NUMO-TR-16-04

TOPAZ ProjectLong-term Tectonic Hazard to
Geological Repositories

Toward practical application of the ITM-TOPAZ methodology

February 2017
Nuclear Waste Management Organization of Japan (NUMO)

2017 年 2 月

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NUMO-TR-16-04

TOPAZ ProjectLong-term Tectonic Hazard to
Geological Repositories

Toward practical application of the ITM-TOPAZ methodology

February 2017
Nuclear Waste Management Organization of Japan (NUMO)

各章の和文要約
1

序論

本報告書は，国際プロジェクトメンバーが作成した原稿を NUMO が編集したものである。NUMO は，地層処分システムに著しい影響を及ぼす火山活動や断層活動な
どの自然現象の発生可能性を確率論的に評価するための手法として，ITM 手法1を開発した。続いて，プレート運動の安定性を必ずしも前提にできない超長期の評価に向けて，ITM-TOPAZ 手法2の開発を進めた。東北地方および中国地方を対象とした
ケーススタディを通じて，ITM-TOPAZ 手法の日本の地質環境への基本的な適用性を確認してきた。今回，ITM-TOPAZ 手法の適用性のさらなる向上を目指し，再び東北地方を対象に，火山活動・断層活動に加えて隆起・侵食の変遷に関するシナリオの拡充，それらの影響の評価方法の高度化などを行った。
2

広域変遷シナリオ（RES: Regional Evolution Senarios）の拡充

これまでに例示した将来 100 万年までのプレート運動の変遷に関する四つのシナ
リオ（RES1: 現在の傾向が継続，RES2: プレート収束速度が増加，RES3: プレート運動の方向が変化，RES4: プレート収束速度が減少）における，東北地方の背弧域（日本海側），脊梁山地，前弧域（太平洋側）での広域的な火山活動，断層活動，
隆起・侵食の傾向について，追加情報に基づき記述を補強した。さらに，最近の科学的知見の進展を踏まえたシナリオの拡充の可能性について述べた。
3

サイト変遷シナリオ（SES: Site Evolution Senarios）の設定

背弧域（日本海側），脊梁山地，前弧域（太平洋側）を代表する数十 km2 程度の検討用サイトとして， WCB（West Coast Backarc），IB（Inland Basin），ECF（East
Coast Forearc）の三地点を設定した。文献情報を基に，各サイトの隆起・侵食速度などを推定し，上記四つの RES の下での火山活動，断層活動，隆起・侵食の変遷を，
ストーリボードを用いて概念的に示した。
4

影響シナリオ（IS: Impact Scenarios）の設定

これまで未実施であった隆起・侵食による地層処分システムへの影響の変遷について検討を行った。廃棄体および人工バリアの状態変遷，ならびに上記 SES の情報に基づき，各サイトで仮に地下 300m に処分場を設置した場合の隆起・侵食に伴う影響（地表との距離の減少，酸化性雰囲気への移行，地表への露出など）の変遷について，ストーリボードを用いて概念的に示した。
5

火山活動および隆起・侵食のハザードマップ作成方法の検討

将来の自然現象の発生可能性の評価結果がサイト選定プロセスに悪影響を及ぼす可能性（プロジェクト・リスク）について，判断指標のしきい値（確率）を設定し，ハザードマップを作成して評価する方法を示した。火山活動については，まず上記

1

International Tectonics Meeting（日本のテクトニクス関連事象の調査・評価に関する国内外の専門家の認識共有・情報発信の場として NUMO 主催の会議体）の海外専門家とともに開発した手法で，数十万 km2 規模のハザードマッ
プにより，数十 km2 程度の領域における事象の発生確率を提示する (Chapman et al., 2009a)。
2
TOPAZ(Tectonics Of Potential Assessment Zones)手法は，将来のプレート運動の変化およびそれに伴い生じる事象の変遷，ならびに地層処分システムへの影響の変遷にかかわるシナリオを設定し，それに対する専門家の確信度とITM 手法の発生確率を統合した評価を行う (Chapman et al., 2012)。

RES における火山活動場の移動を考慮できるように，コックスプロセス法 3を高度化した。海外の事例を参考にしきい値を仮設定して，プロジェクト・リスクを区分し，上記のコックスプロセス法と既存のカーネル法4のモデルに基づくハザードマップを作成した。隆起・侵食については，広域的な隆起速度と，アイソスタシーに伴う隆起・沈降を仮定した重力異常を指標にしきい値を設定し，ハザードマップを作成した。さらに，総合評価に向けた火山活動と隆起・侵食を併せたハザードマップの作成例を示した。
6

断層活動のハザードマップ作成方法の検討ある場所における断層活動の影響シナリオの発生確率を算出し，ハザードマップ
あるいはリスクに基づく安全評価への数値情報として利用する方法を例示した。処分場を含む 5km x 5km x 20km の領域において，マグニチュード 6.5～7.5 の地震を伴う断層の変位が処分場に到達するシナリオを想定し，地表変形，GPS，地震の情報に基づき，年間発生確率を求めた。年間発生確率の逆数から地震の再来期間を算出し，上記同様にしきい値を仮設定することによりハザードマップを作成する方法を示した。
7

ロジックツリーを用いた評価方法の検討

統計解析ソフトウェアを用いた隆起・侵食の影響の確率論的な評価の事例を示した。ストーリボードに整理したシナリオ（RES, SES）や隆起・侵食速度のパラメー
タ5に基づき，地表への接近速度を評価するロジックツリーを構築した。これを基にモンテカルロ・シミュレーションを実施し，100 万年後までの複数の時間枠における処分場の地表への接近量（例えば 300m）の超過確率を算出した。さらに， 異なる
RES と SES の組合せに伴う地表への接近量の確率分布密度の違いや，異なる深度に設置した処分場の地表に露出する確率の時間変化などを提示できることを示した。
8

手法の適用に向けた議論

ITM-TOPAZ 手法を，サイト選定の各段階において適用するにあたっての留意事項などを取りまとめた。その適用方法として，大きくは次の二つが考えられる。①火山活動，断層活動，隆起・侵食などによる著しい影響に関するハザードマップを示
し，対象とする領域やサイトの評価（プロジェクト・リスク評価）に資する。②対象サイトで生じる事象の地層処分システムへの影響に関するシナリオの起こりやすさを定量的に示し，リスク論的な安全評価に資する。ハザードマップは，基本的には複数の領域あるいはサイトの相対比較に適しており，適用に際しては，対象とする領域やサイトの最新情報に基づき段階的に作成・更新していく必要がある。

3

火山の分布密度と地球物理データ（地震波速度構造，重力異常など）との地球統計学的な相関を考慮に入れたモデ
ルのモンテカルロ・シミュレーションにより発生確率を算出する手法(Jaquet et al., 2009, 2012 など)。
4
カーネル関数により火山からの距離や活動頻度に基づく時空間分布モデルを構築し発生確率を算出する米国ユッカマウンテン等で用いられた手法(Connor and Hill, 1995 など)。
5
本検討では時間的制約のため Chapman et al.,(2012)における情報を使用した。

Contents
1

2

3

4

5

6

Introduction ..................................................................................................................... 1
1.1

This report ............................................................................................................... 1

1.2

NUMO site evaluation.............................................................................................. 1

1.3

The ITM Methodology: probability – the likelihood of future tectonic impacts ............ 2

1.4

The March 2011 earthquake - a “natural experiment”............................................... 3

1.5

Timescales and tectonic impacts on a geological repository ..................................... 5

1.6

Going beyond 100 kyr: the TOPAZ Project .............................................................. 7

Regional Evolution Scenarios .......................................................................................... 9
2.1

Major issues to consider in developing future tectonic evolution scenarios ............... 9

2.2

RES 1: Tectonic boundary conditions remain the same ........................................... 9

2.3

RES 2: Rate of upper plate motion doubles ........................................................... 14

2.4

RES 3: Relative plate motion becomes more oblique ............................................. 17

2.5

RES 4: Upper plate shorting decreases to a neutral state ...................................... 20

2.6

Summary of possible impacts on sub-regions (domains) ........................................ 22

2.7

Examples of additional aspects of tectonic evolution for the RES ........................... 23

Site Evolution Scenarios................................................................................................ 25
3.1

Example sites for SES demonstration .................................................................... 25

3.2

Data on regional scale uplift and erosion ............................................................... 26

3.3

Estimation of uplift and erosion rates for the example sites .................................... 28

3.4

Site Evolution Scenarios for the West Coast Backarc (WCB) ................................. 29

3.5

Site Evolution Scenario for the Inland Basin (IB) .................................................... 31

3.6

Site Evolution Scenarios for the East Coast Forearc (ECF) .................................... 33

Introduction to the Impact Scenarios.............................................................................. 35
4.1

Evolution of a HLW repository ............................................................................... 35

4.2

General Impact Scenarios for exhumation (uplift and erosion) ................................ 35

4.3

Uplift, erosion and exhumation scenarios at the example sites ............................... 37

Hazard maps for volcanism and exhumation ................................................................. 39
5.1

Introduction ........................................................................................................... 39

5.2

Probabilistic volcanic hazard assessment .............................................................. 39

5.3

Estimation of volcanic hazard under evolving tectonic conditions at 1 Myr timeframe
49

5.4

Development of volcanic hazard maps................................................................... 60

5.5

Development of exhumation hazard maps ............................................................. 67

5.6

Combined project risk maps .................................................................................. 70

Hazard maps for rock deformation ................................................................................. 72
6.1
Mapping the frequency of a tectonic deformation scenario: future 6.5 ≤ M ≤ 7.5
earthquakes ..................................................................................................................... 72

7

Evaluating site impact hazard probabilities using logic trees and expert elicitation ......... 76
7.1
Explanation of the method: use of logic trees to derive probability weights for site
hazard maps .................................................................................................................... 76

7.2
Expert elicitation approach: updating and extending elicited RES and SES
probabilities ..................................................................................................................... 77

8

7.3

Scenario probability trees: structure and implementation........................................ 79

7.4

Summing up .......................................................................................................... 84

Discussion for practical application ................................................................................ 85
8.1

Practical application of the ITM-TOPAZ methodology ............................................ 85

8.2

Uncertainty in the ITM hazard map ........................................................................ 88

References.......................................................................................................................... 90

1

Introduction

1.1

This report

This report was originally prepared by the TOPAZ international project team members as the
project summary report that described the whole project activities, and was later interpreted
and modified by NUMO under its responsibility. This report aims at supplementing andupdating the basic ITM-TOPAZ methodology (Chapman et al., 2012) for potential practicalapplication, by means of additional case study on the Tohoku region. The study was carriedout to address specific issues in parallel by several groups of the project members, relatively
in a short period. Thus, some of the following chapters may not correlate each other due todifferent boundary conditions and datasets (e.g. Chapter 6 and Chapter 7).
Chapter 2 updates the previous RES with new information. Chapter 3 develops SES for newly
defined three example sites to include uplift and erosion in addition to volcanism and rockdeformation. Chapter 4 introduces the ‘Impact Scenarios’ (IS) for the three sites with respect
to the exhumation hazard due to uplift and erosion. Chapter 5 describes the basic methods ofprobabilistic volcanic hazard assessment, and a newly developed method for evaluating
volcanism in evolving tectonic conditions at 1 Myr timeframe. It then demonstrates how the
information from various models are aggregated to develop hazard maps for both volcanismand exhumation (uplift and erosion). Chapter 6 presents how to map frequency of a tectonic
deformation scenario (occurrence of 6.5 ≤ M ≤ 7.5 earthquakes). Chapter 7 explains how to
use logic trees and expert elicitation approach to assign weights to hazard scenarios and theirprobabilities. Methods for evaluating specific exhumation hazards are also proposed. Chapter
8 summarises points and issues in applying the ITM-TOPAZ methodology to potential
decision-making.
1.2

NUMO site evaluation

NUMO will need to evaluate sites that emerge either from voluntary application bymunicipalities or request made by the national government to municipalities, and select a
preferred site for a repository (NUMO, 2013). They might also need to express preferences
for regions of Japan where stable tectonic conditions would favour repository siting. Siteevaluation will involve the initial Literature Surveys (LS), the surface-based Preliminary
Investigations (PI) and the Detailed Investigations (DI) both on surface and underground(Figure 1.1).
Prior to the field investigations, NUMO will carry out a detailed literature-based evaluation ofsuitability to accept the sites as the Preliminary Investigation Areas (PIAs). A key aspect of
this stage is consideration of the susceptibility of a site to future tectonic activity andtectonically driven processes and events. For repository safety evaluation, the focus ispredominantly on long-term (more than thousands of years) post-closure tectonic processes
leading to progressive perturbations of the repository environment. These may possibly leadto the initiation of disruptive tectonic events which may cause perturbations 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.
Whilst the nationwide evaluation factors for qualification (EFQ) 6 for PIA acceptance aredesigned to remove clearly unsuitable sites from consideration, they cannot guarantee that,over the next hundreds of thousands of years, the risks of tectonic hazard for a chosen PIA
will be acceptable.
This is because large parts of Japan that are potentially geologically suitable for siting aredirectly affected to varying extents by rock deformation, uplift and erosion, the peripheral
impacts of volcanic activity or the possibility of new magma intrusion or volcanic activity. The
EFQ were only intended as preliminary screening guidelines to prevent obviously unsuitable
candidates entering the siting process.

6

The siting factors for selection of PIAs consist of factors relating to legal requirements (evaluation factors for
qualification (EFQ)) and favorable factors (FF) that are not part of legal requirements (NUMO, 2004). The EFQ
include earthquake and fault activity, igneous activity, uplift and erosion, unconsolidated Quaternary sediments andmineral resources.

1Preliminary
Investigation
Areas (PIAs)

Detailed
Investigation
Areas (DIAs)

Preliminary
Investigations (PI)

Detailed
Investigations (DI)• Open solicitation ofvolunteer municipalities• Proposal to municipalities

Literature
Survey (LS)- Geophysical surveys- Borehole investigations,etc.

Selection
criteria

Selection
criteria

1st
stage

Selectionof PIAs

2nd
stage

Repository
Site (RS)- Excavation of test tunnel- Investigations in the test
tunnelSelection
criteria

Selectionof DIAs

3rdstage

Selectionof RS

Figure 1.1: An overview of the NUMO site evaluation stages.
The ITM Methodology: probability – the likelihood of future tectonic impacts

1.3

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.
The ITM (International Tectonics Meeting) project developed such a methodology (the ‘ITM
Methodology’) for the specific conditions of Japan, based upon state-of-the-art approaches
used internationally (Chapman et al., 2009a).
The ITM methodology is essentially probabilistic in nature. A probabilistic approach was seenby the ITM expert group as the only realistic means of addressing the uncertainties in
predicting possible hazards (processes or events that could lead to adverse effects) when
there is marked variability in the spatial distribution, timing, intensity and style of the tectonic
events being evaluated. The consequences of ignoring very low probability events withsignificant impacts were highlighted by the Fukushima accident after the earthquake in March,
2011.
Essentially, two approaches have been adopted internationally to address low probability,
disruptive events and deformation processes that are discontinuous in time and magnitude in
response to continuous regional strain.
7

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 8 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) withdifferent degrees of likelihood (often expressed qualitatively; e.g. ‘likely’, ‘less likely’,‘highly unlikely’).

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
7
8

Health risk is normally defined as the risk of death or serious genetic effects.
Of course, a radiological dose can also be expressed in terms of health risk, by applying accepted dose-to-risk
conversion factors.

2

supported by deterministic models of the underlying tectonic processes that lead to magma
intrusion, volcanism and rock deformation. The ITM methodology is applicable to the following
situations of NUMO’s repository siting programme:in the LS when potential PIAs are being assessed. The ITM methodology will use
currently available information to allow comparison of sites in terms of confidence thatthey are likely to prove acceptable with respect to tectonic impacts.
in the planning of the PIA site investigations, to identify geoscientific information
requirements that will be needed to refine the LS analysis.
at the point where DIAs are being evaluated and compared in order to select a
preferred site (or sites) for disposal.

The ITM project was mainly concerned with the first and second situations, and focussed on
2evaluating comparative hazards of small areas (e.g. 25 km ) within a regional or sub-regional
2
context of 100,000 to 10,000 km . This is partly because the project originally developed tocompare several possible alternative volunteer sites that might arise within a region. However,
it is clear that regional to sub-regional scale assessment of tectonic hazard will also be
required even for single sites.
Application of the methodology in the third situation 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 DIA investigations.
The overall structure of the ITM methodology is described in Chapman et al. (2009a) andconsists of:

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;
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 siting.

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., 2009b). The Tohoku case study looked into the varied strain response of the crustal
plate to subduction of the Pacific Plate (the key current tectonic driver for much of Japan) andthe mechanisms that underlie the apparent clustering of Quaternary volcanoes in much of
Honshu. The Kyushu case study region is among the more dynamic and rapidly changing
plate boundaries in the world, with the tectonic situation being intrinsically more complex thanin 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 the geologicalsetting is evolving more rapidly. The fundamental assumptions in Tohoku, that the plateboundary configurations and plate motions influencing rock deformation and volcanism are
relatively stable over periods up to 1 Myr, are less appropriate in Kyushu. Consequently,
these two case studies allowed development and testing of the ITM Methodology undersignificantly different tectonic conditions that span a significant part of the range of tectonic
environments across the Japanese Islands.
1.4

The March 2011 earthquake - a “natural experiment”

On March 11, 2011, the fourth-largest instrumentally recorded earthquake, a Magnitude (M)9.0 event, occurred offshore northeast Japan. Because of Japan’s history and preparednessfor earthquake shaking, the devastation was primarily a result of the subsequent tsunami. The
Great 2011 Tohoku-oki earthquake was a product of the subduction of the Pacific Platebeneath the edge of the Eurasian plate at a speed of about 8.5 cm/yr along the northeast
Japan trench. This trench segment extends northwards about 800 km from the triple junction

3

intersection with the Izu-Bonin trench and Nankai trough (36°N) and the southern end of theKurile trench (42°N). Along this trench, an earthquake of M≤6 occurs nearly every year. The
historical record indicates M7 or larger events occur every five years with events of M≤7.5
every few decades.
The 2011 M9 event was extraordinary for the Japanese islands as rapid seismogenic slip
occurred along the central 400 km length of this trench segment with as much as 50-60 m ofthrust-type displacement at the toe of the trench wall 85 km seaward of the epicentre. In
retrospect, it is now apparent that a very similar earthquake occurred in AD869. The Jogan
tsunami left a sand layer deposit in the Sendai plain that is remarkably similar in thickness
and extent to that produced in on March 11, 2011. Minoura and Nakaya (1991) discovered
evidence for this event in cores from the Sendai plain. This evidence was investigated morethoroughly and better dated via samples selected from trenches excavated by Minoura et al.
(2001). However, these workers attributed the tsunami to nearby subduction zone earthquakeof only M8.3. Sawai et al. (2008) used cores and trenches to confirm the 869 Jogan event
was the largest tsunami in the past 1,500 years, with only the lesser 1611 Keicho tsunami
leaving evidence in the Sendai plain. They also presented a chart indicating there is evidence
for two other large tsunamis around 1,900 and 2,900 years before present. There is reason tobelieve 2011 M9 Tohoku-scale earthquakes recur over timescales of about 1kyr.
Prior to the 2011 M9 earthquake, the maximum expected magnitude for an event near Japan,
M8.5, was based upon the historical record, considered complete for major events since
AD1600. In 2005, the Japan Headquarters for Earthquake Research Promotion (HERP)
released a formal report that was a “consensus opinion” of a committee of leading Japaneseseismologists (Fujiwara et al., 2006). This report, with updates in 2006, 2007, and 2009
identified 98 major active faults and 15 trench parallel regions of subduction zone seismicity.
Most on-land fault zones were given 30-year probabilities of earthquake-producing ruptures in
the range of M7.2±0.5 of less than 7%. Probabilities as large as 16% were given to six fault
zones, all of which are part of the active deformation belt that delineates the plate triple
junction region in central Honshu. The Nankai Trough subduction zone, long believed as
overdue for a great subduction earthquake, was given a 30 year probability of about 50% fora M8.4 event and about 60% for a M8.1 event, with a M8.5 event listed as possible if the
rupture extends along the entire trenchline.
The 2005 HERP seismic hazard assessment divided the Japan Trench subduction zone into
six segments. The Miyagi-oki area, the epicentre of the 2011 M9 event was given a 99%
probability of a M7.5 event by 2035, the highest probability for a major earthquake anywherein Japan! The North Sanriku-oki segment was given a 30-year probability of 70-80% for a
Mw7.7 event. The entire Japan trench segment from Sanriku-oki to Boso-oki was identified ashaving a 20% potential of a M8.2 tsunami-generating near-trench movement with associatedtsunami inundations up to about 6 m. In retrospect, the activities of the HERP committee
produced an analysis that was correct in terms of probable location of major tectonic activity,
but inadequate in terms of magnitude and tsunamigenic potential.
The expectation of potential M8.3± earthquakes near Japan guided the design of engineered
structures and seawall defences. While the M9.0 earthquake along this subduction zone
segment was about fifteen times larger (in terms of energy release) than the maximum
expected and resultant tragedy from the huge 10+ m high tsunami shocked the highest levelsof the geoscientific community (see e.g., Nature Comment, 2011, “Rebuilding seismology”),
the scientific approach that produced the 2005 HERP report that identified the Miyagi-oki area
as having the greatest probability of a major earthquake for all of Japan can be considered asuccess.
The possibility that subduction zones can produce great M9 thrust-type events is one of thetenants of plate tectonics theory that arose in the late 1960s (shortly after construction beganon the Fukushima Daiichi nuclear power plant reactor #1). In most ways, the 2011 Tohoku
event was very similar to the 1952 Kamchatka M9.0, 1960 Chile M9.5, 1964 Alaska M9.2,
and 2004 Sumatra M9.1 subduction zone earthquakes and tsunami floods. From now on, allprobabilistic hazard analyses near subduction zone settings must consider these kinds ofmulticentury to millennia-scale events.
From the point of view of the long-term (≥100 kyr) disposal of nuclear waste in Japan, the2011 Tohoku M9 earthquake can be viewed as a “natural experiment” because it occurred in4the most intensively monitored (seismograph, GPS) portion of the planet. Already, analysis of
this event has generated hundreds of professional publication that reveal much about earth
behaviour (e.g., Lay et al., 2013). Such events, rare in the historical record, must be viewed
as commonplace over the timescales of concern for the issue of nuclear waste disposal in
Japan.
1.5

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 1.2 shows thedeclining hazard of high-level waste (HLW) and spent fuel as a function of time after
production. Hazard is represented here as the ingestion radiotoxicity of all the radionuclides in
the waste relative to the radiotoxicity of the amount of uranium ore required to manufacture
the fuel from which the HLW was produced.
The hazard from HLW declines rapidly over the first hundreds of years and, after about 3,000
to 4,000 years, reaches the same level as the uranium ore. At this time after disposal, arepository would have a similar hazard potential (by mobilisation of radionuclides into
groundwater to cause exposure to people and the environment) as a rich uranium ore bodyburied at a similar depth.
Worldwide, people live in close proximity to natural deposits of radioactive materials, with noapparent impacts on health. There are large variations in natural background radioactivity,
giving everyday doses to people that far exceed any that might result from a geologicalrepository in the distant future. It can thus be argued that the 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 (100 kyr) – the period addressed in the ITM
project. In several countries, even those dealing with disposal of spent fuel, which can beseen from Figure 1.2 not to reach the natural crossover point with uranium ore until about 100
kyr after disposal, only qualitative statements of safety are required after 100 kyr. Suchstatements are expected to discuss the longevity of safety functions into the distant future, tocompare hazard to natural radiotoxic hazards and to discuss the long-term fate of the wastes
as the repository degrades. The engineered barriers and the waste itself are expected to be
substantially degraded in the period 100 kyr to 1 million years (1 Myr) and beyond.

Figure 1.2: Declining radiotoxicity of HLW and spent fuel as a function of time after
production: see text for description (after NUMO, 2004; Chapman and Hooper, 2012).

5

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 lose credibility as actual indicators ofsafety. 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 1.3.
Returning to tectonic impacts, it can be seen that events and processes that might disrupt a
repository over the first 10 kyr are critical and need to be avoided, so far as possible, bycorrect siting in regions of low tectonic susceptibility. Thus, the probability of occurrence of
events and processes is an extremely important factor. The ITM project develops the tools toassess such probabilities up to 100 kyr – 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.
Major restructuring refers to a change in tectonic regime for parts of Japan.
However, as the regulatory framework for geological disposal is still under development inJapan, NUMO considered it was worthwhile to prepare a measure for providing informationout to longer periods – up to 1 Myr – again in the form of probabilities (or ‘likelihoods’) that
could be used to express degrees of belief to alternative forecasts of the evolution and fate ofa repository.

Figure 1.3: Schematic representation of the progressive degradation of a geologicalrepository 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 righthand side of the diagram.

6

1.6

Going beyond 100 kyr: the TOPAZ Project

Consequently, following the completion of the ITM project, the expert team was proposed tolook at the possibilities and constraints of making forecasts of the likelihood and nature oftectonic impacts for longer periods into the future, from 100 kyr out to 1 Myr.
The TOPAZ (Tectonics Of Preliminary Assessment Zones) project was started to extend themethodology developed in the preceding ITM project for this purpose. The developed
methodology in the TOPAZ project is called the ITM-TOPAZ methodology.
One of the main additional steps of the ITM-TOPAZ methodology involve the development ofalternative conceptual models, ‘Regional Evolution Scenarios’ (RES), which characterise how
the tectonic situation in a region might develop over the next 1 Myr. 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 degrees of belief in these RES andSES are attached using a formal expert elicitation methodology, expressed as probabilities ina logic tree, and can be superimposed on the event probabilities derived from the ITM
methodology to arrive at probabilities of specified impacts in different timeframes.
The basic methodology was tested by means of the Tohoku case study (Chapman et al.,2012). A partial test of the methodology on the different tectonic setting (i.e. monogenetic
volcanism) was made in the Chugoku case study (Connor et al., 2013).
Figure 1.4 summarises the outline of the ITM-TOPAZ methodology. The output of a complete
analysis of a site would be an essential underpinning component of any NUMO safetyassessment and comprise the following 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 ‘Impact Scenarios’ (IS) affecting the repository,
which can be incorporated into a risk-based safety assessment and used to generate
either radiological risks (to compare to regulatory standards) or probabilities ofoccurrence of scenarios to present along with dose consequences in a disaggregateddose-likelihood approach to regulatory standards.

Figure 1.4: The outline of the ITM-TOPAZ methodology.

7

The Project involved the close interactions of Japanese and international experts to gain thenecessary understanding to carry out the methodology development and testing. Theinternational experts came from the organisations in Table 1.1.

Table 1.1: An overview of the International Project Team Members for the TOPAZ Project.
TOPAZ International Project Team Members (as of March 2014)
Dr Kelvin Berryman
GNS Science, New Zealand
Dr Mark Stirling
Dr Nicola Litchfield
Professor Mark Cloos
University of Texas, USA
Dr Laura Wallace
Volcanism
Professor Charles Connor
University of South Florida, USA
Laura Connor
Rocco Malservisi
Professor Steve Sparks
University of Bristol, UK
Dr Sue Mahony
Ellie Scourse
Dr Olivier Jaquet
In2Earth Modelling, Switzerland
Interface with PA
Dr Mick Apted
INTERA, USA
Expert Elicitation
Dr Willy Aspinall
University of Bristol, UK
Project Co-ordination
Professor Neil Chapman
MCM Consulting, Switzerland
Ellie Scourse
Rock Deformation8

2

Regional Evolution Scenarios

In this chapter, we updated the four plausible RES presented in the previous Tohoku casestudy (Chapman et al., 2012) with new discussions on the implications of these scenarios.
Importantly, it is noted that the RES developed here are meant as examples to demonstrate
the approach. The scenarios presented here are based on our current understanding of how
tectonics has evolved in Tohoku in the past. When the RES approach is eventually applied toa specific site, more exhaustive RES should be developed by a panel of experts, who are
expected to back the RES up with more extensive and robust data.
2.1

Major issues to consider in developing future tectonic evolution scenarios

In a regional sense, the first-order features to consider in the future tectonic evolution ofJapan are: the configuration and sense of relative plate motion at the plate boundary, themigration of triple junctions between tectonic plates, and the dynamics of the subducting
slabs and mantle in the region. All of these things will strongly influence the distribution andevolution of rock deformation and volcanism. We have developed four RES for the Tohoku
region in the previous TOPAZ report, each of which assumes a different future tectonic
evolution that we consider to be plausible. One of the most important aspects of future
evolution to consider is whether or not the Sea of Japan convergent zone will evolve into a
subduction margin. Thus, tracking the location and depth of the leading edge of theunderthrusting Sea of Japan crust is one of the features we highlight for the northern Japan
RES. At a more detailed and local-scale level, processes such as fault zone evolution anddevelopment of magmatic centres should be accounted for. In terms of the backbone range
domain, the potential for the development of new volcanic centres and uplift associated withthat is very important to consider, as is the on-going evolution of active faulting in that region.
A suite of geodynamic and numerical models is recommended to inform RES development,
particularly in terms of how active deformation and the distribution might evolve with time, in
response to the evolving tectonic boundary conditions.
To assess future tectonic evolution, we must also consider the possibility that relative platemotions can change over time scales of a few million years; such a scenario will strongly
influence the results for RES developed for the 1 Myr timescales. We know that the PacificPlate has changed its motion (in an absolute sense) numerous times throughout its history.
Most of these plate motion changes arise from forces exerted on the boundaries of the plates.
For example, Wessel and Kroenke (2000) suggested that collision of the Ontong Java Platewith trenches bordering the Pacific Plate in the southwest Pacific initiated a change in PacificPlate motion ~6 Ma. Similar to the Pacific Plate, the Philippine Sea Plate motion has changed
numerous times throughout its history (Sdrolias et al., 2004; Hall et al., 1995). Part of our
RES development involves consideration of scenarios where reasonable changes in relative
plate motion occur at various stages in the future. A recent study (Regalla et al., 2013)
suggest that changes in convergence rates of the Pacific Plate has impacted Pacific slab
dynamics in the past, leading to stages of uplift vs. subsidence. To properly assess theinfluence of changes in plate motion on the future uplift and subsidence scenarios in the
Tohoku region (e.g., Regalla et al., 2013), geodynamic modelling is required. Moreover, formore robust assessment of likely future changes in tectonic plate motion, calculation of theforces acting on the plate boundaries and modelling of how these forces are expected toinfluence future rates of motion of the Pacific Plate are recommended.
2.2

RES 1: Tectonic boundary conditions remain the same

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 (Figure 2.1). As part of this
scenario, we assume that the rate of permanent plate boundary deformation occurring within
the upper plate (>15 mm/yr) remains similar to what occurs today, and that the subducting
Pacific slab maintains its current configuration (configuration defined by Hasegawa et al.,1994) with respect to the upper plate. We also assume that the crust underlying the Sea ofJapan continues underthrusting beneath Honshu at its current rate of ~15 mm/yr, eventually
evolving into a subduction margin (cf. Tamaki and Honza, 1985). The results of this RES forthe various timeframes are shown schematically in Figure 2.2.

9

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 blockmodelling to interpret GPS velocity field in terms 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/yr) show the motion of the Northern Honshu block relative to the Amurian
and Pacific Plates. Interpretation of the GPS velocities also suggest that 3-4 mm/yr ofcontraction occurs in the backbone range of northern Honshu (Miura et al., 2004; Wallace etal., 2009)

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 onto particular structures
could be expected, and/or spatial changes in distribution of active deformation could occur asa result of the natural evolution of the fold and thrust belt there. This could lead to substantialchanges in rock deformation strain and uplift rates in the evolving contractional domain
associated with underthrusting of the Sea of Japan (western contractional domain) and thecurrently active backbone range domain, out to longer time periods. It is also likely that any
optimally oriented bedrock faults (i.e., approximately north-south striking) could become
reactivated. Although we expect the location of volcanism in the arc to be relatively stable
over the timeframes presented, 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 (e.g., Honda et al., 2007).
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, andactive volcanism. We recommend that hazard calculations derived from the ITM methodology
will be reasonable for this timeframe.
10 – 100 kyr scenario. We do not expect this scenario to be significantly different from thepresent day to 10 kyr scenario, although it is possible that some spatial changes in the rates

10and location of volcanism, deformation, and uplift could begin to occur 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 domain (Hasegawa et al., 2000) and along the evolving convergent
margin adjacent to the Sea of Japan. In terms of the backbone range domain, which has
bedrock faults within a few hundred meters of it, it is certainly plausible that bedrock
structures within this domain will become reactivated as the contractional fault systemevolves. This could lead to some combination of uplift or subsidence, depending on whether
or not it is situated within the hanging wall or footwall of the evolving fault system. So for thistime frame, any site located within the footwall of an active reverse fault, continued
subsidence is a reasonable scenario. Volcanism is unlikely to migrate significantly outside of
the current location of active volcanism on this time frame due to the likely stability of magma
supply systems over these timeframes.
100 kyr – 1 Myr scenario. Over longer time periods it is possible that larger spatial andtemporal changes in the distribution of volcanism and rock deformation will occur. However,
we anticipate that the largest rates of rock deformation will still remain focused within the
backbone range and the western part of Honshu. Radiometric dating of volcanism in the
backbone range suggests that the location of volcanic centres there remains stable ontimescales of 5-10 Myr, and the current volcanic centres have been in place since ~4 Ma(Honda et al., 2007). However, the locus of volcanism between the currently active areas andthe “gaps” between the active arc volcanoes appears to flip-flop on 5-10 Myr timescales(Honda et al., 2007). Thus, we cannot rule out the possibility that migration of arc volcanism
into the gaps between the currently active volcanoes could occur during the 1 Myr period ofinterest in this RES. However, given the lack of evidence for significant lateral migration of
volcanism across in the arc in the last 10 Myr or more, it is highly unlikely that active
volcanism will migrate significantly across the margin (for example, into the forearc domain) in
this RES.
If it occurs, migration of volcanism into the “gap areas” between the currently active volcanic
centres will greatly impact the backbone range domain, both in terms of disruption bymagmatic intrusions and potential eruptions, as well as uplift in response to magmatic
injections. However, if the locus of active faulting propagates into the basin, away from therange front, an eventual transition to uplift there is plausible. Overall, given the potential forincreased magmatic influence in the gap between current volcanic centres, as well as
continuing rock deformation in the backbone range domain during this time period, this area
could be prone to increased uplift rates, and higher rock deformation and possibly volcanic
hazard on the 1 Myr timeframe (in contrast to the present situation).
In the western contractional domain, if deformation becomes progressively localized andevolves towards an incipient subduction margin, it is possible that larger portions of theconvergent component of plate motion will be shifted into the western contractional domain.
This could manifest itself by (1) a larger portion of the deformation budget occurring on
structures just offshore, (2) an increase in deformation rates on the already active onshore
faults, or (3) by reactivation of other properly oriented structures onshore, or somecombination of all three options. Potential future increases of the plate motion budget being
focused into the western contractional domain will depend on the details of evolution of theassociated fold and thrust belt, and the distribution of optimally oriented bedrock structures.
Out to longer time periods, we expect continued evolution of contractional deformation(growing anticline), and for continued uplift, erosion, and rock deformation to impact that siteout to 1 Myr.
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 deepened by several km, compared to ~30
km depth that we assume for its current depth (loosely based on seismicity). We note that thelocation of the leading edge of the underthrusting Sea of Japan (and its likely future location)
is not well-constrained, and requires further geophysical investigation. Encroachment of theleading edge of the Sea of Japan crust beneath western Honshu could also impact future
volcanic development in that region (perhaps inhibiting it beneath the west coast, pushing11

currently active west coast volcanoes eastward). Underthrusting Sea of Japan crust may also
perturb the dynamics of the mantle wedge beneath both the western contractional domain
and the backbone range domain, leading to some reorganization of the locus of active
volcanism within the arc andbackarc. Numerical modelling studies to assess the influence of
the underthrusting Sea of Japan crust on the mantle wedge and subsequent volcanic
evolution in northern Honshu is recommended.

12

W

W

W

Figure 2.2: Schematic of major tectonic domains and likely evolution for the three timeframesof RES 1. Light green :evolving contractional domain associated with underthrusting of the
Sea of Japan, light orange: backbone range domain of currently active arc, dashed black
line: current likely leading edge of the underthrusting Sea of Japan crust, red line: likely
position of the leading edge at the end of each timeframe, red triangle: Quaternary volcanoes(Committee for the Catalogue of Quaternary Volcanoes in Japan, 1999).

13

2.3

RES 2: Rate of upper plate motion doubles

In this scenario we assume that the rate of long-term (e.g., >1 kyr) horizontal deformationaccommodated within the upper plate doubles from its current value (~15 mm/yr) in a linear
fashion to ~30 mm/yr at 1 Myr into the future. This is similar to the increase in upper platedeformation that occurred in northern Honshu around 2.4 Ma (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 a 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 due to the evolving slab pull forces.
In this RES (see summary in Figure 2.3) we expect that most of the onshore rock deformationbudget will still be accommodated in the backbone range and within western Honshu, but it isvery likely that any optimally oriented bedrock structures (approximately north-south striking)anywhere in Tohoku (such as the forearc domain) could be reactivated to accommodate
these changes in plate tectonic boundary conditions. In this scenario, it is plausible that rockdeformation rates and uplift rates could double9 in the regions of current active deformation.
Increased uplift and erosion could lead to decompression melting of the crust and triggeringof ignimbrite and caldera formation. For the most part there is unlikely to be a major change inthe position of the volcanic arc in this 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 (Figure 2.3). Perturbation of the flow regime in the mantle wedge is also likely tooccur 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 anywherein the backbone range and western northern Honshu during the longer timeframes of interest(e.g., 100 kyr- 1 Myr).
0-10 kyr timeframe. We do not expect this scenario to be significantly different from what isobserved today, particularly in terms of the location of deformation, uplift, and active
volcanism. Hazard assessments using the ITM methodology will be appropriate for this time
period.
10-100 kyr timeframe. We do not expect this scenario to be significantly different from thepresent day to 10 kyr scenario, although it is possible that some spatial changes in the rates
and location of volcanism, deformation, and uplift could begin occurring later in this 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 domain and along the evolving convergent margin adjacent to the Sea ofJapan. The steady increase in horizontal convergent rates accommodated within the upper
plate during this period (from 15 up to 17 mm/yr) could cause rates of faulting on known
active faults to increase correspondingly, or for optimally oriented bedrock structures to bereactivated to accommodate this. This scenario could begin to impact the westerncontractional domain and the backbone range domain towards the end of the 100 kyr timeframe, leading to a larger rock deformation hazard, and potentially larger uplift rates
depending on how the fault systems near those locations evolve. Volcanism is unlikely to
migrate significantly outside of the current arc on this time frame, although the beginningstages of changes in mantle wedge dynamics (if a flat slab situation has driven the increase
in upper plate deformation rate) could lead to the start of volcanic migration into the gap
between currently active volcanic centres. At the latest part of this time frame, a change inmantle wedge dynamics could lead to increasing volcanic influence at the backbone range
domain, and possible onset of increasing uplift rates due to magmatic contributions. We also
note that the increase in contractional deformation that has been observed over the last few
million years has been accompanied by an increase in uplift rates along the east coast

9

This value was tentatively set for a demonstration purpose, and requires detailed analyses for actual assessment .

14

(Regalla et al., 2013). Under RES 2, we expect the forearc domain to begin to experience anincrease in uplift rates.
100 kyr – 1 Myr timeframe. During this period the convergent component of plate motion
accommodated in the upper plate increases from 17-30 mm/yr. During this time we anticipate
that rock deformation will still remain focused within the backbone range and the western partof Honshu, with some reactivation of optimally oriented bedrock structures across the Tohoku
region. Rates of deformation in those regions will increase dramatically (perhaps doubling by
the end of the timeframe). It is highly likely that migration of arc volcanism into the gaps
between the currently active volcanoes will occur during this period, particularly if the increasein convergence rates is accompanied by a change in slab and mantle wedge dynamics. If aperiod of subducted Pacific slab flattening occurs, then the location of active 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 localized and evolves into an incipient 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
just offshore. Alternatively, larger components of the plate motion budget could beaccommodated on the onshore western contractional domain and backbone range domain
faults, which would greatly increase the rock deformation and uplift and exhumation hazards
at the western contractional domain and the backbone range domain. The future localizationof plate boundary deformation into the Sea of Japan region will depend on the details 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 the underthrusting Sea of Japan crust will havemigrated eastward by ~23 km, and will be ~10 km deeper than today, compared to ~30 km
depth that we assume for the its current depth (rough estimate based on seismicity). The
incipient Sea of Japan subduction scenario would significantly perturb the flow regimes andmagma 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 the underthrusting Sea
of Japan encroaches on and perturbs the magma supply to those regions. This could also
have effects on the location of volcanism in the backbone range domain. The impacts offuture Sea of Japan subduction on mantle wedge dynamics and future volcanism should be
investigated with geodynamic modelling. We also anticipate increasing uplift rates for theforearc domain throughout this time period of enhanced upper plate contraction.
Another consideration for RES 2 in particular (but is also a relevant scenario for all RES
discussed here) is that rifted continental fragments exist within the Sea of Japan, based on Pwave velocity anomalies (e.g., Sato, 2013; No and Kodaira, 2013). As convergence between
the Sea of Japan and northern Honshu progresses, episodes of collision between thesefragments and northern Honshu will occur. Collision of these fragments will lead to more
complex collisional deformation in the collision zone in the western contractional domain.
Moreover, these collisional events are also expected to perturb the locus and style of current
volcanism; in particular, it could also lead to production of collision zone rhyolite or granitic
magmas along the Sea of Japan coast, analogous to what has been proposed for Miocene
partial melting and intrusive complexes that occurred during the Izu-Bonin arc collision withcentral Honshu (Tamura et al., 2010). Assessment of the likelihood of continental fragment
collision with the Sea of Japan coastline is needed for all RES (especially RES 2) via detailed
mapping of the current location of these fragments using existing data, and projection of theirlikely future point of collision (given reasonable convergence rates and kinematic scenarios)
is required.

15

W

W

W

Figure 2.3: Schematic of major tectonic domains and likely evolution for the three timeframesof RES 2. Light green :evolving contractional domain associated with underthrusting of the
Sea of Japan, light orange: backbone range domain of currently active arc, dashed black
line: current likely leading edge of the underthrusting Sea of Japan crust, red line: likely
position of the leading edge at the end of each timeframe, red triangle: Quaternary volcanoes(Committee for the Catalogue of Quaternary Volcanoes in Japan, 1999).

16

2.4

RES 3: Relative plate motion becomes more oblique

Pacific Plate absolute motion has undergone a number of changes throughout the Cenozoic(Wessel and Kroenke, 2007, and references therein). In particular, at ~6 Ma 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 on-going 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 clockwise shift from present to 1 Myr in the future; the effect of this shift is to havea 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, 1992). In Sumatra and the Philippines, thiscritical angle is ~20-25º, while it is as high as 25-45º in Alaska (McCaffrey, 1992; 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 Hatagawa Tectonic Line and the Hizume-Kesennuma Fault (Figure 2.4).

Figure 2.4: Bedrock and fault map from Otsuki (1992). This figure illustrates much of the preexisting 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 ofrelative plate motion.

17

These currently inactive bedrock structures strike NNW, and are in an ideal orientation toaccommodate a more northerly component of Pacific-Eurasia relative plate motion. The
results of this RES for the timeframes are summarized schematically in Figure 2.5. We notethat this RES has a larger impact on the forearc domain, and is unlikely to be very different
from RES 1 for the western contractional domain and the backbone range domain.
0-10 kyr timeframe. We do not expect this scenario to be significantly different from what isobserved today, particularly in terms of the location of deformation, uplift, and active
volcanism.
10-100 kyr timeframe. We do not expect this scenario to be significantly different from thepresent day to 10 kyr scenario, although it is possible that some spatial changes 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 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.
100 kyr – 1 Myr timeframe. As relative plate motion becomes more oblique to the plateboundary 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. 4
and 5) such as the Tanakura fault increases. We expect that this will particularly come intoplay 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 inthe system. Over longer time periods it is possible that some spatial and temporal changes inthe distribution of volcanism and rock deformation could occur, but we anticipate that mostrock deformation 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. Reactivation of NNW trending structures in the forearc could increase rock deformation
hazard at the forearc domain over longer timeframes. It is possible that migration of arc
volcanism into the gaps between the currently active volcanoes will occur during this period
particularly as the overall plate kinematics change, but it is unlikely that active volcanism will
migrate substantially relative to its current position, or migrate across the margin (for example,
into the forearc domain). If deformation in the Sea of Japan convergent zone becomes
progressively localized and evolves towards an incipient subduction margin, it is possible thatlarger portions of the convergent component of plate motion being accommodated within theupper plate could shift into the Sea of Japan region, possibly localizing on structures justoffshore. The future localization of slip in the Sea of Japan region will depend on the details 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 the underthrusting Sea of Japan crust will havemigrated eastward by ~15 km, and will have reached several km deeper, compared to ~30
km depth that we assume for the its current depth (based on seismicity). Overall, the likely
impact of this RES on the western contractional domain and the backbone range domain are
similar to those for RES 1.

18

W

W

W

Figure 2.5: Schematic of major tectonic domains and likely evolution for the three timeframesof RES 3. Light green :evolving contractional domain associated with underthrusting of the
Sea of Japan, light orange: backbone range domain of currently active arc, dashed black
line: current likely leading edge of the underthrusting Sea of Japan crust, red line: likely
position of the leading edge at the end of each timeframe, red triangle: Quaternary volcanoes(Committee for the Catalogue of Quaternary Volcanoes in Japan, 1999), dark green lines:
some of the NNW trending bedrock structures (e.g. Hatagawa Tectonic Line).

19

2.5

RES 4: Upper plate shorting decreases to a neutral state

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 Ma,
northern Japan tectonics was strongly influenced by backarc rifting in the Sea of Japan, while
13-2.4 Ma was a period of transition from upper plate rifting to upper plate contraction. 2.4 Ma
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 Myr. RES 4 (see Figure 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 backarc extension. At the western contractional domain andthe backbone range domain, this would produce a progressive reduction in rock deformation
rates, and most likely, a shift to subsidence (as was observed during the Sea of Japan rifting
phase in the Miocene). This scenario would also impact volcanic patterns, with a likely
eastward migration of the arc out to longer timeframes, possibly into the western part of theforearc domain.
0-10 kyr timeframe. We do not expect this scenario to be significantly different from what isobserved today, particularly in terms of the location of deformation, uplift, and active
volcanism in Tohoku. Rock deformation, volcanic, and surface uplift/exhumation hazards
would be analogous to those estimated using the ITM methodology.
10-100 kyr timeframe. We do not expect this scenario to be significantly different from thepresent day to 10 kyr scenario, although it is possible that some spatial changes 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 the plate tectonic boundary conditions
begin to change. We anticipate that most of the permanent contractional deformationaccommodated in the upper plate during this period will continue to occur within the backbone
range domain and along the evolving convergent margin adjacent to the Sea of Japan,
although rates of rock deformation and uplift may be slightly reduced. Volcanism is unlikely to
migrate significantly outside of the current volcanic arc.
100 kyr – 1 Myr timeframe. During this time, a substantial decrease in the component ofplate motion accommodated as permanent deformation in the upper plate will occur, goingfrom ~15 mm/yr to ~0 mm/yr from 100 kyr to 1 Myr. This will lead to a gradual decrease in themagnitude of contractional deformation in the upper plate. During the latter half of thistimeframe (500 kyr to 1 Myr) the rates of rock deformation and uplift will decrease to nearlyzero compared to the present day, and potentially transition to subsidence. With the shift toan extensional upper plate environment, volcanism could become more voluminous than iscurrently 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 somemigration of the arc and the current location of volcanism in Tohoku. One possibility is that asmall trenchward migration (on the order of a few tens of km) of the volcanic arc into thewestern forearc domain could occur due to slab rollback. The amount of trenchward migration
will depend on the rate 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. Moreover, the occurrence of rhyolitic, caldera-forming
volcanism will be more likely in an extensional tectonic environment, and such features could
develop in the gaps between the currently active volcanoes. This could lead to increased
volcanic impacts at the backbone range domain, as well as increased uplift rates there(relative to surrounding areas), if magmatism migrates significantly into that region. Overall,however, for all domains in Tohoku we expect a shift to subsidence (rather than uplift) as wasobserved during Miocene Sea of Japan opening (e.g., Regalla et al., 2013).

20

W

W

W

Figure 2.6: Schematic of major tectonic domains and likely evolution for the three timeframesof RES 4. Light green :evolving contractional domain associated with underthrusting of the
Sea of Japan, light orange: backbone range domain of currently active arc, dashed black
line: current likely leading edge of the underthrusting Sea of Japan crust, red line: likely
position of the leading edge at the end of each timeframe, red triangle: Quaternary volcanoes(Committee for the Catalogue of Quaternary Volcanoes in Japan, 1999), heavy dashed black
line: current position of the 100 km contour to the surface of the subducting Pacific slab,
heavy line: possible position of this at 1 Myr assuming the decrease in upper plate shortening
and shift to a stress neutral state related to the rollback of the subducting Pacific slab.

21

2.6

Summary of possible impacts on sub-regions (domains)

Overall, under RES 1 and 3, we expect the rock deformation hazard to stay the same at thewestern contractional domain and the backbone range domain, or, potentially to increasedepending on the details of faulting evolution in those domains (and the potential forreactivation of bedrock faults there). RES 3 would produce a potential increase in rockdeformation hazard at the forearc domain. For RES 2, we expect an increase in rockdeformation hazard at the western contractional domain and the backbone range domain out
to the longer timeframes. Under RES 4, there would be a reduction in rock deformation
hazard with time at all three sites, going to very low rates by 1 Myr (although on the 0-100kyrtimescale, we expect the rates to be comparable to today).
In terms of volcanic hazard, all RES would produce a similar scenario at all three sites seen
today out to 100 kyr. Beyond 100 kyr, all RES indicate that it is plausible for volcanism tomigrate into the “gap” between active volcanoes in the arc (e.g., Honda et al., 2007),
producing an increased hazard at the backbone range domain. RES 2 could produce anincreased volcanic hazard at the backbone range domain, if there is a westward migration ofarc volcanism (related to potential slab flattening as part of RES 2). A shift to extension in theupper plate as part of RES 4 could produce more voluminous volcanism in the arc
andbackarc, and we would expect elevated volcanic hazard at the western contractional
domain and the backbone range domain relative to the other RES.
Increased uplift and exhumation at the western contractional domain and the backbone range
domain could be due to an increase in rock deformation rates at those sites, as well as
increased volcanic activity. Increased uplift rates due to changes in rock deformation rates isof most concern out to longer time periods in RES 2 (doubling of upper plate rock deformation
rates), but it is certainly a plausible scenario for RES 1-3. In general, RES 4 should produce ashift to subsidence over longer timeframes at all three sites, unless a magmatic centre
migrated into the western contractional domain and the backbone range domain. A shift to
uplift at the backbone range domain (which includes currently in a fault-bounded basin) would
be of concern during the latter periods (100 kyr- 1 Myr) for all of the RES, if there is a
migration of volcanic activity into the gap areas between the currently active volcanic centres,
or if the thrust front migrates basin-ward. Tables 2.1, 2.2 and 2.3 summarize the possible
impacts of RES 1-4 at different timescales for all three domains.

Table 2.1: Summary of possible impacts on the western contractional domain
0-10 kyr10-100 kyr

100 kyr-1 Myr

RES 1Similar topresent day

Constant or increased
rates of rock deformation
and uplift/exhumation, nochange in volcanism.

Constant or increased rates of rockdeformation and uplift/exhumation, potential
for shifts in the locus of volcanism andincreased volcanic hazard

RES 2

Similar topresent day

Increased rates of rockdeformation anduplift/exhumation, nochange in volcanism

Possible doubling of rates of rockdeformation and uplift/exhumation, potential
for shifts in the locus of volcanism (westward
migration) and increased volcanic hazard.
Role of incoming continental fragments in the
Sea of Japan should be accounted for, as
this will cause local increases in rockdeformation rates, and perturbation of thepresent volcanic regime. The latter could be
an issue for WCB site in all RES.

RES 3

Similar topresent day

Constant or increased
rates of rock deformation
and uplift/exhumation, nochange in volcanism

Constant or increased rates of rockdeformation and uplift/exhumation, potential
for shifts in the locus of volcanism andincreased volcanic hazard

RES 4

Similar topresent day

Constant or decreased
rates of rock deformation
and uplift/exhumation, nochange in volcanism

Decreased rates of rock deformation anduplift/exhumation. Distinct shift to subsidence
There is potential for shifts in the locus of
volcanism and increased volcanic hazard(due to more voluminous volcanism in an
extensional regime).

22

Table 2.2: Summary of possible impacts on the backbone range domain
0-10 kyr10-100 kyr

100 kyr-1 Myr

RES 1Similar topresent day

Constant or increased
rates of rock deformation
and uplift/exhumation, nochange in volcanism.

Constant or increased rates of rockdeformation and uplift/exhumation, potential
for migration of volcanism into the volcanic“gaps” and increased volcanic hazard (andassociated uplift)

RES 2

Similar topresent day

Increased rates of rockdeformation anduplift/exhumation, nochange in volcanism

Constant or increased rates of rockdeformation and uplift/exhumation, potential
for migration of volcanism into the volcanic“gaps” and increased volcanic hazard (andassociated uplift).

RES 3

Similar topresent day

Constant or increased
rates of rock deformation
and uplift/exhumation,
depending on details offault evolution there; nochange in volcanism

Constant or increased rates of rockdeformation and uplift/exhumation, potential
for migration of volcanism into the volcanic“gaps” and increased volcanic hazard (andassociated uplift)

RES 4

Similar topresent day

Constant or decreased
rates of rock deformation
and uplift/exhumation, nochange in volcanism

Decreased rates of rock deformation anduplift/exhumation. Distinct shift tosubsidence. There is potential for shifts in thelocus of volcanism and increased volcanic
hazard (due to more voluminous, potentiallyrhyolitic, volcanism in an extensional
regime).

Table 2.3: Summary of possible impacts on the forearc domain
0-10 kyr10-100 kyr

RES 1

Similar topresent day

Constant or increased
rates of uplift/exhumation

Constant or increased rates of rockdeformation and uplift/exhumation

RES 2

Similar topresent day

Increased rates of
uplift/exhumation

Constant or increased rates of rockdeformation and uplift/exhumation

RES 3

Similar topresent day

Constant or increased
rates of rock deformation
and uplift/exhumation

Constant or increased rates of rockdeformation and uplift/exhumation

RES 4

Similar topresent day

Constant or decreased
rates of rock deformation
and uplift/exhumation

Decreased rates of rock deformation anduplift/exhumation. Distinct shift tosubsidence.

2.7

100 kyr-1 Myr

Examples of additional aspects of tectonic evolution for 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 Ma (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 backarc 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 in to the future, and how this will influence the locations of futuremagmatism and rock deformation. Honda et al. (2007) demonstrate that the distribution of arc
volcanism appears to “flip-flop” between the current location and the gaps in between theactive volcanoes every 5 Myr or so. We discuss this briefly in RES 1, and in practice, thisconcept should be applied more fully to all RES with more precise knowledge of the
distribution of volcanism for the last 10-20 Myr.

23

The 2011 M9 earthquake has led to a reversal in the stress field (to an east-west extensional
regime) throughout Tohoku as documented by numerous seismological studies over the lastyears. In light of this, we should also consider the possibility that any optimally oriented
bedrock faults (generally, north-south trending) could be temporarily reactivated as normal
faults following a giant megathrust earthquake as has been observed following the 2011
event.
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 forthe east coast of northern Honshu by Heki (2004) in order to explain observed subsidence atcontinuously running GPS sites. A more recent study (Regalla et al., 2013) suggests thatchanges in the rate of Pacific Plate convergence (and the impact of this change on slab
dynamics) has led to periods of uplift and subsidence in the past. Assessment of the potential
influence of these and other deeply seated subduction zone processes on the future verticaltectonic motion in the Tohoku region can be incorporated into alternative RES and/or as
options within the suggested RES presented here.

24

3Site Evolution Scenarios

We redeveloped Site Evolution Scenarios (SES) for three example sites, in which uplift anderosion rates were estimated based on regional data, that had not previously been
investigated. The scenarios are shown as the storyboards in Figure 3.2 to Figure 3.4 forexample sites described below. The storyboards are intended to demonstrate visually howeach of the RES might manifest itself as a variety of site evolution scenarios, which may
impact the repository footprint during the time periods of interest. These are largely intended
to demonstrate the use of storyboards to develop these scenarios. These are not considered
as complete or detailed descriptions of the impact of the various RES at any of the example
sites. Undoubtedly, there is a larger variety of SES that could be considered at these sites
than are shown in this demonstration.
3.1

Example sites for SES demonstration

Three example sites for the SES demonstration were selected to represent the three tectonic
domains described in Chapter 2. These are the West Coast Backarc (WCB), Inland Basin (IB)and East Coast Forearc (ECF) (Figure 3.1), based mainly on the availability of data on upliftand erosion.
West Coast Backarc (WCB):
Example site WCB is located within the crest of a growing anticline, on the Sea of Japan
Coast, within the fold and thrust belt comprising a contractional zone. This site is identicalwith the Site C in the previous TOPAZ Tohoku case study (Chapman et al., 2012) and theSite 8 in the ITM Tohoku case study (Chapman et al., 2008).The area is marked by highhistorical seismicity rates and high GPS crustal strain rates. It is located within 500 m of theactive Noshiro Fault (on the hanging wall of the fault), which has a vertical slip rate of 0.4-1.4
mm/yr. Blind thrusts and flexural slip faulting may be widespread in this area. Okamura et al.
(2007) have shown that the folds in the Niigata area to the south are forming above steep
reverse faults that are mostly reactivated normal faults formed during Miocene rifting. The site
is located north-west of the Sengan Cluster, in the low-lying inter-cluster region, and thenearest volcano (Kampu Volcano) is 32 km distant.
The geology of this area is characterized by Quaternary (marine terrace) sediments (mud,sands, and gravel) over Tertiary sediments (mid-late Miocene and Pliocene sandstone andsiltstone). The Tertiary sediments would be the likely host rocks for a 300 m deep repository.
Inland Basin (IB):The IB site is situated within an actively subsiding basin in the backbone range, within a
volcanic ‘gap’ region between the Sengan and Chokai clusters, and near to an active fault.
This site is close to but in between the Site 1 and Site 12 in the ITM Tohoku case study(Chapman et al., 2008). The nearest volcano is 30 km to the northeast (Inai-Takahachi). It is
located in an area of high historical seismicity and high contractional strain from GPS.
The geology of the area consists of Quaternary (alluvial fan) sediments overlying Late
Miocene-Pliocene shales, sands, gravels, and clays, as well as early-middle Miocene tuff,
mudstone, and sandstone.
East Coast Forearc (ECF):
Example site ECF is on the Sanriku (Pacific) Coast in the forearc region of Tohoku, on therelatively stable Kitakami Block. This site is identical with the Site 12 in the ITM Tohoku casestudy (Chapman et al., 2008).This is in an area of no volcanism; the nearest cluster (Sengan)
is 100 km to the west, while the nearest volcano is 88 km to the northwest (Iwate). The
nearest major active fault is 53 km to the northwest, although there is a possibility of
unmapped near-coast submarine faults. A NNE-trending inactive bedrock fault exists adjacentto the site.
The bedrock geology is characterized by Mesozoic crystalline rocks, mostly consisting ofPaleocene to early Eocene dacite, welded tuff, and tuff breccia. The Mesozoic rocks would be
the likely host for a 300 m deep repository. Nearby there are also early Cretaceous dacite torhyolite lavas, pyroclastic rocks, granodiorite, and granite. Relatively thin (a few metres)
Quaternary marine terrace deposits likely overlie the site.

25

Figure 3.1: Example site locations for the SES demonstration. WCB: West Coast Backarc;
IB: Inland Basin; ECF: East Coast Forearc. Blue triangles: Quaternary volcanoes (Committee
for Catalogue of Quaternary Volcanoes in Japan, 1999), solid ines: active faults (AIST, 2013).
3.2

Data on regional scale uplift and erosion

The Tohoku area is undergoing both local and regional scale uplift. Local scale uplift isoccurring on the upthrown side of reverse faults (tectonic uplift), which are often characterisedby fault-associated anticlines, and above volcanic intrusions (volcanic uplift). These localised
uplifts predominantly occur along the (Ou) backbone range (tectonic and volcanic uplift) andalong the Sea of Japan coast (tectonic uplift). Regional scale uplift also appears to beoccurring across the majority of Tohoku and is likely related to the Japan Subduction Zone,
possibly by an isostatic response to crustal thickening (e.g., Matsu’ura et al, 2008, 2009).
Uplift rates along the Tohoku coast are generally well constrained from Pleistocene andHolocene marine terraces (e.g., Miyauchi, 1988; Ota and Omura, 2009; Ota 2010). By
contrast, and like most places around the globe, uplift rates inland are less well constrained.
One dataset that has been used is the vertical spacing between fluvial terraces (Yoshiyama
and Yanagida, 1995; Tajikara and Ikeda, 2005). Tajikara and Ikeda prepared a map of upliftacross Tohoku using a combined marine (125,000 year) and fluvial terrace (≤140,000 year)
dataset (Figure 3.2; Koike, et al., 2005). Although some of the fluvial terrace ages havesubsequently been questioned (Matsu’ura et al. 2008), we consider this the best available
dataset for regional scale uplift rate in the Tohoku area. We have therefore calculated uplift
rates from this dataset by dividing the uplift values by the age of the marine terrace or thetime between fluvial terraces, 125,000 years.
With regard to regional scale erosion dataset for the Tohoku region, Fujiwara et al. (1999)
constructed an erosion rate map for the whole of Japan derived from the relationship between(contemporary) sediment delivery rates to reservoirs and the dispersion of altitude of thedrainage basin (Figure 3.3). However, although this is based on a 6 x 6 km grid (similar to the5 x 5 km grid used in this study) the drainage basin scale means it is too coarse for use in thisstudy.

26

Figure 3.2: Map showing uplift in metres of a normalised 125,000 timeline across thenorthern Tohoku area, derived from marine and fluvial terraces (Koike et al., 2005; preparedby M. Tajikara). The locations of the example sites are also shown.

Figure 3.3: Erosion rate map for the Japanese Islands derived from sediment delivery rates
and the dispersion of drainage basin altitude (Fujiwara et al., 1999). The locations of theexample sites are also shown.

27

3.3

Estimation of uplift and erosion rates for the example sites

West Coast Backarc (WCB):
The mean uplift rate is relatively well constrained from the current altitude (~50 m - Figure 3.2;
also Miyauchi, 1988; Shiraishi et al., 2008) of the 125,000 year old marine terrace strandline(~landward cliff above the terrace), at ~0.4 mm/yr. We infer uncertainties of ± 0.2 mm/yr,
mainly to take into account likely uncertainties in the terrace altitude (from terrace coverbeds
and the fact that the site is not at the strandline).
There are no erosion rate data, but we infer an erosion rate of 0.2 ± 0.1 mm/yr, which is half
the uplift rate and takes into account the soft to moderately hard bedrock beneath the marine
terrace (i.e., the likely host rock for a repository). Over longer timeframes, the erosion rate
could increase through river and coastal erosion.
Inland Basin (IB):The mean uplift (subsidence) rate, rate is -0.15 mm/yr, and is derived from the regional scale
uplift map (Koike, et al., 2005; Figure 3.2). The rate appears to be from the vertical spacing
between fluvial units, which are presumably from drill hole or geophysical data. A relatively
large uncertainty of ±0.1 mm/yr is assigned to reflect the likely uncertainty on the dating of thefluvial units.
There is no erosion (burial) rate data, and so we infer a rate of -0.1 ± 0.05 mm/yr, which isslightly less than the subsidence rate. This rate reflects likely incomplete filling of the basin
and little to no compaction if the sediments are mostly alluvial gravels. It is possible that theerosion rates could increase with time, particularly if the nearby Senya Fault migrates basinwards, leading to uplift at the site, or the rivers migrate to the site.
East Coast Forearc (ECF):
The mean uplift rate is relatively well constrained from the current altitude (~15 m – Figure
3.2; also Miyauchi et al., 2005; Miyauchi, 2011) of marine terraces, at ~0.1 mm/yr. We infer
an uncertainty of ±0.05 mm/yr, mainly to take into account likely uncertainties in the terrace
altitude (from terrace coverbeds and the fact that the site is not at the strandline).
There is no erosion rate data, but the site is situated on relatively hard early Tertiary volcanic
rocks, and so we infer a very low mean erosion rate of 0.05 ± 0.05 mm/yr, which is half the
uplift rate. Over longer timeframes (>100 kyr), lateral coastal erosion could impact the site.

The estimated rates for each site are summarised in Table 3.1.

Table 3.1: Uplift and erosion rates estimated for the three example sites for use in this study.
Site

Uplift rate

Erosion rate

(mm/yr)(mm/yr)Mean

Uncertainty

Mean

Uncertainty

WCB

0.4

0.2

0.2

0.1

IB

-0.15

0.1

-0.1

0.05

ECF

0.1

0.05

0.05

0.05

28

3.4

Site Evolution Scenarios for the West Coast Backarc (WCB)

See storyboard at Figure 3.4

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 400 m of uplift and 200 m of verticalerosion results in exhumation of the repository by 1.5 Myr. Volcanism could also
initiate nearby within longer timeframes (>500 ky).
SES2: As for SES1 except more rapid uplift and erosion rates result in more rapid
rock deformation and repository exhumation—exhumation of the repository occurs by
750 kyr.
SES3: Anticline development and associated deformation and uplift decreases overtime as convergent deformation becomes increasingly localised offshore. Verticalerosion over the 100 kyr - 1 Myr time period leads to 50 m of erosion, and 200 m of
uplift.

RES2




SES1: As for RES1/SES1.
SES2: As for RES1/SES2.
SES3: As for SES2 above, but further increased contraction deformation in thevicinity of the west coast related to possible slab flattening results in even more
intense rock deformation in the repository area. Uplift of up to 1500 m, and verticalerosion of up to 750 m results in repository exhumation by 400 kyr. Possible initiationof volcanism nearby.

RES3




SES1: As for RES1/SES1
SES2: As for RES1/SES2
SES3: As for RES1/SES3

RES4
SES1: As for RES1/SES3

29

Figure 3.4: Storyboard showing the site evolution scenarios for the WCB site.

30

3.5

Site Evolution Scenario for the Inland Basin (IB)See storyboard at Figure 3.5.

RES1






SES1: The repository lies within Miocene sedimentary rocks, and beneath the centreof an actively deepening 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
volcanism migrates into the volcanic gap region. Deposition within the basin increase
the repository burial depth by up to 100 m 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. Increased slip rates on basin bounding faults may cause the basin to
deepen more rapidly, resulting in additional repository burial—up to 200 m by 1 Myr.
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. 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. Burial rates in the basin are much smaller due todecreasing deformation on basin bounding faults, with cumulative burial of the site up
to 25 m by 1 Myr.

31

Figure 3.5: Storyboard showing the site evolution scenarios for the Inland Basin site.

32

3.6

Site Evolution Scenarios for the East Coast Forearc (ECF)

See storyboard at Figure 3.6.

RES1





SES1: The repository lies within Mesozoic bedrock and beneath a stable marine
platform on the east coast. The area continues its low and uniform rates of uplift anderosion over the next 1 Myr. The main potential impacts on repository integrity overtime are bedrock fracturing due to shaking from regional earthquakes, and chemical
weathering in the near-surface bedrock, particularly along fractures. Over the 1 Myrtime period the platform continues to be gradually uplifted and eroded, achieving 100
m of uplift, and up to 50 m of erosion (assuming erosion rates of 0.05 mm/a).
SES2: As for SES1, except with more rapid rates of uplift (up to 500 m over 1 Myr)and consequent erosion (up to 100 m out to 1 Myr).
SES3: Platform uplift decreases to nil by 500 kyr, with an accompanying decrease inerosion rates. This yields up to 50 m of uplift and up to 20 m erosion achieved by 1
Myr.

RES2




SES1: As for RES1/SES1.
SES2: As for RES1/SES1, except greater uplift (up to 300 m) and erosion (up to 100m) 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 due to possible slab
flattening. Uplift of up to 500 m and erosion of up to 250 m suggest that if maximum
uplift and exhumation rates are reached, the repository could be exhumed by ~1.2
Myr.

RES3


SES1: As for RES1/SES1.

RES4



SES1: As for RES1/SES1.
SES2: As for RES1/SES3.

33

Figure 3.6: Storyboard showing the site evolution scenarios for the East Coast Forearc site.

34

4

Introduction to the Impact Scenarios

The purpose of this chapter is to present site-specific considerations that would be factored
into natural-event Impact Scenarios (IS) for the three illustrative sites considered in Chapter 3.
The previous report (Chapman et al., 2012) presented a detailed examination of impacts on arepository arising from faulting and volcanism. Therefore, this section focuses on presenting
illustrations of exhumation scenarios for each of the three sites.
4.1

Evolution of a HLW repository

As previously discussed, there are several aspects of timescale and the evolution of a deep
geological repository that must be kept in mind when examining natural-event scenarios. Firstand foremost, within several thousands of years (a relatively short timeframe compared to
evolving volcano-tectonic conditions in Japan), the radiological hazard of HLW diminishes toextremely low levels.
Furthermore, consider the evolving physicochemical characteristics of the engineered barriers
leading to degradation of their safety functions. After 1 kyr to 5 kyr, the initial mild steel
overpack will have corroded to the point of failure, allowing groundwater to contact and initiatedissolution of the HLW borosilicate glass. The dissolution time for the HLW glass depends ona number of factors, including degree of cracking/enhancement to surface area, but aconservative estimate would be about 70 kyr. Thus, within several tens of thousand years, theinitial safety functions of the EBS barriers (metal overpack and borosilicate waste-form) are
no longer operative or needed. The original EBS materials will have basically converted to athermodynamically more-stable assemblage of insoluble, slightly radioactive oxides,
hydroxides and silicates surrounded by iron hydroxides/ oxides of the corroded overpack, inturn surrounded by a compacted layer of clay. More soluble radionuclides will have beenreleased, yet dispersed and diluted throughout the rock volume surrounding the EBS. At
ambient environmental conditions, the residual phases remain unreactive over the millionyear timescales or more, as evidenced by numerous geological and natural analogue studies.
In essence, the engineered repository will have evolved to a ‘natural state’ that is relatively
inert with respect to plausible thermal-hydrological-mechanical-chemical (THMC) impacts thatmight arise from natural-event scenarios. Off course the isolation provided by the natural rockbarrier of 300m depth will continue beyond this period.
Of particular importance to long-term isolation of radioactivity in a ‘natural-state’ repository is
the low redox potential (Eh) that prevails for deep repository sites. Low Eh assures extremely
low solubilities (retention) and high sorption behaviour (retardation) for many safety-significant
radioelements, such as U, Pu, Np, Tc and Se. Prolonged elevated temperature, for example
from nearby intrusion of magma, could also elevate the solubility of residual radionuclidebearing solids. The leading concern of any longer term (>50,000 years) combined RES/SES
scenario, however, would be if a natural event led to transport of the residual, insoluble
radioactive phases to the near-surface or exposure at the ground surface.
4.2

General Impact Scenarios for exhumation (uplift and erosion)

Figure 4.1 shows a schematic cross section of a hypothetical site in which a HLW repository
has been emplaced at an initial depth of 300 meters at a present day site. For this illustrative
case, it is further assumed that the site has a present-day mean uplift rate of 0.1 mm/year, as
possibly determined through a set of independent, complementary geophysical and geodetic
methods. This illustrative site also has topography, hence erosional processes that operate to
reduce the height and lateral face of terrain as it is uplifted. Uplift rate and erosion rate can beviewed as more-or-less independent processes that depend on a number of site-specificfactors, such as regional strain rate, lithology, existing structural features, climate, topography,
etc. For the purpose of a simple schematic illustration, it is assumed in Figure 4.1 that theerosion rate matches the uplift rate. Therefore, the ‘exhumation rate’, the rate at which theburied repository might approach the surface, would be equal to the uplift rate10.

10

Off course some sites may be experiencing subsidence rather than uplift. For such a site, if deposition (asopposed to erosion) rate equals subsidence, the repository would become more deeply buried, equal to thesubsidence rate integrated over a specific time.

35

yr

kyr

kyr

Figure 4.1: Progressive uplift of the repository at an exhumation rate of 0.1mm/year for time
steps of 10 kyr, 100 kyr and beyond into the future.

36

At each time step, a ‘cartoon’ is included to illustrate the progressive degraded state of the
EBS component barriers (HLW borosilicate glass, mild steel overpack and bentonite clay
buffer) emplaced within a generic host rock.
Also shown in Figure 4.1 is the approximate ‘fate’ and ‘migration’ of various types ofradionuclides. In the first 1 kyr to 5 kyr, the overpack has failed by corrosion, and the HLW
has experienced significant dissolution. As noted previously (Chapman et al., 2012), it is asignificant safety factor that the radiological hazard of HLW significantly decreases over time,within about 3 kyr reaching an equivalent radiotoxicity hazard of the original uranium ore fromwhich the reprocessed radioactive waste was fabricated.
After about 70 kyr the HLW borosilicate glass will have been fully dissolved, and converted
into more thermodynamically stable (hence less soluble) alteration solids. Highly soluble andnon-sorbing radionuclides, if present, will have been released by this dissolution andtransported through the hydrological system of the site to some distant sub-vertical, majorwater-bearing feature (e.g., a fault). These nuclides will then have been transported to thesurface environment, although additional, significant dilution may occur depending on specificdischarge pathway. By contrast, the great majority of radioelements will remain within the
EBS, in the form of insoluble radionuclide-bearing solids that precipitate from the dissolutionof the HLW glass.
Eventually about 3 Myr in the future for this case, a degraded ‘natural state’ repository nears
the surface by means of sustained uplift and erosion. There are two factors to consider at thispoint with respect to enhanced radionuclide release to the accessible environment:

Many radioelements become more soluble and less sorptive under oxidizing
conditions. The depth to which O2-rich surface waters percolate into the sub-surfacehydrological setting depends on many site-specific features, and as a bounding
assumption a 100m depth of oxic surface water is assumed in Figure 4.1.
The transport pathways for radionuclides to reach the surface environment become
shorter (Figure 4.1).



If uplift and erosion is taken to an endpoint, a degraded repository may be exhumed at the
ground surface in this illustration. Obviously this would to significant radiological releasethrough water, and possibly air-borne, pathways. There might also be direct exposure to
future humans, although the radiological exposure-hazard of HLW at 3 Myr would be
extremely low, well below that of naturally occurring uranium ore bodies for example.

4.3

Uplift, erosion and exhumation scenarios at the example sites

WCB Site
The interpreted uplift rate for the WCB site is 0.4 +/- 0.2 mm/yr, while the estimated erosion
rate is 0.2 +/- 0.1 mm/yr. Figure 4.2 shows the time sequence in the evolution of a repository
at such a site at 10 kyr, 100 kyr and 1 Myr and beyond 1.5 Myr into the future. If both rates
are assumed to remain constant into the future, the repository at an initial depth of 300 m
would rise to the level that oxic surface waters might begin to percolate down to the depth ofthe degrade repository. Of course sideways (landward) erosion of the escarpment of the
uplifted terrain might also allow lateral access or even partial exposure of the uplifted residual
repository. At this time period in the far future, however, the radiological hazard of thedegraded HLW repository is significantly reduced, for example far below that for thecomparable uranium ore from which the HLW was fabricated.
IB SiteFor the IB site, the inferred subsidence (rather than ‘uplift’) rate is -0.15 +/- 0.1 mm/yr. A burial(rather than ‘erosion’) rate is estimated to be -0.1 +/- 0.05 mm/yr. Figure 4.2 shows the time
sequence in the evolution of a repository at such a site at 10 kyr and 100 into the future. In
this case, over the next 1 Myr, the repository at an initial depth of 300 meters would become
deeper over time. Certainly the potential for intrusion of oxidizing groundwater from thesurface becomes evermore less likely because of such subsidence. Increasing depth would
increase overall system pressure on the repository, however such a pressure increase would
have a negligible impact on isolation. Change in pressure has only a minor impact on the37solubility of condensed phases, such as the residual radionuclide-bearing solids. Hydrologicalpath lengths for any radionuclide migration might become ever longer with increasing depth.
ECF Site
The ECF site has an estimated uplift rate of 0.1 +/- 0.05 mm/yr, with an inferred erosion rateequal to 0.05 +/- 0.05 mm/yr. This is similar to the general illustrative case presented; with the
exception that erosion rate is lower (half) than the uplift rate. Figure 4.2 shows the time
sequence in the evolution of a repository at such a site at 10 kyr, 100 kyr, 1 Myr, 4 Myr andbeyond 6 Myr into the future. At about 4 Myr, the repository would approach the level of theinitial ground surface, and could experience contact with oxic surface water percolating todepth. Lateral (landward) erosion of the escarpment of the uplifted terrain might allow
sideways intrusion or even exhumation of the repository. At extremely long time periods into
the future the degraded repository might be exposed at the surface, although radioactive
decay will have reduced the radiological hazard of the degrade HLW substantially.

yr

kyr

kyr

Figure 4.2: A demonstrative storyboad for the evolution of a repository at example sites WCB,
IB and ECF at 10 kyr, 100 kyr and beyond 1 Myr into the future.

38

5

Hazard maps for volcanism and exhumation

In the following, we present the developed ITM-TOPAZ methodology could potentially be
used for assessment of project risk. We focus on the methodologies for forecasting two
potentially disruptive phenomena. These are potential volcanism and potential exhumation ornear-exhumation due to uplift and erosion. For both phenomena, a variety of alternativemodels and data may be used to assess the potential for specific events. We present a
method for aggregating these assessments to estimate project risk, recognizing that themethodology may be extended to encompass all potentially disruptive geologic phenomena
for a particular site.
5.1 Introduction
The goal of geologic hazard assessment for HLW repository sites is to provide quantitative,
probabilistic estimates of the occurrence of potentially disruptive geological phenomena.
Potentially disruptive geologic hazards for HLW repositories include volcanism, tectonic upliftand erosion, fault displacement and related processes. In this context, volcanic hazard isdefined as the probability of a specific volcanic event (e.g. volcanic eruption of a specific size)
or volcanic phenomena (e.g. the occurrence of igneous dyke intrusion at a specific location
during a specific time interval). Similarly, faulting displacement hazard for HLW repositories
may be defined as the probability of fault displacement exceeding some magnitude during theassessment period of the site. Uplift and erosion hazard may be defined as the probability
that the repository is exhumed, or reaches a shallow oxic environment during someassessment interval.
The rise of probabilistic geologic hazard assessments and their application to HLW repository
sites during the last several decades has relied on developments in several aspects ofgeoscience. First, vastly improved quality of available data has enabled forecasts. These are
geologic data, such as high-resolution stratigraphy calibrated by radiometric age
determinations, and geophysical data used to assess site stability. Second, numericalsimulation of volcanic and tectonic phenomena was virtually unknown more than 30 years
ago, but has rapidly developed through our understanding of the physics of volcanic andtectonic processes, and our ability to run simulations using ever more sophisticated and fastercodes and computers. These developments have allowed us to better model potentiallydisruptive geologic events for HLW repository performance. Third, statistical models are
evolving at an equally rapid rate, allowing application of improved methods for understanding,for example, uncertainty in hazard, the application of alternative models, and incorporation of
disparate data sets into forecasts using event trees, Bayesian methods and expert elicitation.
The TOPAZ project has partially focused on developing new and efficient ways to assess
aggregate geologic hazard for example sites in Japan, representing a range of tectonic
environments.
Probabilistic geologic hazard forecasts are particularly useful to better understand, compareand communicate the potential for disruptive geologic events on various spatial and temporalscales. In this context, it is possible to use these methodologies to establish thresholds toassess project risk. For instance, probabilistic volcanic hazard assessment (PVHA) may be
used to identify regions in which potential future volcanic activity is highly unlikely to occur
during a given assessment period, or to figure as an issue in siting decisions. Put anotherway, such a site is characterized by an absence of potential for disruptive volcanic events,
and therefore represents low project risk. Similarly, it is practical to use PVHA to identify
regions or sites as potentially unsuitable by developing a threshold probability above which,although unlikely, potential future volcanic activity is sufficient to raise concern, and has
potential to become a siting issue. This approach may be extended to any number ofpotentially disruptive geological hazards, such as fault displacements and uplift. Aggregate
project risk, in this context, is estimated from the union of these assessments. That is, a sitewith low project risk is one in which the assessment indicates that the potential for all
disruptive geological phenomena are below their appropriate threshold values.
5.2

Probabilistic volcanic hazard assessment

The TOPAZ approach involves investigation of potential volcanic hazards using regional data
sets, such as the AIST Quaternary database of volcanoes of Japan, and regional geophysicalstudies as a starting point. Such information, together with additional information from the39

literature, allows for constructing a regional view of rates of volcanic activity and the potential
distribution of volcanism. Most TOPAZ work, particularly with respect to construction ofproject risk maps for volcanism, involves analysis of data at this regional scale. This analysis
is accomplished using alternative models to assess volcanic activity, both in terms of potential
rates of new volcano formation and potential location of future events.
The elements of this PVHA are summarized in a logic tree for the assessment of hazard withensemble models (Figure 5.1). Nodes of this tree represent alternative models for volcanismin the region under consideration. The transitions on the tree give credence, or weights, toparticular models, usually assigned through expert elicitation. In TOPAZ PVHA, constructionof the logic tree considers the following factors:










For regional investigation and comparisons, as is appropriate for project risk maps,
the area under consideration may be the entire volcanic arc. We note that the areafor detailed site characterization in PVHA often extends up to 300 km from the site.
This comparatively large area is appropriate in order to understand and model
regional trends in volcanic activity, especially on time scales of 1 Myr.
For Japan, it is not credible that volcanic arcs are extinct. For example, ITM and
TOPAZ work has focused on developing and evaluating PVHA models for Tohoku,
Kyushu and Chugoku areas. There is no credible possibility that volcanism in theseregions will not occur in the future. Nevertheless, this branch of the logic tree isincluded because there are geographic regions of potential interest in Japan in which
volcanic activity has ceased (e.g., Shikoku). Therefore, node N2,1 represents thepossibility of magmatic activity within the system and node N2,2 assumes themagmatic system is extinct.
Alternative recurrence rate models may be developed for volcanic events in any
volcanic region. Some of these recurrence rate models are based on the known
geologic history of the region and recurrence rate is estimated using statisticalmodels of these distributions. Alternative recurrence rate models are based on RESs
intended to forecast change in average rates of volcanic activity over long periods oftime based on forecast changes in the tectonic setting of magmatism or ingeochemistry and heat flux in the magmatic system. Nodes N3,1 - N3,4 represent
alternative recurrence rate models of volcanism and w2,1 - w2,4 weight their credibility.
Alternative models for the potential locations of future volcanic events may be
developed for any volcanic region. Some of these models are based on developmentof statistical models of potential volcano location based on past patterns of activity.
Examples of such models include the use of kernel density models and Cox processmodels. Such models reflect an RES that the current tectonic setting of volcanism,
and areas of magma production, will persist into the future. Alternative models may
be developed that account for shifts in the location of volcanoes through time, forexample associated with the roll-back of the subducted slab, as portrayed inalternative RES. Nodes N4,4 - N4,12 represent alternative spatial density models andw3,4 - w3,12 weight their credibility.
The probability of a new volcanic event within a <100 km2 area about the site is
calculated by following each individual branch of the logic tree from left to right, andby assigning weights to the transitions that connect each of its nodes, thus,
incorporating and weighting all alternative models represented by the tree to estimate
the aggregate probability.

Thus, the heart of the analysis involves identification of credible alternative models and theirweights. Once these are known, estimating event probabilities associated with each node isstraightforward.

40

Figure 5.1: Logic tree of alternative models for the formation of a new volcano within a given
timeframe and within given area area about a given site. Node N1 represents the current stateof the magmatic system. Node N2,1 represents the possibility of magmatic activity within thesystem and node N2,2 assumes the magmatic system is extinct. Nodes N3,1 - N3,4 represent
alternative recurrence rate models of volcanism and w2,1 - w2,4 weight their credibility (seediscussion of expert elicitation). Nodes N4,4 - N4,12 represent alternative spatial density models
and w3,4 - w3,12 weight their credibility. The probability for the formation of a new volcano withina given timeframe due to one particular recurrence rate model and within a given area due toone spatial density model is represented by each branch, or path, of the tree, followed fromleft to right.

5.2.1

Recurrence rate of volcanic events(1) General model structureLong-term volcanic hazard assessment requires estimation of the recurrence rate of volcanic
activity, defined as the expected number of volcanic events within a given time interval. For
the purposes of HLW repository hazard assessment, a volcanic event is the occurrence of a
new igneous intrusion at, or near repository depths, and the disruption of waste packages by
this intrusion, either through mechanical disruption (e.g., transport of waste package
fragments to the surface in a volcanic conduit) or secondary effects (e.g., development of ahydrothermal system leading to accelerated waste package degradation). Recurrence rateestimates of volcanic events are based upon the frequency with which these events occurred41

in the past for a specific region. These estimates are then used to forecast future volcanic
activity. A key component of long-term hazard assessment is the assumption that future ratesof activity can be extrapolated from the past rate, at least throughout some time scale ofinterest.
Consider a sequence of volcanic events – the formation of new volcanoes within an arc. An
estimation of the current recurrence rate at time t , H (t ) , is derived from the history of
S ,T

volcano formation within the arc, H S ,T , between times S and T . The early limit S , might bedefined as the age of the oldest event in the entire volcanic system, or a readily identified
geologic time interval – such as the Quaternary. The later limit T is typically the time at which
the calculation of the recurrence rate was done, which could be the time of the youngest
event in the sequence, or perhaps the present time. The number of events known to haveoccurred during the interval between S and T is denoted N S ,T . The probability of renewed
eruptions during some time period, u (e.g., the next 1 Myr) is:T u
Pr[ N (T , T  u )  1] = 1  exp   T (t )dt .
 T


(1)

Then the probabilistic analysis becomes a matter of estimating at time T the recurrence rate,T (t ) for times t > T . In practice, this approach must be further generalized so that therecurrence rate depends on the history of the volcanic system, H T = H S ,T :Pr( N (t , t  t ) = 1 | H T )
t 0
t

T (t | H T ) = lim(2)

where the history H T is reduced to the oldest age of each volcano, S  t0 , t1 , t2 ...., tn < T .
Events may additionally be considered in terms of eruption volume, vi , eruption magnitude or
explosivity, mi , or composition, xi . Such time-volume predictable models are often appliedin volcanology, but it is emphasized that for HLW repositories we are generally interested in
the potential occurrence of any event, with less emphasis on volume, magnitude orcomposition.
Although this general model structure is straightforward, there is nearly always ambiguity indefining the time of formation of individual volcanoes, t i , and the start time of the series, S ,are defined. These uncertainties are discussed in the next section.
(2) Data Requirements for Estimating Recurrence Rates
Determining the long-term recurrence rate of new volcano formation in an arc involves large
uncertainty. The most fundamental data required are geologic maps, stratigraphic studies andsamples from boreholes for understanding the thickness of the volcanic section and thetiming of onset of eruptive activity. For many volcanic systems only the most recent volcanic
events are known. For many volcanoes, the most recent eruptive events are the best known,
simply because the products of these eruptions mantle the topography and comprise themost accessible rock outcrops. As a result, in the geological literature and volcano databases,
often only the ages of the most recent activity have been reported. Conversely, the oldest
volcanic units are often not exposed or are unrecognized, so the onset of activity, S , and thetotal duration of volcanic activity are often uncertain. Similarly, eruptive data tends to bebiased toward the largest eruptive events, simply because these large eruptions leave
copious deposits with a greater chance of being preserved in the rock-stratigraphic record. It
is often unclear when the onset of activity at a particular volcano occurred.
For the Tohoku arc, for example, the Quaternary volcano database of Japan (Committee for
the Catalogue of Quaternary Volcanoes in Japan, 1999) lists approximately 65 Quaternary
volcanoes has having formed since 2 Ma. Many additional volcanoes are found in the arc, butthere age of formation is unknown. Some of these ages likely reflect the actual time offormation of the volcano, others suggest age of formation based on stratigraphic relationships.

42That is, some volcanoes are known to have formed before or after a given data, based on
stratigraphic relationships. Thus, even when ages are reported, there is considerable
uncertainty in their meaning.
(3) Steady-state models
Whether or not some statistical models (e.g., Poisson process or renewal models) can beapplied to estimate recurrence rate and probability of new volcano formation within a given
time period depends on determining if this sequence of events is in steady-state. Lack ofsteady state may reflect sampling bias or may reflect the nonstationary nature of volcano
formation in some arcs. If a sequence is in steady state, then the level of activity in the future
is well-represented by activity in the past. Mathematically, this means that estimatedprobabilities of future events will be independent of t , the time at which they could occur. If t
is a factor in the probability forecast, then the system is not in steady state.
The Kolmogorov-Smirnov test may be used to determine if the eruption sequence is steadystate. This test is ideal because usually there are few events comprising the sequence (e.g.,
the 65 known ages of volcano formation in Tohoku) and because the test is simple. For the
cumulative number of volcanic events in the sequence:Fn (t ) =# (ti  t ), i = 1....n, S < t < Tn(3)then the Kolmogorov-Smirnov test statistic is:tS Dn = max | Fn (t ) |.
t[ S ,T ] 
T  S (4)

In other words, in the Kolmogorov-Smirnov test statistic, the maximum deviation from theexpected frequency of events determines whether the process is steady state or not. The
95 % confidence bounds are given as 1.36/ N S ,T .
Consider the history of volcano formation in Tohoku, basing this history on the oldest ages
reported for 65 volcanoes. For this series, N S ,T = 65 , and Dn = 0.38 , at 900 ka (Figure 5.2).
The cumulative distribution function for Tohoku volcanic events shows significant deviationfrom steady state, based on the Kolmogorov-Smirnov test at the 95 % confidence level.
Volcanic events prior to approximately 900 ka apparently occurred at a significantly lower rate
than subsequent events. This suggests that a different recurrence rate applies to the twodifferent regimes, before and after 900 ka. It is conceivable that this result indicates an actual
increase in volcanic events with time in Tohoku (e.g. Umeda et al., 1999), but an alternative
explanation is possible that age determinations for volcanoes formed in the early Quaternary
are less frequently made and are perhaps less reliable than age determinations made on
younger volcanoes. Also, there may be bias in reporting the relative lack of accessibility ofolder strata in these volcanic systems.

43

Figure 5.2: Cumulative distribution function for oldest reported volcano ages for 65
Quaternary volcanoes in the Tohoku arc. The dashed line indicates a steady state model withthe confidence interval for this steady-state model shown by finer dashed lines. The
Kolmogorov-Smirnov test indicates that this series not in steady state, with these volcanic
events occurring at a significantly increased frequency after approximately 900 ka.
A different picture emerges if the series is filtered to include only volcanoes  900 ka. For
volcanoes with oldest ages reported to be  900 ka in the Tohoku arc, N S ,T = 42 , and

Dn = 0.17 , at approximately 300 ka (Figure 5.3). In this case, the series appears to be
steady-state. Changing the start time of the analysis, S , changes interpretation of rate of
new volcano formation. One conclusion is that a steady-state model may be used to model
rates of activity over the last 900 kyr. Note, however, that even on this time scale there is anincrease in activity during the last 50 kyr, presumably the best dated portion of the time series.
Therefore a second conclusion is that there is significant uncertainty in the recurrence rate of
new volcano formation for the Tohoku arc. The most recent time interval yields the highest
recurrence rate, suggesting that additional research on the age of formation of these volcanic
systems will likely lead to a higher estimate of the average recurrence rate overall.

44

Figure 5.3: Cumulative distribution function for oldest reported volcano ages for 42 volcanoes
thought to have formed during the last 900 kyr in the Tohoku arc. The dashed line indicates a
steady state model with the confidence interval for this steady-state model shown by finer
dashed lines. The Kolmogorov-Smirnov test indicates that this series is steady state, but notethe apparent increase in activity during the best dated portion of the series during the last 50
kyr.
(4) Statistical models
The simplest approach to steady-state recurrence rate estimation is to average the number of
events that have occurred during some arbitrary time period. An alternative approach,especially appropriate when the total number of known events is small, is to calculate
recurrence rate using the repose-time method, in which the time range is restricted by theestimated ages of youngest and oldest events:

̂T =

N 1
T S

(5)

where ̂T is the recurrence rate (identical for all t ), N is the number of events, occurring
between S , the age of the first (oldest) event, and T , the age of the most recent (youngest)
event. This is the Poisson process estimate, and is a Maximum Likelihood Estimate (MLE),
provided that an event occurs at time S . If the distribution of repose intervals in a volcanic
system show Poisson behavior, then the probability of a given number of events, k in sometime interval u is: ue T (T u ) k
Pr[ N (t  u )  N (t ) = k ] =,k!

k = 1...n(6)

for all t (the definition of stationarity), and the probability of zero events in the time interval, u,
is:
 u

Pr[ N (t ) = 0] = e T(7)

indicating that repose intervals between eruptive events have an exponential distribution, thememoryless property of which implies that there is no possibility of using patterns of activity inthe past history to improve on the forecast.
Using volcanoes thought to have formed during the last 900 kyr in the Tohoku arc, theinterval which appears to be steady-state based on available data, ˆT = 5 10

45

5

yr

1

. If weconsider even the comparatively short-term assessment period of a HLW repository in the
Tohoku region, u = 10 kyr, the probability of no new volcano forming in that period of time isabout 60%. When longer assessment periods are considered there is a much greater chanceof a new volcano forming in the Tohoku arc during that period. For example, whenassessment periods are estimated for 100 kyr, the probability of a new volcano forming in the
Tohoku arc is approximately 99%. Put another way, based on these rates it is expected that
5–6 new volcanoes will form in the Tohoku region within the next 100 kyr, and 50–60 new
volcanoes will form in the Tohoku region within the next 1 Myr. To summarize:







There is great uncertainty in the rate of new volcano formation in the Tohoku arc
because of difficulty resolving the timing of onset of eruptions at a particular volcano
or volcanic system. No age estimates for the onset of activity is available for many
volcanoes in the Tohoku arc.
Nevertheless, the time series of new volcano formation can be assessed using 62
volcanoes for which the data are available. This series is not steady-state, showing
an increase in rate of new volcano formation for times less than approximately 900 ka.
This change may reflect a change in activity in the arc, but likely reflects a change indata quality. For example, the onset of activity for any volcanoes in the arc is reported
to be approximately 1 Ma. In many cases this is likely an educated guess based on
available geological data.
For the 42 volcanoes thought to have formed since 900 ka, the series is steady-state.
Because the series is steady-state, it is possible to estimate recurrence rate using a
MLE model. The estimated recurrence rate of new volcano formation in the arc,
approximately ˆT = 5 10 yr , is biased because some volcanoes in the arc
have no date for the onset of activity. Nevertheless, it is interesting to note thatassuming all Quaternary volcanoes found in the arc formed in the Quaternary yields
51ˆT = 6.5 105 . This range of values appears to do a reasonable job of bounding
the regional recurrence rate of volcano formation in Tohoku.
As the probability of a volcanic event in the region is relatively high for the long assessment
period of a HLW repository, it is necessary to consider the potential locations of these
volcanoes. Alternative models for potential volcano location is considered next in the ITMTOPAZ PVHA methodology.
5.2.2

Location of eruptive vents

Alternative models for the potential location of new eruptive events were developed as part of
the ITM-TOPAZ methodology. These include kernel density methods and Cox processmodels. Both nonparametric statistical models of potential volcano distribution are based on
the assumption that the distribution of volcanism in the past is a guide to future volcano
distribution. For each type of model may be modified to include additional data, using
Bayesian methods, such as the distribution of seismic tomographic anomalies. Each method
may be modified to account for shifts in the location of volcanism with time. Thus, many
alternative models of spatial density are considered as part of the PVHA.
A simple approach relies on the distribution of mapped vent locations that have erupted in thepast. Like in probability models of the timing of future eruptions, this assumption assumes a
steady-state model for the distribution of new volcanic vents is appropriate. Kernel density
estimation is a non-parametric method for estimating the spatial density of future volcanic
events based on the the locations of eruptive vents. Two important parts of the spatial density
estimate are the kernel function and its bandwidth, or smoothing parameter. The kernel
function is a probability density function that defines the probability of future vent formation at
locations within a region of interest.
A two-dimensional elliptical kernel with a bandwidth that varies in magnitude and direction is
given by:ˆ(s) =

1

N

 1



 exp  2 b b
2N | H |
T

(8)

i =1

where,

46

b = H 1/2x.
The local spatial density estimate, ˆ (s) is based on N total events (vent locations). The
bandwidth, H , is a 2 2 element matrix that specifies the bandwidth as an ellipse, withmajor and minor axis lengths, and a rotation. This bandwidth matrix is both positive anddefinite, important because the matrix must have a square root; x is a 1 2 distance matrix,
b is the cross product of x and H 1/2 . The resulting spatial density at each point location, s ,is usually distributed on a grid that is large enough to cover the entire region of interest.
A special case of the kernel function is a two-dimensional radially-symmetric Gaussian kernel:ˆ (s) =N 1  di 2 
1exp
  2  h  2h 2 N i =1



(9)

that depends on the distance, d i , to each event location, N , from the point of the spatial
density estimate, s , and the smoothing bandwidth, h that is constant in all directions.
The bandwidth is selected using some criterion, often visual smoothness of the resulting
spatial density plots. Bandwidths that are narrow focus density near the locations of mappedvents. Conversely, a large bandwidth may over-smooth the density estimate, resulting in
unreasonably low density estimates near clusters of past events, and overestimate density far
from past events. This dependence on bandwidth can create ambiguity in the interpretation of
spatial density if bandwidths are arbitrarily selected. Several methods have been developedfor estimating an optimal bandwidth matrix based on the locations of the event data, such as
the least-squares cross validation (LSCV), smoothed asymptotic mean squared error(SAMSE) and modified asymptotic mean integrated squared error (AMISE) methods. Such
optimization methods are extremely useful because, although they are mathematicallycomplex, they find optimal bandwidths using the actual data locations, removing subjectivity
from the process. That said, alternative optimization methods result in alternative spatial
density maps (Figure 5.4), so an important part of the hazard assessment is evaluation of thesensitivity of vent spatial density to different model assumptions. It is also important to selector set priorities of models considering the scale of evaluation (e.g. accross the arc, a specificregion or a volcanic cluster).
A potential disadvantage of these kernel functions and similar methods is that they are notinherently sensitive to geologic boundaries and to ancillary data. For example, the distributionof earthquake hypocenters or zones of high strain observed during magma intrusion should
inform and update the spatial density estimate as this information becomes available. Various
schemes have been developed to weight spatial density estimates in light of geological or
geophysical information in a Bayesian framework. A difficulty with such weighting is the
subjectivity involved in recasting geologic observations as density functions. Furthermore,
geologic insight is not always consistent with event distributions. Developing such weighting
schemes remains an important area of research. In addition, note that this statistical structure
uncouples the temporal forecasting problem from the spatial forecasting problem. That is,estimates of the recurrence rate are not mathematically linked to estimates of the spatial
density. This assumption may be inappropriate on some volcanoes where there is migrationof activity. An area of significant future research is the formulation of spatio-temporal models,
which make this link explicitly.

47

-3

2.5x10-3

6.8x10-3

2.0x10-3

4.8x10-3

1.4x10-3

2.9x10-3

8.0x10-3

1.6 x10-4-4

2.4x10-4

7.4x10-4

4.3 x101.2x10Figure 5.4: Alternative spatial density models of Quaternary volcano distribution in the
Tohoku arc, northern Honshu Japan (modified after Chapman et al., 2008). Both maps areprepared using kernel density estimation, with alternative kernel bandwidths estimated using
different optimization algorithms. An isotropic, short bandwidth by LSCV (left) emphasizes
clustering along the volcanic arc, possibly in response to along-arc variations in magma
productivity and heat transfer in the mantle wedge. An anisotropic, long bandwidth by SAMSE(right) smooths volcano distribution, and emphasizes relatively uniform melt production along
the arc. Hazard assessments need to consider such alternative models, which are linked to
our understanding of volcanological processes on various scales.

48

5.3

Estimation of volcanic hazard under evolving tectonic conditions at 1 Myr
timeframe

In order to become fully TOPAZ compatible, the ITM probabilistic volcanic hazard
assessment methodology needs to be extended to account for timeframes up to 1 Myr.
Accordingly, new developments were performed for the stochastic model to account forvarious tectonic evolution scenarios likely to occur at the million-year time scale. With theresults that volcanic hazard maps at 1 Myr timeframe were produced using the region of
Tohoku as an illustration example.
5.3.1

Migration scenarios

Kondo (2009) has evaluated the evolution of volcanism by investigating the distribution of
Middle Miocene to Quaternary igneous rocks for the northern part of Tohoku. The cumulative
maps of igneous rocks produced for four periods show significant trends in the distribution of
volcanism over time (Figure 5.5). It appears that at the million-year time scale, volcanism
migration has occurred; in particular, towards the west in the backarc and as well between the
volcano clusters of Quaternary age. Such long term patterns in the distribution of volcanism
can be described using the hot finger model of Tamura et al. (2002); i.e. some variability isadded in terms of the distribution of volcanism along the fingers or in relation to local lateralmigration of volcanism towards the gap regions. The general 3-dimensional structure of the
hot fingers model remains as it is likely to last for millions of years (Tamura et al., 2009).

Figure 5.5: Northern Tohoku: cumulative distribution of volcanism for four periods after
Kondo (2009).

Estimation of long-term volcanic hazard relies on Regional Evolution Scenarios (RES). They
constitute the conceptual description of the expected behaviours of tectonic evolution. A
representative range of four plausible RES was produced with various tectonic conditions
likely to capture the main evolution trends in terms of rock deformation and volcanism for
Tohoku (See Chapter 2). The four RES provide a description of the future behaviour of arc
volcanism in Tohoku region at 1 Myr timeframe (Figure 5.6).

49

W

Figure 5.6: Example of Regional Evolution Scenario for Tohoku: RES 2 where the amount ofrelative plate convergence accommodated in the upper plate doubles from present to 1 Myr in
the future (See Section 2.3 for detailed description).

Although the above evidences and models support that the current location of volcanism
could continue for the next 1 Myr, the potential of migration in the four RES cannot be ruledout. The three potential Migration Scenarios (MS) in relation to arc volcanism are thus defined
as below:

MS1: migration of volcanism likely occurring towards the west.
MS2: migration of volcanism likely into gaps between Quaternary volcanoes.
MS3: migration (small) of volcanism likely occurring towards the trench.

The relationships between the Migration Scenarios and the RES are given in Table 5.1.

Table 5.1: Characteristics of Migration Scenario in relation to Regional Evolution Scenarios
Regional Evolution Scenarios

Migration direction

Migration Scenarios

Westward

Gaps

Trenchward

–

X

–

MS2

(ca. 20–40
km/Myr)*X–

MS1, MS2

RES 3(relative plate motion oblicity ↑)

–

X

–

MS2

RES 4
(plate motion rate ↓)

–

X

X

MS2, MS3

RES 1(present tectonic condition ↔)RES 2(upper plate motion rate ↑)X(small)*Migration velocity.

5.3.2

Stochastic model

The developed stochastic model for the long term estimation of volcanic hazard relies on thefollowing hypotheses: (i) the potential of volcanism is considered as evolving (non stationary)in the space-time domain, (ii) the potential can be simulated using an evolution equation witha constant velocity, and (iii) the translated distribution of volcanic events conserves thepotential and remains depicted by a Cox process.

50Within the framework of the Cox process, the potential of volcanism (Jaquet et al., 2009;2012) – representing the propensity of a given region to be affected by volcanic events – is nolonger considered as stationary. The following evolution equation is chosen for the descriptionof the initial potential of volcanism, Z 0 x,t , in the space-time domain:Z 0 x,t  =Z 0 x + vt 

(10)where:

v : displacement (migration) velocity of volcanism.
The cumulated (integrated) potential of volcanism, Z c x  , is related to the initial potential
using the following expression:tp

Z c x  = Z x + vt dt

(11)

0

t0

where:

t 0 : initial time (e.g. beginning of Quaternary)t p : present time.
A deconvolution of eq. (2) is required to derive the initial potential of volcanism knowing the
cumulated potential estimated from the data. A statistical minimisation technique is chosen forsolving the following linear system of equations:



tp




Z 0 x + vt dt 
 Z c x  
t0







2

dx  minimum(12)

where:Z0=z1 ,..., z n  : vector of unknowns representing the initial potential.

The discrete form of the linear system is written as follows:Z c x i  
i 


2


a ij z j jF z 1 ,..., z n  =F z 1 ,..., z n  = Z x  +  a a z z



i



2
c



i

ij

j

(13)

ik

j

k



 a z  2Z c xi 

k

ik

k

k



The linear system is minimized with respect to z k :

F
z k

= 0

(14)

And can be expressed using the following symmetric matrix form:

AT AZ =
where:AT Z ca  :
Z = z  :
A =

(15)

ij

interpolation matrix for the potential (migration velocity dependent)jsolution vector for the initial potential

51

Zc

=

Z c xi  : input vector for the cumulated potential: Z c xi  =

a z
ij

j

j

The resolution of the system is performed using a non-linear transformation (Gaussiananamorphosis) for the cumulated potential:

Z c xi  =  z Yc xi (16)

Yc xi  =  z1 Z c xi 
where:

z: monotonic increasing function (i.e. Gaussian anamorphosis function; Chilès
and Delfiner, 1999).

Such deconvolution in the Gaussian space (this method is also applied in Oceanography;
Simon and Bertino, 2009) allows overcoming physical limitations linked to constraints ofpositiveness when solving the linear system:Z c x i    z1  Y c xi   numerical resolution  yj  Yf xi    z  Z f  x i (17)where:

Yc xi  :Gaussian cumulated potential

yj :Gaussian initial potential

Y f x i  = a y : Gaussian future potential
f
ij

j

j

Z f  xi  :future potential.

The principles of evolution for the potential of volcanism are illustrated in Figure 5.7. It must
be emphasized that the potential remains stationary in space when fixing the time step.

Figure 5.7: Schematic evolution for the potential of volcanism in space-time domain, where
space is one-dimensional along a W–E line; and, with a migration velocity oriented towards
the west.

The numerical resolution of the linear system provides the initial potential needed for thederivation of the future potential. Knowing this potential enables using the Cox process tosimulate the volcanic events likely to occur in the future. The required stochastic andnumerical modelling steps for the generation of probabilistic volcanic hazard maps are shownin Figure 5.8.

52

Figure 5.8: Methodology for the estimation of volcanic hazard using Cox simulation withdeconvolution and Monte Carlo method.

5.3.3

Application to Tohoku data

The objective is the estimation of the probability for the formation of new polygenetic
volcanoes over a proposed period of 1 Myr for Tohoku region. All Quaternary polygenetic
volcanoes (those active within the last 2 Myr) of the Tohoku region are considered as
representing potentially active volcanic areas for the future. The Tohoku volcanic arc is
located in northern Honshu, Japan and consists of more than 100 volcanic edifices erupted
during the Quaternary (Committee for the Catalogue of Quaternary Volcanoes in Japan,1999). A volcanic edifice corresponds to the volcanic event considered for the Tohoku casestudy; i.e. a polygenetic volcano is composed of one or more edifices. Polygenetic volcanoes
are characterized by a geomorphology and geologic structures that are created by many
episodes of eruptive activity likely to affect broad areas. The formation of new polygenetic
volcanoes can take place at locations up to tens of kilometres away from sites of previous
eruptions; i.e. with likely future locations based on the Migration Scenarios described in thesection above. The proposed probabilistic methodology is illustrated using a subset of thedatabase of Quaternary volcanoes of Japan (Mahony et al., 2009).
According to Tamura et al. (2002; 2009), magma production may be controlled by locallydeveloped hot regions within the mantle wedge beneath the Tohoku volcanic arc having theshape of inclined fingers with an average width of 50 km. These structures of the mantle
wedge were revealed by various seismic tomography studies (Zhao et al., 1992; Zhao et al.,
1994; Zhao et al., 2000; Hasegawa and Nakajima, 2004). They correspond to inclined
seismic low-velocity zones located at depths shallower than about 150 km with an orientation
sub-parallel to the down-dip direction of the slab. Such low-velocity zones are likely related tothe upwelling–flow portion of the convection mechanically induced by slab subduction(Hasegawa and Nakajima, 2004). This upwelling flow of mantle materials encountering
aqueous fluids is expected to produce partial melting in the low-velocity zones. The spatial
distribution of volcanic events from the Tohoku volcanic arc is characterized by clusters,
which are located above these zones of partial melting. The spacing of volcano clusters isgoverned by temperature variations along the Tohoku arc, ascent velocities and/or melt
content in the upwelling flow of the mantle wedge underneath (Hasegawa and Nakajima,
2004; Hasegawa et al., 2005).
The cumulated potential of volcanism for the Tohoku region is believed to assimilate
geological data related to Quaternary volcanic events as well as seismic tomography data53

indicating zones of partial melting at depth (Jaquet et al. 2009). The latter consists of S-wave
velocity perturbations along the inclined low-velocity zone in the mantle wedge of NE Japan(Hasegawa and Nakajima, 2004).
The values of the reference parameters for the simulation of the cumulated potential aretaken from Jaquet et al. (2009). The main parameters are: a) the coefficient of correlation
between the potential and the seismic tomography data and b) the correlation scale of thevariogram, estimating the scale of the (randomly structured) spatial patterns for the potential.
Sensitivity analysis is performed for these parameters as well as for the module of the
migration velocity. The three scenarios of the migration direction are considered based on the
Migration Scenarios in Section 5.3.1:


MS1: east → west
MS2: south → northMS3: west → east

5,000 Monte Carlo simulations were performed for obtaining probability maps with stableprobability estimates at 1 Myr timeframe and a domain scale of 10 x 10 km; i.e. the cell sizeused for gridding.
Using scenario MS1, with a migration velocity of 40 km/Myr and reference parameter values,
space-time dependent probability maps are produced for various timeframes. The increase
effect in probability towards the west, related to the migration of volcanism, can be observed
when comparing hazard maps at timeframes at 100 kyr, 500 kyr and 1 Myr (Figure 5.9).

Figure 5.9: Volcanic hazard maps for Tohoku for scenario MS1 at timeframes 100 kyr (left),
500 kyr (centre) and 1 Myr (right) with migration velocity of 40 km/Myr towards the west, usingreference parameter values. Triangles: location and number of Quaternary edifices ( △: 0<n≤1,△: 1<n≤2, △ : 2<n≤3, △: 3<n≤5, ▲: 5<n≤20). Grey dots: location of the example sites (WCB at
the upper west coast, IB at the upper-middle inland, ECF at the east coast).

Hazard maps at 1 Myr timeframe were produced for scenario MS1 with different velocities of
migration using reference parameter values. From a hazard point of view, it is moreconservative to select velocity of migration in the range 20 – 30 km/Myr. Since a velocity of 40
km/Myr corresponds to a likely too rapid migration of volcanism for the region of Tohoku(Figure 5.10).

54

Figure 5.10: Volcanic hazard maps for Tohoku for scenario MS1 at timeframe 1 Myr with
migration velocity of 20, 30 and 40 km/Myr towards the west, using reference parametervalues. Triangles: location and number of Quaternary edifices ( △: 0<n≤1, △: 1<n≤2, △: 2<n≤3,△: 3<n≤5, ▲: 5<n≤20). Grey dots: location of the example sites (WCB at the upper west
coast, IB at the upper-middle inland, ECF at the east coast).

Therefore, solely the velocity values of 20 and 30 km/Myr were considered for the sensitivity
analysis with respect to volcanic hazard estimation for Tohoku at 1 Myr timeframe. 18 cases
were simulated based on parameter increase (Table 5.2): (i) in the correlation scale in the NS direction (by a factor of 2); and, (ii) in the correlation between potential and seismic
tomography data (by a factor of 2), the higher values mean stronger assumption that thepresent correlation will continue for 1 Myr. These parameter variations aim at investigating theeffects of volcanism migration towards the gaps; and the impact of giving more weight to theseismic tomography, when estimating volcanic hazard at the 1 Myr timeframe.

55

Table 5.2: Cases with their parameters values and corresponding scenarios; 41 km and -0.3
are respectively the reference values for correlation scale and seismic correlation. The mostplausible case for each Migration Scenario is given in red.
Case

Migration
Scenario

Velocity[km/Myr]

Directionof migration

Correlation
scale [km]Seismic
correlation

Timeframe

1

MS1

20

E→W

41–41*

-0.3

1 Myr

2

MS1

20

E→W

41–82**

-0.3

1 Myr

3

MS1

20

E→W

41–41

-0.6

1 Myr

4

MS1

30

E→W

41–41

-0.3

1 Myr

5

MS1

30

E→W

41–82

-0.3

1 Myr

6

MS1

30

E→W

41–41

-0.6

1 Myr

7

MS2

20

S→N

41–41

-0.3

1 Myr

8

MS2

20

S→N

41–82

-0.3

1 Myr

9

MS2

20

S→N

41–41

-0.6

1 Myr

10

MS2

30

S→N

41–41

-0.3

1 Myr

11

MS2

30

S→N

41–82

-0.3

1 Myr

12

MS2

30

S→N

41–41

-0.6

1 Myr

13

MS3

20

W→E

41–41

-0.3

1 Myr

14

MS3

20

W→E

41–82

-0.3

1 Myr

15

MS3

20

W→E

41–41

-0.6

1 Myr

16

MS3

30

W→E

41–41

-0.3

1 Myr

17

MS3

30

W→E

41–82

-0.3

1 Myr

18

MS3

30

W→E

41–41

-0.6

1 Myr

*Isotropic correlation scale.
**Anisotropic correlation scales with major axis orientated N-S.

The effect of parameter variations on the hazard maps are demonstrated for scenario MS1
using different values for the velocity of migration. An increase in correlation scale leads tothe likelihood of more volcanism in the gap regions (Figure 5.11). And, an augmentation in
the correlation between the potential and the seismic tomography data generates morevariability in volcanic hazard away from Quaternary volcanic activity (Figure 5.12).

56

Figure 5.11: Cases 1 & 2: volcanic hazard maps for Tohoku for scenario MS1 at timeframe 1
Myr with migration velocity of 20 km/Myr (left), the effect of increased correlation scale (right).
Triangles: location and number of Quaternary edifices ( △: 0<n≤1, △: 1<n≤2, △: 2<n≤3, △:3<n≤5, ▲: 5<n≤20). Grey dots: location of the example sites (WCB at the upper west coast,
IB at the upper-middle inland, ECF at the east coast).

Figure 5.12: Cases 4 & 6: volcanic hazard maps for Tohoku for scenario MS1 at timeframe 1
Myr with migration velocity of 30 km/Myr (left), the effect of increased seismic correlation(right). Triangles: location and number of Quaternary edifices ( △: 0<n≤1, △: 1<n≤2, △: 2<n≤3,△: 3<n≤5, ▲: 5<n≤20). Grey dots: location of the example sites (WCB at the upper west
coast, IB at the upper-middle inland, ECF at the east coast).

57

Using the simulated results for each Migration Scenario, the most plausible case is selected
in terms of migration characteristics and hazard variability (Table 5.2) for the region of Tohoku
at the 1 Myr timeframe (Figure 5.13). The selected cases are associated to the following
Regional Evolution Scenarios (RES):

RES 1: Case 11 (MS2).
RES 2: Case 6 (MS1) and Case 11 (MS2).
RES 3: Case 11 (MS2).
RES 4: Case 11 (MS2) and Case 13 (MS3).

Due to the differences in migration velocity and parameter values, the three plausible cases
display remarkable diversity in terms of variability patterns when comparing their respective
hazard maps (Figure 5.13).

Figure 5.13: Cases 6, 11 & 13: volcanic hazard maps for Tohoku for scenarios MS1, MS2
and MS3 at timeframe 1 Myr. MS2 belongs to all RES; MS1 and MS3 are respectively solely
part of RES 2 and RES 4. Triangles: location and number of Quaternary edifices ( △: 0<n≤1, △:1<n≤2, △: 2<n≤3, △: 3<n≤5, ▲: 5<n≤20). Grey dots: location of the example sites (WCB at
the upper west coast, IB at the upper-middle inland, ECF at the east coast).

Hazard estimates at the following examples sites (Figure 5.13) were extracted from thesimulated hazard cases:

WCB: West Coast Backarc.
IB:
Inland Basin.
ECF: East Coast Forearc.

The range of hazard estimates, expressed in probability at 1 Myr timeframe, for the three
example sites are given with respect to Migration Scenarios (Table 5.3) and regarding
Regional Evolution Scenarios (Table 5.4). The comparison of hazard range values (high &low) for Regional Evolution Scenarios (Table 5.4) shows that WCB site (high) values are
generally higher by a factor of 2 compared to IB site values. Regarding the (low) site values,
the opposite is observed; IB site values are generally higher by a factor of 3 in comparison to
WCB values and a factor of up to 20 can be reached in case of RES 4. Such behaviour is
explained by hazard range (high & low) values that are dominated by scenario MS2, with amigration direction towards the gaps, likely to occur for all RES.

58

Table 5.3: Volcanic hazard estimate ranges at Tohoku examples sites at 1 Myr timeframe, in
relation to Migration Scenarios (MS).
Migration Scenario
Site

MS1

MS2

MS3

WCB

3.4·10 – 1.5·10-3-1

2.9·10 – 2.2·10-2-1

3.8·10 – 9.4·10

IB

3.2·10 – 1.6·10-3-2

7.7·10 – 1.3·10-2-1

3.6·10 – 2.1·10

ECF*< 10−5< 10−5< 10−5

-3

-3

-1

-1

*Site located outside of model domain.

Table 5.4: Volcanic hazard estimate ranges at Tohoku examples sites at 1 Myr timeframe, in
relation to Regional Evolution Scenarios (RES).
Regional Evolution Scenario
Site

RES 1

RES 2

RES 3

RES 4

WCB

2.9·10-2– 2.2·10-1

3.4·10-3– 2.2·10-1

2.9·10-2– 2.2·10-1

3.8·10-3– 2.2·10-1

IB

7.7·10-2– 1.3·10-1

3.2·10-3– 1.3·10-1

7.7·10-2– 1.3·10-1

7.7·10-2– 2.1·10-1

ECF*< 10−5< 10−5< 10−5< 10−5*Site located outside of model domain.

5.3.4

Conclusions and perspectives

The need of estimating volcanic hazard up to 1 Myr timeframe has led to new developments
for the stochastic model. At such long time scale, arc volcanism migration is likely to occur inrelation to the various tectonic evolution scenarios. Therefore, the proposed stochastic model
enables to estimate long-term volcanic hazard under evolving tectonic conditions in order toproduce hazard maps at 1 Myr timeframe. The mapped space-time probability of volcanic
events, using Tohoku region as illustration, were obtained for three Migration Scenarios(linked to RES) with assimilation of Quaternary volcanic events and seismic tomography data.
Actually for such a long timeframe, datasets that extend beyond the Quaternary, possibly up
to the Middle Miocene, are required for describing the volcanic patterns likely to occur inspace and time.
Remarkable diversity in volcanic hazard patterns were obtained when comparing the 1 M yr
maps associated to the Migration Scenarios. Sensitivity calculations have enabled estimationof volcanic hazard ranges at examples locations for Tohoku region.
Additional developments are needed as to include various types of volcanic activity in form of
selected scenarios in order to estimate their specific hazards. Using a Boolean approach
coupled to a stochastic model would allow hazard estimation for explicitly described scenarios(e.g. caldera events).

59

5.4

Development of volcanic hazard maps

5.4.1

Consequence of volcanic events

Once temporal and spatial estimates have been made for event timing and location,
respectively, it becomes possible to forecast potential event impacts and to prepare hazard
curves (also known as exceedence curves or complementary cumulative density functions)for specific sites. For HLW repositories, the consequences of volcanic events include thefollowing:






Direct disruption of waste packages by intruding magma and transport of the HLW tothe surface and dispersion. Development of volcanic conduits within the footprint of
the repository would subject waste and the engineered barrier system to high6
-1
temperatures (e.g., >1000°C) and high mass flow rates (e.g., >10 kgs ), for extended
periods of time (e.g., >1 yr). HLW fragments may be transported in erupted tephra
plumes, in lava flows, pyroclastic density currents and by related volcanic
phenomena.
Igneous dykes and sills may intrude into the repository without necessarily venting to
the surface. Such intrusions may destroy or partially destroy the engineered barriersystem.
Nearby igneous intrusions may subject the repository to high rates of deformation.
Uplift and fault displacement (e.g., reactivation of long-inactive faults or slip along
joint surfaces) commonly accompany igneous intrusions.
Nearby igneous intrusions may subject the repository to changes in the level of thegroundwater table, changes in groundwater flow, hydrothermal activity, and gas flux
through the repository.

The ITM-TOPAZ methodology has not involved modeling most of these processes explicitly.
Rather, a conservative offset distance was developed for volcanic events. For ITM-TOPAZ,
the probability of new volcano formation was considered for a 100 km 2 area about the site.
Any activity within this footprint was considered to be a disruptive event, without assessing its
radiological consequences through a full performance assessment.
5.4.2

The aggregate probability of volcanic events

The aggregate probability of volcano formation within a given area and timeframe about apotential HLW repository site may be calculated by combining the recurrence rate and spatial
density models. Probability of volcano formation can be estimated for any given scenario –any branch of the logic tree shown in Figure 5.1 – by setting the models weights along thisbranch equal to unity. As an example, consider the 14 sites shown in Figure 5.15, used to test
and compare volcano and tectonic hazard models. At any one of these sites, probability is
estimated using:P[volcanicdisruption] = P[numberevents  1]P[volcanicdisruption | events]

P[numberevents  1] = 1  e ̂t uˆP[volcanicdi sruption | Events ] = 1  e ( s ) A(18)

where u is the assessment period (e.g., 1 Myr), A is the area encompassing the site andthe disruptive volcanic event (e.g., 100 km ), ̂t is the estimated recurrence rate and ˆ ( s )is the estimated spatial density. For example, at WCB site, the estimated recurrence rate forvolcano formation, given an RES in which the tectonic setting remains unchanged and the2

MLE recurrence rate, is ˆt = 5 10

5volcanic events per year. Considering the spatial

density estimate using kernel density estimation and the SAMSE bandwidth optimization,

ˆ(s) = 6.6 106 per square kilometer. With these values, the probability of disruption is:P[volcanicdi sruption, u = 1yr ] = 3.3 108

P[volcanicdisruption , u = 100 kyr] = 3.3  10 360

P[volcanicdisruption , u = 1Myr ] = 3.3  10 2.

(19)

WC
B

EC
F

IB

Figure 5.15: Example sites at which aggregate probabilities are calculated and thresholdvalues are compared (yellow and white boxes) with respect to the location of Quaternary
volcanoes (green circles) (modified after Chapman et al., 2009). Active faults are shown bydashed lines. Topography from Shuttle Radar Topography Mission (Jarvis et al., 2008).

This probability is estimated assuming one model scenario, that the expected values forrecurrence rate and spatial density are appropriately chosen given the average recurrence
rate estimated by MLE and the kernel density estimate of spatial density with SAMSE
optimization. As shown in Table 5.5, other model scenarios result in other probabilities. In thisexample, an ensemble of 16 models results from four alternative recurrence rate models andfour alternative spatial density models. The two recurrence rate models correspond to
estimates of recurrence rate based on estimated t during the last 900 kyr using volcanoes
with estimated ages of formation (5 x 10-5/yr for Senarios 1 to 4), and using all volcanoes-5
formed in the Quaternary (6.5 x 10 /yr for Senarios 5 to 8) under RES 1. Alternatively, the-5recurrence rate may increase over time (10 x 10 /yr for Senarios 9 to 12) under RES 2, or-5decrease over time (2.5 x 10 /yr for Senarios 13 to 16) under RES 4. Four alternative spatial
density models are used:-5 kernel density estimate using SAMSE (large, anisotropic bandwidth; 3.5 x 10 /yr for
Senarios 1, 5, 9 and 13), kernel density estimate using LSCV (short, isotropic bandwidth; 3.5 x 10-5/yr for Senarios
2, 6, 10 and 14),
 Cox process model Case 6 (volcanism migrating toward the west; 5.2 x 10-5/yr for
Senarios 3, 7, 11 and 15), and Cox process model Case 11 (volcanism migrating along the arc into gaps; 5.1 x 10-4/yr
for Senarios 4, 8, 12 and 16).

61

These alternative models can be aggregated by assigning weights to each branch of the tree
(Figure 5.1). Using the weights for the recurrence rate models (wt RR) and spacial density
models (wt SD) in Table 5.5 yields an aggregate probability of disruption of WCB site of
approximately 1.6 x 10-8/yr. It is emphasised that these weights were temporarily set for
demonstration purpose, which should be assigned through expert elicitation process in the full
application of the ITM-TOPAZ methodology.
It is notable that in this example the range of annual probability of disruption of WCB site
represented by these models is about one order of magnitude, providing a sense of theepistemic (model) uncertainty associated with these ensemble models. Alleatoric uncertainty
could be better represented by assigning a probability distribution of recurrence rates to eachmodel, based on uncertainty in parameter estimation.

Table 5.5: Summary of estimation for of the aggregate probability of new volcano formation at
the example site WCB (site 8). Value represent mean weights, wt and modeled probabilities
for 16 scenarios. The aggregate probability, AP, is the sum of the weighted probabilities.
Scenario

1

RR model

2

RR

3

wt RR

4

SD model

5

6

SD

wt SD

7

2.5x10wt Pr

8Pr |Scenario

1~900 ka (RES 1)

5

0.4

SAMSE

5.1x100.25

2~900 ka (RES 1)

5

0.4

LSCV

3.5x10-5

0.25 1.8x10-10

1.8x10-93~900 ka (RES 1)

5

0.4

Cox (Case 6)

5.2x10-5

0.25 2.6x10-10

2.6x10-94~900 ka (RES 1)

5

0.4

Cox (Case 11) 5.1x10-4

0.25

2.5x10-9

2.5x10-8

5Quaternary (RES 1)

6.5

0.4

SAMSE

5.1x10-4

0.25

3.3x10-9

3.3x10-8

6Quaternary (RES 1)

6.5

0.4

LSCV

3.5x10-5

0.25 2.3x10-102.3x10-97Quaternary (RES 1)

6.5

0.4

Cox (Case 6)

5.2x10-5

0.25 3.4x10-10

3.4x10-98Quaternary (RES 1)

6.5

0.4

Cox (Case 11) 5.1x10-40.25 3.3x10-10

3.3x10-89increase (RES 2)

10

0.1

SAMSE

5.1x10-4

0.25

1.2x10-9

5.1x10-8

10increase (RES 2)

10

0.1

LSCV

3.5x10-5

0.25 8.8x10-11

3.5x10-9

11

increase (RES 2)

10

0.1

Cox (Case 6)

5.2x1012

increase (RES 2)

10

0.1

Cox (Case 11) 5.1x10-4

13decrease (RES 4)

2.5

0.1

SAMSE

5.1x10-4

14

decrease (RES 4)

2.5

0.1

LSCV

3.5x1015

decrease (RES 4)

2.5

0.1

Cox (Case 6)

16

decrease (RES 4)

2.5

0.1

-4

-5

-9

-8

2.5x10-10

5.2x101.3x10-9

5x10-80.25 3.2x10-10

1.3x10-8

0.25 1.3x10
0.25

-5

0.25 2.2x10

5.2x10-5

0.25 3.3x10

Cox (Case 11) 5.1x10-40.25 3.1x10

-9

-11

8.8x10-10-11

1.3x10-10

1.3x10-9AP
1.6x10-8
dataset or assumption for recurrence rate model under the RES
2-5
estimated recurrence rate (events per 10 year)3
weight assigned to the recurrence rate4
spatial density model
5
estimated spatial density (events per 100km2)
6weight assigned to the spatial density model
7
weighted probability of each scenario
8
conditional probability given each scenario (unweighted probability of each scenario = RR*SD)
1

62

-8

5.4.3

Hazard maps for volcanic disruption

The logic tree serves two roles. First, as already illustrated, the weights and scenario
probabilities may be used to estimate aggregate probability, or a range of aggregate
probabilities given uncertainty in model input parameters. Such weighting through expert
judgment gives the best estimate of aggregate probability and uncertainty. It is, however, adifferent goal to assess project risk due to a particular geologic hazard, or set of geologic
hazards. Although the aggregate probability for a particular site may be acceptable in terms ofgeologic hazard, different scenario models for the site may yield large differences inprobability ( P[disruption | scenario ] in Table 5.5). A project has greater risk if somebranches of the logic tree have high probabilities, even if the aggregate probability is
acceptable. Rather than aggregating probability to assess project risk, credible scenarios
should each be investigated. A site has low project risk if individual models for hazard indicate
an absence of potential for disruptive geologic events. Note that this approach does not resultin a robust probabilistic assessment for any particular site. Instead, it provides a metric forcomparing relative project risk among potential sites using a range of models. In that sense,
the method is a project tool rather than a siting criterion.
Thus, the estimation of project risk for volcanic disruption of a given site depends on
establishing threshold probabilities to define “absence of potential for disruptive geologic
events" for each scenario. For volcanism, we established two thresholds to illustrate themethodology. Internationally, an annual probability of 1 x 10-8 for disruption due to volcanism
has often used as a threshold – below which there is an absence of potential for volcanic
events. Sites that have annual probabilities of <1 x 10-8 for each of a variety of alternativemodels are considered to carry low project risk with respect to potential for volcanism. A
second threshold is established at an annual probability of 1 x 10-7. Sites that have annual
probabilities of >1 x 10-7 for any one of a variety of alternative models are considered to carryhigh project risk with respect to potential for volcanism. Intermediate sites, 1 x 10-8 ≤P[volcanism]≤1 x 10-7 carry moderate project risk. That is, detailed analysis may reveal thesite carry in fact low project risk through estimation of aggregate probability, especially after
detailed site investigation, but credible scenarios for potential volcanic disruption exist andmust be evaluated. Potential volcanism may remain a siting concern for such sites.
For example, Scenarios 5 to 8 (Table 5.5) were used to evaluate project risk for 15 example
sites in the Tohoku region (Figure 5.15) to illustrate the methodology. The results of thisassessment are shown in Table 5.6. Sites with very low project risk (sites 7, 9, ECF/12) arecharacterized by annual probabilities <1 x 10-8 for all of the alternative models. The two sitesof high project risk (IB, 2) are characterized by one or two scenarios yielding annual
probability >1 x 10-7. It is notable for these sites that at least one scenario produces
probabilities <1 x 10-8. Nevertheless, it is clear that potential for volcanic disruption of arepository at these two sites would create high project risk. Similarly, intermediate sites arecharacterized by a range of probabilities for different scenarios. A good example are the “gap"sites (sites 1, 13). Spatial density models that emphasize tight clustering of volcanoes (Coxand LSCV methods) produce lower probabilities than the smooth spatial density model(SAMSE). This difference reflects differences in models for volcanism in the arc. The classic
subduction zone model emphasizes uniform volcano distribution along the arc, and rapidchanges across the arc. This tends to increase the modeled spatial density in gaps between
clusters of volcanoes in Tohoku. The “hot finger" model of volcanism emphasizes ventclustering. Thus the Cox and LSCV models tend to reduce probability between clusters. For
these sites, it may be that the aggregate probability, estimated after detailed investigation andexpert elicitation, may indicate such sites carry low project risk. But, there are geologic
models extant in the literature that support higher probabilities. In other words these sites
carry some, if moderate, project risk.
Maps may be constructed for the Tohoku region for each scenario to be compared. Four suchmaps are constructed for illustration, using the SAMSE, LSCV and Cox spatial density
models (Figures 5.16). These maps may then be combined into a single project risk map forvolcanism (Figure 5.17) to take all uncertainties in the scenarios into account.

63

Table 5.6: Probabilities of volcanism and project risks for the 15 example sites based on thefour alternative spatial density models.
Spatial Density Model
5

Site
1

SAMSE

IB

9.9 x 10-8

1

7.7 x 102

1.1 x 102LSCV

8.1 x 10-10-8

6.4 x 10-7

8.4 x 10-7

3Case 6

3.2 x 10-9

-11

8.4 x 10-99.8 x 10-7

4

1.1 x 10-7

H

-8

M

-7

H

-7

-9

8.4 x 10-9

2.5 x 10

-7

Project Risk

Case 113

1.2 x 10

6.2 x 105.1 x 10

3.8 x 10H

4

4.4 x 10-8

2.8 x 10-8

2.5 x 10-7

2.7 x 10-7

H

5

-8

9.2 x 10-8

9.0 x 10-7

2.1 x 10-7

1.1 x 10H

6

5.8 x 10-8

9.0 x 10-8

1.1 x 10-7

1.3 x 10-7

H

7

-9

7.2 x 100

* <1 x 10-11*<1 x 10L

WCB/8

3.3 x 10-8

2.3 x 10-9

3.4 x 10-93.3. x 10-8

M

-11

-11

-9

-11

9

4.3 x 100

* <1 x 10*<1 x 10L

10

5.3 x 10-8

2.0 x 10-8

3.2 x 10-8

2.3 x 10-7

H

11

-8

9.3 x 10-7

3.4 x 10-7

6.2 x 10-7

5.1 x 10

H

ECF/12

1.8 x 10-15

0* <1 x 10-11* <1 x 10-11

L

-9

-8

4.6 x 10M

14
4.4 x 10-8
8.8 x 10-13
1.1 x 10-8
3.6 x 10-8
SAMSE method for optimal bandwith determination
2
LSCV method using a radial bandwith of 60 km
3
migration of volcanism from E to W during a 1 Myr timeframe
4
migration of volcanism into the “gap” during a 1 Myr timeframe5
L: low; P<1x10-8 /yr for all models
M: moderate; 1x10-8 /yr ≤P≤1x10-7 /yr
H: high; P>1x10-7 /yr for any one model* outside the model domain for these spatial density estimates

M

13

-8

8.7 x 10-8

3.4 x 10

4.4 x 101

64

Project Risk

Project Risk(a) SAMSE(b) LSCV

Prepared explicitly
for demonstrationof methodology

Prepared explicitly
for demonstrationof methodology

H

H

M

M

L

L

Project Risk

Project Risk(c) Cox Case 6(d) Cox Case 11

Prepared explicitly
for demonstrationof methodology

Prepared explicitly
for demonstrationof methodology

H

H

M

M

L

L

Figure 5.16: Project risk based on spatial density models (a) SAMSE, (b) LSCV, (c) Cox
process Case 6 and (d) Case 11 for an area of 100 km2 ,using a recurrence rate of 6.5x10-5/yr. Green areas are characterized by very low estimated rates of volcanic activity (P<1x10-8/yr). These areas comparatively carry low project risk. Red areas correspond to relatively high-7
rates of volcanic activity (P>1x10 /yr). Sites in these areas may have a relatively high projectrisk due to concerns about long term rate of volcanic activity.

65Project Risk
(Volcanism)
Prepared explicitly
for demonstrationof methodology

H
M
L

Figure 5.17: All four maps in Figure 5.15 are combined into a single project risk map forvolcanism. Green areas are characterized by very low estimated rates of volcanic activity(P<1x10-8 /yr). These areas comparatively carry low project risk. Red areas correspond to-7
relatively high rates of volcanic activity (P>1x10 /yr). Sites in these areas may have arelatively high project risk due to concerns about long term rate of volcanic activity. activity
based on this model. Yellow areas have 1x10-8 /yr ≤P≤1x10-7 /yr and carry moderate projectrisk due to volcanism.

66

5.5

Development of exhumation hazard maps

This section explores how the same approach to develop hazard map (project risk map) by
assigning threshold values as for volcanism could be applicable to the evaluation of uplift anderosion. It is noted that this work was carried out in parallel with the scenario development
described in the previous chapters, and thus has not been directly correlated each other.
5.5.1

Models of uplift

Like volcanism, relatively rapid uplift and erosion of a site may result in release ofradionuclides into the biosphere. There are essentially two pathways leading to this release.
The first involves such rapid uplift and erosion that a repository is “daylighted" within theassessment period. Assuming a repository is constructed at 500 m depth and has a 1 Myr
assessment period, uplift of approximately >0.5 mm/yr is required to bring this repository to
the surface during the assessment period. Here, the erosion rate is assumed to be equivalent
to the uplift rate for a simple demonstration of methodology. In addition, smaller amounts of
uplift may place the repository in the oxic zone, potentially creating a plume of redox-sensitive
radionuclides. Two types of uplift models are used for this assessment: geological markers ofregional uplift and isostatic gravity anomalies.
It is important to realize that different processes lead to long-term uplift, including
compression along subduction zones, transpression, igneous intrusions and underplating of
the crust, and glacial isostatic adjustments. This implies that multiple models are necessary inorder to assess alternative scenarios for long term uplift. In TOPAZ, this problem has been
explored using two data sets. Geomorpholocial data on uplift over the last 125 kyr provides a
relatively short-term direct record of uplift resulting from any one of these processes (e.g.
Koike et al., 2005, see Section 3.2). Isostatic gravity anomalies regional gravity data compiledby the Geological Survey of Japan, AIST (Komazawa et al., 2004) suggest a potential for
future uplift, based on the long term compensation of topography at relatively longwavelengths. Alternative models, for example finite element models of uplift in response tocompression, might certainly be developed.
We note that quantifying the disruptive potential of uplift and erosion in terms of measured
vertical uplift and isostasy thresholds does simplify the problem. For a full probabilistic hazard
assessment of uplift and erosion, factors need to be considered, including: (1) the averageregional rate of uplift or subsidence; (2) the magnitude of differential uplift and erosion, inother words, the horizontal derivative of uplift; (3) the potential for localized extreme uplift orerosion, for example associated with igneous intrusion; (4) the balance between uplift anderosion, subsidence and burial, due to factors such as climate. Here the models used are
only sensitive to the first factor.
5.5.2

Geological markers of regional uplift

Geologic markers of long term uplift and erosion include marine geologic markers including
marine terraces, paleoshoreline indicators, coral reefs and marginal marine sediments. Away
from coastlines, uplift markers include differential uplift across faults, flights of fluvial terraces,
rates of incision, and similar criteria. In high uplift environments, geochronometry andgeothermometry, for example, the annealing of fission tracks in zircon and apatite minerals,
can be used to constrain long term rates.
A model of uplift and subsidence during the last 125 kyr has been constructed for part of
Tohoku based on these types of data (Koike et al., 2005). Although there is considerable
uncertainty in extrapolation of the uplift rates measured at local outcrops across the Tohoku
region, the map can be used as one model of potential uplift to assess project risk.
Consequently, we can define two thresholds for project risk maps related to uplift and erosion.
The first threshold is defined as 0.5 mm/yr – potential sites characterized by uplift exceeding
this threshold are considered to carry high project risk. The second is 0.25 mm/yr – sites
characterized by uplift <0.25 mm/yr are considered to be highly suitable from the standpointof uplift and erosion. Sites characterized by intermediate values carry moderate project risk,both in the sense that there is relatively low probability of moving a repository into the oxic67zone and there is relatively low chance that the uplift at such a site is drasticallyunderestimated.
Figure 5.18 shows uplift data for a part of Tohoku contoured using these threshold values.
Roughly 50% of the area is associated with zones of subsidence or low uplift (<0.25 mm/yr).
These areas are considered to be highly suitable from a project risk standpoint in terms of theabsence of potential for rapid uplift. Tohoku is characterized by relatively narrow zones ofrelatively high rates of uplift (>0.25 mm/yr) and broad regions are characterized byintermediate values. This likely reflects uncertainty associated with measured uplift in theseareas. That is, from a project risk perspective it would likely be necessary to gather additional
data via detailed site investigation to characterize uplift at a site in located in these areas, andthere is some potential that uplift and erosion would be of concern.

Project Risk
(Uplift rate)
Prepared explicitly
for demonstrationof methodology

H
M
L

Figure 5.18: Project risk based on measured and interpolated uplift for part of Tohoku basedon geomorphological data for the last 125 kyr (Koike et al., 2005). The uplift rates (mm/yr) arecolor-coded using threshold values. Green areas are characterized by subsidence or low
rates of estimated uplift (<0.25 mm/yr). These areas carry comparatively low project risk. Red
areas correspond to relatively high estimated uplift. Sites in these areas may have a relatively
high risk due to concerns about long term tectonic uplift.

5.5.3

Isostatic models of uplift

Isostatic gravity anomalies result from magma underplating and differentiation at the Moho orConrad discontinuities, or elsewhere in the crust. This process results in a negative Bouguer
anomaly, no Free Air Anomaly if the crust is in isostatic equilibrium, and positive topography.
Therefore, the extent of the isostatic gravity anomaly (or corresponding elevation anomaly,
g=2πGρh, indicates potential for uplift. An absence of a negative isostatic anomaly
corresponds to an absence of potential for rapid uplift as a disruptive event. Therefore, a
topographic compensation, h>0 m is a highly suitable site using this uplift model. Uplift is
proportional to topographic compensation. If h<-30 m, then the area is may experience rapid

68

uplift to compensate this topography in the future (Figure 5.19). This map and Figure 5.18 are
combined into a single project risk map for uplift and erosion (Figure 5.20).

Project Risk
(Isostatic anomaly)
Prepared explicitly
for demonstrationof methodology

H
M
L

Figure 5.19: Project risk based on measured and interpolated uplift for part of Tohoku based
on topographic compensation derived from the isostatic gravity anomlay map by the
Geological Survey of Japan, AIST (Komazawa et al., 2004). The topographic compensation(m) is color-coded using threshold values. Green areas are characterized by subsidence or ahigh degree of topographic compensation (h≥0 m). These areas carry comparatively low
project risk. Red areas correspond to relatively high negative topographic compensation (h≤30 m). Sites in these areas may have a relatively high project risk due to concerns about longterm uplift.

69

Project Risk
(Uplift/erosion)
Prepared explicitly
for demonstrationof methodology

H
M
L

Figure 5.20: Figures 5.18 and 5.19 are combined into a single project risk map for uplift anderosion. Green zones are characterized by low observed uplift rate over the last 125 kyr andlow potenital for uplift based on the isostatic model. Red correspond to relatively high rates of
uplift based on at least one of the two models. Yellow areas carry moderate project risk due
to uplift and erosion.

5.6

Combined project risk maps

Figure 5.21 shows relative project risk combining the volcanism and uplift maps. It is notedthat this map was presented to demonstrate the idea of showing hazards (risks) of different
phenomena in a single map. It is also emphasised that evaluation for exhumation hazard
should be linked with the long-term evolution scenarios as same as for the volcanism, which
has not been achieved in this case study.
Naturally the combined project map is rather smooth. Relatively high project risk occurs near
Quatrnary volcanoes. The smooth SAMSE spatial density and the Cox process, case 11,
models forecast higher rates of volcanism in gaps between current Quaternary volcano
clusters. Relatively high project risk zones extend into thebackarc, especially in areas west ofactive Quaternatry volcano clusters. Even using these highly conservative threshold values
most of Tohoku is located in zones of moderate to low project risk. Away from areas of active
volcanism, sites with moderate project risk are associated with areas that have experienced
uplift in the last 125 kyr. Detailed site investigations could improve the evaluation if these
uplift values are the results of relatively sparse geologic data. Finally, there are large areas
extending 10–50 km from the east coast of Tohoku that have very low project risk from theperspective of rapid uplift and volcanism (Figure 5.21).

70

Project Risk
(Volcanism/Uplift)
Prepared explicitly
for demonstrationof methodology

H
M
L

Figure 5.21: The volcano and uplift potential maps are combined into a single project risk
map for Tohoku. Green zones on this map represent low project risks with respect to volcano
and uplift disruptive hazards, as probabilities of disruptive events are negligible for all
scenarios analyzed. Yellow zones are characterized by one or more slightly higher probability– these sites carry moderate project risk. Red zones are areas in which one or more scenario
indicates high project risk.

71

6

Hazard maps for rock deformation

This chapter describes how the hazard maps of specific Impact Scenarios (IS) of rockdeformation at any location can be developed. The example IS here is that of a large
earthquake discussed in the previous report (Chapman et al., 2012), in which crustal strain isreleased in a 6.5 ≤M≤7.5 earthquake on a concealed deep active fault below the repository,causing shaer that could penetrate upwards into the repository.
6.1

Mapping the frequency of a tectonic deformation scenario: future 6.5 ≤ M ≤ 7.5
earthquakes

In this section we develop maps that illustrate the annual frequency of hazardous tectonic
deformation across Tohoku. We use the ITM Tohoku strain rate models for surface
deformation, GPS and seismicity (Chapman et al., 2009) to demonstrate how tectonic
deformation can be quantified in map form. Our measure of tectonic deformation for the
purposes of this demonstration model is the annual frequency of an earthquake of magnitude
M6.5 to M7.5 occurring in a 3-dimensional grid cell of dimensions 5x5km, and 20km thickness.
M6.5 approximates what might be the lower limit of hazardous tectonic displacements (about
0.5m displacement, depending on the magnitude scaling relationship used). We assume
M7.5 is the maximum magnitude, based on the earlier ITM modelling of the region (Chapman
et al. 2009).
Demonstration 6.5≤M≤7.5 maps are produced for each of the surface deformation (fault
displacement, uplift and tilt), GPS and seismicity strain rate models (Figure 6.1 to Figure 6.3),
for the equally weighted average of the three models (Figure 6.4), and for the maximum valueof the three models (Figure 6.5). The following sections describe the method used to convert
strain rates into 6.5≤M≤7.5 earthquake rates, and the resulting calculations, illustrated as aseries of maps.
The method is described in Chapman et al. (2012), and summarised as below. The procedure
to develop 6.5≤M≤7.5 rates is to first convert the ITM strain rates from the highest-weighted
branch of the ITM logic trees for Surface Deformation, GPS and Seismicity strain rate models
into seismic moment rates for a map grid of 5x5x20km resolution via the method of Kostrov(1974). The seismic moment rates are then distributed across the magnitude range of
6.5≤M≤7.5 according to a Gutenberg-Richter distribution: logN = a – bM, in which N is thenumber of events of magnitude M, and a and b are empirical constants. The GutenbergRichter relationship (Gutenberg and Richter, 1944) well-describes the size distribution ofearthquakes across regions, and the globe.
The maps in Figure 6.1 to Figure 6.3 show the distribution of 6.5≤M≤7.5 rates across the
Tohoku region for the surface deformation, GPS and seismicity models. Also shown are
combination maps for the three models; that is the average of 6.5≤M≤7.5 rates for eachmodel, and the highest value of 6.5≤M≤7.5 rates from each model. All maps are produced forthe same 5x5x20km grid cell dimension. The colour scheme is used to distinguish between
high (>10-4/yr), medium (10-4-10-5/yr), and low (<10-5/yr) 6.5≤M≤7.5 rates. These divisions canalso be thought of as the time span in which a 6.5≤M≤7.5 earthquake could be expected tooccur (return time). There values are for demonstration purposes because it is important toconsider consequences prior to defining what level of activity truly represents hazard. We
have developed a threefold division to match with the number of divisions used for the
volcanic hazard maps (although the rates are markedly different and cannot be compared interms of true hazard (this would have to be done by considering consequences). Here, we
refer to the three rate categories as low, medium and high tectonic hazard where highcorresponds to one or more 6.5≤M≤7.5 earthquakes occurring in a 10 kyr period in that grid
cell; medium corresponds to a 10-100 kyr timeframe for the occurrence of a 6.5≤M≤7.5
earthquake in that grid cell; and low corresponds to a > 100 kyr timeframe for a 6.5≤M≤7.5
earthquake occurring in that grid cell.
The surface deformation map (Figure 6.1) shows a roughly equal mix of low and medium
tectonic hazard across much of the region, except for zones of high tectonic hazard along
major faults in the west.

72

Figure 6.1: Map of 6.5<M<7.5 rates from the ITM Surface Deformation strain rate model, fora grid of cells with dimensions 5x5x20km.

The GPS map (Figure 6.2) shows largely medium tectonic hazard across much of the region,
whereas the seismicity map (Figure 6.3) shows medium hazard largely confined to the central
ranges (backbone ranges) of the region.

Figure 6.2: Map of 6.5<M<7.5 rates from the ITM GPS strain rate model, for a grid of cellswith dimensions 5x5x20km.

73

Figure 6.3: Map of 6.5<M<7.5 rates from the ITM Seismicity strain rate model, for a grid ofcells with dimensions 5x5x20km.

The equally weighted (average) map for the three models (Figure 6.4) shows most of theregion characterised by medium tectonic hazard, with some isolated areas of high hazard in
the west, and low hazard scattered in the northwest and east. The maximum value map(Figure 6.5) shows almost no area of low tectonic hazard, but significantly more areas of highhazard where major active faults are located.

Figure 6.4: Map of 6.5<M<7.5 rates from the equally-weighted average of the three ITM
Surface Deformation strain rate models, for a grid of cells with dimensions 5x5x20km.

74Figure 6.5: Map of 6.5<M<7.5 rates from the maximum of the three ITM Surface Deformation
strain rate models, for a grid of cells with dimensions 5x5x20km.

In the context of tectonic hazard modelling from diverse datasets, we consider the maximum
value map (Figure 6.5) to be the most appropriate representation of the three datasets,
particularly with respect to the areas of highest hazard, which come from active fault data andare obviously muted by the averaging process seen in Figure 6.4. The GPS and seismicity
datasets typically do not show elevated rates near the fault sources, which is due to seismic
quiescence of the faults. This is a common observation in seismic hazard modelling, and thejustification for combining datasets to build composite source models. We emphasise that theanalyses developed here are to demonstrate the method rather than producing the “right”
answer. The calculations we have done use early logic trees developed for each of theunderpinning surface deformation, GPS and seismicity hazard models. Additionally, we have
simply assumed a hazard event (the 6.5≤M≤7.5 earthquake) as a measure of “high” hazard
and an implication for low prospectivity as a candidate repository site. In reality, thresholds
will be very uncertain until consequence studies were undertaken for site-specific conditions.
One methodological issue associated with the hazard realisations shown in Figures 6.1-6.5 isin the grid-based approach to modelling the 6.5≤M≤7.5 rates. Each 5x5x20km grid cell ismodelled independently, with no ability for grid cells to interact when large sources havedimensions outside the cell volume. This is of particular concern for the fault sources, where
the 6.5≤M≤7.5 rate for a fault source that crosses N grid cells is the fault rate divided by N.
The net effect would be to underestimate the tectonic hazard at grid cells on or close to major
active faults, and possibly in some other areas as well (e.g. near areas of high seismicity
rates or GPS strain), possibly by a factor of three or more. Future work on this topic area
should consider the interaction of grid cells for sources that are larger than the unit cellvolume. This potential underestimation reinforces the existing NUMO criteria of avoidance ofknown active faults as candidate repository sites.

75

7

Evaluating site impact hazard probabilities using logic trees andexpert elicitation

Incorporating expert opinion and uncertainties into the ITM-TOPAZ methodology is done via aformal process of expert elicitation, which was developed and tested in the first stages of the
TOPAZ project (Chapman et al., 2012). The process is based on the use of logic trees thatare designed to include alternative interpretations and modelling approaches and to generatescenario probabilities. This chapter updates the methodology with detailed explanation oflogic trees and expert elicitation approach, followed by examples of how to use the scenario
probabilities for evaluating exhumation hazard of a site.
7.1

Explanation of the method: use of logic trees to derive probability weights forsite hazard maps

A Logic Tree is a useful framework for assessing probabilities of possible alternative future
geophysical or geological scenarios and outcomes. Such trees are used extensively forvolcanic hazard assessments (for example, Neri, A., et al. 2008; Newhall and Hoblitt, 2002).
There are two main sub-types of Logic Tree: Event Trees and Probability Trees, depending
upon the way they are evaluated in probabilistic terms. An Event Tree is a graphical, tree-like
representation of a series or sequence of events in which branches are logical steps from astartpoint (prior) event through increasingly specific subsequent events (with intermediateoutcomes) to final events and their outcomes. For each event through the tree, a probability is
assigned for its likelihood of occurrence. The product of probabilities along any one path,multiplied together, gives the probability of that endpoint event and outcome. At the most
simple conceptual level, events at any particular level of the tree need not be mutually
exclusive or exhaustive.
A Probability Tree is also a graphical, tree-like representation of alternative scenarios orevents, very similar to an Event Tree, but formally the events on different branches at a
specific level in the tree are exhaustive and mutually exclusive. Thus there is a requirementthat the individual probabilities of these events, at that level, will sum to 1.0. This summation
constraint is required if one wishes to know, for example, the total probability of a particularoutcome that might occur as a result different events happening along several different
possible paths.
If a situation is encountered with multiple scenarios, processes and sequential occurrences tobe represented on such a tree, it is very easy for the tree to grow to have hundreds or
thousands of branches and, without a structured formalism, the outcome probability
evaluation problem rapidly becomes intractable. The solution is to make use of a suitablecomputer-based graphical package, designed for the purpose. A key benefit of this approachis that the scenario model can be made as comprehensive, or as simple, as needed, and thealternative possibilities and probability calculations become traceable and auditable – animportant attribute for developing any case for geological disposal under a regulatory
framework.
However, there is another important aspect to the challenge of moving from simplistic
qualitative or deterministic descriptions of potential radiological impacts associated withgeological disposal to more quantitative estimates. That is the ubiquitous and unavoidable
presence of uncertainty in our knowledge and understanding of long-term geologicalprocesses, and how this should be tackled and characterized for site hazard evaluation. If a
logic tree formulation is adopted, then it is feasible (with some software packages) torepresent uncertainty on the variable values or probabilities on the tree in the form of
appropriate probability distribution functions, rather than as (deterministic) single pointestimate values. These distributions can be parameterized from statistical data, where this isadequate, from conceptual model findings, or from elicitation of expert judgement, when data
are too sparse and models are of limited validity. With such uncertainty distributions it is thenpossible to apply Monte Carlo sampling techniques to determine estimated confidence levels
associated with the hazard impact or outcome assessment problem.
In the TOPAZ study, a probability tree sub-type of the logic tree concept has been adopted
and implemented to create hazard scenario trees. For whichever particular form of geologicalor geophysical hazard is of concern, each site-specific hazard scenario tree starts from a prior76

(i.e. current) condition or state at the given site, with the tree configured to depict pathways
through alternative branchings that represent different defined Regional Evolution Scenarios(RES) and Site Evolution Scenarios (SES); the tree culminates in a range of hazard levelvalues and their associated probabilities for the specific site and hazard. For example, theoutcome of interest might be the total amount of exhumation of a repository that could occur
at the particular site, over a number of future timescales.
In TOPAZ, hazard scenario trees have been developed for representing selected plausible
regional and site vicinity patterns of geological evolution, through time and spatially, treating
volcanic activity, tectonic strain and seismogenicity, and uplift and erosion (exhumation). The
goal is to provide a methodology by which these strain, volcanic intrusion and uplift
assessments can inform probabilistic hazard maps which then can be combined into a single
tectonic hazard map, showing the overall susceptibility of a site to tectonic events over time
periods of interest. In TOPAZ, the latter comprise the next 10 kyr, 100 kyr and 1 Myr.
Because the problem concerns long-term future evolutionary trends and patterns andbecause there are no viable forecasting models or data for this problem, in conducting aprobabilistic scenario analysis the only recourse to expert judgement. The basis on which thishas been done in TOPAZ, and preliminary findings in support of the hazard scenario trees arepresented in the next section.
7.2

Expert elicitation approach: updating and extending elicited RES and SES
probabilities

As a part of the ITM-TOPAZ methodology, the concept of incorporating differing expert views,
alternative interpretations and models using a formal Expert Elicitation (EE) approach wasintroduced to Japanese tectonics and radioactive waste experts during the project.
This was done by means of an initial workshop, held on 25th - 26th October 2011, in Tokyo.
In that workshop, selected RES and SES probabilities were elicited from a group of experts,
after first calibrating the attending experts in terms of their ability to provide informative
judgements for establishing probabilities and ranges of probabilities in the face of
uncertainties. The outcomes of that first workshop are reported in more detail in Chapman etal. (2012).
A further elicitation was undertaken in June 2013 to update some of the RES and SES
probabilities obtained at the October 2011 workshop, and to extend the scope to include
further scenarios which were omitted from the first elicitation because of constraints on timeand analysis resources. Further details of this follow-on exercise are provided below.
The basis of both these TOPAZ elicitations is a structured questionnaire addressing specificscenarios, influences and factors that have been identified as pertinent for characterizing
future geological processes in certain areas of Japan where a proposed geological repository
for radioactive wastes might be sited. The questionnaire is completed by each participating
expert, and their responses analysed with the Cooke Classical Model (below) and the
EXCALIBUR software package. The main goal of the procedure is to provide a rational anddefensible basis for ascribing uncertainty distributions to parameters, variables andprobabilities in relation to long-term future volcanic and tectonic hazards and risks. For manyof the factors addressed by the expert panel, the scientific uncertainties associated with theserisk factors are large, and their quantification is a crucial element for supporting decision
making related to hazards, repository siting and risk management.
Using expert judgment in this formalized manner furnishes objective quantitative estimates for
identified hazard and risk factors - crucial for informing policy decision-making underconditions of severe scientific uncertainty. In addition, the elicitation exercise also helps
identify major knowledge gaps, topics where definitions and terms may need clarification, andcritical areas where additional research and investigation may be worthwhile.
The goal of the approach is to develop a ‘rational consensus’ on scientific questions ofconcern or significance for hazard assessment by utilizing a formalized approach based on
performance-based scoring rule optimization for differentiating between the views of different
experts. The group of experts involved agree that the method will produce representations ofparameter uncertainty - for the purposes for which the panel was convened - without knowing77

a priori the outcomes of this method. However, it is not required that each individual member
adopts the results as his personal scientific belief.
To be rational, the method must comply with necessary generic conditions devolving from thescientific method; Cooke (1991) formulates the necessary conditions or principles which any
such elicitation method should satisfy. These principles are implemented by adopting theClassical Model, a performance-based weighted opinion pooling model. The expert weights
are derived from individual calibration and information scores, measured empirically on
related subject-matter seed variables. Differentiating experts’ performances with these seed
items enables optimized combinations of expert uncertainty distributions to be determined fortarget questions that have t rely on their judgements.
In a typical application, each expert gives quantile information for his or her uncertainty
distributions, such that for each calibration variable there are four distribution intervals definedththth
by three quantiles (in TOPAZ, the 5 , 50 and 95 percentiles, respectively). The statisticalaccuracy (‘calibration’ score) of an expert is measured by looking at how far the equivalent
empirical distribution given by the seed items differs from that given by the expert. The
analyst wishes to identify those experts for whom the corresponding statistical hypothesis iswell supported by comparison with that known distribution. Thus, 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 – a low p-value converts to a highcalibration score.
Another criterion for an expert’s performance is relative information (as noted earlier,sometimes referred to as ‘informativeness’). Relative information represents the degree towhich the expert’s distribution is concentrated, relative to some user-selected backgroundmeasure. The information score is a positive number, with increasing values indicating
greater information relative to the background 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, ‘good expertise’ in this context - i.e. judging
uncertainties - corresponds to high statistical likelihood and high information. In order todetermine the performance-based weight that an individual expert gets, their information andcalibration scores are combined together as a product. In this situation, the calibration scores
are the more important: calibration dominates over relative information (or informativeness),while the relative information score serves to adjust between equally calibrated experts.
The seed questions used in the TOPAZ Project to score participating experts and the resultsof the calibration exercise are referenced in Chapman et al. (2012), and are not presented
here.
In order to generate weights or probabilities for each hazard scenario tree, in June 2013 theTOPAZ experts were invited to provide central estimate and 90% confidence bound quantiles
for those elements of the following key questions that were not elicited in 2011:







for the four RESs, what weight should be given to each in expectation of it being anappropriate future scenario;
given each RES, what is the probability that strain rates at the site location increase,decrease or stay the same over the next 1 Myr;
what weight should be given to each strain dataset (in the ITM methodology) in termsof reliability for indicating strain rates;
given each RES, what would be the expected long-term average rate of formation of
new volcanoes in the Tohoku region: an increase, decrease or stay the same overthe next 1 Myr;
what weight should be given to each spatial density model (in the ITM methodology)
for estimating formation rates of new volcanoes at the site location;
what weight should be given to different spatial density models for representing an
increase in long-term average rate of formation of new volcanoes.

For the target questions summarised above, performance-based weighted solutions andequal weights solutions are both reported to allow comparison, together with the facilitator’scommentary on each item finding.

78

As with the judgments elicited from the experts, each pooled target item solution provides
three values that can be used to define a unique uncertainty distribution for that particularthitem: the median (50 percentile) and two values marking a corresponding 90% credibleinterval about the central value.
After considering the nature and spread of these quantile values, a suitable functional form forthe uncertainty distribution is chosen, and used for Monte Carlo analysis in concert with therelevant hazard scenario tree – see next section.
7.3

Scenario probability trees: structure and implementation

In this section, the structure and implementation of a scenario tree are exemplified byexhumation risk at ‘Site C’ in the previous report (Chapman et al., 2012), which is defined as
WCB site in this report (see Chapter 3). The analyses were based on the uplift and erosion
rates defined in the Site C Storyboard (Chapman et al., 2012; Fig. 2.11), that wereindependent of the updated scenario for the WCB site, because of time constraints. The
software
PrecisionTree[http://www.palisade.com/precisiontree/]
and
@Risk
[http://www.palisade.com/risk/] were jointly used for the following analyses.
Figure 7.1 presents an example of the structure of the exhumation scenario probability tree
for Site C using the PrecisionTree and @Risk. The exhumation risk was quantified bystandard Monte Carlo technique using re-sampling of the various associated uncertainty
distributions. Here, the assumption was made that the extent of exhumation, in metres, is the
net result of tectonic uplift and surface erosion, with different levels of correlation between thetwo processes over the different timeframe, i.e. the next 10kyr; 100kyr and 1Myr. All branchesof the tree incorporate elicited uncertainty distributions for scenario probabilities and indicative
uncertainties for uplift and erosion rates.

79

UPLIFT RATEURT1EROSION RATE
100.0%
0.50

Nodes N1_x

Nodes N2_x

0.55
ERT1

20.9%

SES1

EXHUMATION RATE
and Prob (above)

T1 ER mm/yr
100.0%

0.04560.05

0.55

100.0%

0.04560.05

0.55

100.0%

0.04560.50

1.00

100.0%

0.12800.05

1.05

100.0%

0.12800.05

1.05

100.0%

0.12800.10

1.10

100.0%

0.04490.05

0.55

100.0%

0.04490.05

0.29

100.0%

0.04490.25

0.49

SES1 UR mm/yr

0
URT2

100.0%T2 ER mm/yr

0.50

0.55
ERT2

URT3

100.0%T3 ER mm/yr

0.50

1.00
ERT3

RES1

65.5%

SES

0
URT1

100.0%T1 ER mm/yr

1.00

1.05
ERT1

58.6%

SES2

SES2 UR mm/yr

0
URT2

100.0%T2 ER mm/yr

1.00

1.05
ERT2

URT3

100.0%T3 ER mm/yr

1.00

1.10
ERT3

URT1

100.0%T1 ER mm/yr

0.50

0.55
ERT1

20.5%

SES3

SES3 UR mm/yr

0
URT2

100.0%T2 ER mm/yr

0.24

0.29
ERT2

URT3

100.0%T3 ER mm/yr

0.24

0.49
ERT3

RES2

12.3%
0

Exhumation Site C

SES

+

0.86

RES

RES3

12.7%
0

RES4

SES

+

9.5%
0

0.71
SES

+

0.37

Figure 7.1: Example of the structure of scenario probability tree produced using
PrecisionTree and @Risk.
Figure 7.2 presents an example of parameter set used in the analysis using @Risk. The
column Node3_x contains Beta distributions for Uplift Rate matching the three quantiles foreach RES/SES scenario, and column Node4_x does the same for Erosion Rate; these
distributions are linked to corresponding Node branches on the Exhumation Tree (Figure 7.1)
where sample rate values are multiplied by timeframes and summed to estimate net
Exhumation over the time period. For illustration, the column with caption ‘T1 T2 T3 rates
Corr Coeffs’ indicate Erosion Rate samples are presumed positively correlated with Uplift
Rate values with the following coefficients: over 10 kyr +0.6; over 100 kyr +0.4; over 1 Myr
+0.2.

80

UpliftRate

URdistrib

ErosionRate

Scenario
Rn.Sn.Tn

5%ile

50%ile95%ileNode3_x

Label3

5%ile

50%ile95%ileNode4_x

Label4

T1, T2, T3 rates
CorrCoeffs

R1S1T1

0.250

0.500

0.750

0.5000N3_1

0.0001

0.050

0.100

0.0530N4_1

0.60

R1S1T2

0.250

0.500

0.750

0.5000N3_2

0.0001

0.050

0.100

0.0530N4_2

0.40

R1S1T3

0.250

0.500

0.750

0.5000N3_3

0.2500

0.500

0.750

0.5000N4_3

0.20

R1S2T1

0.500

1.000

1.500

1.0000N3_4

0.0001

0.050

0.100

0.0530

N4_4

R1S2T2

0.500

1.000

1.500

1.0000N3_5

0.0001

0.050

0.100

0.0530

N4_5

R1S2T3

0.500

1.000

1.500

1.0000N3_6

0.0500

0.100

0.150

0.1000

N4_6

R1S3T1

0.250

0.500

0.750

0.5000N3_7

0.0001

0.050

0.100

0.0530

N4_7

R1S3T20.0001

0.250

0.450

0.2560N3_8

0.0001

0.050

0.100

0.0530

N4_8

R1S3T30.0001

0.250

0.450

0.2560N3_9

0.0001

0.250

0.500

0.2653

N4_9

R2S1T1

0.250

0.500

0.750

0.5000N3_10

0.0001

0.050

0.100

0.0530

N4_10

R2S1T2

0.250

0.500

0.750

0.5000N3_11

0.0001

0.050

0.100

0.0530

N4_11

R2S1T3

0.250

0.500

0.750

0.5000N3_12

0.0001

0.250

0.500

0.2653

N4_12

R2S2T1

0.500

1.000

1.500

1.0000N3_13

0.0001

0.050

0.100

0.0530

N4_13

R2S2T2

0.500

1.000

1.500

1.0000N3_14

0.0001

0.050

0.100

0.0530

N4_14

R2S2T3

0.500

1.000

1.500

1.0000N3_15

0.0001

0.100

0.200

0.1061

N4_15

R2S3T1

0.500

1.000

1.500

1.0000N3_16

0.0001

0.050

0.100

0.0530

N4_16

R2S3T2

1.200

1.500

2.000

1.5368N3_17

0.0001

0.050

0.100

0.0530

N4_17

R2S3T3

1.000

1.500

2.000

1.5000N3_18

0.0001

1.000

2.000

1.0612

N4_18

R3S1T1

0.250

0.500

0.750

0.5000N3_19

0.0001

0.050

0.100

0.0530

N4_19

R3S1T2

0.250

0.500

0.750

0.5000N3_20

0.0001

0.050

0.100

0.0530

N4_20

R3S1T3

0.250

0.500

0.750

0.5000N3_21

0.0001

0.500

0.750

0.4845

N4_21

R3S2T1

0.500

1.000

1.500

1.0000N3_22

0.0001

0.050

0.100

0.0530

N4_22

R3S2T2

0.500

1.000

1.500

1.0000N3_23

0.0001

0.050

0.100

0.0530

N4_23

R3S2T3

0.500

1.000

1.500

1.0000N3_24

0.0001

0.500

1.000

0.5306

N4_24

R3S3T1

0.250

0.500

0.750

0.5000N3_25

0.0001

0.050

0.100

0.0530

N4_25

R3S3T20.0001

0.250

0.450

0.2560N3_26

0.0001

0.050

0.100

0.0530

N4_26

R3S3T30.0001

0.250

0.500

0.2653N3_27

0.0001

0.025

0.050

0.0265

N4_27

R4S1T1

0.250

0.500

0.750

0.5000N3_28

0.0001

0.050

0.100

0.0530

N4_28

R4S1T20.0001

0.250

0.450

0.2560N3_29

0.0001

0.050

0.100

0.0530

N4_29

R4S1T30.0001

0.250

0.500

0.2653N3_30

0.0001

0.025

0.050

0.0265

N4_30

Notes:

Uncertainty quantiles are illustrative

R1 = RES1

S1 = SES1

T1 = 10ka

R2 = RES2

S2 = SES2

T2 = 100ka

R3 = RES3

S3 = SES3

T3 = 1000ka

R4 = RES4

S4 = SES4

Cols E,J have cell names as per adjacent labels for ease of reference on nodes in main tree
Quantile values on this sheet are dummy placeholders, pending expert elicitation

Figure 7.2: Tabulated uncertainty quantiles for uplift and erosion rates, used to define
probability density functions on the scenario tree for Monte Carlo analysis using @Risk.
Net exhumation and its associated probability is re-calculated 10,000 times over the whole
tree by the Monte Carlo technique and the sampled evaluations stacked to provide statisticalexpression to the results.
Figure 7.3 illustrates the sort of exhumation exceedance probability curves that can bederived from a Monte Carlo analysis of this type.

81

Site C: total exhumation exceedance curves
1.0
10
10
kyr
kyr
10ka

0.9

100ka
100
kyr
1 Myr
1Ma

Exceedance probability

0.8
0.7
0.6
0.5
0.4
0.3

X = 300

0.2
0.1

10000

Exhumation [m]

1000

100

10

1

0

0.0

Figure 7.3: Example exhumation exceedance probability curves for three future periods(10 kyr; 100kyr; 1Myr).
To exemplify the risk indications that are provided by exceedance curves, such as thoseshown in Figure 7.3, the analysis suggests there is a probability < 0.0001 that exhumation at
Site C will exceed 300m in 100 kyr. But, by contrast, the results also suggest that exhumation
will exceed 300m a probability of 0.97, in 1Myr.
One of the strengths of the TOPAZ approach to site risk quantification - using uncertainty
distributions on a scenario probability tree - is that the contributions of different scenarios,
models or assumptions can be disaggregated and scrutinized. In Figure 7.4, for instance, thepossible extent of exhumation over 100kyr is evaluated for two scenario combinations: RES1
& SES3, versus RES2 & SES3. The former has median 31m [90% range 8 – 53m], whereas
the latter indicates 155 m median exhumation [90% range 123 – 208 m].
Such analysis capabilities can provide a reasoned stimulus for further investigation into
contributory factors or other model aspects if results like these were considered critical or
intolerable, or even just marginal.
The analysis based on the scenario tree, outlined above, need not stop there: having
computed exhumation exceedance probabilities for three different periods into the future (e.g.
Figure 7.3), those findings can be interpolated to determine, probabilistically, when any
specific amount of exhumation may be experienced.
Thus, on Figure 7.5 the time to surface exhumation for three different repository burial depths
can be quantified – conditional on the models incorporated on the scenario tree.

82

Exhumation PDFs: scenarios RES1 SES3 -v- RES2 SES3
100ka timeframe end member cases
0.030

0.025

RES2 SES3
RES1 SES3

PDF

0.020

0.015

95% = 208

Median = 155

5% = 123

95% = 53

5% = 80.005

Median = 31

0.010

300

250

200

150

100

50

0

0.000

Exhumation [m]Figure 7.4: Example probability density function spreads for two end-member
exhumation scenarios for 100 kyr timeframe.

kyr

Figure 7.5: Illustrative curves for time to exhumation for three different burial depths[red=200m; blue=300m; green=400m].

83Some implications of Figure 7.5 might be summarized as follows:
Time to exhumation for 200m burial (rounded values):Minimum* = 50 kyr
1 percentile = 90 kyr
5 percentile = 125 kyr
Median = 225 kyr

Time to exhumation for 300m burial:

Minimum* = 75 kyr
1 percentile = 120 kyr
5 percentile = 190 kyr
Median = 340 kyr

Time to exhumation for 400m burial:

Minimum* = 95 kyr
1 percentile = 170 kyr
5 percentile = 250 kyr
Median = 450 kyr* ‘Minimum’ is the shortest time from a Monte Carlo simulation with 10,000 iterations; i.e.,here it approximates to 0.01% or 10-4 probability.

7.4

Summing up

The TOPAZ Project has developed a state-of-the-art probabilistic methodology for quantifying
geological hazard risks for a geological repsoitory using conceptual hazard scenario treesthat can represent plausible regional and site vicinity patterns of geological evolution, throughtime.
Separate trees can be configured to treat the different processes involved, for instance thosethat could engender future volcanic activity, tectonic strain and seismogenicity, or uplift anderosion (exhumation) at the site.
One important feature of this approach is that it incorporates capabilities for full treatment ofmodel and parameter uncertainties, specifically including in exceedance calculations thosethat are obtained by structured expert elicitation, for which a formalized, and scientificallydefensible, technique is identified.
The mode of implementation of this methodological approach is illustrated here by theexample of a basic risk analysis for the exhumation, by tectonic uplift and erosion, of a buried
facility. Application to a real site safety case, for this and other long-term geological processes,
would almost certainly involve altogether more comprehensive and exhaustive models, andmore detailed characterizations of associated uncertainties. This demonstration shows thatsuch conceptual models can be developed, that elicitation protocols can be designed, andthat software tools exist for executing the necessary probabilistic calculations.
The final step is to combine tectonic strain, seismicity, volcanic intrusion and uplift
assessments into a single tectonic hazard map that evinces the overall susceptibility of a site
to tectonic events over future time periods of interest. The methodology outlined in thepresent section allows the probabilities to ascribe to the different hazards and their maps tobe determined in a tractable and auditable way, whence a joint hazard and risk evaluationcase can be made.

84

8Discussion for practical application

This chapter summarises points and issues in applying the ITM-TOPAZ methodology to thereal site. It is, however, emphasised that how NUMO will actually apply this methodology
requires further discussions in the future.
8.1

Practical application of the ITM-TOPAZ methodology

The ITM-TOPAZ methodology can produce quantitative information on the likelihood of
specified tectonic events for a range of different timescales in the future: 1 ky, 10 kyr, 100 kyrand 1 Myr.
The user can specify the nature and magnitude of the events for which probability estimates
are made. For development and demonstration purposes, a specific set of volcanic intrusion,
fault movement and uplift/erosion events was identified, which provides a good general basis
from which to assess project risk.
The overall aim of the methodology is to provide decision-makers with a scientifically based
approach to evaluating geological stability that is based on managing and incorporating theinevitable uncertainties. The output of the methodology can be used by scientific/technicalstaff involved in site evaluation and performance assessment.
It is also intended to provide quantitative guides to management team on project risk – therisk to programme success of accepting a site for detailed study as a DIA, and can also be
used as an effective communication tool.
The aim is to provide users with a tool that will assist with two important activities that will be
carried out as part of the geological repository site selection programme:
Application 1: Identify potentially suitable regions and sites with respect to their longterm tectonic hazards
 this is done using regional hazard maps that show susceptibility to disruption by
future volcanic events, rock deformation and uplift and erosion
Application 2: Quantify the likelihood of specified tectonic impacts at any location overdifferent timescales this allows risks to be incorporated into safety case development and into quantitative
safety assessment
Eventually, the methodology will be used to carry out increasingly detailed studies of tectonic
hazards at a site during the later stages of PIA assessment and in the DIA stage, when
detailed site investigation data become available. Application of the methodology in the runup to site (PIA) selection will provide confidence in site choice from the perspective ofgeological stability. Application 2 might also have used to support preliminary PA studies of asite.
During the detailed surface investigations of a site, it will be possible to address uncertainties
that have been identified by ITM-TOPAZ work. For example, uncertainties in age
determinations of volcanic events in the region will affect the spatial mapping of future event
probabilities and uncertainties in active fault histories in the region will affect the strain rate
forecasts. These uncertainties can be overcome by gathering more data, not only within a PIA,
but also locally and regionally. This type of fieldwork should plan to carry out, informed by
ITM-TOPAZ regional evaluations, and making use of local geological knowledge.
With enhanced datasets, it will be possible to reapply the methodology to produce betterconstrained probability values for risk estimation in later stages of safety assessment (e.g. foreventual licensing of construction of a repository). It is inevitable that the methodology willneed to be reapplied several times over the whole period of PIA and DIA investigation, as
improved safety assessments are carried out and as the safety case is being developed.
An important consideration in any of these applications concerns the time scales or relevanceto assuring repository safety, as discussed in Section 1.4. Here, it is again emphasised thatthe critical period of concern to HLW disposal is the first thousand to 10 kyr. The ITM-TOPAZ
methodology allows forecasting well beyond this period – out to 1 Myr – but decision-makers

85need to be aware that not only does the methodology contain less uncertainty for theseshorter times, but these times should also be at the forefront of their considerations andcommunications with the public. Factors that are of importance in different timeframes andthe way in which ITM-TOPAZ provides support in each are shown in Table 8.1.

Table 8.1: Timescales in safety evaluation for a geological repository for high-level waste andhow the ITM-TOPAZ methodology supports evaluation and decisions-making in these periods.
Period0 – 1kyr

1kyr –
10kyr

10kyr –
100kyr100kyr– 1MyrWhat matters?

What is needed?

What does ITM-TOPAZ do?

The waste is extremely
hazardous and people mustbe isolated from any
radionuclides released fromit into the rock. The
engineered containment in
the repository will begin todegrade.

The waste and the engineered
containment need to be protectedfrom natural tectonic disruptive
events. This must be done by
avoidance: selecting a stable sitewhere the probability of volcanic
activity or penetration of therepository by faulting (e.g. from ahidden active fault deep beneath it)
is extremely small over this period.

Provides quantitative
probabilities of disruptive
events of any specified
magnitude, which must be so
low that they can be
discounted.

Well within this period, thehazard from any releases
from the waste has declined
to levels of a natural ore
body. The engineered
containment has broken
down and radionuclides areentering the deep
groundwater system.

Because potential hazards fromreleases from the repository are still
relatively significant, although likely
within the ranges of global naturalradiation exposures, it remains acentral objective to protect thedegrading repository from tectonic
disruption over this period too. Thisis again achieved by siting in areasof low probability of such events –by continued avoidance of tectonic
impacts.

This is arguably the principal
period over which assuranceof protection from tectonic
events is required and whenthe ITM methodology is mostvaluable.

The isolation and containment
functions of the repository are
essentially fulfilled by the end of thisstage.

Allows quantitativeevaluations of dose and risk
from disruptive tectonic
scenarios

The repository system issubstantially degraded andremaining longer-lived
radionuclides are dispersed
though the deep geologicalenvironment in very low
concentrations. Potential
doses to people from thesedispersed and diluteconcentrations are
extremely low and well
below natural backgroundradiation levels.

Over this timescale the longevity ofsafety functions can be qualitatively
discussed to compare hazards tonatural radiotoxic hazards anddiscuss the long-term fate of wastes
as the repository degrades.

Quantitatively assesses theprobabilities of major tectonic
events and processes thatmight disrupt a repository overthis timeframe.

The waste materials aredegraded, refractory
residues with someradionuclides bound into
newly formed minerals. The
system has degraded almost
completely and is releasing
only small amounts ofradioactivity, with potential
doses to people beingorders of magnitude below
natural background.

The isolation and containment
functions of the repository arecomplete. It is expected that
tectonic processes and events will
have significant impacts on therepository. Uplift and erosion may
have brought the system closer tothe surface.

Although there areconsiderable uncertainties,
semi-quantitative probabilistic
forecasting is possible, tosupport major event anduplift-exhumation evaluations

The probability that large magnitude
volcanic events could disperse theremnants of the wastes into theenvironment are of interest to safetyassessors, even though the relative
hazard compared to that of such an
event are trivial.

Forecasts can be presented in
the framework of alternative
conceptual models
characterising how thetectonic situation in a region
might develop.

Because there are still relatively
high concentrations of someradionuclides immobilised in thewastes, scenarios where major
tectonic events cause major
disruption to the repository remainof interest in safety assessments,
so quantitative estimates ofprobability are required.

86

Identifies areas and regionswith a very high probability ofproviding stability over thisperiod.

Quantitatively assesses theprobabilities of events andprocesses that might disrupt a
repository over this timeframe.
Allows siting decisions.

Provide probability input toquantitative performanceassessment to allow
calculation of risks.
This timeframe is the limit offorecasting with any
reasonable level of certainty

This is the timeframe
considered by TOPAZ.

A specific use of the methodology in Application 1 is emphasised here: the use of hazard
maps to assist decision makers by illustrating project risk. This is essentially a management
tool, rather than a tool that is related directly to safety evaluation, in that it allows qualitative
comparison of areas and sites, and highlights where problems might emerge as a projectprogresses. Complex hazard maps can be simplified to produce project management
information by defining high, medium and low ranges of hazard. The implication of theseterms is shown below.

High Project Risk

Sites located in these areas on the map would present considerable project management
difficulties, such that it could be extremely difficult to make a credible safety case for arepository owing to tectonic hazards.

Medium Project Risk

Sites located in these areas on the map would present moderate project management
difficulties. It would be possible to make a credible safety case for a repository that takes
tectonic hazards into account, but difficulties may be encountered as more information is
obtained about the site and its local area and more detailed modelling is undertaken.

Low Project Risk

There appear to be few tectonic hazards that would present a challenge to building asafety case, although detailed site characterisation and safety assessment modelling
would be needed to confirm this.

Selecting the most appropriate numerical values of probability or hazard to define theboundaries between these categories will be a matter for users to discuss. It is also importantto recognise that project risk will depend upon many more factors than just tectonic stability.
For example, a site that carries low project risk from the viewpoint of tectonic stability might
be unsuitable as a result of other bedrock features, such as deep groundwater flow properties.
Figure 8.1 (taken from Chapter 5) shows an example of a project risk map for Tohoku thatcombines volcanic hazard (using the probability of new volcanism) with uplift hazard (based
on uplift rates) to show areas with high, medium and low risk with respect to these two factors.
High project risk is shown in red. The boundaries used to distinguish, for example, high risk
from medium risk (yellow) are a probability of volcanism of 10-7 per year and an uplift rateover 125,000 years of 0.5 mm/year.

87Project Risk
(Volcanism/Uplift)
Prepared explicitly
for demonstrationof methodology

H
M
L

Figure 8.1 (re-presentation of Figure 5.21): The volcanism and uplift hazard maps are
combined into a single project risk map for Tohoku. It is emphasised that this maps wasdeveloped for demonstrating the methodology for evaluation the potential project risk. Green
zones on this map represent site areas that carries low risk with respect to volcano and uplift
disruptive hazards, as probabilities of disruptive events are negligible for all scenarios
analyzed. Yellow zones are characterized by one or more slightly higher probability – it islikely many of these sites carry moderate project risk. Red zones are areas in which one ormore scenario indicates high project risk.

8.2

Uncertainty in the ITM hazard map

Uncertainty is a critical aspect of prognosis for natural hazards. In applying the ITM
methodology (e.g. Chapman et al., 2009) for developing hazard maps, it is essential tocharacterise uncertainty in all its manifestations. Characterisation is much more thandescribing statistical errors in key parameters and includes deep understanding of epistemic
and aleatory uncertainties as well recognition that some of the assessment will require formal
expert judgement, particularly when it comes to weighting alternative conceptual models andforecasting hazards in deep time as is required for a geological disposal facility. Uncertainty
assessment is not something that can be added on at the end of a hazard assessment andneeds to be embedded and consciously done right from the outset. More detailed discussionof uncertainty in the context of hazards and risk assessment can be found in Rougier et al(2013).
Much of ITM hazard maps take place at a regional scale and it was originally developed toenable the relative hazards at different sites to be compared. Uncertainties grow greatly withtime and there thus needs to be due regard how they change. When volunteer communities

88emerge and the time comes to make real comparisons users need to recognise that thesources of the uncertainties are understood in terms of the nature of the evidence andunderstanding of the processes. We do not recommend using the results that are in thisreport once sites have been specified because knowledge and understanding in tectonics
hazards of Japan will have advanced. Indeed a re-application of the methodology will give asense of how stable the results of tectonic hazards assessment are and likely increase
confidence in the method. We note that important components of tectonic hazards anduncertainty, for example random errors, will shift systematically at all sites if values change.
So differences will not change even though absolute hazard changes. Other aspects oftectonic hazards may change as new knowledge or understanding is developed so thatdifferences between sites may change.
Our case studies have shown that volcanic hazards vary by many orders of magnitude acrossJapan and so we are confident that ITM methodology allows a robust evidence-based
approach to identify sites of very low or negligible volcanic hazard and to compare sites.
Some of the output (e.g. Mahony et al. 2009) assesses spatial uncertainty in volcanic hazard
and will enable the user to evaluate whether two or more sites have demonstrably different
hazard. Seismic hazard can also be compared between sites, although the range ofmagnitude of hazard is typically less for earthquakes.
Users need to be much more circumspect in using hazard maps for a single site. Of course asingle site can be compared with regional variations to establish that hazard is suitably low ina relative sense. Hazard assessment then needs to focus on a smaller scale and hazard
maps should only be used in an indicative sense that the site shows promise. Much moredetailed high-resolution site investigation is likely to change the hazard and the uncertainty inthe hazard. There should be no assumption that more information, better understanding andhigh resolution study will reduce uncertainty.

89

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原子力発電環境整備機構(略称：原環機構)
Nuclear Waste Management Organization of Japan(NUMO)