement).

The rationale for the selection of an M 6.5-7.5 earthquake on an undetected fault was
discussed at a TOPAZ workshop on concealed active faults, held in Christchurch, New
Zealand in January 2012, a note of which is included as Appendix to this report.
The rock deformation hazard component estimates the likelihood of an earthquake Mw 6.5 orlarger (up to a prescribed Mmax value, which varies from region to region) impacting therepository. If an extremely large (M9 or similar) earthquake is deemed plausible within agiven region, this possibility can be easily accounted for by raising the Mmax value to 9 orlarger. However, current understanding is that M9 earthquakes are probably restricted to
subduction interface faults, which have the extremely large surface area required for such a
giant event. Most, if not all, onshore crustal faults probably do not have enough surface area
to support the occurrence of such a huge earthquake.
Effectively, the demonstration work in TOPAZ has identified ISs in advance, rather than
generating all potentially relevant ISs via the expert elicitation process, as is shown in Figure1.5. This identification has been done from first principles, informed by known sensitivities ingeological repository safety cases. It is emphasised that, at a ‘real site’, the morecomprehensive approach involving Japanese geoscientists would be required. The‘demonstration approach’ for a specified IS in Stage 3 is illustrated in Figure 1.6, which shows
the important interplay between the team developing the scenario and impact probabilities
and the team carrying out safety assessment.

8

Figure 1.6: How an impact scenario probability (here, defined as the probability of occurrence of a
specified magnitude of event) is derived by combing RES and SES weights with ITM-derived
probabilities.

As noted above, the ITM hazard maps are based on estimated probability values in each 5km x 5 km square. This roughly represents the possible size or ‘footprint’ of the geologicalrepository. For some tectonic events, safety assessors will be interested in a ‘near miss’
event on the repository – that is, the probability that an event could occur close enough to the
5 km x 5 km repository footprint to impact its performance. Figure 1.7 shows an example ofthe formation of a new volcanic vent close enough (15 km in this example: equivalent to
NUMO’s EFQ exclusion factor for proximity to an existing volcano) to the repository that it
could affect its performance. As discussed in NUMO (2004), it is generally considered thatperipheral thermal, hydrogeological, gas and minor intrusion impacts of a new vent that could
occur outside this radius would not be significant enough to consider in the first stage ofrepository siting – hence the value has been used as an initial Nationwide Evaluation Factor.
It can be seen that, to avoid the 15 km exclusion zone use din this example, performanceassessors would need to know the probability of a new vent being formed with an area about
50 times larger than the repository footprint. This can be defined as an interest in probability
values that are 1.5 orders of magnitude (50 times) lower than the probability estimated for asingle 5 km x 5 km area.

9

Figure 1.7: The increased area of interest for probability estimation (white square) that would be
required to account for the peripheral impacts of a new volcanic vent formation (see text for further
description).

1.4This report

This report presents a demonstration of the methodology described in Section 1.3. It isemphasised that this is a partial demonstration, as it was not possible to explore all aspects of
the approach. For example, only a sub-set of safety-relevant ISs was addressed and expert
elicitation exercises were not carried out on all steps in the methodology. This was also thefirst time that a structured expert elicitation methodology had been used by the Japaneseradioactive waste community, so this approach was novel in its own right. Also, it should be
noted that the team did not have any of the site-specific stratigraphic, lithographic or structural
information that would be obtained during an initial literature survey site investigation phase.
The output of a complete ITM-TOPAZ analysis of a site will be an essential underpinningcomponent of any NUMO safety assessment and will comprise the following main elements:••••a complete description of the tectonic situation of a site;
alternative conceptual models of how that situation might evolve over the period ofinterest to the safety assessment;
a comprehensive list of impact scenarios for transfer to repository consequence
analysis;the relative probabilities of each IS affecting the repository, which can beincorporated into a risk-based safety assessment and used to generate either
radiological risks (to compare with regulatory standards) or probabilities ofoccurrence of scenarios to present along with dose consequences in a disaggregateddose-likelihood approach to regulatory standards.

Regardless of the direction that regulatory standard development takes in Japan, thisinformation will be a central part of NUMO’s safety case.

1.5

TOPAZ International Project Team Members

The Project involved the close interaction of Japanese and international experts to gain thenecessary understanding to carry out the methodology development and testing. Theinternational project team members all worked on the ITM project.
The international experts came from the following organisations (the acronyms used in thisreport are indicated):

10

MCM
UBR
USF
UTX
GNS
INT
I2E

MCM Consulting, Switzerland
University of Bristol, UK
University of South Florida, USA
University of Texas, USA
GNS Science, New Zealand
INTERA, USA
In2Earth Modelling Ltd, Switzerland

TOPAZ International Project Team Members
Rock Deformation

Dr Kelvin Berryman
Dr Mark Stirling
Dr Laura Wallace

GNS Science, New ZealandProfessor Mark Cloos

University of Texas, USA

Professor Charles Connor
Laura Connor
Koji Kiyosugi

University of South Florida, USA

Professor Steve Sparks

University of Bristol, UK

Dr Olivier Jaquet

In2Earth Modelling, Switzerland

Interface with PA

Dr Mick Apted

INTERA, USA

Project Co-ordination

Professor Neil Chapman
Ellie Scourse

MCM Consulting, Switzerland

Volcanism

112

Regional and Site Evolution Scenarios

As described in Section 1, the TOPAZ methodology first establishes a range of plausible
Regional Evolution Scenarios (RES) considered to capture the range of evidence-based
expert views on the principal evolution trends for the region of interest and then applies theseat the site scale to develop an equivalent set of Site Evolution Scenarios (SES) in response to
the different RESs. The RESs developed here are intended as examples to demonstrate the
RES approach. We base the example RESs on plausible future scenarios for Tohoku
tectonics, based on our understanding of how tectonics have evolved in Tohoku in the past.
When the RES approach is eventually applied to a specific site, more exhaustive RESs
should be developed by a panel of experts, who would also ensure the provision of more
extensive and robust supporting data for the scenario development. This Section describesthat process for the Tohoku region and the four example sites selected for demonstration
purposes.

2.1

Tectonic Setting of Tohoku

Since the advent of plate tectonic theory, the Tohoku region, in northern Japan, from 35°N to
43°N, has been widely recognised as the type example of an active subduction zone. The
northern Japan subduction zone, like nearly all others, can be subdivided into the trench axis,
trench slope, forearc, volcanic arc, and back-arc regions. The tectonic activity associatedwith subduction comes in the form of faulting, magmatic intrusions and extrusions and verticalmovements that range from folding to tilting of fault blocks to regional warping.
The Tohoku region forms most of northern Honshu. This region is an area of land primarily
because of the interaction between the Pacific, North American, and Eurasian plates(Takeuchi et al., 1970; Taira, 2001). Tohoku and Hokkaido Island are on a prong of the North
American plate that projects southward from the Bering Sea region. Geologic studies longindicated and geodetic studies recently confirmed (Wei and Seno, 1998; Heki et al., 1999)
that the prong of the North American plate is moving slightly differently (generally a few mm/a
differential motion) than the main part of the plate. These small differential movements resultin the recognition of this plate segment as the Okhotsk subplate. Similarly, the segment of
the Eurasian plate containing the crust of the Sea of Japan has a small differential movement,
resulting in the recognition of the Amur subplate. From east to west, the Pacific platesubducts beneath the Okhotsk subplate, which in turn converges with the Amur subplate. A
mountainous belt, including several large volcano clusters runs roughly along the centerline of
Tohoku and is commonly referred to as the Ou Backbone Range.
The present phase of westward dipping subduction began at about 130 Myr (Sugimura andUyeda, 1973; Engebretson et al., 1985). Cretaceous and Cenozoic intrusives and volcanicsof the northern Honshu island were emplaced into and on top of several different basement
terranes. Early Cretaceous plutonics are most abundant near the eastern coastline and occur
offshore (Finn, 1994). Late Cretaceous arc magmatism appears to have been concentrated
near the west coast of Honshu. These magmatic rocks were down-dropped during the rifting
associated with the opening of the Sea of Japan and mostly buried by sediments (Finn et al.,1994). The volcanic arc appears to have maintained its present position since about 20 Myr.
Crustal thickness beneath most of northern Honshu is between 30 to 35 km.
2.1.1

Basement of Tohoku

The basement of Japan is largely composed of Paleozoic and Mesozoic rock terranes
created during westwards subduction along the eastern edge of Asia (Taira, 2001). The
northernmost part of Honshu is underlain by the Oshima Belt, a Jurassic accretionary prism.
Central Honshu is underlain by the Ashio Belt, largely composed of Triassic to Jurassic rocks
that are variably deformed and little metamorphosed accretionary prism materials and the
Abukuma Belt which is thoroughly metamorphosed under high-temperature / low-pressure
conditions. Between these two Mesozoic terranes is the Kitakami Belt, a varied mixture of
sedimentary, igneous and metamorphic rocks of Paleozoic age. This juxtaposition makes it
appear that the early Mesozoic history of subduction and accretionary prism growth was
followed by an episode of transform faulting (Taira, 2001). Strike-slip faulting shuffled the12accretionary prism - forearc basin terranes (Abakuma-Ashio and Oshima belts) with the arcbasement terrane (Kitakami Belt) that had formed along the edge of the Asian continent.
2.1.2

Forearc Region

The subducting Pacific plate directly underlies the forearc region. The geology of the forearc
region of northeast Japan records a long history of subsidence and volcanogenic sedimentaccumulation. Because the upward flow of heat from the descending plate and through theforearc block is slower than the speed of subduction, geothermal gradients are very low
across the forearc region (Honda, 1985).
High-pressure/low-temperature blueschist
metamorphic conditions are present at depth and the crystalline basement underlying theforearc region is cold and relatively strong. This forearc region has a rather smooth andcontinuous slope down to the Japan Trench and the forearc basin sediments as old as
Cretaceous is little deformed.
One of the major discoveries, only possible because of the DSDP/ODP coring program, isthat the forearc region of Northeast Japan is largely one of non-accretion during the Cenozoic
with slow subsidence resulting from subduction erosion of the base of the hanging wall block
above the descending Pacific plate (von Huene and Lallemand, 1990; Heki, 2004).
2.1.3

Volcanic Arc Region

The area around active volcanic arcs is a region of high heat flow because heat is advected
to shallow levels in the subduction-generated magmas. Recent seismic tomography studies
indicate most of the active volcanoes in northeast Japan are centred above areas with slow
seismic velocities in the upper mantle and crust (Tamura et al., 2002). The current volcanicarc position was established at about 20 Myr (Kondo et al., 2004). The lifespan of typicallarge arc-type composite volcanoes appears to be about 1 to 2 million years (Davidson and
De Silva, 2000) and numerous volcanic edifices have formed and decayed since the middle
Miocene. Volcanic arcs are noted for giant composite volcanoes, but surrounding clusters ofsatellite cones and flows are typical. Along the northeast Japan arc, ten volcanic clusters
about 50 km wide are separated by gaps between 30 to 75 km wide (Tamura et al., 2002).
Volcanic arc regions are areas of relatively thin, weak lithosphere. Many are areas with little
non-volcanic tectonism, but some are areas with episodes of shortening or extension.
Northern Honshu has a mountainous spine, the Backbone Range, that is largely a product ofconvergent deformation since the early Pliocene, but perhaps somewhat earlier (Nakajima etal., 2006).
2.1.4

Backarc Region

The geology of the backarc and arc region of northern Japan records a history of a recent
profound change. Until mid-Cenozoic time, the crust forming the bulk of the basement ofJapan was the edge of the Asia continent. The Japan Sea formed by continental rifting which
evolved into seafloor spreading with ocean crust formation between 23 to 14 Myr (Jolivet etal., 1994). This rifting event occurred along the line of late Cretaceous volcanism, which
paralleled the western edge of Honshu.
Additionally, backarc areas commonly have a few volcanoes that are tens of kilometres
behind the line of the arc. The backarc area of Tohoku contains several active volcanic
clusters. They are volumetrically minor, but others probably existed since the current phaseof arc stability began in the Miocene.
Volcanic arcs that became a region of divergence that evolved into seafloor spreading haveoccurred at several sites along the western margin of the Pacific basin. Major episodes ofbackarc spreading have occurred along Izu-Bonin-Mariana and Tonga-Kermadec subduction
zones. In the backarc regions of these oceanic subduction zones, lines of extinct volcanoes
parallel the current volcanic arc with a region of new ocean crust in between.
It is evident that rifting is localised to the part of the lithosphere that was sufficiently weakened
by the ascent of magmas that a change in the force balance near the subduction zone led tolithospheric divergence that was centred on the arc. Backarc spreading is a subordinate
process to subduction. During the time seafloor spreading created a 200 km width of new13

ocean crust beneath the Japan Sea, an approximately 2000 km width of Pacific plate wasconsumed by subduction at the Japan Trench.
In contrast to the low heat flow of the Tohoku forearc region, the backarc region is an area ofhigh heat flow because of magmatism and the rifting that thinned the lithosphere. The
Miocene phase of rifting that led to the phase of seafloor spreading that formed the Sea ofJapan created many normal faults in the basement. Some of these high-angle faults havebeen reactivated since the latest Pliocene as reverse faults generating large folds in theoverlying Miocene and younger strata. A long period of slow subsidence and sedimentaccumulation has become a recent period of uplift and erosion.
2.1.5

Active Tectonics

Most of the active tectonic movements in the region are directly a response to subduction
along the plate interface zone that surfaces at the Japan Trench. This movement, at a speedof 9 cm/yr, causes the frequent earthquakes that form the inclined Wadati-Benioff seismiczone beneath northeast Japan and the belt of arc volcanism along the Backbone Range. The
disastrous March 11, 2011, M9.0 earthquake is the most recent manifestation of subduction
along the Japan Trench and is probably the largest seismic event in the area since 869AD(Minoura et al., 2001). As slip at the focus of the earthquake was on the order of 30 toperhaps 50 m, this event accounts for about 300 to 500 years of plate convergence (Simons
et al., 2011). Whether several centuries of plate motion must occur before mechanicalasperities lock a subduction zone and begin to build up large elastic strains that can lead tothe next large ‘megaslip’ event is a matter of much interest.
A significant secondary interaction that has affected the Tohoku region results fromconvergent deformation that has been distributed between the eastern margins of the Sea ofJapan and the Backbone Range. This ~200 km wide region accommodates significant
folding with reverse faulting at several areas near the west coast of northern Honshu and abroad region of uplift and high-angle faulting forming the Backbone Range along the line ofthe volcanic arc. Across this region, convergence is occurring at speeds of at least 10 mm/a
to perhaps 15 mm/a (Tamaki and Honza, 1985; Okamura et al., 1995; Sagiya et al., 2000;
Miura et al. 2002). Folded and faulted strata indicate the current phase of crustal shortening
began at ~4 Myr and that the zone of deformation appears to have widened from west to east.
The westward limit of shortening in the Sea of Japan is well defined as a deformation front.
Most earthquakes in the region of the volcanic arc and backarc occur at depths less than 15
km (Zhao et al., 2000). Seismicity is sparse and shallow in areas of high heat flow and nearlylacking beneath volcanoes. A distinct belt of seismicity and surface faulting extends along thewestern coast of northeast Japan north and south of Niigata. Scattered reverse-slip
earthquakes as large as M7.8 nucleate at depths of 10 to 20 km (Okubo and Matsunaga,
1994). Faulting and folding in the belt is a result of > 1 cm/yr convergence between Amur andOkhotsk subplates. This area of active folding, reverse faulting, uplift and erosion was anarea of subsidence during the Miocene phase of rifting.
Normal faults created during Miocene rifting were buried by volcanogenic sediment shedwestward into the widening depression. The convergent deformation in the Niigata region
began in the late Pliocene, at about 3 Myr (Okamura et al., 1995). The crystalline basementin this region primarily deforms by reversing the movement on normal faults created during
the rifting phase – "inversion tectonics." The overlying sediments respond to the deformation
primarily by drape folding over the rising fault blocks. Geometrical complexities and new
breakouts occur in many areas because the old fault system does not everywhere have
optimal orientations for the imposed movements.
Some convergence is occurring within the region of the volcanic arc. These movements havecreated the Backbone Range, a topographic divide that has profoundly affected regional
drainage and sedimentation patterns (Sato, 1994). Between volcanic clusters, some of theshortening occurs by episodic earthquake-generating stick-slip movement along reverse
faults (Zhao et al., 2000). Directly beneath most volcanic clusters, the temperatures are high,making much of the crust ductile, hence, there is little seismicity. Especially rapid uplift of the
Backbone Range began at about 2 Myr (Sato and Amano, 1991).
In summary, the tectonic environment of northern Japan is dominated by subduction at theJapan Trench causing arc magmatism with second-order convergent motions that are
14

concentrated in the backarc region, but extend into the area of the arc. The current mode oftectonism in northern Honshu began at about 4 Myr and has been well established since 2
Myr (Taira, 2001). The initiation of this tectonic regime is probably a manifestation of thecreation and movement of the Amur and Okhotsk subplates. Why these convergent
movements began, which are secondary compared to fast, long-term subduction at the Japan
Trench, is not known. They may be a result of a modest reorientation of plate motions about
the Pacific basin that occurred between 5 to 3 Myr (Cox and Engebretson, 1986; Pollitz,1986).

2.2

Overview of Regional Evolution Scenarios for Tohoku

For the purposes of demonstrating the RES approach, we assess Regional Evolution
Scenarios for three time periods of interest: 0-10 kyr, 10-100 kyr, 100 kyr – 1 Myr. Other
alternative Regional Evolution Scenarios could also be developed for the Tohoku region,
although we suggest that the four presented here are a representative range of plausible
scenarios for the future tectonic evolution of Tohoku.
2.2.1

RES 1: Tectonic boundary conditions are the same from the present to 1 Myr

In this scenario, we assume that the overall tectonic boundary conditions in the Tohoku
region remain the same as they are today for the next 1 Myr (Fig. 2.1). As part of this
scenario, we assume that the rate of permanent plate boundary deformation occurring within
the upper plate (>15 mm/a) remains similar to what it is today, and that the subducting Pacificslab maintains its current configuration (configuration defined by Hasegawa et al., 1994) withrespect to the upper plate. We also assume that the crust underlying the Sea of Japan
continues underthrusting beneath Honshu at its current rate of ~15 mm/a, eventually evolving
into a subduction margin (cf. Tamaki and Honza, 1985). The results of this RES for thevarious timeframes are shown schematically in Figure 2.2.

Figure 2.1: Tectonic setting and GPS velocity field in Northern Honshu (shown relative to a fixed
Eurasian plate). Thinner grey lines are active fault traces. Ellipses show uncertainty at 65% confidence
level. Velocities are derived from daily solutions provided by Geographical Survey Institute of Japan.
Block boundaries (heavy grey lines) used in the elastic block modelling to interpret GPS velocity field interms of long-term block motion (Pacific, Amurian, and northern Honshu Block) and fits to the GPS
velocities (red arrows) from the best-fitting model (black arrows) (from Wallace et al., 2009). Arrows on
the boundaries with numbers beside them (mm/a) show the motion of the Northern Honshu block
relative to the Amurian and Pacific Plates. Interpretation of the GPS velocities also suggest that 3-4mm/a of contraction occurs in the Backbone Range of northern Honshu (Miura et al., 2004; Wallace etal., 2009).

15

Although this scenario assumes relatively constant tectonic boundary conditions, it could leadto some degree of temporal and spatial change in the distribution of faulting and volcanismwith time. For example, as the convergent margin and associated fold and thrust belt evolves
near the west coast of Tohoku, more localization of deformation could be expected onto
individual structures, and/or spatial changes in distribution of active deformation could occur
as a result of the natural evolution of the fold and thrust belt there. This could lead to
substantial changes in rock deformation strain and uplift rates at particular locations out tolonger time periods. It is also likely that optimally oriented bedrock faults (i.e., approximately
north-south striking) could become reactivated. Although we expect the location of volcanismin the arc to be relatively stable, it is possible that volcanism could move into the “gap” region
between volcanoes, possibly driven by temporal and spatial changes in magmatic supply andmantle wedge flow regimes over longer timeframes (although we consider a stable location
for the magma supply and subsequent volcanism more likely).
•

Present day – 10 kyr scenario. We do not expect this scenario to be significantly
different from what is observed today, particularly in terms of the location of
deformation, uplift, and active volcanism.

•

10 – 100 kyr scenario. We do not expect this scenario to be significantly different
from the present day to 10 kyr scenario, although it is possible that some spatialchanges in the rates and location of volcanism, deformation, and uplift could begin tooccur during this longer time period, as fault and volcanic systems evolve. We
anticipate that most of the contractional deformation accommodated in the upper
plate during this period will continue to occur within the Backbone Range
contractional domain (Hasegawa et al., 2000) and along the evolving convergent
margin adjacent to the Sea of Japan. Volcanism is unlikely to migrate significantly
outside of the current location of active volcanism due to the stability of magma
supply systems.

•

100 kyr – 1 Myr scenario. Over longer time periods it is possible that some spatial
and temporal changes in the distribution of volcanism and rock deformation could
occur, but we anticipate that rock deformation will still remain focused within the
Backbone Range and the western part of Honshu. It is possible that migration of arc
volcanism into the gaps between the currently active volcanoes will occur start during
this period, but it is unlikely that active volcanism will have migrated significantly
within or across the margin (for example, into the forearc domain). If the Sea of Japan
convergent zone deformation becomes progressively localised and evolves towards
an incipient subduction margin, it is possible that larger portions of the convergent
component of plate motion will be shifted into the Sea of Japan region, possibly
localizing on structures just offshore. The future localization of slip in the Sea ofJapan region will depend on the details of evolution of the associated fold and thrustbelt, and the distribution of optimally oriented bedrock structures. If the Sea of Japan
is evolving into a subduction margin (e.g., Tamaki and Honza, 1985), we expect that
1 Myr into the future the leading edge of the underthrusting Sea of Japan crust will
have migrated eastward by ~15 km, and will have reached ~37 km depth, comparedto ~30 km depth that we assume for its current depth (note that the location of theleading edge of the underthrusting Sea of Japan crust is highly uncertain).

16

Figure 2.2: Schematic of major tectonic domains and likely evolution of faulting and volcanism into the
future for all 3 stages of RES 1. The light green highlighted region indicates the evolving contractional
domain associated with underthrusting of the Sea of Japan, while the light orange highlighted areashows the currently active arc and the Backbone Range contractional domain. The dashed black line
represents schematically the current likely leading edge of the underthrusting Sea of Japan crust, while
the red line shows the likely position of the leading edge at the end of each timeframe. Red triangles are
Quaternary volcanoes and black lines are active faults.

17

2.2.2

RES 2: The amount of relative plate convergence accommodated in the upper
plate doubles from present to 1 Myr in the future

In this scenario we assume that the amount of permanent horizontal convergenceaccommodated within the upper plate doubles from its current value (~15 mm/a) in a linear
fashion to ~30 mm/a at 1 Myr into the future. This is similar to the increase in upper platedeformation that occurred in northern Honshu around 2.4 Myr (Sato and Amano, 1991).
There are a number of potential triggers for this scenario, including: (1) an increase in theconvergent component of Pacific/Eurasian plate motion in the Tohoku region (this could occur
by an anti-clockwise shift in Pacific-Eurasia relative motion vector, or by acceleration of the
motion between the two plates); (2) the subducting Pacific slab flattens its subduction angle(similar to what currently occurs along parts of the Andean margin), causing transmission of alarger component of plate boundary strain into the upper plate; (3) the Sea of Japan
convergent zone evolves into a self-sustaining subduction plate boundary with a negatively
buoyant slab, allowing a progressively larger amount of the plate motion budget to beaccommodated in the Sea of Japan region. In this RES (see summary in Fig. 2.3) we expectthat most of the rock deformation will still be accommodated in the Backbone Range andwithin western Honshu, but it is very likely that any optimally oriented bedrock structures(approximately north-south striking) anywhere in Tohoku could be reactivated toaccommodate these changes in plate tectonic boundary conditions. In this scenario, it isplausible that rock deformation rates and uplift rates could double in the regions of current
active deformation. Increased uplift and erosion in the areas of most intense shortening could
lead to decompression melting of the crust and triggering of ignimbrite and caldera formation.
For the most part there is unlikely to be a major change in the position of the volcanic arc inthis RES during the time period of interest. However, if an onset of flat slab subduction occurs,
the volcanic arc could migrate westward of its current position (Fig. 2.3). Perturbation of theflow regime in the mantle wedge is also likely to occur in the flat slab scenario, as well as the
scenario where the Sea of Japan evolves into an early stage subduction margin. This should
be considered in the context of the volcanic evolution of the region. This RES could open up
the possibility of future volcanism anywhere in the Backbone Range and western northern
Honshu during the longer timeframes of interest.
•

0-10 kyr timeframe: We do not expect this scenario to be significantly different fromwhat is observed today, particularly in terms of the location of deformation, uplift, andactive volcanism.

•

10-100 kyr timeframe: We do not expect this scenario to be significantly different
from the present day to 10 kyr scenario, although it is possible that some spatialchanges in the rates and location of volcanism, deformation, and uplift could begin
occurring during this longer time period, as fault and volcanic systems evolve. We
anticipate that most of the contractional deformation accommodated in the upper
plate during this period will continue to occur within the Backbone Range
contractional domain and along the evolving convergent margin adjacent to the Sea
of Japan. The steady increase in horizontal convergent rates accommodated within
the upper plate during this period (from 15-17 mm/a) could cause rates of faulting on
known active faults to increase correspondingly, or for optimally oriented bedrock
structures to be reactivated to accommodate this. Volcanism is unlikely to migratesignificantly outside of the current arc.

18

Figure 2.3: Schematic of major tectonic domains and likely evolution of faulting and volcanism into the
future for all 3 stages of RES 2. The light green highlighted region indicates the evolving contractional
domain associated with underthrusting of the Sea of Japan, while the light orange highlighted areashows the currently active arc and the Backbone Range contractional domain. The dashed black line
represents schematically the current likely leading edge of the underthrusting Sea of Japan crust, while
the red line shows the likely position of the leading edge at the end of each timeframe. The heavy
dashed black contour shows the current position of the 100 km contour to the surface of the subducting
Pacific slab, while the heavy line shows the possible position of this at 1 Myr in the future if the increase
in upper plate shortening is related to a westward advance of the subducting slab.

19•

100 kyr – 1 Myr timeframe: During this period the convergent component of platemotion accommodated in the upper plate increases from 17-30 mm/a. During thistime we anticipate that rock deformation will still remain focused within the Backbone
Range and the western part of Honshu, with some reactivation of optimally oriented
bedrock structures across the Tohoku region. It is possible that migration of arc
volcanism into the gaps between the currently active volcanoes will occur during thisperiod. If a period of subducted Pacific slab flattening occurs, then the location ofactive arc volcanism could be expected to migrate westward from its current position(probably by no more than 20-40 km during this period). If the Sea of Japan
convergent zone deformation becomes progressively localised and evolves into anincipient subduction zone, it is possible that larger portions of the convergent
component of plate motion being accommodated within the upper plate could shift
into the Sea of Japan region during this time, possibly localizing on structures justoffshore. The future localization of slip in the Sea of Japan region will depend on thedetails of evolution of the associated fold and thrust belt, and the distribution of
optimally oriented bedrock structures. If the Sea of Japan is evolving into a
subduction margin, we expect that 1 Myr into the future the leading edge of theunderthrusting Sea of Japan crust will have migrated eastward by ~23 km, and will
have reached >40 km depth, compared to ~30 km depth that we assume for the its
current depth (note: this is not well-constrained).
The incipient Sea of Japansubduction zone scenario would significantly perturb the flow regimes and magma
supply within the mantle wedge, and we expect that this scenario could lead to a shut
down of active volcanism near the west coast of northern Honshu as theunderthrusting Sea of Japan encroaches on and perturbs the magma supply to thoseregions.

2.2.3

RES 3: Pacific-Eurasia relative plate motion becomes more oblique frompresent to 1 Myr in the future.

Pacific Plate absolute motion has undergone a number of changes throughout the Cenozoic(Wessel and Kroenke, 2007, and references therein). In particular, at ~6 Myr Pacific absolute
plate motion (APM) underwent shift in direction of motion, leading to more rapid northerly
absolute motion of the Pacific Plate, although in northern Japan the result was to shift PacificPlate APM to a more southerly direction (by ~15 degrees) (Wessel and Kroenke, 2000). It issuggested that this change occurred due to a change in plate boundary forces related to
Ontong Java Plateau collision with the Pacific/Australia Plate boundary in the western Pacificand ongoing subduction at the northern and western boundaries of the Plate (Wessel andKroenke, 2000, 2007). This change was not likely to be abrupt, and we expect that it occurred
more gradually over a period of ~1 Myr.
In this RES, we assess the possibility that Pacific Plate motion could take a similar shift in the
opposite direction over the next 1 Myr period, leading to more oblique relative motion withmore northerly-directed Pacific Plate motion in the northern Japan region. In RES 3, we
assume that the relative motion vector between the Pacific and Eurasian Plates undergoes a
15 degree anti-clockwise shift from present to 1 Myr in the future; the effect of this shift is tohave a larger northward component for the motion of the Pacific Plate relative to Eurasia.
This would shift the angle between relative plate motion and the direction perpendicular to thetrench (e.g., obliquity angle) from 10º (currently) to 25º at 1 Myr. At subduction plateboundaries such as Sumatra, partitioning of the margin-parallel component of relative platemotion onto strike-slip faults within the upper plate begins to occur when the obliquity angle
exceeds some critical angle (dependent on the strength of the upper plate, and the shear
stresses acting on the plate boundary; McCaffrey, 1991). In Sumatra and the Philippines, thiscritical angle is ~20-25º, while it is as high as 25-45º in Alaska (McCaffrey, 1991; McCaffrey etal., 2000). One of the major outcomes of an increase in plate motion obliquity suggested inthis RES is the potential for reactivation of bedrock structures in the Tohoku region such as
the Tanakura Fault and the Hizume-Kesennuma Fault (Fig. 2.4). These currently inactive
bedrock structures strike NNW, and are in an ideal orientation to accommodate a more
northerly component of Pacific-Eurasia relative plate motion. The results of this RES for thetimeframes are summarised schematically in Figure 2.5.

20

Figure 2.4: Bedrock and fault map from Otsuki (1992). This figure illustrates much of the pre-existing
bedrock faulting, including extensive NNW striking faults in the forearc region. It is possible that such
faults could become reactivated if there is a change in orientation of relative plate motion.

•

0-10 kyr timeframe: We do not expect this scenario to be significantly different fromwhat is observed today, particularly in terms of the location of deformation, uplift, andactive volcanism.

•

10-100 kyr timeframe: We do not expect this scenario to be significantly different
from the present day to 10 kyr scenario, although it is possible that some spatialchanges in the rates and location of volcanism, deformation, and uplift could begin
occurring during this longer time period, as fault and volcanic systems evolve. We
anticipate that most of the contractional deformation accommodated in the upper
plate during this period will continue to occur within the Backbone Range
contractional domain (Hasegawa et al., 2000) and along the evolving convergent
margin adjacent to the Sea of Japan. Volcanism is unlikely to migrate significantly
outside of the current volcanic arc.

21

Figure 2.5: Schematic of major tectonic domains and setting between 100 kyr – 1 Myr into the future for
RES 3. The light green highlighted region indicates the evolving contractional domain associated with
underthrusting of the Sea of Japan, while the light orange highlighted area shows the currently active
arc and the Backbone Range contractional domain. The dashed black line represents schematically the
likely leading edge of the underthrusting Sea of Japan crust at present, while the red line shows thelikely position of the leading edge at the end of each timestep. The dark green lines in the forearc
schematically show the location of some of the NNW trending bedrock structures, such as the Tanakura
fault.

22•

100 kyr – 1 Myr timeframe: As relative plate motion becomes more oblique to theplate boundary through this time period (the obliquity will increase from just above 10º
to 25º by 1 Myr), the potential for reactivating NNW trending bedrock faults in the
Tohoku region (Figs. 2.4 and 2.5) such as the Tanakura fault increases. We expectthat this will particularly come into play during the latter half of this time period (>500
kyr into the future). The point at which slip partitioning in the upper plate begins will
depend on the details of the strength of the upper plate and the shear stresses acting
on the plate boundary interface and other major faults in the system. Over longer
time periods it is possible that some spatial and temporal changes in the distributionof volcanism and rock deformation could occur, but we anticipate that most rockdeformation will remain focused within the Backbone Range and the western part ofHonshu, and along optimally oriented NNW trending bedrock faults such as the
Tanakura fault. It is possible that migration of arc volcanism into the gaps between
the currently active volcanoes will occur during this period, but it is unlikely that active
volcanism will migrate substantially relative to its current position, or migrate acrossthe margin (for example, into the forearc domain). If deformation in the Sea of Japan
convergent zone becomes progressively localised and evolves towards an incipient
subduction margin, it is possible that larger portions of the convergent component ofplate motion being accommodated within the upper plate could shift into the Sea ofJapan region, possibly localizing on structures just offshore. The future localization ofslip in the Sea of Japan region will depend on the details of evolution of theassociated fold and thrust belt, and the distribution of optimally oriented bedrock
structures. If the Sea of Japan is evolving into a subduction margin, we expect that 1
Myr into the future the leading edge of the underthrusting Sea of Japan crust will
have migrated eastward by ~15 km, and will have reached ~37 km depth, comparedto ~30 km depth that we assume for the its current depth.

2.2.4

RES 4: The amount of plate convergence accommodated within the upper platedecreases linearly to a neutral state at 1 Myr

Tectonics in northern Honshu throughout the Cenozoic have been dominated by alternating
periods of extension and contraction (Sato and Amano, 1991, among others). From 22-13
Myr, northern Japan tectonics was strongly influenced by back-arc rifting in the Sea of Japan,
while 13-2.4 Myr was a period of transition from upper plate rifting to upper plate contraction.
2.4 Myr to present has seen a shortening dominated regime in Tohoku upper plate tectonics.
Extension in the upper plate of the Sea of Japan region and northern Honshu was most likely
driven by the initiation of a phase of rollback of the subducting Pacific slab, as suggested forother plate boundaries worldwide (cf. Schellart and Lister, 2004, and references therein).
Oceanic crust subducting at the Japan Trench is Cretaceous in age, and is among the oldest
oceanic crust on earth. It is certainly plausible that the slab could founder and that an episodeof slab rollback could occur again sometime in the next few million years. RES 4 (see Fig.
2.6) considers the possibility that a phase of slab rollback and a shift to upper plate extension
could occur in the near future, and that over the next 1 Myr we could see a transition from thecurrent upper plate shortening regime to a more or less neutral state of stress in the upper
plate in the lead up to a phase of back-arc extension.
•

0-10 kyr timeframe: We do not expect this scenario to be significantly different fromwhat is observed today, particularly in terms of the location of deformation, uplift, andactive volcanism in Tohoku.

•

10-100 kyr timeframe: We do not expect this scenario to be significantly different
from the present day to 10 kyr scenario, although it is possible that some spatialchanges in the rates and location of volcanism, deformation, and uplift could begin
occurring during this longer time period, as fault and volcanic systems evolve and theplate tectonic boundary conditions begin to change. We anticipate that most of thepermanent contractional deformation accommodated in the upper plate during thisperiod will continue to occur within the Backbone Range contractional domain andalong the evolving convergent margin adjacent to the Sea of Japan. Volcanism isunlikely to migrate significantly outside of the current volcanic arc.

23

Figure 2.6: Schematic of major tectonic domains and likely evolution of faulting and volcanism into the
future for all 3 stages of RES 4. The light green highlighted region indicates the current contractional
domain associated with underthrusting of the Sea of Japan, while the light orange highlighted areashows the currently active arc and the Backbone Range contractional domain. The dashed black line
represents schematically the current likely leading edge of the underthrusting Sea of Japan crust, while
the red line shows the likely position of the leading edge at the end of each timeframe. The heavy
dashed black contour shows the current position of the 100 km contour to the surface of the subducting
Pacific slab, while the heavy line shows the possible position of this at 1 Myr in the future if the decrease
in upper plate shortening and shift to a stress neutral state is related rollback of the subducting Pacific
slab.

24

•

100 kyr – 1 Myr timeframe: During this time, a substantial decrease in thecomponent of plate motion accommodated as permanent deformation in the upper
plate will occur, going from ~15 mm/a to ~0 mm/a from 100 kyr to 1 Myr. This willlead to a gradual decrease in the magnitude of contractional deformation in the upper
plate. During the latter half of this timeframe (500 kyr to 1 Myr) the rates of rockdeformation and uplift will decrease compared to the present day. With the shift to an
extensional upper plate environment, volcanism could become more voluminous thanis currently observed. It is also likely that the gradual onset of a phase of slab rollback
would perturb the flow regime within the mantle wedge substantially, and this could
cause some migration of the arc and the current location of volcanism in Tohoku.
One possibility is that a small trenchward migration of the volcanic arc (resulting fromslab rollback) could occur. The amount of trenchward migration will depend on therate of slab rollback. Perturbation of the mantle wedge by the onset of slab rollback
would also disrupt the “hot fingers” (Tamura et al, 2002) in the mantle wedge that are
feeding the currently active volcanoes, and could lead to volcanism within the gap
areas between the active arc volcanoes.

2.2.5

Examples of additional aspects of tectonic evolution that could be integrated
into the RES.

The well-developed volcanic arc in central northern Honshu (coinciding with the Backbone
Range) has remained roughly in its current location since at least 10 Myr (e.g., Finn et al.,1994). Conceptual models have been developed to explain the distribution of arc volcanism innorthern Honshu, such as the “hot finger” model (Tamura et al., 2002). The hot finger model
helps to explain the clustering of volcanic centres along the arc (with non-volcanic gaps in
between) as well as some volcanism in the back-arc region of western northern Honshu. An
important consideration for Tohoku RES is how models such as the “hot finger model” might
be expected to evolve into the future, and how this will influence the locations of futuremagmatism and rock deformation.
It is also important to consider the impact that deep-seated processes could have on rates oftectonic uplift in Tohoku. For example, processes such as deep underplating of subducted
sediment can lead to rapid uplift at subduction margins (e.g., Walcott, 1987). In contrast,
deep basal subduction erosion can lead to subsidence of the margin, as has been suggested
for the east coast of northern Honshu by Heki (2004) in order to explain observed subsidence
at continuously running GPS sites. Assessment of the potential influence of these and otherdeeply seated subduction zone processes on the future vertical tectonic motion in the Tohoku
region can be incorporated into alternative RES and/or as options within the suggested RES
presented here.

2.3

Site Evolution Scenarios

We develop potential Site Evolution Scenarios for each of the 4 RESs (discussed in section
2.1) at four example sites (Figure 2.7). The Site Evolution Scenarios are shown as
storyboards in Figures 2.9 to 2.12 for example locations A-D. The site evolution scenarios
are intended to demonstrate visually how each of the RESs might manifest itself as a variety
of site evolution scenarios, which may impact the repository footprint during the time periodsof interest. These are largely intended to demonstrate the use of storyboards to developthese scenarios, and are not to be considered as complete or detailed descriptions of the
impact of the various RES at any of the example sites. Undoubtedly, there is a larger varietyof SESs that could be considered at these sites than are shown in this demonstration. Notethat location C is the only example that is used in the Expert Elicitation demonstration.
The SESs for this region are based upon geological studies that indicate the convergent
differential motion between the Eurasian/Amur plate/subplate and North American/Okhotskplate/subplate are at least 10 mm/a and may be 15 mm/a (Figure 2.8). Over a 10 kyrtimescale, this corresponds to differential horizontal movements across the region totalling
100 to 150 m. The resultant folding and faulting will cause surface uplift, perhaps highly
localised. As subplates are fragments of larger plates that are moving in ways that differ at
speeds measured in mm/a from the parent plate, it is probable that they can speed up or slow
down over timespans that are much less than 1 Myr.

25

2.3.1

Description of sites used in the SES demonstration

Figure 2.7: Example site locations in northern Honshu that are developed in the storyboard exercise.
The green dots show recent volcanic edifices, and the dashed lines are known active faults. The
numbers in the yellow boxes are the sites used in the ITM Tohoku Case study.

Location A: Site is in the forearc region of Tohoku, within early Cretaceous mafic volcanic
rocks and volcanogenic sedimentary material. Site A is within a relatively stable region ofslow uplift that has produced marine platforms cut into the previously deformed andfractured/faulted rocks. Coastal uplift suggests the possibility of an active fault offshore. Preexisting NNW trending inactive bedrock structures cut through the 5 x 5 km footprint. There isno nearby volcanism.
Location B: This example location is within the northern part of the stable forearc block,where the bedrock is composed of early Cretaceous, shale and sandstone, with thin layers ofchert. The site is within a relatively stable region of slow uplift that has produced marine
platforms cut into the previously deformed and fractured/faulted rocks. Coastal uplift suggests
the possibility of an active fault offshore.
Location C (used for the remainder of this methodology demonstration): Example
location C is located within the crest of a growing anticline, on the Sea of Japan Coast, within
the fold and thrust belt comprising the Sea of Japan contractional zone. The area is marked
by high historical seismicity rates. The location is north-west of the Sengan Cluster, in the
low-lying inter-cluster region, and the nearest volcano (Kampu Volcano) is 32 km distant. The
bedrock in this region is Mid-Miocene hard mudstone, with sandstone and acid tuff.
The strata exposed in the folds are mostly hard, Mid-Miocene mudstone, with beds ofsandstone and felsic tuff. The crystalline basement is probably variably deformed (bothfolding and faulting) and metamorphosed Mesozoic accretionary prism materials. The
Miocene rift fill near location C was as much as 6 km. The major folds extend for several tensof kilometres. Okamura et al. (2007) have shown that the folds in the Niigata area to thesouth are forming above steep reverse faults that are mostly reactivated normal faults formed
during Miocene rifting. A close association of deep-seated fault slip with near surface folding
is probable near location C. The region near location C has high seismicity. The nearest26

mapped active fault is located about 3 km north of the site. The style of faulting and folding in
the region indicates that blind thrusts are associated with flexural slip faulting.
Location D: is situated within the Backbone Range, within a volcanic ‘gap’ region and near toan active fault. Bedrock faults also exist within a few hundred metres of the location. The site
is within an actively deepening tectonic basin. It is situated within bedrock comprising Upper
Miocene tuff and sandstone.

Figure 2.8: Regional tectonic setting of the four example locations, with respect to major plateboundaries.

27

Figure 2.9: Storyboard for the SES for example location A.

28

Figure 2.10: Storyboard for the SES for example location B.

29

Figure 2.11: Storyboard for the SES for example location C.

30

Figure 2.12: Storyboard for the SES for example location D.

31

2.3.2Site Evolution Scenarios for Site A

See storyboard at Figure 2.9.
RES1•

SES1: Over the 1 Myr time period the platform continues to be gradually uplifted anderoded, achieving 30-300 m of uplift, and 2.5-25 m of erosion (assuming erosion
rates of 0.025 mm/a). Possible fracturing related to regional earthquakes, andchemical weathering promote degradation of the repository host rock over longer time
periods.

•

SES2: As for SES1, except with more rapid rates of uplift and consequent erosion.

•

SES3: Platform uplift and consequent erosion decreases to nil by 500 kyr, with only
30-90 m of uplift and less than 10 m erosion achieved by 1 Myr.

RES2•

SES1: As for RES1/SES1.

•SES2: As for RES1/SES1, except greater uplift of marine platform achieved by 1 Myr.

•

SES3: As for SES 2 above, except for even more rapid rates of uplift and erosion atlater stages, due to higher upper plate deformation rates and possible slab flattening.
If maximum uplift and exhumation rates are reached, the repository could be
exhumed.

RES3•

SES1: As for RES1/SES1.

•SES2: As for RES1/SES1, except that bedrock faults are reactivated in the 100 kyr -1
Myr timeframe to accommodate increasing plate motion obliquity. The repository ispotentially displaced by motion on these faults later in this time frame.

RES4•

SES1: As for RES1/SES1.

•SES2: As for RES1/SES3.

2.3.3

Site Evolution Scenarios for Site B

See storyboard at Figure 2.10.
RES1•

SES1: The repository lies within early Cretaceous bedrock and beneath a stablemarine platform on the east coast. The area continues its low and uniform rates of
uplift and erosion over the next 1 Myr. The main potential impacts on repository
integrity over time are bedrock fracturing due to shaking from regional earthquakes,
and chemical weathering in the near-surface bedrock, particularly along fractures.

•

SES2: As for SES1, except an upper plate fault offshore drives higher uplift andhigher consequent erosion rates.

•

SES3: Uplift and consequent erosion progressively slows to nil by 500 kyr. Regional
earthquakes and chemical weathering gradually weaken the bedrock surrounding/above the repository.

RES2•

SES1: As for RES1/SES1, except slightly higher uplift rate and total uplift of theplatform by 1 Myr.

•

SES2: As for RES1/SES1, except greater uplift of marine platform achieved by 1 Myr.

32

•

SES3: As for SES 2 above, except for even more rapid rates of uplift and erosion atlater stages, due to higher upper plate deformation rates and possible slab flattening.
If maximum uplift and exhumation rates are reached, the repository could be
exhumed.

RES3•

SES1: As for RES1/SES1.

RES4•

SES1: As for RES1/SES1.

•SES2: As for RES1/SES3.

2.3.4

Site Evolution Scenarios for Site C

See storyboard at Figure 2.11.
RES1•

SES1: The repository lies within a growing anticline in mid Miocene sedimentaryrocks. Over the 1 Myr timeframe the anticline becomes tighter, and fault displacement
accumulates at the margins, as well as secondary faulting, and possible flexural slip
faulting within the repository footprint. Up to 350 m of vertical erosion results inexhumation of the repository within the 100 kyr - 1 Myr timeframe. Volcanism could
also initiate nearby within that same timeframe.

•

SES2: As for SES1 except more rapid uplift and erosion rates result in more rapid
rock deformation and repository exhumation.

•

SES3: Anticline development and associated uplift decreases over time asconvergent deformation becomes increasingly localised offshore. Vertical erosion
over the 100 kyr - 1 Myr time period reduces the anticline by up to 250 m.

RES2•

SES1: As for RES1/SES1.

•SES2: As for RES1/SES2.

•

SES3: As for SES2 above, but slab flattening in the vicinity of the west coast resultsin even more intense rock deformation in the repository area. Uplift of 100-1500 m,
and vertical erosion of up to 1400 m results in repository exhumation in the 100 kyr 1 Myr timeframe. Possible initiation of volcanism nearby.

RES3•

SES1: As for RES1/SES1•

SES2: As for RES1/SES2•

SES3: As for RES1/SES3

RES4
•

2.3.5

SES1: As for RES1/SES3

Site Evolution Scenarios for Site D

See storyboard at Figure 2.12.
RES1•

SES1: The repository lies within Miocene sedimentary rocks, and beneath the centreof an actively developing basin. Over the million-year timeframe the basin slowly
deepens as a result of progressive fault displacement at the basin margins. Basin
margin volcanism could also initiate in the 100 kyr - 1 Myr timeframe, if the arc

33

volcanism migrates into the volcanic gap region. Negligible erosion rates do not alter
repository depth throughout the 1 Myr time frame of basin development.
•

SES2: As for SES1, except that accelerating rock deformation rate over time
produces secondary fault displacement within the basin and potentially through therepository. Vertical erosion of between 100 and 200 m occurs above the repository in
the 100 kyr - 1 Myr time period.

•

SES3: As for SES1 except that volcanism develops more rapidly and intensely due tolateral migration of volcanism into this ‘gap area’.

RES2
SES1: As for RES1/SES1
SES2: As for RES1/SES2
SES3: As for above SES2 except that deformation rates, including secondary faulting within
the basin, become even more intense. Also, vertical erosion of between 100 and 200m
occurs above the repository in the 100-1000 kyr time period, and lateral migration of
volcanism could occur earlier due to possible large-scale tectonic perturbations on the system.
RES3•

SES1: As for RES1/SES1.

•SES2: As for RES1/SES2.

•

SES3: As for RES1/SES3.

RES4•

SES1: As for RES1/SES1.

•SES2: Rate of development of the basin decreases over time, with cessation of allrock deformation by 1 Myr. Vertical erosion of between 100 and 200 m occurs in the
100 kyr - 1 Myr time period.

34

3

Estimating Scenario Probabilities Using Scenario Logic Trees

The concept of using logic trees to incorporate scenario probabilities in the form of expert
degrees of belief was introduced in Section 1.3.
Logic, or scenario, trees are increasingly applied in volcanology, both for long-term and shortterm eruption forecasts and for mitigation of volcanic hazards (e.g., Newhall and Hoblitt,
2002; Marzocchi et al., 2004; Aspinall et al., 2003). Logic and scenario trees provide amechanism for organizing the approach to the complex problem of assessment of thepotential for different forms and intensities of volcanic activity and the potential consequences(impacts) of such events. In volcanology generally, a probabilistic approach is used because
any volcanic action is a complex process, characterised by many unknowns. Except in a few
cases, this complexity means that future evolution of specific areas is intrinsically variable.
The evolution of volcanic hazards at specific sites and over the long term cannot be forecast
within a deterministic framework and, therefore, deterministic models for characterising future
volcanic activity are inadequate. More generally, the key drawback to deterministic
approaches is that there is no way to achieve uniformity or comparability across different
hazards or risks, and hence no way to rank or prioritise them without some expression of theirprobabilities. Therefore, the volcanological community has developed logic and scenario trees
as a means of estimating probabilities of different forms of future volcanic activity. Such an
approach is required for quantification of the uncertainty related to imperfect knowledge of
non-linear volcanic processes, to space-time variability of distribution and intensity of volcanic
events, as well as to a limited amount of information. For example, the International Atomic
Energy Agency recommends a probabilistic approach to volcanic hazard assessment for
nuclear facilities of all types. For assessment of hazards at HLW facilities, volcanic hazard
assessments have also been conducted using probabilistic approaches and logic trees
specifically (Coppersmith et al., 2009).
For the TOPAZ methodology development, three scenario logic trees were developed (upliftand exhumation, rock deformation and volcanic intrusion), although only the latter two wereanalysed for demonstration. All three are described in the following sections.

3.1

The Rock Deformation and Uplift/Exhumation Logic Trees

The logic trees in Figure 3.1 were developed to incorporate the impact of various RES for (a)the rock deformation hazard and (b) hazard due to uplift and exhumation of the repository.
Probability weightings for many of the branches (see elicited branches in red) on the rockdeformation logic tree were elicited during the October 2011 expert elicitation workshop.
However, we did not address the uplift and exhumation tree during the workshop (it was not
feasible to consider all of the SESs in the time available), and only show it here as an
example of what could be developed for the uplift/exhumation problem.

35

36

Figure 3.1: Scenario logic trees for rock deformation (left) and uplift/repository exhumation (right).

On both logic trees, the left-most branches on both trees involve weighting each of the four
different Regional Evolution Scenarios (see Section 2.1), while the second row of branches
addresses the consequences at the candidate site for each RES.
In the rock deformation tree, the final set of branches includes the three alternative
deformation datasets (e.g., GPS, active fault/surface deformation, and seismicity). During theexpert elicitation workshop, weightings were assigned for RES 1-4, on the first set ofbranches. However, given the limited time available for the workshop, weightings for
subsequent branches were only assigned for RES 1 and 2 – additional time would have been
needed to give proper, detailed consideration to further scenarios.

3.2

The Volcanism Logic TreeIn application to volcanic hazard assessment for HLW facilities, a scenario tree is a branchinggraph representation of possible events. Individual branches represent alternative
progressive steps from one event, or condition, to the next. For example, an early node on
the tree may represent the state of volcanic unrest, in which geophysical evidence suggeststhat magma intrusion has occurred. Later nodes may represent the state of eruption, or
alternatively the state of no eruption, two states that generally follow from the state of volcanic
unrest. As this example illustrates, in scenario trees these events are often ordered insequence, such that time progresses from early events to later events as the scenario tree
branches. Eventually, these branches terminate at final outcomes that represent specifichazardous events, such as the probability of dyke injection into a HLW repository. In practice,
the branches between states are weighted with probabilities. Often these probabilities are
determined using data, models, and expert judgment. Such scenario trees are now commonlyused in volcanology to estimate the probability of specific events at active volcanoes,
especially in light of monitoring data (Marzocchi et al., 2006).
Here, we differentiate between logic trees and scenario trees. Whereas scenario trees areconstructed around events, logic trees are constructed around models. A set of logic tree
branches represents exclusive and exhaustive alternatives (with probabilities summing tounity), whereas event trees can have incomplete branch sets, with weights or probabilities
that may not sum to unity (e.g. if two or more events can happen in one event). In logic trees,
nodes consist of specific geoscientific models that are used to estimate the probability of
specific events, rather than consisting of the events themselves. For example, ITM and
TOPAZ work has produced a number of different models of the spatial density of volcanism.
These models include the Cox process model, the kernel density model, empirical models
and homogeneous spatial density models. These models, or their aggregates, represent
nodes on the volcanism logic tree. On the logic tree, the transition probabilities represent theweights that experts assign to specific models. As in scenario trees, logic trees arehierarchical, leading to increasing detail required to estimate hazard. The models themselves
rely on data and calculations to produce a probability of the specific volcanic event.
There are several advantages to using logic trees in the assessment of volcanic hazards.
First, a complex sequence of events or models is described explicitly using a logic tree. This
lends transparency to the entire analysis, which is essential in both internal and external
review of the volcanic hazard assessment. Second, experts estimating the weights to give toeach model node need not be directly concerned with the probability calculation, but only withthe geological relevance of the individual models. Third, it is possible to frequently assess
impacts of new data on estimated probabilities as these data become available. In long-termsite investigations, it is inevitable that new data will emerge and will be discussed in terms ofimpacts on all types of hazard models, including volcanic hazards. Fourth, logic trees canprovide an efficient means of assessing both epistemic (model) uncertainty and aleatoric(data) uncertainty on the estimated probability of specific volcanic hazards. Just as new data
may emerge during prolonged site investigations, new data, models and interpretations (e.g.
RES and SES models, and data constraining these models) may emerge as well. Thesemodels are readily incorporated into the logic tree and their impact (or not) on probability
estimates can be assessed. Again, this has proven to be essential in siting of HLW facilities
because new models invariably emerge (Coppersmith et al., 2009).
In the following, the specific logic tree used in the assessment of models in TOPAZ isdescribed in detail (Figure 3.2). N1 is the current state of the volcano-tectonic system. Our

37

goal is to consider all credible models of how this system will evolve during the performance
period of the site, out to 1 Myr and to assign weights, based on expert judgment, to thesealternative scenarios. We create a logic tree to perform this evaluation. The logic tree
contains states, represented by nodes on the tree. These states represent alternative models
for the system that are required in order to estimate the probability of a credible site evolution
scenario. Transition probabilities on the logic tree represent the weights assigned toalternative models of igneous activity. The first part of the tree represents our understandingof the regional tectonic setting of volcanism and the evolution of this tectonic setting during
the next 1 Myr. This ends with weighting of alternative RESs. The second part of the tree
represents the site-specific processes that may give rise to igneous activity at the site. By
following a single branch through the logic tree, a single site evolution scenario (SES) is
specified. Each SES branch gives rise to a probability of igneous activity affecting the site.
The weights assigned to each branch are multiplied to estimate the probability that a given
SES will occur. The aggregate probability of igneous activity affecting the site is determinedby summing the weighted SES probabilities for the entire logic tree.

38

Figure 3.2: The volcanism logic tree.

For a proposed HLW facility, the main issue is estimation of the probability and consequencesof volcanic activity during the performance period of the facility (e.g., up to the next 1 Myr).
Therefore, impact scenarios scenarios must be developed for non-zero branches of the logic
tree (SES probability > 0). The impact scenarios consider alternative models of impact (e.g.,
direct disruption of the repository barrier system by igneous dyke injection or indirect thermal,hydrological, chemical, and mechanical effects).
For volcanism, states N2,1 and N2,2 represent alternative models for the credibility ofmagmatism occurring at any time within the next 1 Myr in the site region, where the site
region is taken to be a circle 100 km in radius centred on the site. This radius is considered tobe sufficiently large to provide a clear picture of the tectonic setting of the site and also large
enough to encompass any nearby changes in the tectonic setting, for example a transition
from forearc to arc tectonic environment, which might indicate that change in the tectonic
setting is possible during the 1 Myr performance period of the site (e.g., maximum rate of39

change on order 1 mm/a). The transition probability, p1 indicates the weight assigned themodel that no credible scenario can give rise to magmatism in this region during the next 1
Myr. The transition probability 1-p1 indicates the weight assigned to the model thatmagmatism is credible in the site region within the next 1 Myr. If p1 can be demonstrated toequal one, then no further analysis for the site is required. If p1 < 1 then alternative
conceptual models for volcanism need to be developed in the RES and SES stages.
For the volcanism scenario tree, this node is essential because there are numerous sites
where there is no credible potential for future volcanic activity, even in a 1 Myr timeframe.
Therefore, there are some locations, (e.g., example location A) for which no reasonable RES
can give rise to volcanic activity in the next 1 Myr. It is not efficient to debate alternative RESs
for such sites. For example, locations in the Shikoku region may have a complete analysis forvolcanism simply by demonstrating that p1=1 (location A, in the forearc of Tohoku, isconsidered to be in this category).
If p1 < 1 then alternative conceptual models for the evolution of the site region need to beconsidered. The RESs summarise the range of conceptual models of the tectonic evolution of
the site, represented by the states N3,1 – N3,4 on the logic tree. Weights are assigned to thealternative RESs using p2,1 – p2,4. These weights must sum to unity. These weights weredeveloped using an informal process in the March 2011 Topaz workshop.
For volcanism, the site evolution scenario process involves considering the regional
recurrence rate of volcanism and the spatial density of volcanism, based on each RES. For
methodology development, a range of alternative models is considered. States N4,1– N4,12(not all shown on the logic tree) represent recurrence rate models, based on the RESs. Threebasic recurrence rate models are shown. For example, RES1 (tectonic conditions persist as
they are today) is assigned to N3,3. N4,7 represents a state of increasing recurrence rate ofigneous activity, N4,8 represents a state in which recurrence rate persists as it is currently,
and N4,9 represents a decrease in recurrence rate of igneous activity. For the TOPAZ
methodology, we should consider a probability distribution of recurrence rates for each ofthese states. For example, for N4,8 (recurrence rate remains as today), the model should
specify a mean recurrence rate and standard deviation in recurrence rate, based on TOPAZ
and VOGRIPA data. Similar distributions should be developed for N4,7 and N4,9. Then, thesesame distributions should be used for alternative RESs. Individual experts may choose toweight the recurrence rate models differently on different branches, but would not choose therange of recurrence rates represented by the nodes.
Given an RES and recurrence rate of volcanism, nodes N5,1 – N5,24 (not all shown)represent alternative spatial density models. These are the cluster models, represented byspatial distributions based on the Cox or kernel methods, and the empirical model (based on
distance from existing volcanoes and a threshold cut off value). In addition, models can be
developed that are completely spatially random, such as homogeneous Poisson models. In
such models, a zone is defined, perhaps based on some geological criteria, and the
distribution of volcanism within this zone is considered to be completely spatially random. In
this case, the estimate of the recurrence rate of activity within this zone using this model issimply the average density of volcanoes within the zone. Weights are assigned to thedifferent spatial density models by the experts. It follows that, for expert assessment,
individual experts were asked a simple question: Should a tight cluster (Cox) model orsmooth (Kernel) model or homogeneous Poisson model be used to estimate spatial density
and, by elicitation, what weight should be assigned to each type?
The results comprise an aggregate probability, the sum of the weighted SES probabilities.
Each SES probability may be a range, e.g., based on a range of recurrence rate and spatial
density models, represented by high (H) and low (L) values, or as a distribution. This brings
us to the conclusion of the SES stage and sets the scene for Impact Scenarios (ISs), where
the tree is further developed to consider alternative site impacts, if necessary.

40

4Using Expert Elicitation to Derive Scenario Probabilities

The concept of incorporating differing expert views and alternative interpretations and models
using a formal expert elicitation approach has not been used before in the Japaneseradioactive waste programme. Consequently, as well as using the technique as part of the
TOPAZ methodology, it was being introduced generically for the first time to Japanese
tectonics and radioactive waste experts as well. This was done by means of a single
workshop, where selected RES and SES probabilities were elicited from a group of experts,
after first calibrating experts in terms of their ability to provide informative judgements forestablishing probabilities and ranges of probabilities in the face of uncertainties.
The workshop demonstration of a structured expert judgment (SEJ) elicitation was held onthth
25 - 26 October 2011 in Tokyo. About thirty people attended the elicitation workshop, withtwenty-one experts and specialists participated actively in the elicitation exercise. Because oftime constraints on processing data, the responses of a subset of twelve experts wereselected at random to exemplify the process, comprising eight Japanese experts and fourinternational experts. The workshop is reported in detail in Aspinall, 2011, and only a brief
outline is provided in the current report.
The elicitations were conducted using a structured questionnaire and analysed with the
Cooke Classical Model and the EXCALIBUR software package. Before the Workshop took
place, a set of draft Target Item questions were prepared by members of the TOPAZ group,
and a set of Seed Items devised.
Overall, the exercise was judged successful, and achieved its main goal of providing ademonstration of a rational and defensible structured expert judgment basis for ascribing
initial uncertainty distributions to parameters, models and variables in relation to long-term
future volcanic and tectonic hazards and risks at proposed geological repository sites forradioactive wastes in Japan.
For many of the factors addressed by the expert panel, the scientific uncertainties associatedwith these risk factors are large, and their quantification is a crucial element for supporting
decision making related to hazards, repository siting and risk management. The elicitation
showed how, using expert judgment in a formalised manner, objective quantitative estimatesfor certain hazard and risk factors could be derived - crucial for supporting policy decisionmaking under conditions of severe scientific uncertainty. In addition, the exercise also
illustrated how the structured elicitation framework can help identify major knowledge gaps,
topics where definitions and terms may need clarification, and critical areas where additional
research and investigation may be worthwhile.

4.1

Background to Expert Elicitation

When expert advice is needed as support to critical, science-based decision-making, astructured way of eliciting a variety of opinions is helpful. In this context, seeking a “rational
consensus” refers to a group decision process in which a formalised approach is followed,
based on performance-based scoring rule optimization. The group of experts involved needs
to agree on a method according to which representations of parameter uncertainty will be
generated for the purposes for which the panel was convened, without knowing a priori theoutcomes of this method. However, it is not required that each individual member adopts theresults as his personal degree of belief.
To be rational, this method must comply with necessary generic conditions devolving from thescientific method. Cooke (1991) formulates the necessary conditions or principles, which any
method warranting the designation “scientific” should satisfy, as:•

Scrutability/accountability: All data, including experts' names and assessments, andall processing tools are available for peer review and results must be open andreproducible by competent reviewers.

•

Empirical control: Quantitative expert assessments are subjected to empiricalquality controls.

41•

Neutrality: The method for combining/evaluating expert opinion should encourage
experts to state their true opinions, and must not bias results.

•

Fairness: Experts’ competencies are not pre-judged, prior to processing the resultsof their assessments.

Thus, a method is desired which satisfies these conditions and to which the parties commit,
beforehand. The principles outlined above have been implemented for expert elicitation in theso-called “Classical Model”, a performance-based linear pooling or weighted averaging model(Cooke 1991). The weights are derived from experts’ calibration and information scores, asmeasured on seed variables. Seed variables serve a threefold purpose:•to quantify experts’ performance as subjective probability assessors,•

to enable performance-optimised combinations of expert distributions, and•to evaluate and hopefully validate the combination of expert judgments.

The name “Classical Model” derives from an analogy between expert calibration
measurement and classical statistical hypothesis testing.
In the Classical Model,
performance-based weights use two quantitative measures of skill with respect to thestatement of uncertainty: calibration and information. Loosely, calibration measures thestatistical likelihood that a set of measurable or experimental results correspond, in astatistical sense, with the expert’s assessments. Relative information measures the degree towhich an expert’s uncertainty distribution is concentrated around the true answers to a set of
seed questions – this latter evaluation is sometimes referred to, in the context of expert
elicitations, as the expert’s “informativeness”.
However, it is essential to stress here that the connotation of “expert informativeness” and theuse of the adjective “informative” can be different in this respect from their vernacularmeaning in the “real world”. In some societies, “informative experts” - in any situation otherthan a formal elicitation - are those who are recognised by their peers or judged by others to
be authoritative in what they say. In the context of a structured elicitation, informative experts
are taken to be those who demonstrate an ability to express an appropriate range of
uncertainties over a combination of variables, and thus assist decision-making.
The EXCALIBUR procedure implements a method for combining experts’ subjective
probabilities based on mathematical and statistical theory, and is therefore more rigorous
than other, less formalised, approaches. The aim is to find some way of combining several
distributions, given by different experts, into one distribution, representative of the spectrum of
their opinions. What is frequently used is a linear opinion pool, which is just a weighted
arithmetic mean of the distributions provided by the experts. This pooling can use either
simple equal weights or it can use a performance-based weighting scheme. The aim of thelatter is to create a basis for achieving rational consensus. Since each individual has his/her
own subjective probability, it is necessary to find a way of achieving this convergence. In
EXCALIBUR, weights are determined from the participating experts’ performances on‘calibration’ questions, questions whose answers are known to the analyst but not to theexperts.
Suppose the experts are asked for their uncertainty ranges over a number of calibration
variables. In a typical application, each expert gives quantile information for his or her
uncertainty distributions, such that for each calibration variable there are four intervals:•

0 to 5%; 5% to 50%; 50% to 95%; 95% to 100%

The quality of calibration of an expert is measured by looking at how far the empirical
distribution given by the calibration variables differs from that given by the expert. The analyst
wants to identify those experts for whom the corresponding statistical hypothesis is well
supported by the data obtained from the calibration variables. The calibration score is theprobability that the divergence between the expert’s probabilities and the observed values of
the calibration variables might have arisen by chance. An illustrative example of how
calibration via seed questions works is shown in Figure 3.3.

42

Figure 3.3: Schematic chart showing how experts’ responses are calibrated against (multiple) seedquestions at given quantiles to produce performance-based weights, which are then used to pool theexperts’ opinions for the corresponding quantiles of target items.

Calibration is not the only way to measure the quality of an expert opinion. Another criterion isrelative information (as noted earlier, sometimes referred to as “informativeness” in thecontext of expert elicitation). Relative information represents the degree to which an expert’s
distribution is concentrated, relative to some user-selected background measure. The relative
information is calculated for each expert at each query variable and then averaged over all
the query variables to get the overall information score of expert e. The information score is apositive number, with increasing values indicating greater information relative to thebackground measure.
Experts with distributions closest to the realization values are given higher calibration scores,
and experts with smaller uncertainty bands are given higher information scores. The “ideal”
expert would have predicted close to the true value with little uncertainty – thus, “goodexpertise” corresponds to high statistical likelihood and high information.
In order to determine the performance-based weight that an individual expert gets, theirinformation and calibration scores are combined together as a product. In this situation, thecalibration scores are the more important: calibration dominates over relative information (orinformativeness), while the relative information score serves to adjust between equallycalibrated experts.
The seed question used and the results of the calibration exercise are described in the
FY2011 report for NUMO and are not presented here.

4.2

Questions Posed in the Expert Elicitation

The experts were asked to provide their best estimate and confidence bounds to the following
questions (note: these are much abbreviated: the full details of the questions are not provided
here but were outlined in an internal briefing note and thus provide weights for each option:•

which of the four RESs was considered most appropriate (or whether another model
might apply)

43

•
•
•
•
•

given each RES, would strain rates at Location ‘C’ increase, decrease or stay thesame over the next 1 Myr;
which strain dataset (as used in the ITM methodology) is most reliable for indicatingstrain rates;
given each RES, would the long-term average rate of formation of new volcanoes in
the Tohoku region increase, decrease or stay the same over the next 1 Myr;
which spatial density model (as used in the ITM methodology) is most reliable for
estimating volcanic hazard at Location ‘C’;
which spatial density model is most appropriate for the situation that the long-term
average rate of formation of new volcanoes increases.

The following sections show only the full results of the expert elicitation for the first question,as an example of the type of output derived from expert elicitation, with a summary table of
the results of the other questions.

4.3

Evaluation of RESs

The results of the RES questions are presented in full below. Each diagram shows the
responses of the experts, plus a group-averaged solution and a performance-based weighted
solution where, in the latter, responses of the experts are weighted according to theirstatistical calibration and informativeness and then pooled, as described above. The label“Perf” is used for a performance-based weighted solution of opinions, with individual weights
determined from the analyst’s choice of model constraints. “Equal” is used to denote anequal weights solution. As noted above, the questions are given in shortened format here.
Experts were asked to provide three values: a median (50%ile) value and their view on the
90% credible interval values about their central value (i.e. their judgment of the 5%ile value
and the 95%ile value). The spread of each expert and the median value are indicated on the
diagrams for the four RESs and an ‘any other RES’ alternative in Figures 4.1 to 4.5. Also
shown is the facilitator’s commentary on each result.

44

Figure 4.1: For characterizing the likely Regional Evolution over the period 100 kyr to 1 Myr, what
relative weight would you give to the RES 1 model option?
Facilitator’s comments:
There are wide-ranging uncertainties in individual judgments; credible intervals vary, but mostly
overlap across the group.
The Performance Weights solution credible interval is narrower than the Equal Weights solution.

45

Figure 4.2: For characterizing the likely Regional Evolution over the period 100 kyr to 1 Myr, what
relative weight would you give to the RES 2 model option?
Facilitator’s comments:
There are wide-ranging uncertainties in individual judgments; credible intervals vary, but mostly
overlap across the group.
The Performance Weights solution credible interval is narrower than the Equal Weights solution.

46

Figure 4.3: For characterizing the likely Regional Evolution over the period 100 kyr to 1 Myr, what
relative weight would you give to the RES 3 model option?
Facilitator’s comments:
There are wide-ranging uncertainties in individual judgments; credible intervals vary, but mostly
overlap across the group.
The Performance Weights solution credible interval is narrower than the Equal Weights solution.

47

Figure 4.4: For characterizing the likely Regional Evolution over the period 100 kyr to 1 Myr, what
relative weight would you give to the RES 4 model option?
Facilitator’s comments:
There are wide-ranging uncertainties in individual judgments; credible intervals vary, but mostly
overlap across the group.
The Performance Weights solution credible interval is narrower than the Equal Weights solution.

48

Figure 4.5: If you think an alternative model, distinctly different from models RES1, RES2, RES3 and
RES4, is feasible for characterizing the likely Regional Evolution over the period 100 kyr to 1 Myr, what
relative weight would you give to this other model?

Facilitator’s comments:
The disparities in individual judgment credible intervals are marked for this Target Item, withseveral of the group providing zero or negligible weights to the possibility of an alternativemodel; others ascribe real non-zero weights to this possibility.
The Performance Weights solution credible interval is narrower than the Equal Weights solution.

49

4.4

Weights for the SESs

Figure 4.6 summarises the numerical values for three quantiles (5%ile; 50%ile; 95%ile) on
each of the SES questions outlined in Section 4.1, obtained by weighted pooling of theexperts’ judgments. Three decimal places are displayed below in order to report low values,
but limited significance should be ascribed to this level of precision and values should berounded appropriately.
Item
Nr.

Item Id.

5%ile

50%ile95%ileDescription

T1

WtRES1model

0.425

0.688

0.897

Relative weight to RES 1 model for 100 kyr to 1 Myr future

T2

WtRES2model

0

0.105

0.291

Relative weight to RES 2 model for 100 kyr to 1 Myr future

T3

WtRES3model

0

0.109

0.302

Relative weight to RES 3 model for 100 kyr to 1 Myr future

T4

WtRES4model

0.002

0.084

0.218

Relative weight to RES 4 model for 100 kyr to 1 Myr future

T5

WtOtherModel

0

0.027

0.198

Relative weight to different model for 100 kyr to 1 Myr future

T6

RES1Strain2x

0.009

0.204

0.361

Rel. weight strain rate increases by 2x for RES1 over 1 Myr

T7

RES1Strain=

0.395

0.556

0.884

Rel. weight strain rate remains same for RES1 over 1 Myr

T8

RES1Strain0.5x

0

0.201

0.350

Rel. weight strain rate decreases by 2 x for RES1 over Myr

T9

RES2Strain2x

0.215

0.582

0.992

Rel. weight strain rate increases by 2x for RES2 over 1 Myr

T10

RES2Strain=

0

0.241

0.585

Rel. weight strain rate remians same for RES2 over 1 Myr

T11

RES2Strain0.5x

0

0.025

0.276

Rel. weight strain rate decreases by 2x for RES2 over 1 Myr

T12WtRES1_GPS

0.068

0.267

0.498

Rel. weight for GPS-based strain rate model under RES1

T13

WtRES1_Seism

0.067

0.261

0.496

Re. weight for seismicity-based strain rate model under RES1

T14

WtRES1_Fault

0.151

0.358

0.644

Rel. weight for active faulting-based strain rate model under RES1

0

0

0.303

Rel. weight for different strain rate model

T14b WtRES1_Other
T15

WtRES2_GPS

0.051

0.130

0.394

Rel. weight for GPS-based strain rate model under RES2

T16

WtRES2_Seism

0.044

0.129

0.389

Rel. weight for seismicity-based strain rate model under RES2

T17

WtRES2_Fault

0.215

0.669

0.989

Rel. weight for active faulting-base strain rate model under RES2

0

0

0.303

Rel. weight for different strain rate model

T17b WtRES2_OtherT18

RES1Birth_3x

0.066

0.166

0.340

Rel. weight volcano birth rate increases 3x under RES1

T19

RES1Birth=

0.384

0.525

0.944

Rel. weight volcano birth rate unchanged under RES1

T20

RES1Birth_0.3x

0.066

0.166

0.340

Rel. weight volcano birth rate decreases 3x under RES1

T21

RES2Birth_3x

0.086

0.271

0.498

Rel. weight volcano birth rate increases 3x under RES2

T22

RES2Birth=

0.118

0.444

0.599

Rel. weight volcano birth rate unchanged under RES2

T23

RES2Birth_0.3x

0.077

0.270

0.495

Rel. weight volcano birth rate decreases 3x under RES2

T24

RES1_Tight

0.076

0.512

0.747

Rel. weight tight cluster model under RES1

T25

RES1_Smooth

0.069

0.340

0.500

Rel. weight smooth cluster model under RES1

T26

RES1_Random

0.000

0.058

0.397

Rel. weight random density model under RES1

0

0

0.097

Rel. weight for different cluster model

T26b RES1_VolcMap
T27

RES2_Tight

0.063

0.278

0.518

Rel. weight tight cluster model unde RES2

T28

RES2_Smooth

0.077

0.446

0.600

Rel. weight smooth cluster model under RES2

T29

RES2_Random

0.077

0.258

0.470

Rel. weight random density model under RES2

0

0

0.092

Rel. weight for different cluster model

T29b RES2_VolcMap

Figure 4.6: Numerical values for three quantiles (5%ile; 50%ile; 95%ile) on each of the SES questions
outlined in Section 4.1, obtained by weighted pooling of the experts’ judgments.

4.5

Scenario tree weights leading to SES probabilities

The following series of eight chart panels show the two scenario sub-trees which were theframework for the elicitation exercise, with central value weights for branch nodes added
sequentially, as derived from the elicitation exercise.

50

Where, in the range graph results presented above, elicited results for combinations ofweights on companion branches do not sum exactly to unity these have been normalised
before being entered on the sub-tree plots that follow.
The purpose of these particular plots is simply to illustrate how the results from a structured
elicitation can be used to populate such probability trees. The tree plots also allow ease ofcomparison across the various Target Items.
In a formal quantitative hazard or risk assessment, the corresponding distributional spreads
would be utilised also, in order to express related ranges of scientific uncertainty.
Tectonic Deforma tion Scena rio Tree
Event: a M6.5 ea rthquake will occur on an unknown faul t a t the site

T1

T2

RES 1
RES 2

T3

RES 3

T4

RES 4

T5

Al terna te/
Other

T6
T7
T8

Strai n x 2
Strai n - median
Strai n x ½

T12
T13
T14

GPS s train
EQ s train
fa ul t s train

T9

Strai n x 2
Strai n - median

T15
T16

GPS s train

T10
T11

Strai n x ½

T17

fa ul t s train

EQ s train

Figure 4.7: The two scenario sub-trees, which were the framework for the elicitation exercise, withcentral value weights for branch nodes added sequentially, as derived from the elicitation exercise.

Tectonic Deforma tion Scena rio Tree
Event: a M6.5 ea rthquake will occur on an unknown faul t a t the site

0.68

RES 1

0.11

RES 2

0.11

RES 3

0.08

RES 4

0.03

Al terna te/
Other

T6
T7
T8

Strai n x 2
Strai n - median
Strai n x ½

T12
T13
T14

GPS s train
EQ s train
fa ul t s train

T9

Strai n x 2
Strai n - median

T15
T16

GPS s train

T10
T11

Strai n x ½

T17

fa ul t s train

Figure 4.8: (same as above).

51

EQ s train

Tectonic Deforma tion Scena rio Tree
Event: a M6.5 ea rthquake will occur on an unknown faul t a t the site

0.68

RES 1

0.11

RES 2

0.11

RES 3

0.08

RES 4

0.03

Al terna te/
Other

0.21

Strai n x 2

T12

GPS s train

0.57

Strai n - medianT13

EQ s train

0.21

Strai n x ½

T14
T14b

fa ul t s train

0.76

Strai n x 2

0.32
0.03

Strai n - median
Strai n x ½otherGPS s train

T15
T16

EQ s train

T17

fa ul t s train

T17b

other

Figure 4.9: (same as above).

Tectonic Deforma tion Scena rio Tree
Event: a M6.5 ea rthquake will occur on an unknown faul t a t the site

0.68

0.11

RES 1
RES 2

0.11

RES 3

0.08

RES 4

0.03

Al terna te/
Other

0.21

Strai n x 2

0.30

0.57
0.21

Strai n - median

0.30
0.39

Strai n x ½

0.76

Strai n x 2

0.32
0.03

Strai n - median
Strai n x ½

Figure 4.10: (same as above).

52

GPS s train
EQ s train
fa ul t s train

0

other

0.14
0.14

GPS s train

0.72
0fa ul t s trainother

EQ s train

Vol cano Scena rio Tree
Event: a M≥5 vol cani c event will occur in the si te a rea

T1

T2

RES 1

T18 Ra te x 3
T19 Steady ra te
T20 Ra te x 1 /3

RES 2

T3

RES 3

T4

RES 4

T5

Al terna te/
Other

T21 Ra te x 3

T24

tight

T25
T26

smoothrandomtight

Steady ra te

T27
T28

smoothRa te x 1 /3

T29

random

T24
T25
T26

tight
smooth

T27
T28

tight

0.44 Steady rate
0.27 Rate x

T29

random

T22
T23

Figure 4.11: (same as above).

Volcano Scenario Tree
Event: a M ³ 5 volcanic event will occur in the site area
0.20 Rate x 3
0.68

0.11

RES 1

0.60 Steady rate
0.20 Rate x 1/3

RES 2

0.11

RES 3

0.08

RES 4

0.03

Alternate/
Other

0.27 Rate x 3
1/3

Figure 4.12: (same as above).

53

randomsmoothVol cano Scena rio Tree
Event: a M≥5 vol cani c event will occur in the si te a rea

0.68

0.11

RES 1

0.20 Ra te x 3

T24

tight

0.60 Steady ra te

T25
T26

smooth

T26b

other

0.20 Ra te x 1 /3

RES 2

0.27 Ra te x 3
0.44 Steady ra te
0.27 Ra te x 1 /3

0.11

RES 3

0.08

RES 4

0.03

Al terna te/
Otherrandom

T27

tight

T28

smoothT29

random

T29b

other

0.56
0.37
0.07

smooth0.0

otherFigure 4.13: (same as above).

Vol cano Scena rio Tree
Event: a M≥5 vol cani c event will occur in the si te a rea
0.20 Ra te x 3
0.68

0.11

RES 1

0.60 Steady ra te
0.20 Ra te x 1 /3

RES 2

0.27 Ra te x 3
0.44 Steady ra te
0.27 Ra te x 1 /3

0.11

RES 3

0.08

RES 4

0.03

Al terna te/
Other

tight
random

0.28

tight

0.45

smooth0.26

random0.0

otherFigure 4.14: (same as above).

4.6

Discussion of the Expert Elicitation

In this demonstration exercise, the usual procedures for a structured elicitation with analysis
using the Cooke Classical Model were followed. The exercise – as an expert elicitation – canonly be regarded as a very preliminary attempt at teasing out where significant knowledge
gaps exist in relation to the problem of forecasting long-term scenarios for hazards and risks
for proposed geological repository sites of radioactive wastes in Japan, at quantifying certain
factor uncertainties in present models, and at identifying issues and factors that might beamenable to targeted further work.
In terms of experts’ responses to the Seed Item questions, the general pattern was one ofwide-ranging uncertainties in individual judgments. This said, individual credible intervals
54almost invariably overlapped across the group, and there was little indication of systematic
dichotomies due to question ambiguity or misunderstanding. The credible intervals derived
from the Performance Weights pooling solutions were almost universally narrower than thoseof the corresponding Equal Weights solutions and, with one exception, realization values fell
consistently within the credible ranges of both the Performance Weights and Equal Weights
credible ranges, with the Performance Weights solutions doing marginally better, overall.
In similar vein, experts’ judgments on nearly all the scenario questions had large credibleintervals associated with them, indicating extensive uncertainty in relation to the questions
posed. While there are varied uncertainties in the individual judgments for all the scenario
questions, collectively the credible intervals usually overlapped across the group. With just a
few marginal exceptions, the Performance Weights solution credible intervals wereconsistently narrower than those of the Equal Weights solutions.
Taking the elicitation scenario questions outcomes overall, the scientific uncertainties on
many factors are seen to be large and quite difficult to quantify precisely. This said, theexercise served to show that a formalised expert elicitation can be useful for providing rational
constraints on the ranges of parameter or probability variations, and for promoting discussionof key issues at the heart of scientific uncertainties.

4.7

Output of the Expert Elicitation

The end result of the Expert Elicitation is a set of SESs for Location ‘C’ with probabilities
attached to each in the form of relative degrees of belief of the group of experts involved.
These SESs are propagated to the next stage of the TOPAZ methodology, in Section 5.

55

5

Estimation of IS probabilities for Location ‘C’

Following the approach outlined in Section 1.3, the SES probability results of the Expert
Elicitation exercise were integrated with the results of the ITM hazard mapping to determine
probabilities that the two selected Impact Scenarios (ISs) could occur over the next 1 Myr.
The first step is to provide a clear, specific ‘event definition’ of credible tectonic-deformationand volcanism ISs for a given RES-SES branch. Based on such ‘event definitions, it ispossible to use various geological and geophysical data to set initial probabilities for any
specific IS. In the following section two illustrative ‘event definitions’ for ISs of a M6.5-7.5
earthquake and a M5 volcanic event at Location C are described.

5.1

Demonstration of the Quantitative Estimation of the Probability of a
Tectonic Deformation Scenario for a M6.5-7.5 Future Earthquake

We use the results of the expert elicitation demonstration exercise together with the ITM
strain rate models (Chapman et al., 2009) to demonstrate how tectonic deformation can bequantified. The purpose is to show how the range of RESs, and choice of ITM deformation
model (active fault and surface deformation-based, seismicity-based and GPS-based) can be
combined and used to quantify the probability of hazardous tectonic events at a site. Ourdefinition of a hazardous event for the purposes of this demonstration is a range ofearthquake sizes from M6.5 to M7.5 occurring in the upper 10 km of the 5 x 5 x 20 km crustal
volume defining example Location C. The upper several hundreds of metres is the crustal
depth within which a repository would potentially be hosted, and M6.5-7.5 ruptures below 10
km would be less likely to impact the shallower repository depths with permanent deformation(see Appendix 1). The earthquake magnitude range is modelled according to a GutenbergRichter distribution: logN = a – bM, in which N is the number of events of magnitude M, and a
and b are empirical constants. We assume the same maximum magnitude defined for theregion surrounding location C in the ITM modelling (M7.5; Chapman et al. 2009) and theminimum is chosen largely for demonstration purposes in this study.

56

Figure 5.1: Seismicity map with longitudinal and latitudinal depth sections in the vicinity of Location C.
The earthquakes are from 1980 to 2011, and M>3. Clearly, the majority of seismicity lies at depths
greater than 10 km. Source http://wwweic.eri.u-tokyo.ac.jp/db/jma/

It is important to note that the epistemic uncertainties have not been fully quantified in thisdemonstration exercise. The expert elicitation exercise was limited to populating only a few
branches of the logic tree. These are: (1) RES1, Deformation constant over 1 Myr, and choiceof ITM active fault/surface deformation, seismic, and GPS deformation models, and; (2) RES2,
upper plate shortening increasing by a factor of two in 1 Myr, and the same choice of ITM
deformation models. The expert elicitation exercise provided performance-based weights forthese sections of the logic tree.
Examination of approximately 30 years of seismicity around Location C to a depth of 20 km(Figure 5.1) shows that approximately 25% of earthquakes lie at 0-10 km, and 75% in thelower 10 – 20 km of the 5 x 5 x 20 km crustal volume. This is the basis for modelling theseismicity applied in the tectonic deformation modelling, in that any deformation rateestimates (i.e. seismic moment rates and resulting earthquake rates) originating from the ITM
deformation models (based on a 5 x 5 x 20 km crustal volume) are factored down to 25% of
the original rate estimates.
5.1.1

Methodology

Our procedure is to use the ITM strain rate-based tectonic deformation models for active
fault/surface deformation, seismicity and GPS as input to the expert elicitation logic tree (seelogic tree in Section 3). These three deformation models are assigned the weights defined
during the expert elicitation procedure, as are the choices of deformation rate increase in the
1 Myr timeframe for RES1 and RES2.
For the purposes of this demonstration we simplify the input from the three ITM deformation
models by condensing the earlier histograms of numerous strain rate models (Chapman et al.,2009) down to a single seismic moment rate for each ITM deformation model data type. In
this way, we can clearly show the epistemic uncertainties arising directly from the choice of
RES, rate increase and ITM deformation model in the demonstration (Figure 5.2).

57

This involves converting the strain rates into seismic moment rates via the method of Kostrov(1974) and then summing the incremental seismic moment rates. The summed seismic
moment rate is then increased by a factor of two for RES2, and maintained at a constant for
RES1 according the limited logic tree branches populated by the expert elicitation exercise(see Section 3).

Figure 5.2: The discrete histogram for the 5 x 5 x 20 km location C block, in which the seismic moment
rate is factored down to 25% of original moment rate to take account of low seismicity rates in the upper
10 km (Figure 5.1). Particular combinations of RES, rate increase factors for 1 Myr (RF), and tectonic
deformation models (GEOL, SEIS, GPS) clearly dominate various parts of the histogram (see labels),
despite the Gutenberg-Richter distribution producing a diversity of magnitudes and rates. The red dot
shows the position of the arithmetic weighted mean of the entire distribution.

The combinations of RES, deformation rate change and deformation model are then used to
distribute the seismic moment rates into Gutenberg-Richter distributions of earthquakes from
M6.5 to 7.5. The results of 10,000 Monte Carlo logic tree simulations of M6.5-7.5 GutenbergRichter-distributed earthquake rates are shown by the histograms in Figure 5.2 and 5.3.
Figure 5.2 shows the discrete normalised frequency (count per bin/summed count for all bins)of the 10,000 simulations on the y-axis, and the M6.5-7.5 rate bins in the x-axis. Magnitudes
within this range are not distinguished in the figures.

58

Figure 5.3: The cumulative histogram for the 5 x 5 x 20 km location C block, in which the seismic
moment rate is again factored down to 25% of the original moment rate to take account of low seismicity
rates in the upper 10km (Figure 5.1). Three “bulk cumulative rates” for M6.5-7.5 earthquakes (i.e. single
rate for the sum of all earthquakes from M6.5 to 7.5) derived from the three original ITM deformation
models are shown for comparison as red diamonds (seismicity and GPS-derived rates plot on top of one
another at the left-most diamond). The rates are calculated for each deformation model by dividing thesingle condensed seismic moment rate for that dataset (see Section 5.1.1) by the seismic moment
integrated over M6.5-7.5. The cumulative histogram shows a broad range of rates to the right of the
diamonds, due to the assumption of the Gutenberg-Richter distribution producing a great diversity ofrates across the magnitude range.

An alternative, but very simplistic histogram is shown in Figure 5.4. The histogram shows theresults of a further 10,000 simulations, but this time assuming that all seismic moment releaseoccurs only by M6.5 scenario earthquakes within the 5 x 5 x 20 km crustal volume. This
extreme model is for illustrative purposes and would be relevant in the situation where only
one active source is known to exist within the crustal volume. The rates are again scaled
down to 25% of original, to take into account the depth distribution of seismicity in the vicinityof the site.

59

Figure 5.4: Discrete histogram showing the range of rates for a scenario M6.5 earthquake in a 5 x 5 x
20 km crustal volume. The rates are again scaled down to 25% (see above). The particular
combinations of RES, rate factor increase and deformation model responsible for the spikes on thehistogram are also shown.

5.1.2

Observations

Our methodology of condensing each of the three original ITM strain rate model histogramsfor sites (Chapman et al. 2009) down to single seismic moment rates allows us to observe theepistemic uncertainty due solely to the choice of RES, rate factor increase, and ITM
deformation model in Figures 5.2 to 5.4. We label the discrete histograms accordingly toshow the particular combinations of these parameters that produce some of the histogram
spikes.
The spikes on this histogram translate to changes in slope on the cumulative plot (Figure 5.3).
In general, the frequency of M6.5-7.5 rates fall steeply from about 1e-7/a to about 3e-6/a in
Figure 5.2, but with some higher rates encountered in the right of the histogram (3e-5/a). The
weighted arithmetic mean of the distribution is 2.8e-6/a. This low rate is a consequence of thevery small crustal volume being considered, and the effect of distributing the seismic moment
rates across a Gutenberg-Richter distribution from M6.5 to 7.5.
The large earthquakes of the Gutenberg-Richter distribution (M7-7.5) will accommodate eighttimes the seismic moment release of the moderate earthquakes (M6.5-6.9), so fewer large
events are required to accommodate the seismic moment release relative to moderate events.
This effect is further illustrated by comparing Figure 5.2 to the simplistic earthquake histogramin Figure 5.4, in which earthquakes are assumed to occur only as M6.5. In this case, theweighted mean is about 8e-5/a, a much higher rate than in the Gutenberg-Richter-based
histogram (Figure 5.2). This is because all of the seismic moment rate is assigned tomoderate (M6.5) earthquakes in Figure 5.4.

60

5.2

Demonstration of the Quantitative Estimation of the Probability of a M5
Volcanic Scenario

In the following sections we look first at the probability of any magnitude of new volcanic
activity affecting location C (effectively, the SES probability), using the spatial density
modelling approaches developed as part of the ITM methodology, weighted according to theresults of the expert elicitation exercise reported in Section 4. The Expert Elicitation
developed relative degrees of belief in these different evaluation approaches. We then look at
the specific probability of occurrence of the defined IS at location C, which is an event ofgreater than M5 on the VOGRIPA explosive volcanism magnitude scale.
5.2.1

Estimation of SES probability at Location C

The aim of this exercise is to illustrate how expert elicited probabilities are combined withestimated probabilities obtained from probabilistic volcanic hazard assessment models. For
the Tohoku region, the ITM methodology was only applied for scenario RES I (Chapman et al.,2009), so only estimated probabilities in relation to that scenario are available for location C.
The data available for the application are given in Table 5.1; the elicited median probabilities
correspond to the weighted pooling of the experts judgments on the related target items (seethe table in Section 4.4). The estimation of volcanic hazard of any type and magnitude
occurring within a 5 x 5 km area at the location C was obtained over a 100 kyr period using
the ITM probabilistic methodology (Chapman et al., 2009). Since the period of interest for
TOPAZ extends out to a million years, the volcanic hazard values given by the Cluster (Cox)and the smooth Kernel (SAMSE) spatial density models were linearly extrapolated to cover
the period of interest. For the Homogeneous Poisson model, simple analytical probabilistic
calculations were applied to estimate volcanic hazard at location C over 1 Myr period using
the data given in Chapman et al. (2009). In addition, the long-term average rate of formationof new volcanoes in the Tohoku region was assumed to stay the same as the average
Quaternary rate.
Therefore, the application is targeted at estimating site evolution scenario (SES) probability inrelation to the regional evolution scenario I (RES I) with a recurrence rate model under steadystate conditions of volcanic activity (i.e. the recurrence rate model named Model_=). The
applied data are displayed in red in Table 5.1.
Table 5.1. Volcanism example: elicited probability and estimated volcanic hazard in relation to RES I.
Volcanism results from expert elicitation (cf. section 4.4)
Scenario[Prob 50%ile]

RES I [0.688]

ITM methodology(Chapman et al.
2009)

Recurrence rate model[Prob 50%ile]

Spatial density model[Prob 50%ile]

Volcanic hazard
at site C

Model_3x [0.166]

Cluster (Cox) [0.512]

3.7∙10-3

Model_= [0.525]

Kernel (SAMSE) [0.340]

7.2∙10-3

Model_1/3 [0.166]

Homogen. Poisson [0.058]

2.6∙10-2

RES II [0.105]...

RES III [0.109]...

RES IV [0.084]...

Other model(s)[0.027]...

...

The specification of a SES scenario is obtained by following a single branch through the logic
tree. A given SES scenario is then associated to the following states: i.e. (i) RES scenario, (ii)61recurrence rate model, (iii) spatial density model and (iv) volcanic hazard at location C.
Therefore, the probability of igneous activity for a SES scenario is estimated by multiplying
the probability values related to his given states (Table 5.2). Using this approach, elicitedprobability and estimated volcanic hazard are combined in order to estimate values for SES
probability of igneous activity at location C, given the occurrence of RES I, specific models
and their associated results.
The variability of the SES probability values at location C given in Table 5.2 seems rather low;
this effect is likely related to the linear extrapolation performed for the estimation of volcanic
hazard over a 1 Myr period. The probabilistic assessment of volcanic hazard for such a longer
period requires datasets that extend beyond the Quaternary, possibly up to the Miocene, inorder to describe the volcanic patterns likely to occur in space and time.
The key advantage of this approach lies in the capability of assimilating various sources of
uncertainty, related to scenarios, models and data, when performing probabilistic hazard
assessments.
Table 5.2. Volcanism example: estimation of probability of igneous activity at site C for three SES.
RES IModel=

Cluster
(Cox)

0.688

0.525

0.512

0.688

0.525

0.688

0.525

5.2.2

Kernel(SAMSE)

Homogeneous
Poisson

0.340
0.058

Volcanic
hazard at
location C

SES
Probability
at location C

3.7∙10-3

6.8∙10-4

7.2∙10-3

8.8∙10-4

2.6∙10-2

5.4∙10-4

Estimation of IS probability of a VOGRIPA M5 event at Location C

Potential for large magnitude volcanic eruptions is particularly relevant for the siting ofgeological repositories. Such eruptions, should they occur directly through a repository, could
result in the release of a large fraction of the inventory into the biosphere. Although theprobability of such eruptions is extremely low (Connor et al., 2009) their potential effects arelarge, so estimation of risk associated with these rare events should be considered as part ofthe siting process.
Volcanologists have developed at least two methods for characterizing the size of volcanic
eruptions. The volcano explosivity index (Newhall and Self, 1982), generally termed VEI, uses
an integer scale from 0 to 8 and is based on both the volume of erupted material and theheight of the explosive eruption column. For HLW facilities VEI => 5 is of most concern
because of the high impact and frequency of events in the VEI 5 range. These eruptions (VEI3=> 5) have eruptive volumes of tephra greater than 1 km and typically have eruption column
heights that reach maximum height of greater than 25 km. Like many indices of natural
hazards, the VEI scale is approximately logarithmic. Consequently, the rarest and largest
magnitude explosive eruptions have scales of effects on the order of thousands of square
kilometers. Indeed, the largest known volcano eruptions have continental scale effects (e. g.,
the Yellowstone eruption of approximately 600 kyr). The global frequency of VEI => 5 isapproximately 0.03 per year, based on the known record of activity in the Holocene (Seibert-3
et al, 2010). Globally, rates of VEI 7 eruptions occur with an annual frequency of 10 per-4year; VEI 8 eruptions occur with an annual frequency of approximately 10 per year.
A second method for classifying eruptions is the magnitude scale proposed by Mason et al.
(2004) that is based on the magnitude of the total volume of erupted products. This scale
relaxes the assumption of explosivity, including effusive eruptions as potentially large
magnitude events. The largest volume effusive eruptions, often referred to as flood basalts
because of the large areas inundated by lava flows, are extremely rate, occurring on Earth ontime scales of tens of millions of years. Therefore, the largest magnitude eruptions generally

62are explosive in nature, producing copious ash fall deposits over vast regions. In the following,
we consider the relative frequency of large volume explosive eruptions by tectonic setting
using this magnitude scale, acknowledging that large volume effusive events may also occur
but are exceedingly rare, even on the time scales of geological repository performance
periods.
We have developed an explosive eruptions database of the Japanese islands. This Japan
database forms part of the Large Magnitude Explosive Eruptions database of Volcano Global
Risk Identification and Analysis (VOGRIPA) project. The database contains information about
the age of eruptions, pyroclastic ejecta volume, VEI (Volcanic Explosivity Index), magnitude
and data source of volcanic records. The database attempts to include all known explosive
eruptions during the Quaternary and VEI magnitude ~4 or greater. We use this database to
explore the preservation of products of explosive eruptions and patterns of volcanic activity inJapan and to provide data to help assess the probability of the largest magnitude eruptions
occurring at or near a geological repository in Japan during its performance period.
5.2.2.1 The Japan VOGRIPA database
The Large Magnitude Explosive Eruptions database of VOGRIPA has been systematicallycompiled from primary and secondary sources. Major data sources of explosive events inJapan include:
•

•
•
•

The one million year tephra database for the Japanese islands and the Japan 2000
year eruption database. Both of these databases can be found at the website:
http://gunma.zamurai.jp/database/ maintained by Professor Hayakawa at the Gunma
University, Japan.
The
activevolcanodatabaseof
Geological
SurveyofJapan(http://riodb02.ibase.aist.go.jp/db099/index-e.html)
The Quaternary volcano catalogue of the Volcanological Society of Japan(http://www.geo.chs.nihon-u.ac.jp/tchiba/volcano/index.htm)
The atlas of tephra in and around Japan (Machida and Arai, 1993).

Information from these references was cross-validated and discrepancies addressed where
possible.
The database contains a total of 696 explosive eruptions. Half of these eruptions occurred
within the last 65 kyr; 77% of the total eruptions occurred since 200 kyr; the oldest eruption in
the database is 2.25 Myr. In addition, percentages by eruption magnitude are: VEI 4 (40%),
VEI 5 (42%), VEI 6 (13%) and VEI 7 (5%). Given the generally accepted order of magnitudedecrease in eruption frequency with order of magnitude increase in eruptive volume (aGutenberg-Richter style relation for eruption frequency-magnitude), it appears that large
magnitude eruptions are over represented in the database. That is, VEI 5 eruptions are nearly
as abundant as VEI 4. Similarly, VEI 6 and 7 eruptions are over representative if they truly
occur at order of magnitude lower frequencies. Part of this result may be attributable to thepreservation potential of volcanic deposits. Because larger volume eruptions are more readilypreserved, they may be disproportionately represented in the geologic record. Furthermore, it
may be more likely that larger volume explosive eruption deposits will be identified in thegeologic record and these deposits described. Thus, there is a clear need to develop astatistical model of the frequency of large magnitude events, given our imperfect knowledgeof this frequency based on the geologic record alone. Such models will provide key input
parameters to spatial density models (ITM methodology) and will allow improvements ofhazard maps linked to specific volcanic events.
5.2.2.2 Development of Statistical Models
Figure 5.5 illustrates empirical survivor functions for eruptions of VEI magnitude 4 and greater
as a function of time. This plot is created by normalizing the total number of events in each
VEI category, and plotting the complementary cumulative distribution as a function of time.
Several features of the plot indicate that eruptions are under-recorded as a function of time.
For each VEI category, the number of known eruptions decreases approximately
exponentially as a function of eruption age. The half-life of each empirical survivor function(time since the present to 50 percent cumulative number of eruptions) depends on VEI
category. VEI 4 eruptions have much shorter “half-life” than VEI 5; VEI 7 eruptions have the63longest “half-life”, as expected given the large magnitude of these events. If eruptions of any
age to 2.2 Myr, the total duration of the Quaternary record, were equally likely to be preserved,
the expected values of the half-life would be approximately 1.1 Myr. Given that all VEI
categories have significantly shorter half-lives than 1.1 Myr, it is clear that many older
eruptions are missing from the geologic record. Finally, it is clear that the preservation ofrelatively small magnitude eruptions (e.g., VEI 4) is much less likely than the preservation of
the relatively large magnitude eruptions.

Figure 5.5: Empirical survivor functions for eruptions of VEI magnitude 4 and greater, as a function oftime.

Note that the change in half-life as a function of VEI category (Figure 5.5) is a strong
indication that eruption frequency has not changed with time. Because VEI 4 eruptions are
smaller volume than VEI > 4, they are much more likely to disappear from the geologicalrecord in a shorter period of time. Hence the half-life of VEI 4 eruptions is much less than thatof the larger eruptions. The most important result of this simple analysis is that estimates ofthe frequency of explosive eruptions will be underestimated if they are based on average
Quaternary rates. That is, we can be certain that the number of Quaternary large-magnitude
explosive volcanic eruptions in Japan greatly exceeds 696, the total number identified in the
VOGRIPA database.
We developed a double exponential model to describe the change in observed (known)eruptions as a function of time. This model has the form:
R(t) = R1 exp (-l1 t) + R2 exp (-l2 t)
where R(t) is the rate of preserved (recognised) eruptions as a function of time, t. R and l are
estimated from the distribution of explosive eruptions of a given VEI category as a function
time in the VOGRIPA database for the Japanese islands. We use a double exponential model
because, intuitively, two separate processes may be operating to produce the observed
eruption frequency distribution. The first term represents the ability of a deposit to bepreserved in the geologic record. The second term represents the ability for geologists torecognise the deposit, given that it is preserved.
The average frequency (recurrence rate) of large magnitude explosive volcanic eruptions in
the Japanese islands can be estimated from these fitted distributions, as a function of VEI
category. Figure 5.6 illustrates this fit using category VEI 6 eruptions from the VOGRIPA
database. Observations from the VOGRIPA database suggest that the current recurrence

64

rate of VEI 6 eruptions in the Japanese islands is approximately 0.2 events per one thousand
years. This value appears to be somewhat low compared to the global estimated rate of VEI 6
eruptions of approximately 1 per one hundred years, based on the global Holocene record.
However, detrending this record using the double exponential model yields an average
recurrence rate of VEI 6 eruptions of 0.6 per thousand years. Using this average value, theexpected number of VEI 6 eruptions in the Japanese islands during a 2.2 Myr period is 1320
eruptions. The record < 100 kyr also gives an indication of the fluctuation in recurrence rate of
VEI 6 eruptions, and hence the uncertainty in this estimate. It can be seen (Figure 5.6) thatrecurrence rate in this period appears to have fluctuated between approximately 0.2 – 0.9 VEI
6 eruptions per thousand years.

Figure 5.6: Observed (light blue) and modelled (light red) recurrence rate of VEI 6 eruptions from the
VOGRIPA database. The integral of the observed recurrence rate, yields 90 VEI 6 eruptions in theJapanese Islands during the Quaternary (last 2.2 Myr). The detrended recurrence rate is much higher,
and indicates that on order 1000 VEI 6 eruptions actually occurred in the Japanese islands during thisperiod (average recurrence rate of approximately 0.6 VEI 6 eruptions per 1000 years).

Overall, the results suggest that 97% of VEI 4 events are missing from the record after only
100 kyr, whereas 40% for VEI 5 to 7 are missing after this time period. These results indicatethat eruption probabilities based on long term recurrence rate must account for the potential
for even large eruptions to be missing from, or unidentified in, the geologic record.
We can also use these data to investigate the frequency-magnitude relationship of large
volume explosive eruptions in the Japanese islands as a function of tectonic setting. In orderto accomplish this, the Japanese islands were subdivided into tectonic domains. These
domains included: Hokkaido, Tohoku, Central Japan, Chugoku, Kyushu, and the Izu-Boninarc. As with earthquakes, one might expect that the frequency-magnitude relationship, or bvalue, of large magnitude explosive volcanic eruptions can vary with tectonic setting. These
relationships are shown using detrended recurrence rates for three areas in Figure 5.7.
In Tohoku, the Pacific plate is subducting beneath the North American plate. Results of ouranalysis suggest that smaller eruptions are much less frequent than expected for a b-value =1. A b-value of 0.7 is estimated using all of the detrended data. One explanation is that small
volume magma batches solidify as intrusions more frequently than expected (compared with
Hokkaido and Kyushu), possibly due to thicker or cooler crust. In Hokkaido, the Pacific plateis subducting beneath the North American plate, as in Tohoku. In this region, however, a bvalue of approximately one, or slightly less, seems to characterise the frequency magnitude
distribution of large-volume explosive volcanism. Similarly, in Kyushu, where the Philippine
Sea Plate is subducting below the Eurasia plate, a b-value=1 appears to characterise
volcanism. Thus, b-value appears to be different in Tohoku than in these other areas,suggesting that Tohoku is characterised by larger magnitude eruptions than expected using
global averages.

65

Figure 5.7: Frequency-magnitude relationships for large volume explosive volcanic eruptions in three
regions (Tohoku, Hokkiado, and Kyushu) based on analysis of the VOGRIPA database for theJapanese islands.

5.2.3

Implications for potential repository sites

Clearly, estimates of volcanic hazard in active regions cannot simply be based on average
numbers of events. Analysis of the VOGRIPA database indicates that such an approach (e.g.,using average Quaternary recurrence rates of volcanism) vastly underestimates the potential
for large magnitude explosive volcanic eruptions. This is relevant, given typical regulatory
requirements to assess repository performance over periods of 100 kyr to 1 Myr (even though
the hazard potential of HLW diminishes rapidly over a much shorter timeframe). Assessment
periods of the order 100 kyr or 1 Myr require correspondingly long time periods for estimationof recurrence rates. The analysis of the VOGRIPA database indicates that bias invariably
occurs in recurrence rate estimates for geologically long periods of time due to the variable
preservation potential of deposits.
One approach to estimating the potential for large magnitude explosive eruptions (e.g., VEI=> 5) would be to multiple the spatial density of volcanism (derived from Cox process orkernel density models) with the appropriate recurrence rate based on the VOGRIPA-derived
frequency-magnitude relationships. Caution is needed in this regard, however, because thesefrequency-magnitude relationships are derived for existing volcanic systems. For volcanic
hazard assessment at a potential geological repository site, no volcanism currently exists atthe site, so one needs to consider the potential development of volcanic systems and the typeof volcanic system that develops. For example, Mahony et al. (2009) demonstrated that the
distribution of dominantly explosive volcanoes in Tohoku is different from the distribution of
dominantly-effusive volcanoes. Consequently, the frequency-magnitude relationship cannot
be uncoupled from the spatial density estimate, at least for the Tohoku region.
Furthermore, evidence appears to exist that volcanic systems evolve from dominantly effusive
to dominantly explosive through time (e.g., Druitt et al., 2002; Connor et al., 2006). Therefore,
considering performance periods of 1000 – 10,000 years, the potential for large magnitude
explosive volcanism at the site is completely different than would be estimated for a 1 Myr
performance period. One thousand years is too short a time period for a volcano to develop
and evolve a magma chamber capable of sustaining large volume eruptions. On the other
hand, 1 Myr is a perfectly reasonable time period for such developments. Additionally, since
the probability of large magnitude events through the repository within the first and critical
1000 years of performance is much less than the average rate (given the evolutionary natureof volcanic systems), radiological consequences and risk associated with large volume
explosive eruptions might be overestimated using an average approach.
These factors indicate that considerably more detailed analysis is required to estimate thehazard and risk of large-volume explosive volcanic eruptions at a potential HLW repositorysite. Nevertheless, the analysis of the VOGRIPA database for the Japanese islands does
indicate that recurrence rates of large-magnitude eruptions in the Quaternary must be
modelled using statistical methods. Otherwise, hazard rates potentially will be underestimated.

66

Additional work is required to determine which statistical model is most appropriate,especially for integration into a performance assessment.

67

6

Repository Relevance of the Selected Impact Scenarios

The TOPAZ methodology described in the previous sections uses expert judgment andstatistical analysis to evaluate aggregate probabilities of plausible RES and SES for a givensite. Each non-zero branch of the RES-SES logic tree (probability > 0) leads to an ‘event
definition’ in the form of Impact Scenarios (IS) that are relevant to the long-term performanceof a deep geological repository. As discussed in Section 1, for a given RES-SES branch,
there can be multiple possible ISs that might arise. Eventually, expert judgments must also
be used to establish probabilities of individual ISs with such sets of credible ISs. The resultsof the TOPAZ logic-tree approach, with associated RES, SES and IS probabilities, can thenbe used by NUMO‘s performance assessment team to inform, guide and focus their safety
analyses based on expert geoscientific judgments.
The preliminary development of ISs by TOPAZ is discussed here. Section 6.1 examines thebasic time-dependent evolution and performance of a generic HLW repository. As discussed
in Section 1.2, within a relatively short timeframe (of the order of 10,000 years) compared to
evolving volcano-tectonic events in Japan, the radiological hazard of HLW diminishes toextremely low levels and the engineered barriers of a HLW reach the end of their design
functions. These engineered materials return to a ‘natural state’ that are relatively inert tothermal-hydrological-mechanical-chemical (T-H-M-C) changes in the deep geologicalenvironment. Thereafter the greatest safety concerns are possible ISs leading to the directtransport of radioactive material to the ground surface.
The following sections describe basic attributes of three groups of impact scenarios (IS) for(1) rock deformation, (2) volcanism and (3) repository uplift and exhumation (noting that thelatter has not yet been evaluated in the TOPAZ demonstration work, although a preliminary
logic tree has been developed to do so when required). Within each IS group there are, ofcourse, a wide number of variants with different impacts, which are briefly reviewed.
Recommendations for preliminary aggregation of representative variants for each IS groupare made for further consideration by NUMO.

6.1

Evolution of a HLW Repository

Figure 6.1 shows the basic engineered barriers for a generic geological repository for HLW(JNC, 2000). Borosilicate glass is the reference waste form for encapsulation and disposal of
reprocessed HLW in Japan. The borosilicate glass is fabricated within a stainless steel
vessel that is placed into a massive mild-steel overpack (sometimes called “canister”) that isdesigned to provide initial containment. The HLW plus overpack is called a ‘waste package’,and the waste package will be emplaced into deposition holes or tunnels within a suitable
repository host rock. A bentonite-based buffer barrier containing a large fraction of smectite
clay (that swells on contact with water) is placed between the surface of the deposition holes
and the waste package. This buffer assures diffusion-only release of any radionuclides fromthe EBS into the host rock, aided by further retardation of transport by radionuclide-sorption
on buffer minerals. Taken together, the buffer, waste package and any other seals or plugs
placed into the underground openings of a repository are called the engineered barriersystem (EBS).
There are numerous variants to EBS concepts under consideration by NUMO, including
different orientations, different dimensions of materials, and different compositions of barriermaterials. While it will be important to link and revise the TOPAZ methodology to any specificfuture repository concept (RC) developed by NUMO, at this preliminary stage the basic
generic H12 repository concept shown in Figure 6.1 is selected for this exercise.
After emplacement and sealing of a HLW repository, the initial post-closure phase is often
referred to as the “Containment Period” in which the EBS, notably the overpack, acts toprevent contact of the HLW by groundwater. Slow, predictable corrosion of the mild steel
overpack prevents any radionuclides releases during the time that radiogenic heating byradioactive decay by short-lived radionuclides elevates both the near-field temperature andtemperature gradients from ambient values. Within 300-500 years, the initial thermal pulse
has largely dissipated, with ambient thermal conditions returning within several 1000’s ofyears depending on site-specific conditions (JNC, 2000, Volume 3). The ‘containment’ design

68function of the overpack is, therefore, on the order of 1000 to 2000 years (see Figure 3),although somewhat longer containment times may occur due to variable environmental
conditions or use of conservative overpack thicknesses.

Figure 6.1: Natural and Engineered Barriers for a Generic HLW Repository (from JNC, 2000).

Once the overpack has ‘failed’, groundwater will contact the HLW borosilicate glass, leading
to sustained dissolution and release of radionuclides over a prolonged “Release Period”.
Under the expected reducing conditions of a deep geological repository, most of the released
radionuclides will be incorporated into low-solubility phases that precipitate, thereby limiting
the release rate of these radionuclides from the EBS into the host rock. Radionuclides will betransported by groundwater flow to a hypothetical major water-bearing fracture zone thatenables advective transport to the surface environment, assumed here to be along a coast as
a representative location for a repository (Figure 6.2). A few radionuclides, such as Cs-135,
may be extremely soluble under likely repository environmental conditions, so that theirrelease from the EBS will be controlled by the dissolution rate of the borosilicate glass. The
time for total dissolution of HLW glass can vary depending on factors, such as the assumed
degree of surface cracking, presence and interaction with other engineered barriers andprevailing geochemical conditions, with a range on the order of approximately 10,000 and
70,000 years, and perhaps longer for extremely favorable disposal conditions (e.g., JNC,
2000, H12 report, Table 6.1.2.1-1; US National Academy of Sciences, 2011). The transport
time for radionuclides from the repository to the nearest major water-bearing fracture zone isalso relatively short because of the assumed relatively short distance (~50-100 metres) to thesub-vertical fracture zone (Figure 6.2).
A key point to understand with respect to assessing impacts from future natural events is thatafter the combined Containment and Release Period (on order of approximately 50,000100,000 years after repository closure, see also Figure 6.3), the overpack will likely havebeen breached by corrosion and mechanical buckling in approximately the first 1000 years(JNC, 2000, Table 6.1.2.1-1), leading to transformation of the initial glass waste form into an
assemblage of low solubility alteration phases (JNC, 2000, Table 5.3.1-6; US National
Academy of Sciences, 2011).
The original repository has basically converted to a thermodynamically more-stableassemblage of insoluble, slightly radioactive oxides, hydroxides and silicates surrounded byiron hydroxides/ oxides of the corroded overpack, in turn surrounded by a layer of compacted
bentonite buffer. At ambient environmental conditions, these residual phases are unreactive
over timescales of a million years or more based on numerous geological and natural
analogue studies. Soluble radionuclides have been released, retarded, dispersed, diluted inaquifers, and diminished by radioactive decay throughout the rock volume surrounding the69

repository, as shown in JNC, 2000 (Chapters 5 and 6, e.g., Figures 5.3.3-2 and 6.1.1-1). Low
solubility radionuclides will be isolated and retained in alteration phases of the initial HLW
glass (JNC, 2000, Table 5.3.1-6; see also Figure 6.3 of this report). Thus, the engineered
barriers of the repository evolve toward a ‘natural state’, in closer equilibrium to thesurrounding host rock environment. The leading concern for any longer term (>50,000 year)
combined RES/SES scenario would be if a natural event led to transport of the residual,
insoluble radioactive phases to the near-surface oxidizing environment (see Section 6.2.3) or
exposure at the ground surface.

Figure 6.2: Schematic representation of radionuclide release from a deep geological repository for HLW.

70

0Time after 10 ydisposal

Over Pack

101y

102y

104y

103y

Physical containment

105y

Physical containment

Chemical buffering

Glass

物理的な閉じ込め

Slow nuclide release

Slow nuclide
releaseLow permeability
Low elemental solubility

Buffer

Filtration of colloids,
microorganisms and organics

Sorption/diffusion
Retardation ad dilution and dispersion

Host rock（slow groundwater flow, sorption, path length）

Conditions for EBS emplacement and long-term stability

Basic function （expected function in reference case of H12 report）
Potential function （expected function as back-up in case of losing other functions unpredictably）
Reserve function （consideration of data uncertainty case and alternative design case in H12 report）

Figure 6.3: Schematic diagram of the assumed safety functions of engineered and natural barriers overtime following closure of a generic H12-type repository. Sources for this schematic are JNC (2000) as
interpreted by Ebashi et al (2011).

6.2

Illustrative Impact Scenarios

As a preliminary demonstration of the use and utility of the TOPAZ methodology, a
representative set of Impact Scenarios that might arise from combined RES/SES scenarios
are considered here. These illustrative ISs could serve to provide the necessary link between
NUMO’s set of geological experts and its performance assessment (PA) team. These
representative ISs are based on the following assumptions:
•
•
•
•
•
•

the generic H12 HLW repository concept (Figure 6.1);
a generic hard-rock site for the repository;
a repository depth of 300 metres;
saline, reducing and near-neutral pH groundwater conditions at the repository depth,
avoiding placing any waste package across rock discontinuities or fracture zones;
consideration of the types of volcanic and rock-deformation events that mightsignificantly compromise the expected isolation performance of the engineered andnatural barriers of the repository.

The following sub-sections explain the basic variants of volcanism, rock-deformation anduplift and erosion ISs that might arise from future changes in the tectonic setting of Japan.
One illustrative variant for each IS is described and proposed to be used when eventuallyconducting a full expert elicitation on TOPAZ’s RES-SES-IS methodology: that is, extending
the expert elicitation exercise presented in this report to include the ISs as well as the RES
and SES. It is noted that the October 2011 TOPAZ workshop did not include consideration of
ISs and did not explore how to assign expert-judgment probabilities or likelihoods to ISs.

71

6.2.1Rock Deformation Impact Scenario

TOPAZ has previously used strain rate as a proxy metric for considering how long-term,
sustained tectonic forces might lead to some form of rock deformation at a candidate
repository site. Figure 6.4 schematically illustrates a range of possible mechanisms by whichsustained strain rate in a repository host rock might manifest rock deformation impacts. At
this stage for an unexplored, generic hard-rock site, there is no possibility to sensibly judge
the likelihood of one mechanism over another.

Figure 6.4: Schematic representation of different types of rock deformation that may arise fromsustained elevated strain rate.

A leading rock-deformation IS considered for hard rock sites, being considered in Sweden
and Finland for example, is a future earthquake causing the “Fault Shear” of one or morewaste packages (see Figure 6.5). The physical displacement of the surrounding rock may
cause failure of the containment function of the overpack, or failure of the diffusion andsorptive functions of the clay-based buffer. The threshold for fault displacement leading tosignificant impairment of the functions of engineered barriers is on the order of 5-10 cm,
depending on the overall thickness of the buffer and material properties of the overpack/canister. Of course, the orientation and displacement of the fault, the number of sympathetic
rock displacements that may occur surrounding the main fault, and the timing of the faulting
event are all important aspects to consider in evaluating the actual impact on repository
safety.

72

Figure 6.5: Illustrative cross section of the rock-deformation variant case of fault shear of a waste
package. “pc” is the length of the waste package, “ds” is the length of the fault displacement assumed tooccur normal to the axis of the waste package.

As noted in the previous section, however, the containment function for the overpack is a few
1000’s years, and the diffusion-sorptive function of the buffer becomes unimportant to limiting
peak release rates from the repository system after the HLW has fully dissolved, a period on
the order of approximately 10,000 to 50,000 years (Apted and Ahn 2010, also see ‘Glass’ inFigure 6.3, which shows glass dissolution complete at approximately 50,000 years). This isbecause the remaining key dose-contributing radionuclides still present within the EBS (e.g.,
Se-79, Cs-135, Np-237, see JNC, 2000, Figure 6.1.3.2-1, for example) are so long-lived that
their sorption-retarded diffusive-transport time across the buffer would be less than one halflife. Hence, no significant decay would occur during diffusive transport, even if the buffer
remained intact (Apted and Ahn. 2010). Therefore, the rock shear IS for the buffer is ofsignificant concern only for earthquake events occurring in the about 50,000 years (Apted and
Ahn 2010) within the rock block containing the repository.

6.2.2

Volcanism Impact Scenario

The major factors to consider in possible volcanism ISs (Figure 6.6) are:••the type and magnitude of the volcanic event (e.g., monogenetic, polygenetic, or
caldera eruption);
the location of the event with respect to the repository: whether the volcanic event
directly or indirectly intersects the repository itself.

73

Figure 6.6: Schematic of a volcano either directly or indirectly intruding through a HLW repository(shown as red rectangles) (image from Steve Sparks and Thea Hinks).

The type of volcanic event defines important characteristics such as duration of the volcanism,
the lateral extent of the conduits of the volcanic system at repository depth, type and volumeof erupted material, magmatic temperature and volatile contents, and the evolution in thesecharacteristics over time.
TOPAZ has not yet made a systematic analysis of either direct or indirect impacts of a
volcanic event on repository performance, although recognizing the clear need to do soeventually to aid and guide NUMO’s PA efforts.
As a prelude to conducting formal IS analyses, TOPAZ has considered, the following
categories or groupings for volcanic ISs:•••

direct intrusion of a repository by a polygenetic or caldera event,direct intrusion of a repository by a monogenetic event or single dike, or
nearby intrusion of a repository by a volcanic event, with indirect perturbation of
conditions of the repository.

Of course, there can be a wide number of possible variant cases within each IS category. At
this time, with no specific site under consideration and no defined set of RES-SES constraints,
it would premature to attempt to document the wide range of possible types and74characteristics of variants to volcanic ISs. Aspects and difficulties regarding future, detailed
analyses of the three IS categories are outlined below.
The IS category of a large-scale volcanic directly intruding a repository has the potential totransport a significant fraction of the radioactive and non-radioactive materials of therepository to the surface, with significant impacts on repository performance and safety.
There are several challenges to devising ISs for large-scale eruptions, however. First, it will
be necessary to consider how (and how much of) the radioactive portion of this transported
material would be intimately mixed and diluted with both the non-radioactive portion of therepository and the pristine volcanic material erupted. Further aspects of this process are
discussed in the following paragraph. Second, radiological exposure analysis of any erupted
radioactive material would be complicated by consideration of partitioning of the radioactivecontaminated material among erupted lava, volcanic bombs, and tephra. Third, while
geotechnical models for evaluating air-borne dispersal of tephra exist, it is not evident if andhow such evaluations would be compared to Japanese radiological safety regulations for a
HLW repository, which are based on an assumed groundwater pathway (see Figure 6.2).
Fourth, the non-radiological safety impacts from such large-scale volcanic events could
conceivable dwarf radiological impacts, especially when the large volume-dilution ofradioactive waste by volcanic ejecta is considered. Therefore, this category of volcanic ISs
will eventually require considerable assessment to credibly demonstrate the application andlinkage of the TOPAZ methodology to NUMO’s PA efforts.
The IS category of a monogenetic or single dike directly intersecting a repository presents
similar challenges to building competence in defining volcanic ISs. First, it will be necessary
to evaluate how ascending magma could directly interact with repository materials, andwhether these repository materials (including radionuclide-bearing phases) would be in theirinitial state or degraded to an evolved ‘natural state’. Magmas generally are close to theirsolidus temperature, meaning the heat necessary for incorporation of repository materials by
melting would require a corresponding degree of crystallization of the magma itself.
Hydrodynamic incorporation of repository materials in ascending magma as solid “xenoliths”
would be an additional mechanism for bringing any radiological material to the surface. The
generic HLW repository, whether in an initial or evolved state, is expected to be fully sealed
and backfilled, so no lateral re-direction of ascending magma along open-pathways would be
expected. Second, and different than consideration of a large-scale eruptive event, it will benecessary to evaluate for a monogenetic event how portions of the repository adjacent to thevertical magmatic conduits might undergo indirect impacts on T-H-M-C conditions. The spatial
extent and duration of such indirect T-H-M-C changes will be much shorter for a monogenetic
event or single dike intrusion, and there are available models and field evidence to support
bounding estimates of such impacts. Third, the possible transport of radionuclide-bearing
material to the surface of a monogenetic event also presents the same difficulties as a largescale vent to NUMO’s PA team of how to consider and model such a consequence withrespect to Japanese radiological safety standards. Therefore, this category of volcanic ISs
also will require further resources to demonstrate the application and linkage of the TOPAZ
methodology to NUMO’s PA efforts.
The third basic IS category involves intrusion of a volcano nearby the repository, perhaps
within a distance of 15 km, the current NUMO exclusion value for siting a repository awayfrom present-day volcanic centres (see discussion in Section 1.3). Analysis of such a volcanic
IS is far more tractable (although not necessarily more likely) than the first two, directintrusion IS categories. First, such an event would have only indirect T-H-M-C impacts on arepository, with no transport of radiological material to the surface. While there arechallenges to this analysis as well, the modelling of indirect T-H-M-C impacts can beaccomplished with existing multi-phase T-H and T-H-C models, as well as T-M models.
Second, this category of indirect volcanic ISs results only in impacts on the release
performance of a repository via groundwater pathways. This is an analysis for which the
NUMO PA team has existing models, and even supporting data because of similar thermalimpact modelling needed to evaluate initial radiogenic heating of a HLW repository. Of
course, the consequences of a nearby intrusion of a volcano will be significantly less severe
than consequences on repository performance compared to either of the direct intrusioncategory ISs.

75

As a way forward, it is recognised that a step-wise approach to building up competence on
volcanic ISs can be based on the fact that the three IS categories are, in effect, nested subsets. That is to say, analyses of T-H-M-C impacts for the indirect volcanic ISs are the mostbasic (and currently tractable) analyses. Such indirect impacts are a sub-set of the analyses
necessary to evaluate the monogenetic/ single-dike ISs, which eventually requiresconsideration of how a relatively limited portion of radioactive and non-radioactive repositorymaterials might interact with ascending magma. And in turn, analyses of the issues for themonogenetic/ single-dike ISs are a sub-set of range of analyses needed for the large-scalevolcanic ISs, in which there would be long-term and spatially extensive disruption to therepository system

6.2.3

Uplift, Erosion and Exhumation Impact Scenarios

While the radiological hazard of HLW does significantly decrease over time, the expected
reducing conditions of a repository at depth leads to localised retention of most radionuclides
within the EBS, either by precipitation or strong sorption. This spatial retention of insoluble
radionuclides within a deep repository is a favourable safety feature. This same feature,
however, represents a vulnerability, if sustained uplift and erosion were to bring thisconcentrated mass toward the near-surface environment (Figure 6.7).
If a repository, even in a degraded ‘natural state’, nears the surface, enhanced radionuclide
release to the accessible environment can occur.
This occurs because (1) many
radioelements become more soluble and less sorptive under oxidizing conditions, and (2) thetransport pathways for radionuclides to reach the surface environment become shorter(Figure 6.8). If taken uplift and erosion is taken to an endpoint, a repository may be exhumed
at the ground surface, leading to significant radiological exposure through multiple water andair-borne pathways as well as direct exposure to future humans.

Figure 6.7: Schematic illustration of uplift and erosion impact scenario.

There are numerous possible variant cases for the uplift and erosion IS, some of which areillustrated in Figure 6.7. One variant is the erosion of the overburden above a repository,
leading to depressurization and an increase in bulk permeability of the rock. An increase inpermeability, as well as influences of surface topography on hydraulic head and formation of
new or extended fractures in the rock surrounding a repository (Figure 6.8), could lead to76

faster groundwater flow and shorter transport pathways, hence faster radionuclide transport
to the surface. Another variant is based in a boundary layer of oxic surface water penetrating
down into a repository that is being uplifted (Figure 6.8). In this variant, the intruding
boundary layer of oxic conditions would first lead to enhanced mobilization of previously
released redox-sensitive radionuclides sorbed on mineral surface of the rock. Eventually, as
the oxic boundary layer intruded into the EBS portion of the repository, the solubility of many
redox-sensitive radioelements would greatly increase, further enhancing radionuclide
mobilization. This ‘shrinking core’ variant case, based on gradual intrusion of oxygenatedwater into a disposal site. A third variant, with the most extreme radiological consequences,would be the endpoint of uplift and erosion in which the repository is exhumed at the surface.
There are existing, verified models that could be used to address the impacts on radionuclide
release via groundwater for both the ‘increased permeability’ and ‘shrinking core’ IS variants.
It will be important to carefully describe the evolution in hydrological, mechanical andchemical conditions over discrete time and spatial intervals, to avoid overly conservative
estimates of impacts on radionuclide release (as, for example, in JNC, 2000). At the sametime, caution will need to be used to avoid overly optimistic assumptions regarding theapproach-rate, orientation and characteristics of a repository as it reaches the near-surface
environment in the far future. As for the ‘exhumation’ variant IS, NUMO’s PA team would
need to consider in what way to model and evaluate radiological consequences for not only
groundwater pathways, but also air-borne pathways and direct external exposure.

Figure 6.8: Schematic illustration of sequential impacts from uplift and erosion. Path lengths forradionuclide transport to major water-conducting faults decrease, while ascent of repository anddeveloped release plume (shaded red) into oxidizing zone leads to increase in radioelement solubilities
and decrease in radioelement sorption.

6.3

Aggregation of Impact Scenarios

A potential issue in applying the TOPAZ methodology is that geoscience experts may identify
a large number of possible variants for each of volcanism, rock-deformation and uplift-erosion
impacts scenarios (ISs).
The conceptual basis of the TOPAZ methodology canaccommodate any number of ISs, however, the computational burden can become acute forexpert elicitations. This is because a large set of ISs would need to be defined, discussed77

with respect to site-specific and region-specific data, applied, and probabilities evaluated foreach of the previous RES-SES branches for a given site. In order to allow a more accessible
demonstration and testing of the full TOPAZ methodology, therefore, an effort was made in
the current project to consider the aggregation of volcanism, rock-deformation and uplifterosion ISs in a limited set that could be broadly applied to any hypothetical candidate
repository site (Table 6.1). It is recognised, of course, that when real candidate repositorysite were to be evaluated in the future, a much fuller range of ISs would need to be needed inorder to confidently assure that all credible variants had been considered.
Table 6.1 shows one possible formulation of volcanism, rock-deformation and uplift ISs.
Three combined rock-deformation and uplift-erosion ISs are listed down the side of Table 6.1.
•••

no new faulting within the repository block and no significant uplift;
formation or re-activation of faults intersecting part of the repository but no significant
uplift;re-activation of faults intersecting part of the repository and significant uplift.

Volcanism ISs are listed across the top of Table 6.1:•••

no volcanic event occurring within 15-km of the repository;
volcanic event occurring 15-km of repository, but no direct intrusion of the repository;
volcanic event directly intruding the repository.

It is proposed that these nine ISs would capture all credible impacts that might arise fromfuture tectonic and volcanic events at a site, while implicitly aggregating the large number ofpossible variant cases within each of the ISs. Another advantage is that these nine ISs canbe used to highlight for NUMO’s PA team what issues and capabilities they may need to beconsidering for safety assessment of natural event scenarios. While careful and prudent site
selection and characterization may reduce or eliminate the possibility of one or more IS (forexample, future volcanism may have a vanishingly small probability at certain locations inJapan), it is unlikely that past safety assessment models applied to the static Reference Casewill be fully capable of safety assessment for other ISs shown in Table 6.1.

6.4

Summary

The TOPAZ effort on Impacts Scenario (IS) analysis to date has focused on identification of
categories for volcanism, rock-deformation and uplift-erosion ISs, as well as specific variants
within each IS. The October 2011 TOPAZ workshop did not have the opportunity to elicit
specific views from Japanese geoscience experts as to possible impacts. A key result of theworkshop, however, was recognition that the large number of potential variant cases for each
IS category could quickly overwhelm the ability to initially demonstrate and test the TOPAZ
methodology.
The TOPAZ methodology can conceptually accommodate any number of ISs. The difficulty is
the computational burden arising from multiplying the large number of possible ISs by a large
number of RES-SES branches for a given site. In order to allow a more straightforward
future demonstration of the TOPAZ methodology, therefore, an initial suggestion is made toaggregate volcanism, rock-deformation and uplift-erosion ISs. A preliminary set of nine ISs is
proposed that can be broadly applied to any hypothetical candidate repository site andassociated RES-SES assessments. In the future, when real candidate repository site are tobe evaluated, a much fuller range of ISs variants could be considered within the same
TOPAZ methodology in order to assure decision makers and other stakeholders that allcredible ISs have been considered.

78

Table 6.1: Possible aggregation of impacts scenarios, with associated issues regarding
performance assessment analyses.

No future volcanic
event

Volcanic event; nointrusion into
repository

Volcanic event;intrusion into
repository

No new faulting

IS-1

IS-2

IS-3

No uplift to near-surface• Reference Case

• groundwater pathway
only• IS-2 analysis• current PA models
applicable

Fault reactivationintersecting repository
No uplift to near-surface• current T-H-M-C codes
probably suitable• need to consider
radiological exposure
pathways at surface• need for careful IS
description• need to evaluate magma
impacts

IS-4

IS-5

IS-6

• need to evaluate
number of waste
packages damaged• combined IS-2 and IS-4
analyses• combined IS-3 and IS-4
analyses

IS-7

IS-8

IS-9

• IS-4 analysis• combined IS-2 and IS-7
analyses• combined IS-3 and IS-7
analyses• need model for loss of
buffer safety functions

Fault reactivationintersecting repository
Uplift to near-surface• groundwater pathway
analysis for ‘shrinking
core’ from oxic water• groundwater pathway
analysis for reduced
path length/ higher flow
• radiological assessmentof exhumed repository?

79

7Conclusions

The TOPAZ project is a continuation and development of the ITM project that extends theprobabilistic methodology of ITM to provide information on the likelihood and nature oftectonic impacts for timescales up to 1 Myr in the future. As with the ITM project, TOPAZ has
developed, but only partially tested, the methodologies that will be needed as potential
repository sites emerge and require evaluation. Refinements and some further development
will undoubtedly be needed as the methodology is applied.
The TOPAZ project team considers that probabilistic approaches are fundamental to any
evaluation of tectonic hazards to long-term repository safety in Japan. The Fukushimadisaster has highlighted graphically how, even on a much shorter-term basis, the use ofmultiple geoscientific data sets and probabilistic techniques is necessary to characterise
hazards to facilities for quantitative risk assessments. Although the regulatory approach togeological disposal is still under consideration in Japan, the TOPAZ project team considers
that it will be vital to be able to present estimates of event likelihood when discussing hazards
and potential health impacts of a repository. These may be in the form of actual risk estimates(as used in the regulatory guidelines and requirements for geological disposal in several
countries), or in the form of disaggregated estimates of radiation doses and their likelihoods.
Whichever direction regulatory development takes, it will be necessary to present hazard
potential and uncertainty estimates clearly to various audiences in a probabilistic as well as adeterministic sense. The ITM-TOPAZ methodology will be an effective tool for doing this. In
particular, the approach presented in this report addresses the key issue of uncertainty overlong future time periods.
Increasing uncertainty with time in the tectonic evolution of any region in Japan is captured in
the ITM-TOPAZ approach by applying elicitation techniques to incorporate the range of expert
opinion on alternative tectonic evolutionary models. This range of uncertainty is weighted andconverted to quantitative estimates of likelihood (probabilities) of different evolutionary paths.
As different paths will have different outcomes in terms of tectonic events that may impact ageological repository, and the time at which they might occur, these quantitative forecasts canbe transferred to probabilities of occurrence of certain safety relevant impacts. This provides
safety assessors with quantitative information both to compare alternative sites with respect
to relative tectonic hazard and to provide regulatory authorities with either risk-based hazard
forecast or deterministic estimates of health impacts accompanied by relative likelihoods ofoccurrence of specified exposure scenarios.
In the TOPAZ project reported here, only a sample of all the requisite steps in the applicationof expert elicitation has been tested. Using the results of the previous ITM Tohoku Cases
Study and focussing on one of the example locations from that study, the TOPAZ project has
demonstrated the application of expert elicitation, principally to the RES stage, and thenprovided a condensed demonstration of how it could be used at the SES and IS stage, bysimplifying the ‘SES set’ considerably and by modelling only two example ISs. Neverthelessthese two ISs will be critical components when developing a safety case. The TOPAZ project
has not yet illustrated the full range of information that can be derived from the elicited
credible intervals on each probability, as we have only used the median values in thedemonstration.
When the methodology is applied to a ‘real’ prospective siting location, then elicitation will
have to be applied to scenario definition in more detail at both the SES and IS level. The
TOPAZ project has shown how this can be done and prepared the ground for a real site
evaluation.
An important outcome of TOPAZ is that it shows how the wide range of geosciences data andexpert opinion that exists in Japan can be incorporated into a safety analysis. NUMO
considers it important to be able to explain a logical approach to the complex problem anduncertainties of tectonic impact forecasting and to take account of differing views in thegeosciences community. TOPAZ not only provides a method for involving and informing thecommunity, but also for sampling these opinions and actively including them in its
assessment and site suitability work.

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84

Appendix: Concealed Active Faults Workshop
Notes on the Concluding Discussionsstnd

A three-day workshop was held in Christchurch, New Zealand from 31 January to 2
February 2012, hosted by GNS Science staff and involving NUMO staff and consultants, plus
some of the other members of the TOPAZ team. The aim was to discuss the characteristics
of concealed active faults, particularly in the light of the ongoing series of earthquakes in the
Canterbury Plains area, which had resulted in devastating damage and loss of life in the cityof Christchurch in 2011. The workshop participants were able to discuss the nature of thebasement faults in the area and see evidence for deformation at the surface. This enabled a
lively discussion on the concerns being addressed by NUMO with respect to identifying
environments where active faults may be concealed and means of tackling this issue in theirsiting programme. This note provides a short summary of the concluding discussions.

1Context

Concealed active faults (CAFs) have been considered a potential problem in NUMO’s siting
programme as (a) they are legally required to reject sites that lie on active faults and (b), if
they failed to locate a deep active fault beneath a repository during site investigations (SI)there is a risk to the safety case. Therefore, the interest in discussing them is to find out
whether:

2•

There is some possibility that their potential presence can be indicated in advance ofcommitting to a site (to avoid problem (a) above);•

There is good prospect that a detailed SI will identify all deep, blind faults (those thathave no surface expression and whose activity cannot be inferred with confidence)that have the potential for reactivation as AFs in the future, allowing repository
location to be closely linked to the deep structural geometry and thus avoid problem(b) above.

What do we mean by CAF?

There are certainly many AFs that are not on the AF Map of Japan – they have simply not
been recognised and mapped, as they may be in inaccessible terrain or simply not have been
examined in sufficient detail to determine whether they are active (i.e. have evidence of
movement in the last c.200 kyr) or not. Provided that these faults have a surface expression,then detailed mapping and sampling/trenching carried out in an area of interest, should locate
these ‘missing’ AFs.
AFs should be assumed to be present anywhere, unless exhaustive geological investigations
have eliminated the possibility of their presence.
A CAF is ‘concealed’ because there is no surface expression of displacement by a fault that is
not exposed at the surface. The range of tectonic and geomorphic conditions that could leadto concealment of AFs are:
1. Rapid sedimentation rates relative to tectonic rates (e.g., away from a plate boundary
in areas of rapid glacial sedimentation, such as the Greendale Fault beneath theCanterbury Plains). If the sediments covering the basement rocks that host the AF
are thick (hundreds of metres) weak, generally homogeneous and have little internal
structure, then even high resolution reflection seismic techniques may be unable toindicate whether an observed basement fault has been active during the period overwhich the sediments were deposited.
2. Rapid denudation rates in soft rock, relative to tectonic rates leading to erosion of
surficial displacement evidence for the presence of a deep AF (e.g. mountainous soft
Tertiary sediments in humid terrains such as central NZ North Island environments).
3. A contractile tectonic environment is most likely to produce blind thrust CAFs thathave no/subtle geomorphic expression.

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3

When is a CAF a threat to a repository?

Even under the circumstances of (1) above, an old, mature AF is likely to have evolved to a
sufficiently large average annual displacement that it could host earthquakes large enough tohave broken the surface (M6.5; rule of thumb). Even if this has not yet occurred, repeated
events of lower magnitude should produce surface geomorphological deformation indicativeof the presence of a fault at depth. However, it is also rather unlikely that such a geologicalenvironment would ever be considered suitable for a repository as the sediments would be apoor host formation and the basement may be too deep to be a host (e.g. >1000 m).
A repository could be threatened with direct shear if a deep basement fault is activated orreactivated and displacement penetrates upwards to repository depth (typically about 500 m).
In this context, whilst the 2010 Darfield M7.1 EQ broke surface, the February 2011
Christchurch M6.1 EQ probably did not rupture to within 1000 m of the surface.
The EQ magnitude threshold for surface rupture varies greatly according to tectonic
environment. This parameter, and the associated uncertainties, should be established in ahazard modelling effort, and priority given to identifying CAFs with potential magnitudes
greater than the magnitude of surface rupture. There is no sense in searching for CAFs on
the ground surface for a magnitude range less than that associated with surface rupture.
A real issue is that faults capable of hosting a M6.5 event are on the verge of being‘repository significant’ whilst, at the same time, being on the border of being discoverable.
A volume of basement rocks that contains immature AFs and is ‘blinded’ by sediments
present perhaps the largest problem of identification, although again the complex fault pattern
may make the basement an unattractive host for a repository. With an immature fault patternand evidence of stress accumulation that points to potential activity in the future, thecomplexity of the system may make it very difficult to forecast which fault will move, in what
sense, or by how much. A Bayesian, evidence weighting, approach (related to the expert
elicitation approach being developed in TOPAZ) may be a valuable tool in this respect. In
particular, it is unlikely that all the stress accumulation will be relieved by movement along asingle fault, so complex displacements may occur in a network of capable fractures. Thissituation can be seen in the current series of Christchurch EQs across the Canterbury Plains.

4

How can we identify CAFs?

The ITM methodology that develops strain maps and budgets at a regional and local scale
provides a first indication of areas where CAFs might be suspected: essentially, the presenceof an obvious anomalous stress accumulation in a region where no fault activity has beenrecorded indicates that stress relief is likely to occur on features that have not been observed,
or on reactivated ‘inactive’ faults (that have not been active since before stress accumulation
began and might thus have been regarded as inactive). Comparison of different datasets in a
multidisciplinary ITM analysis can reveal areas of significant activity not otherwise evident
from active fault datasets. The probabilistic approach allows frequency information to be partof the overall assessment.
The ITM methodology will be informed by geomorphic evidence: is the region of interest
characterised by any of the three tectonic and geomorphic conditions listed in Section 1? Ifthese conditions exist and if the strain mapping from other approaches (e.g., GPS, seismicity)
indicates accumulated strain potential with no clear features that would allow stress relief,then there is a high likelihood that CAFs may be present.
Moving to the site scale, a standard sequence of investigations focussed on seismic
surveying linked to borehole data will identify AFs if they break the surface (trenching will be amajor tool to help confirm their activity) and potentially active faults at depth in the basement(CAFs). A prerequisite is high quality geological mapping. As noted above, faults that arecapable of hosting less than a M6.5 EQ are likely to be difficult to isolate from all otherfractures with similar dimensions and geometries. Consequently, a safety case is likely toneed to treat a network of such features as potentially capable and will then need to forecast
how displacement magnitudes might evolve (and perhaps focus onto one feature alone) overthe next 10,000 and 100,000 years. The Christchurch earthquake sequence provides avaluable example of how low-activity faults can be virtually impossible to detect in areas ofhigh sedimentation and considerable geomorphic modification.

86

The CHCH earthquake sequence has further demonstrated the value of developing a
background or distributed seismicity model for tectonic hazard analysis. The backgroundmodels provide no information on the geometry of CAFs, but do provide first-order informationon the magnitude-frequency of future hazardous events. Efforts to develop realistic,
defensible background models should be supported, particularly using multiple approaches(seismicity and GPS) and constraining them into a systems approach to modelling (e.g. withall source models consistent and matching plate motion constraints). Ideally, candidate faults
identified from seismic reflection, inference from offshore areas etc, could be used to reassign
an otherwise grid-based background seismicity model.
Moreover, GPS data suggested ~15 nanostrain/yr of strain accumulation across theCanterbury Plains prior to the sequence of earthquakes in Christchurch (Wallace et al., 2007).
The GPS data clearly highlighted the strong likelihood of CAFs in the Christchurch region,and emphasises the need for inclusion of datasets such as GPS when assessing CAF hazard.
Once identified, CAFs often prove to have long recurrence intervals so, for hazard analyses,
they would best be characterised in time-dependent models (i.e. taking account of theelapsed time since the last earthquake as well as the long-term recurrence interval). This isdifficult information to obtain for the vast majority of fault sources, but such information should
be sought wherever possible.

5

Conclusions

Concealed active faults do not need to be regarded as an intractable siting problem, for two
main reasons:
1. They tend to occur in geological and tectonic environments that, for a number ofreasons, would probably not, in any case, be considered suitable for a geologicalrepository (weak sediments down to repository depth; high erosion rates; immatureand complex tectonic setting;).
2. We have ITM and SI techniques to identify regions where CAFs might be expected to
be present and we have a toolbox of techniques that can then be deployed to assist
in identifying potentially capable faults (capable of M6.5 or greater EQs) at depth.
Through ITM, NUMO has developed strain budget mapping techniques for identifying regionsthat may have a higher potential for concealing AFs, which, if combined with basic
geomorphic analyses of environments that could hold CAFs, allow initial decisions to be taken
about potential site suitability. Existing SI programmes have the capacity to identify CAFs andto classify them in terms of their capability to host repository-relevant displacement in the 10
kyr to 100 kyr timeframe.

87