KYT - Seminarkyt2018.vtt.fi/rakoseminaari2017/Johannes Suikkanen.pdf · and post closure safety KYT –Seminar 1.12.2017 2017-11-01 Suikkanen Johannes 4. Public Triggering mechanism

KYT - Seminar

POST - Project & Deterministic

Earthquake modelling

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Contents of the presentation

Defining the relation between seismic event and consequential risk in post closure analysis of a spent nuclear waste repository.

Reducing uncertainties in earthquake analyses through site characterization program An example project for determining scale dependency of fracture properties

(POST – Project / KARMO)

How does site characterization of fractures propagate onwards in seismic analyses concerning post closure period?

Deterministic modelling of large seismic event within repository footprint What is the philosophy behind, and how are they connected to repository

site understanding?

Summary & discussion

2017-11-01 Suikkanen Johannes 2

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Terminology for the presentation

Primary fault: Deformation zone on which the

earthquake hypocentre is located at, or from

modelling perspective the EQ event initiated at.

Secondary (target) fracture: Adjacent or nearby

fracture on which co-seismic shear movement

potentially could be induced.


Total zone width

Core

Da

ma

ge z

on

e

Da

ma

ge z

on

e

Ho

stro

ck

Ho

stro

ck

Core width

1 23

Possible fault

slip planesGround surface

Defining the relation between seismicity

and post closure safety

KYT – Seminar 1.12.2017


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Triggering mechanism for large scale

earthquakes in stable continental regions

Long term safety analyses are required to be extended to reach up to 1 Ma period.

During this period, it is estimated that multiple cycles of glaciation will go over Olkiluoto, causing transient disruption in the prevailing in situ stress state.

Suikkanen Johannes 52017-11-01

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Seismic risk and SNF repository design

Earthquakes occurring on small local Deformation Zones or Brittle Fault Zones, possibly identified and geometrically characterized during site investigations or during early construction stages, or on larger more distant, known or unknown, deformation zones have been concluded to be the only mechanism by which host rock fractures would be loaded sufficiently to slip in excess of the 50 mm with the velocity of 1 m/sthus causing damage to the canister insert.

At the spent nuclear fuel repository site located in Olkiluoto, layout protocols have been designed to minimize the risk of canister damage caused by fracture shear displacements along fractures intersecting potential canister positions. This is achieved by avoiding Full Perimeter Intersecting (FPI) fractures of which the fracture size is unknown.


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Relation of fault slip and EQ magnitude In general, no valid way of estimating slip that could

occur on connected secondary fractures during an EQ

event exist. (TR-08-11)

However, for a slip magnitude of 5cm or larger to be

induced on secondary fractures, the maximum slip of

the primary fault in which the EQ event occurs needs to

be significantly larger than the induced 5cm on

secondary features.

5cm of slip in primary fault would correspond to an

earthquake of moment magnitude Mw ~5.5 – 6.0 or

larger according to Wells & Coppersmith 1994

regression of recorded earthquakes.

Then again, a 5.5 moment magnitude earthquake

requires a rupture area of ~20 km2, or for the sake of

conservativness 10 km2.

For a shallow eartquake with 10 – 20 km2 rupture area,

a minimum of 3 km trace length is required, as this is the

upper bound estimate for full surface rupture.

Consequently, for a deformation zone with a trace length

of 3 km or more, a Mw > 5.5 earthquake can not be

completely excluded. Canister should not be placed in

the intersections of such zones, which are nevertheless

avoided by the repository design.


9

8

7

6

5

40.01 0.1 1 10

Strike Slip

Reverse

Normal

80 EQs

Mo

men

t M

agn

itu

de

Maximum Displacement (m)

M = 6.69+0.74*log(MD)

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Primary causes for slip on canister

intersecting secondary fractures The slip in secondary fractures occurs due to two

differing mechanicsms during & after seismic event: 1. Due to the dynamic force wave emitted during earthquake rupture

2. Static redistribution of stress after deformation zone slips during

earthquake, thus releasing the prevailing stress state


1. 2.

POST – Project

Determining realistic properties

and parameters for fractures

2017-11-01Suikkanen Johannes 9

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Parametrisation of Structures (POST)

Objective of the project was to develop a strategy or methodology for determining parameters of large fractures and deformation zones to a degree of confidence.

Currently, all fracture parameters are estimated by CNL testing, while in nature the fractures are experiencing “spring like” conditions consisting of CNS boundaries.

By increasing the confidence in fracture specific parameters (τ, ks, kn) through CNS –style of testing, we could see that thefractures are much more resistant towardsdeformation and thus say that the slip of 5 cm during EQ is unlikely!


Τ

σn=(kn,δv)

Τ

F

CNS CNL

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How were the objectives to be achieved

The principal research method of the POST – project was to find large structures in two experiment locations at Äspö & Olkiluoto, which could be shear tested under in situ conditions.

The normal stress (σn) of the structures would have been released by slot drilling, thus natural stress state would have acted as a driving force.

The resulting displacement in the features would have beenmonitored during destressing, and following back calculationthe properties of the large features could have beenestabilised!


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Synthetic shear box modelling

methodology A modeling methodology in

3DEC was consturcted, bywhich synthetic shear testscould be conducted.

The initial hypothesis and impact of difference between shear behaviour of undulating surfaces under CNS and CNL conditions could be quantified.

In addition, to see howobserved fracture data fromeither photogrammetry ortrace observations could betaken into account in estimation of shearproperties


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Input from tunnel mapping for the

synthetic shear test methodology Fractures can either be observed as surfaces

or as traces in tunnel walls.

The impact of how input is handeled, eithersurface captured through photogrammetry orlaser scanning, in comparison to constructedsurface from fractal decomposition of a tracewas asessed.

It was analyzed, how the input data is handled and how coarse can the input data be so it starts to have an effect on actualshear properties!

An example fracture in ONKALO (P424) wasselected as a study case!


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Shear 57 mm CNS 5 Mpa /mm

Synthetic shear modelling methodology

Shear 57 mm CNS 20 Mpa /mmShear 0.57 mm CNS 20 Mpa /mm

Shear 0.57 mm CNS 5 Mpa /mm

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Shear test methodology benchmarks

Sheardiscplacement of non-planar surfacesunder CNS –boundary conditionsresults on remarkable increaseof normal stress and shear resistancebetween sheardisplacement of 0 and 50 mm

The increase in bothnormal stress and shar resistance is function of appliedCNS boundary butnot in 1:1 ratio!

Fractal surfacesbased on tracedecomposition show similar behaviour as scanned surfaces!


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Observations from ÄSPÖ during

planning Once a discontinuity is large

enough, it is consisted of multiple splays instead of one clear surface.

During the planning of the in situ test, it was concluded that it could not be controlled on which splay the displacement would occur during slot drilling.

Thus the experiment execution was considered too risky and abandoned, as it could be not controlled on which splay and where would the monitored displacement occur.


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– – – –

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– –

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Outcomes of the POST – project in situ

test design phase

After the initialidea a) wasabandoned due to limited control, more sophisitacedsetups b) & c) were proposed.

The resultingdesigns withimproveddisplacementcontrol started to resemble more of laboratory testsconducted in tunnelenvironment, thuswhy not move to lab?


a) In situ slot drilling induced shear test setup b) In situ shear test setup with reaction beam

c) In situ shear test setup d) Laboratory shear test setup

Figure Virhe. Tiedostossa ei ole määritetyn tyylistä tekstiä.–1. Schematic shear test

setups that were considered in the project, for details

Slot drillings

Shear

a) d)Degree of disturbation

Increased control

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Recommendations from POST – project

for continuation studies

1. A large scale shear box with a stiff frame should bedeveloped

2. The shear box must be capable of shearing samples upto the shear displacement of 50 mm

3. The shear equipment should have the capacity to applymaximum normal stress of approximately 10 MPa

4. The equipment must be able to restrict dilation, onceshearing commences to simulate CNS boundarycondition

5. A methodology should also be developed for replicationof fractures to facilitate testing under various loadingconditions with identical fracture surfaces

6. Further development of fracture mapping methodology to better capture the complex nature of observed criticalstructures.


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Fracture vs Deformation zone

Fracture composes of two opposing surfaces, and in

case of Olkiluoto their size is in order of m2

Deformation zones are in order of km2, incredibly

complicated structures composed by swarms of

fractures formed around fault core & influence zone

over the geological history.


Deterministic modelling of large

hypothetical earthquakes

Impact of hypothetical earthquakes

on secondary fracture

displacement

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Earthquake model construction in 3DEC 3-dimensional dynamic earthquake rupture simulations are carried out using models, where the primary fault and

target fractures are modelled as planar discontinuities embedded in an elastic continuum.

The target fractures, which can have different orientations, are located at repository depth and at different positions relative to the primary fault.

Earthquake ruptures are propagated, and fault slip is driven by site specific or synthetic stress fields.

The target fractures are subjected to both dynamic and quasi-static stress effects generated by earthquake rupture and fault slip.

100

100

50

(km)

BFZ021

BFZ020

BFZ099

Target fracture region

#1

#2

2017-11-01 21Suikkanen Johannes

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Conservative assumptions listed for the

assessment of large earthquake impact

Within the model construction, the dimensions of faults zones are increased relative to those given by the geological model

The frictional properties of the fault zones are set such that they generate moment magnitudes that are high, given the modelled rupture areas.

Within the numerical 3DEC models, faults are assumed to fully rupturefrom their entire area while in nature only proportion of the fault area is ruptured.

Within the models, zero damping is applied.

Rock mass is treated as elastic, which results that all of the energy is directed to maximally displace the “target fractures” instead of energy being dissipated to plasticity etc.

The target fractures are assumed planar

Synthetic stress state in large depth is applied, which keeps the deformation zone of interest on its stability limit.

Purpose of the deterinistic models is to provide insight on what couldhappen in secondary fractures during a large scale hypotheticalearthquake, so that we can see the effect of extreme and unlikely events!


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Synthetic stress state applied within the

models In Olkiluoto, no stress measurement

data exist below the depht of 800m.

In addition. stress state in deep partsof earth crust is not well known.

As the model geometry extends to depths of ~20 - 50km, a syntheticstress model respectingmeasurement data for first 1km and then extrapolated to depths is applied.

This stress state is created so, that it keeps the fault zones in their stabilitylimit when glacial stresses aresummed on the earth crust.


-20000

-18000

-16000

-14000

-12000

-10000

-8000

-6000

-4000

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0

0 100 200 300 400 500 600 700 800

Dep

th (

m)

Stress (MPa)

σHσhσv

Depth Trend0 63°500 77°2000 85°7000 89°20000 89°

-6000

-5000

-4000

-3000

-2000

-1000

0

-55 -50 -45 -40 -35 -30 -25 -20 -15 -10 -5 0 5

Dep

th (

m)

CFS (MPa)

FB, Strike-slip

EG, Strike-slip

P-D, Strike-slip

FB, Reverse

EG, Reverse

P-D, Reverse

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Model calculation sequence and EQ

initiation The earthquake rupture is initiated

at a pre-defined hypocentre and propagated at a speed in accordance with observations (0.7-0.9 Vs).

During the rupture process, the fault frictional strength is ramped down to a residual value. This residual fault friction value determines the stress drop, the seismic efficiency and fault slip. Lower values tend to give higher slip velocities, as shown by the example plots to the right.

The rupture reaches all positions on the fault plane.


Case 1

2 s

3 s

4 s

5 s

6 s

8 s

10 s

Case 2

0

1

2

3

4

5

6

7

m/s

#1 #2 #3 #4 #5 #6

18 s

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How are the models benchmarked?

The magnitude of theearthquake initiated withinthe fault zone is controlledby seismic efficiency η.

Seismic efficiency is the relative difference of how much seismic energy (or stress drop) is being released during the seismic event ranging from 0 – 1.

If no seismic energy is being released, thus no stress drop achieved resulting in no earthquake eg. τ1= τ2 -> thus η = 0.

It can be seen that themodel does not requirerealistic parameters per se, as it is really controlled byrelative difference!

What is required is a set of parameters in the realms of reality that create Physicallyviable earthquakescompared against knownobservations!

Olkiluoto, extended BFZ214 Cases 1, 2, 6

Olkiluoto, BFZ100, WR 2012-08

Example model, Fälth et al., 2015

Olkiluoto, BFZ021, WR 2012-08

Olkiluoto, extended BFZ214 Case 3



Forsmark, ZFMA2, Fälth et al., 2015

Forsmark, ZFMA2, Fälth and Hökmark, 2013 (thermal)

0,1 1 10 100 1000 10000

4

4,5

5

5,5

6

6,5

7

7,5

8

Rupture Area (km2)

Mo

men

t M

agn

itu

de

WC Earthquake Catalogue, non-SCR

WC Earthquake Catalogue, SCR

Synthetic 3DEC Earthquakes

Leonard regression

WC regression


1 2 1 2 1 2/ ( ) ( ) / ( )

Case Rupture

area

(km2)

Average

fault disp.

(m)

Peak

slip (m)

Peak slip

velocity*

(m/s)

M0 (Nm) Mw Average

stress drop

(MPa)

1 720 6.5 9.1 7.8 1.2∙1020 7.3 11

2 720 3.1 4.6 3.3 5.7∙1019 7.1 5.1

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Notes about the planar representation of

features Planar features with homogeneous properties do tend to

form circular displacement profiles, as the displacement of the feature is resisted by the fracture surface & surrounding matrix.

This is not observed always in nature due to heterogenity, but this is not the controlling aspect of EQ – secondaryfracture shear we are interested in.

Planar features are conservative since an undulated feature has higher shear resistance (given that one assume similar frictional properties)


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Discussion on seismic efficiency on

fracture slip The seismic efficiency (η) is, directly coupled to the stress drop, but also to slip and

seismic moment of the primary fault.

In the table at slide 25 you see that stress drop, slip and seismic moment in Case 2 all are about 50% of those in Case 1. You also see the same ratio between the average seismic efficiencies of both cases presented below.

As we see in slide 25, the slip velocities, and thereby the dynamic secondary stress effects, depends on the stress drop (and seismic efficiency). This is reflected in the target fracture displacement results below for two cases in which only seismic efficiency of the primary fault is varied


Region #1

Region #2

Fault trace

Region #1 Region #2

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40

Cu

mu

lati

ve d

istr

ibu

tio

n (

%)

Induced displacement (mm)

300 m

700 m

1500 m

2500 m

4500 m

dd/dip 90/80dd/dip 180/80dd/dip 195/77dd/dip 150/30dd/dip 300/30dd/dip 90/30

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40

Cu

mu

lati

ve d

istr

ibu

tio

n (

%)


300 m

700 m

1500 m

2500 m

4500 m


Region #1

Region #2

Fault trace

Region #1 Region #2

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40

Cu

mu

lati

ve d

istr

ibu

tio

n (

%)


300 m

700 m

1500 m

2500 m

4500 m


0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40

Cu

mu

lati

ve d

istr

ibu

tio

n (

%)


300 m

700 m

1500 m

2500 m

4500 m


Case 1: η = 0.18 Case 2: η = 0.091 2 1 2 1 2/ ( ) ( ) / ( )

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What happens on secondary fractures

during an earthquake?


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-2

-1

0

1

2

3

4

5

6

7

0 2 4 6 8 10 12 14

CFS

(M

Pa)

Time (s)

dd/dip = 0/30

180 m

400 m

520 m

840 m

1000 m

1170 m

1330 m

0

5

10

15

20

25

0 2 4 6 8 10 12 14

Shea

r d

isp

lace

men

t (m

m)

Time (s)

dd/dip = 0/30

180 m

400 m

520 m

840 m

1000 m

1170 m

1330 m

-2

-1

0

1

2

3

4

0 2 4 6 8 10 12 14

CFS

(M

Pa)

Time (s)

dd/dip = 0/30

200 m

400 m

600 m

800 m

1000 m

1500 m

0

1

2

3

4

5

6

7

0 2 4 6 8 10 12 14Sh

ear

disp

lace

men

t (m

m)

Time (s)

dd/dip = 0/30

200 m

400 m

600 m

800 m

1000 m

1500 m

( )nCFS P Hanging wall Foot wall

Both normal- and shearstress of the secondaryfractures varies during thedynamic phase of theearthquake event

This results from the pointof ΔCFS at the fractureplane in a momentary lossof stability followed byincreased stability.

The slip of the secondaryfractures mainly occursduring this dynamic phase.

Fractures located on thehanging wall side of theprimary fault slip morethan the ones on the footwall.

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Same as previous, steeply dipping target

fractures:

Hanging wall


Foot wall( )nCFS P

-16

-14

-12

-10

-8

-6

-4

-2

0

0 2 4 6 8 10 12 14

CFS

(M

Pa)

Time (s)

dd/dip = 144/80

180 m

400 m

520 m

840 m

1000 m

1170 m

1330 m

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 2 4 6 8 10 12 14

Shea

r d

isp

lace

men

t (m

m)

Time (s)

dd/dip = 144/80

180 m

400 m

520 m

840 m

1000 m

1170 m

1330 m

-16

-14

-12

-10

-8

-6

-4

-2

0

0 2 4 6 8 10 12 14

CFS

(M

Pa)

Time (s)

dd/dip = 144/80

200 m

400 m

600 m

800 m

1000 m

1500 m

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0 2 4 6 8 10 12 14Sh

ear

dis

pla

cem

ent

(mm

)Time (s)

dd/dip = 144/80

200 m

400 m

600 m

800 m

1000 m

1500 m

In comparison to theprevious slide, it canbe observed that thefractures with leastinitial stability (highCFS) slip more thanthe ones with highstability

This in case of allfracture sets remainingthe same, is mainlycontrolled by the initialstress field & dip/orientation of thesecondary fractures

Summary

What have we learned so far from

conceptual point of view?


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Discussion

What additional value for an EQ related issue would a shearexperiment of size in the order cm2 yields, when we are dealingwith structures ranging from m2 to km2?

The problem of earthquake is not dictated much by the details of fracture properties, but rather from their orientation in regardsto the prevailing stress field, distance from the fault zone & hypocentre location & wheter located on the footwall or hangingwall side of primary fault.

In the majority of the simulations, the synthetic earthquakes are assumed to generate high stress drops and thus high seismic efficiencies, large fault slip and high moment magnitudes relative to the rupture areas when compared to real earthquakes. Given this, the 50 mm canister failure criterion is exceeded only by way of exception on large planar fractures. If fault slip and resulting earthquake instead is simulated according to database regressions, the secondary fracture slip is minimal.


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Conclusions It is often ignored during fracture characterization projects, such as

POST & KARMO in which the fracture slip is related to seismic loads, that the generated slip on secondary fractures is caused by dynamic loading occurring due to nearby large seismic events. Such events do contain considerable amount of uncertainty such as location- and magnitude of the possible seismic event which are the dominating uncertainties instead of details of fracture specific parameters.

It was already well known before POST / KARMO project that undulating surfaces have increased shear resistance compared to planar.

The uncertainties, associated conservativness and their effect on slipmagnitude of secondary fractures in EQ models are listed below:


1. Realistic earthquake magnitude in repository footprint, given realisticsize estimates of deformation zones in Olkiluoto

2. Less conservative synthetic stress state to be used in the deterministicearthquake models.

3. Apply damping on the EQ – slip assesment models, as common in such an earth science applications.

4. Location of the secondary fracture in relation to the earthquake source

5. Orientation and dip of the secondary fracture in relation to the loading, such as prevailing in situ stress state

6. Strength and deformation properties of the secondary fractureunder scrutiny

Kiitos

Thank you


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References Fälth, B., Hökmark, H. (2010). Effects of large earthquakes on a KBS-3 repository. Evaluation of

modelling results and their implications for layout design. SKB Technical Report TR-08-11. SKB AB, Stockholm, Sweden.

Fälth, B., Hökmark, H. (2015). Effects of Hypothetical Large Earthquakes on Repository Host Rock Fractures. Posiva Working Report 2015-18. Posiva Oy, Eurajoki, Finland.

Leonard, M. (2010). Earthquake fault scaling: Self-consistent relating of rupture length, width, average displacement, and moment release. Bulletin of the Seismological Society of America, 100(5A), 1971-1988.

Lönnqvist M and Hökmark H, 2015. Assessment of method to model slip of isolated, non-planar fractures using 3DEC. ISRM Congress 2015 Proceedings - Int’l Symposium on Rock Mechanics -ISBN: 978-1-926872-25-4.

McGarr, A. (1999). On relating apparent stress to the stress causing earthquake fault slip. Journal of Geophysical Research-Solid Earth, 104(B2), 3003-3011. doi:10.1029/1998jb900083

Stigsson, M. (2015). Parametrisation of Fractures – Methods and Evaluation of Fractal Surfaces. Posiva Working Report 2015-27. Posiva Oy, Eurajoki, Finland.

Scholz, C.H. (2002). The Mechanics of Earthquakes and Faulting, 2nd edition. Cambridge University Press.

Siren, T., Hakala, M., Valli, J., Christiansson, R., Mas Ivars, D., Lam, T., Mattila, J., Suikkanen, J. (2017). Parametrisation of Fractures – Final report. Posiva Report 2017-1. Posiva Oy, Eurajoki, Finland.

Valli, J., Hakala, M. (2016). Parametrisation of Fractures – Model Generation Methodology and Prediction Calculations. Posiva Working Report 2016-32. Posiva Oy, Eurajoki, Finland.

Wells, D. L., Coppersmith, K. J. (1994). New empirical relationships among magnitude, rupture length, rupture width, rupture area, and surface displacement. Bulletin of the seismological Society of America, 84(4), 974-1002.


Documents

KYT - Seminarkyt2018.vtt.fi/rakoseminaari2017/Johannes Suikkanen.pdf · and post closure safety KYT –Seminar 1.12.2017 2017-11-01 Suikkanen Johannes 4. Public Triggering mechanism