intraplate earthquakes and postglacial rebound in eastern...
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Intraplate Earthquakes and Postglacial Reboundin Eastern Canada and Northern Europe
Patrick Wu
Dept. of Geology & Geophysics, University of Calgary,Calgary, Alberta T2N-1N4, Canada,
Keywords: Earthquakes, Faulting, Mantle viscosity, Seismotectonics, Stress distribution
Abstract. The causal relationship between postglacial rebound and intraplate earthquakes inEastern Canada and Northern Europe is investigated with the finite element model. Prominentfeatures of this analysis are the inclusion of: i) a stratified, viscoelastic mantle, ii) a realisticdeglaciation model and iii) the ambient tectonic stress and overburden stress contributions inthe calculation of the total stress field. It is demonstrated that the spatial distribution of currentseismicity in these areas cannot be explained by the strain rate distribution due to rebound. Inorder to explain the observed spatial distribution of earthquakes, the mode of failure and thetiming of the pulse of earthquake/faulting activity following deglaciation, both postglacial re-bound and tectonic stress are required. In addition, the observed orientations of the contempo-rary stress field and the rotation of stress in E. Canada since deglacial times can be explained bya viscoelastic Earth with uniform 1021 Pa-s mantle. The effect of a high viscosity lower mantlehas also been investigated. It is demonstrated that a high viscosity lower mantle will introducezones in E. Canada where earthquake activities will increase in the future. Also any significantrotation in stress orientations since the last deglaciation is prevented. Otherwise, it has no effecton the mode of failure and does not significantly affect the onset time of intraplate earthquakes.
1. Introduction
Large intraplate earthquakes are found in Eastern Canada (Laurentia) and Northern Europe(Fennoscandia). In order to mitigate more effectively the hazards associated with these earthquakesand to plan for safe storage of nuclear toxic-waste in underground repositories, it is vital to understandthe spatio-temporal variation of the state of stress, the fault potential and the cause of such earth-quakes.
A fundamental question in the study of intraplate earthquakes in Laurentia and Fennoscandiais the relative importance of plate tectonics and postglacial rebound (including both glacial loadingand unloading) in earthquake generation. There are geological and geophysical evidence that supportpostglacial rebound as the dominant cause of these intraplate earthquakes but there are other evidencethat favors tectonic stress as the dominant cause. These evidence will be reviewed in the next section.To resolve the issue, Wu & Hasegawa [51, 52] and Wu [49, 50] have used the finite element method tomodel the spatio-temporal distribution of stress and changes in fault stability in E. Canada. The aim ofthis chapter is to review their work and extend the analysis to N. Europe.
Prominent features of the present series of studies [49, 50, 51, 52], which distinguish themfrom previous ones [20, 35, 40, 41, 48] are the inclusion of : (1) a viscoelastic mantle and thus themigration of stress associated with viscoelastic relaxation; (2) a realistic ice sheet deglaciation historyand sawtooth cycles of loading and unloading; (3) glacial/deglacial induced stress, tectonic stress andoverburden pressure contributions in the calculation of the total stress field; and (4) deviatoric stressand mean stress in the computation of fault instability [21, 22].
The plan of the chapter is as follows: After a review of the observations and the evidence foror against rebound stress as the trigger to intraplate earthquakes, we will describe the model moreclosely. Then we shall show that with a uniform viscosity mantle, most of the observations can bepredicted. Finally, we shall explore the effect of a high viscosity lower mantle on the predictions ofthe model before giving the conclusion.
BU
BA
BB
LS
SLV
OBG GB
Hudson Bay
Baffin Island
Canadian Shield
M > 6(1663-1990)
6 > M > 4.5(1980-1990)
4.5 > M > 3.0(1980-1990)Seismotectonic
trendsST
Earthquake location and magnitude
Figure 1a. Spatial distribution of the larger earthquakes and the seismotectonic trends in E. Canada.Symbols are as follows: BU- Bothia Uplift; BA- Bell Arch; BB - Baffin Bay; LS-LabradorSea; GB - Grand Banks; SLV- St. Lawrence Valley; OBG - OttawaBonnechere Graben.Reprinted with permission from [52] © Blackwell Science Ltd.
II. Geological and Geophysical Evidence
Here we wish to review the geological and geophysical evidence on intraplate earthquakes inLaurentia and Fennoscandia. Some of these observables will be calculated and compared with theobservations which act as constraints for our model.
There are two pieces of observational evidence that suggest plate tectonics may be the domi-nant cause of these earthquakes: First of all, the spatial distribution of recent earthquakes in E. Canadaand N. Europe shows little correlation with the center of postglacial rebound - instead, seismic activitymainly lies along three pre-weakened tectonic zones (Fig. 1a) in Eastern Canada [4, 52] while in N.Europe most of the larger earthquakes (magnitude > 4) are distributed along the coastal regions (Fig.1b) while the interior is relatively nonseismic with most magnitudes less than 4 [10, 38, 39, 47].
Figure 1b. Spatial distribution of earthquakes in Fennoscandia from 1965-1995. ( Courtesy ofInstitute of Seismology, University of Helsinki)
The second piece of evidence is from the contemporary stress field whose orientation does notappear to be dominated by the effects of past glaciation - in fact, for most parts of continental E.Canada (east of the Cordillera), the maximum horizontal stress component SHmax is almost uniformlyaligned along an ENE-NE azimuth (Fig.2a) [1, 5, 55, 56] and can be most readily explained by stressesassociated with spreading at the mid-Atlantic Ridge [36]. In Fennoscandia, the first order SHmax
orientation below 300 m depth is along NW-SE, in agreement with the direction of ridge-push fromthe Mid-Atlantic (Fig. 2b), while that in Norway is affected by the Caledonides [11, 16, 42, 43]. Thus,rebound stress has little influence on contemporary stress orientation - again indicating that the domi-nant stress is tectonic.
On the other hand, the good correlation between the onset time of postglacial faults/earth-quakes in E. Canada and Fennoscandia with the end of deglaciation, indicates that postglacial reboundmay have played a much more important role in earthquake generation at early postglacial time. Forexample, the small thrust faults in southeastern Canada offset scour marks left by the glaciers -indicating that their time of formation is postglacial. The dating of some earthquake-triggered mudslumps in S.E. Canada [37] further indicates that earthquake activity was generated at the end of
Figure 2a. Map of Eastern Canada showing: (i) orientation of the contemporary regional stress field(bold inward-pointing arrows) on land and off-shore Labrador. Question mark near thearrows indicates that the orientation is uncertain since only a few observations are avail-able; (ii) orientations of an earlier stress field duduced from postglacial faults (thin inwardpointing arrows); (iii) the location of the ice margin at the last glacial maximum and (iv)the location of some sites in Fig.7 where the evolution of of stress and dFSM are shown.
Great Lakes
Labrador
40N
50N
60N
120W
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0W
60W40W
Current Orientation
SHmax Orientation at early Postglacial time
LAU
RE
NTID
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AR
GIN
AT GLACIAL MAXIMUM
?Mel
ville
P
en.
Ungava Pen.
Cartwright
NW New-foundland
Charlevoix
Indiana
Oslo
Orebro
Hels ink i
Ves tera len
Gal l i va re
Angermanland
55N
60N
70N
65N
40E
30
E
20
E
10
E
0E10
W
40E
20
E
0E
Figure 2b. Map of Fennoscandia showing (i) locations and orientations of some postglacial faults.Paleostress orientations are approximately perpendicular to these postglacial faults; (ii)orientations of the contemporary stress field; (iii) location of some sites in Fig. 7.
deglaciation around 9,000 years ago. The dating of earthquake-induced liquefaction features in theWabash Valley bordering Indiana and Illinois (USA) also gives a postglacial time of 8 to 1 ka BP [30].In N. Europe, postglacial faults (Fig.2) were also formed between late glacial to early postglacialtimes 8,000 to 9,000 years ago [6, 26] although their displacements are much larger - averaging about15 meters and they may have fractured the whole crust [6].
All of the postglacial faults in E. Canada and N. Europe are thrust faults and are consistentwith the fault mechanism predicted by postglacial rebound. Further suggestion of a possible link topostglacial rebound is from the orientations of the postglacial thrust faults in southeastern Canada [3]which indicate that the maximum horizontal principal stress (SHmax) orientation of the paleo-stressfield was mainly in the NW-SE direction (Fig.2a) - perpendicular to the ice margin at glacial maxi-mum and consistent with the direction of ice retreat [1]. Note that this orientation of the paleostressfield is almost perpendicular to the SHmax orientation of the contemporary stress field indicating thatlarge stress rotation occurred during postglacial time. In Fennoscandia, the postglacial faults are gen-erally NNE-SSW trending [6, 24, 25, 27, 28] and are also subparallel to the ice margin at glacialmaximum. However the orientation of the paleostress field implied by these postglacial faults is alsosubparallel to the tectonic stress caused by spreading of the North Atlantic Ridge. Thus, it is not clear
if paleostress orientation in N. Europe was determined by postglacial rebound or by plate tectonics. Furthermore, Ekman [12,13] found that there is a high spatial correlation between
microseismicity, boulder caves and the maximum curvature of uplift. Basham et al. [8] and Hasegawa& Basham [17] also found that the steep gradients in postglacial rebound contours in Baffin Islandcorrelate well with bands of intense seismic activity. However, earthquake clusters are not found in allplaces with steep gradients in postglacial uplift.
Finally, the focal mechanisms of the larger earthquakes in E. Canada today are dominantly ofthe thrust type and are consistent with that predicted by postglacial rebound. However, there are someexceptions along the northeast coast of Baffin Island, where focal mechanisms are of the normal-faulttype, and in the northeastern United States, where strike-slip faulting [18, 19, 44, 55] are interspersedwith thrust faulting [19, 29]. In N. Europe, some of the larger recent events show thrust faulting [47],however, the mode of failure of the majority of smaller earthquakes are a mixture of strike-slip, thrustand normal faulting [6, 39, 47], with strike-slip motion being consistent with the notion that tectonicstress is dominant.
To resolve the issue of which is the dominant cause of intraplate earthquakes in E. Canada andN. Europe, it is proposed here that both tectonic and rebound stress are required: past tectonic proc-esses are responsible for creating the pre-weakened tectonic zones and the ridge-push forces at theMid-Atlantic bring the faults close to failure. On the other hand, the stress induced by glacial unload-ing, although not large enough to dictate the location of new faults [35], can reactivate those pre-existing faults that have optimal orientations. However, the stress due to glacial unloading graduallydiminish with time, so that at present, tectonic stress difference is large enough to dominate the stressorientations. However, this does not mean that rebound stress cannot trigger intraplate earthquakestoday. In fact, rebound stress is still responsible for most of the seismicity in E. Canada and some ofthe activities in N. Europe today.
III. The Model
Due to the loading and unloading of the earth (by ice sheets and melted ice-water loads) andthe relaxation of stress associated with the creep of mantle rocks, the state of stress inside the earthconstantly changes even if it is assumed that tectonic stress and all other stresses remain constant.(This latter assumption is valid for time scales of a few glacial cycles.) As the state of stress changes,the Mohr circle moves closer to and/or away from the line of failure according to a time-dependentquantity called dFSM [Wu & Hasegawa 1996] which is defined by:
dFSM (t ) = 12 σσ1(to) – σσ3(to) – σσ1(t ) – σσ3(t )
+ µµ ββ σσ1(t ) + σσ3(t ) – σσ1(to) + σσ3(to) (1)
where
ββ = sin arctan µµ / 2 µµ (2)
and µ is the coefficient of friction taken to be 0.6; t is the time under consideration; t0 is the time beforethe onset of glaciation; σ1, σ2 and σ3 are the maximum, intermediate and minimum (compressive)principal stress respectively. The physical meaning of dFSM is the change in time of the Fault Stabil-ity Margin (FSM) [21, 22], which is the shortest distance between the Mohr circle and the line offailure (see Fig.3a). Equation (1) therefore states that FSM will decrease if the increase in deviatoric
stress, which increases the radius of the Mohr circle and makes the Mohr circle closer to failure (e.g.Fig.3c), is able to dominate over the increase in mean stress, which moves the center of the Mohrcircle away from the line of failure (e.g. Fig.3b). Therefore, if the pre-existing faults are initially closeto but not at failure, then, a negative value of dFSM would mean enhanced likelihood of faultingwhereas a positive value of dFSM indicates that fault stability is enhanced.
The Mode of Failure depends on which of the principal stresses is closest to the vertical (seeFig.3a). If σ1 is nearly vertical, then the mode of failure is normal; if σ3 is close to the vertical, then
b) Fault Stability Margin increases at glacial maximum
τ
τ = τ + µ σ0
n FSM'
τ0
σ1σ3 σ'1σ'3 σn
c) Fault Stability Margin decreases after deglaciation
a) Fault Stability Margin & Mode of Faulting:
FSMτ
τ = τ + µ σ0
n
τ0
σnFault Type:Normal: Thrust:S t r i k e - S l i p :
v e r th o r i zh o r i z
h o r i zv e r th o r i z
h o r i zh o r i zv e r t
σ1σ3 σ2 (σ + σ ) / 21 3
r = (σ − σ ) / 21 3
2θ
τ
τ = τ + µ σ0
n FSM'
τ0
σ σ'3 3σ' σnσ11
Figure 3. a) Mohr circle, line of Failure, definition of Fault Stability Margin and the mode of failure.b) During glacial loading,the increase in mean compressive stress moves the Mohr circleaway from failure. c)After removal of the ice, the minimum (vertical) principal stress returnto its initial value, but the maximum (horizontal) principal stress relaxes with time. Theincrease in deviatoric stress and thus the radius of the Mohr circle, moves it closer to failure.Reprinted with permission from [52] © Blackwell Science Ltd.
thrust faulting occurs; On the other hand, if σ2 is nearly vertical, then strike-slip motion occurs .For simplicity, the total stress is assumed to be composed of the rebound stress (due to loading
and unloading of the ice sheets), tectonic stress and overburden pressure - the last two can be taken astime-invariant in the time period under consideration. Although fluctuation of pore fluids do affectfault stability, their time dependence is more difficult to model and thus will be neglected in thispreliminary study.
The orientation of the tectonic maximum horizontal principal stress is assumed to be deter-mined by ridge-push at the mid-Atlantic Ridge. Thus, it is taken to be along the N60oE direction inNorth America [3, 5, 55, 56], and along the N60oW direction in Northern Europe [11, 42, 43]. Al-though the magnitude of the maximum and minimum horizontal tectonic stresses are largely un-known, Wu & Hasegawa [51, 52] have shown that tectonic stress magnitudes have little effect ondFSM. Furthermore, stress orientation only depends on horizontal stress differences alone [50]. In thischapter, the maximum horizontal tectonic stress is taken to be 150 MPa - this is much larger than the50 MPa assumed in Wu [50] and Wu & Hasegawa [52] and give stress levels closer to that observedin Europe [54]. For the minimum horizontal tectonic stress, a range of values will be considered.
Rebound stress is calculated with the finite element method. The earth model considered hereis a compressible, stratified flat earth that consists of an elastic lithosphere on top of isotropic, viscoelastic
Figure 4. Elastic structure and viscosity profiles of the stratified earth models L1, L2 & L3. Thelithosphere is 100 km thick under N. America but 80 km thick under N. Europe.
0
5000
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15000
ρ (k
g/m
^3)
, V
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), V
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)
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B) Viscosity Models
Depth (km)
Maxwell layers in the mantle that, in turn, overlie an inviscid fluid core. The elastic structure andviscosity profiles of the earth models considered are shown in Fig.4. Model L1 has a uniform 1x1021
Pa-s mantle. Models L2 is different from L1 only in the lower mantle (below 670 km depth), wherethe viscosity is 1x1022 Pa-s. For both earth models, the lithosphere is assumed to be 100 km thickbeneath N. America, but 80 km beneath N. Europe. However, it can be demonstrated [50] that theconclusions of this paper is not affected by the thickness of the lithosphere.
The initial state of the Earth is assumed to be deglaciated and without any bending stresses -only tectonic stress and overburden pressure exist in the initial state. Here, loading and unloading ofthe Laurentian, Cordillera, Innuition, Greenland and Fennoscandian Ice Sheets plus eustatic loading/unloading in the ocean floor are included (see [52] for details). The ice model is adopted from theICE3G model of Tushingham & Peltier [45, 46]. The adopted ice thickness variation in North Americaand Greenland is given in an earlier chapter by Wu & Johnston [53], and its thickness in Fennoscandiais contoured in Fig.5. Thirty Glacial cycles are included before the final deglaciation which beganaround 19,000 BP. For simplicity, the glacial cycle are assumed to have a saw-tooth shape, with slow
1400
1400
200
200
1000
1200
t = 12 KaBPt = 18 KaBP
300
600
300
1500
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2100
2100
900
t = 10 KaBP
200800
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t = 8 KaBP
Figure 5. Adopted version of ICE3G deglaciation history in Northern Europe . Thickness of ice iscontoured in meters.
buildup time of 90,000 years but a rapid deglacial time of 10,000 years.
IV. Results
In the following, all results of fault stability and related calculations are shown at a representativedepth of 12.5 km depth. However, all stress orientation and stress rotation calculations are for theEarth’s surface - otherwise the large overburden pressure will make SHmax indistinguishable fromShmin. It can be shown that the conclusions of this paper are not affected by depth in the top 40 km[51].
IV.a Model L1 - uniform viscosity mantleFirst consider the earth model that has a uniform 1x1021 Pa-s mantle. Figure 6 shows the
spatio-temporal evolution of dFSM in Laurentia and Fennoscandia predicted at a depth of 12.5 km.Inspection of Fig.6 shows that maximum fault stability of 14 MPa in Laurentia and 8 MPa inFennoscandia are promoted underneath the load and in the surrounding areas at glacial maximumaround 18 ka BP (thousand years before present). Fault stability is promoted inside the ice marginbecause the increase in the mean stress due to the weight of the ice has a more dominant effect thanthat due to the increase in the deviatoric stress (see Equation 1).
At 9 ka BP, stability is still promoted underneath the existing ice and around the peripheralbulge in Laurentia while instability is promoted in deglaciated areas. Instability is promoted indeglaciated areas because the horizontal flexural (rebound) stress, which adds to the maximum princi-pal stress, remains large at 9 ka BP. However, the minimum principal stress, which is nearly vertical,returns to value in the initial state since the ice load is now removed. Thus the increase in radius of theMohr circle (relative to the initial state) has a more dominant effect than the increase in mean stress -thus moving the Mohr circle closer to the line of failure. On the other hand, stability is promoted inareas around the peripheral bulge despite of the absence of ice loads there. This is because reboundstress, which is tensional at 9 ka BP around the bulge, causes the (compressive) maximum principalstress to decrease while the minimum principal stress, which is nearly vertical, remains the same asthe initial state. Thus, the radius of the Mohr circle decreases (relative to the initial state), and faultstability increases.
The situation is different in N. Europe: at 9 ka BP, when about 300-500 meters of ice stillcovered the center of Fennoscandia, instability of about 2 MPa is promoted underneath the ice, whilestability is promoted in the surrounding peripheral bulge. This early promotion of instability beforecomplete deglaciation is due to the amplification of stress as the wavelength of the load approachesthe thickness of the lithosphere [23] and the characteristic length of the Fennoscandia load is closer tothe thickness of the lithosphere than the one in Laurentia.
For the present (0 ka BP), fault instability of about 1 MPa is promoted in the center of reboundin Eastern Canada and northern Europe. This magnitude of fault instability is probably too low tocause fracture, but is large enough to reactivate optimally oriented pre-existing faults. (It is larger thanthe stress level that has triggered the Landers earthquake [15]). Therefore, to explain the observedspatial distribution of earthquakes (Fig. 1), one needs to assume that the initial Fault Stability Marginis everywhere greater than 1 MPa, except in the pre-weakened tectonic zones of E. Canada and thecoastal areas in Fennoscandia, where the initial value of FSM is close to zero.
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(e)
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- 1 . 0- 1 . 0
dFSM at 0 ka BP
( f )
Figure 6. Spatio-temporal variation of dFSM in N. America and Northern Europe at three time peri-ods at a seismogenic depth of 12.5 km depth. Model L1 is used. Contours are in MPa.
2
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The evolution of dFSM, the total stress (expressed in terms of horizontal principal stressesHmax, Hmin and the vertical principal stress PrinZ) and the mode of failure at several sites in NorthAmerica and Northern Europe are shown in Fig.7. Inspection of Fig.7a shows that for the sites alongthe Boothia Uplift-Bell Arch (Melville Pen. and Ungava Pen.) and the St. Lawrence Valley-OttawaBonnechere Graben (Charlevoix), dFSM becomes negative around 7-9 ka BP and reaches a minimumvalue around 7-4 ka BP. The mode of failure is predicted to be thrusting since the least principal stressis nearly vertical. This is consistent with the observation that postglacial faults in southeastern Canadaare of the thrust type [2]. In Newfoundland, where melting is early, instability is predicted to havebegun much earlier at around 13 ka BP - however, due to the loading of the nearby ocean floor by a
Figure 7a. Evolution of the horizontal principal stress (Hmax, Hmin), the vertical principal stress(PrinZ), dFSM and the mode of failure for six sites in Fig.2a - all at a seismogenic depthof 12.5 km. The Earth model L1 is used in this calculation.
4 0 0
4 5 0
5 0 0
5 5 0
6 0 0
NW NewFoundland
0
5
- 1 5 - 1 0 - 5 0
Time (ka BP)
- 505
1 0
- 1 5 - 1 0 - 5 0
Time (ka BP)
4 0 0
4 5 0
5 0 0
5 5 0
6 0 0
Indiana
- 2- 1
01
- 50
51 0
dFSM
4 0 0
4 5 0
5 0 0
5 5 0
6 0 0
Ungava Pen.
4 0 0
4 5 0
5 0 0
5 5 0
6 0 0
Melville Pen.
HmaxHmin
PrinZ
- 505
1 0
dFSM Thrust
Hmax
Hmin
PrinZ
Thrust
4 0 0
4 5 0
5 0 0
5 5 0
6 0 0
&
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tres
s (M
Pa)
Labrador Seanear Cartwright
Hmax
Hmin
PrinZ
- 1012
dF
SM
dFSM Thrust
HmaxHmin
PrinZ
Thrust
dFSM
4 0 0
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Charlevoix, Que.
HmaxHmin
PrinZ
dFSM Thrust
Hmax
Hmin
PrinZ
dFSM Thrust
3 0 0
3 5 0
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Orebro, S. Sweden
0
5
- 1 5 - 1 0 - 5 0
Time (ka BP)
- 505
1 0
- 1 5 - 1 0 - 5 0
Time (ka BP)
3 0 0
3 5 0
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5 5 0
He ls ink iFinnland
3 0 0
3 5 0
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AngermanlandSweden
- 6- 3
036
- 50
51 0dFSM
3 0 0
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5 5 0
Oslo, Norway
Hmax
Hmin
PrinZ
- 505
1 0dFSM
Hmax
Hmin
PrinZ
Thrust
3 0 0
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Ves te ra lenNorway
Hmax
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PrinZ
- 20246
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SM
dFSM Thrust
Hmax
Hmin
PrinZ
ThrustdFSM
3 0 0
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Ga l l i va reSweden
HmaxHmin
PrinZ
dFSM Thrust
Hmax
Hmin
PrinZ
dFSM Thrust
Thrust
Figure 7b. Same as Fig.7a except for six sites in Fig.2b.
rapid influx of melted ice water, instability is suppressed temporarily from 10-7 ka BP. The loading ofthe nearby ocean floor also causes delay in the onset of instability along the Labrador coast (e.g.Cartwright). In areas beyond the ice margin in the northern United States, seismic and faulting activ-ity is predicted to have occurred later at around 8 ka BP. The predicted onset time of instability inIndiana also coincides with the observed time of a very large (MW7.5) Wabash Valley earthquakediscovered and dated by paleo-liquefaction research [30]. The mode of failure is again predicted to bethrust faulting. Examples of pure thrust-fault earthquakes are the MW5.4 southern Illinois earthquakeof 1968 [19] and the mb5.1 Goodnow earthquake in New York State [29].
For Fennoscandia, the early promotion of instability due to stress amplification results in an
onset time around 11-9 ka BP and maximum instability is reached around 10-7 ka BP. Thus, thepredicted onset time of Gallivare coincides with the observed time of formation of the nearby postglacialfaults [6, 26]. Again the mode of failure is predicted to be thrusting and this is precisely what isobserved for the postglacial faults. However, the mode of failure of current earthquakes is not re-stricted to thrusting [6, 7] - indicating that tectonic stress may have played a comparatively moresignificant role today than at postglacial times (due to the relaxation of the rebound stress). Inspectingthe magnitudes of instability, it can be seen that in Angermanland and Oslo, dFSM were as low as -4MPa and in Gallivare -3 MPa was also achieved during early postglacial time. These predicted mag-nitude are at least twice as large as those in Laurentia during postglacial time. If these values of faultinstability is an indicator of the magnitude of rebound stress available to trigger earthquakes, then thislarger magnitude of dFSM may result in a larger throw of the postglacial faults in Fennoscandiaprovided that rock friction for fault reactivation in Laurentia is comparable if not higher than that inFennoscandia. This may partly account for the much smaller throw seen in Laurentia.
Figure 7 also shows that the compressive stress within the ice margin generally decreases inamplitude from glacial maximum to the present (e.g. Melville Pen., Ungava Pen., Charlevoix, Oslo,Orebro, Angermanland, Gallivare and Helsinki). However, for sites near the ice margin (Cartwright,Newfoundland and Vesteralen), the compressive horizontal stress increases slightly from around 9 kaBP to the present. This is due to the inward migration of the peripheral bulge characteristics of deepflow models.
Fig.8 shows the change in orientation and magnitude of the horizontal principal stress from 9ka BP to the present if tectonic stress difference (SHmax-Shmin) is 5 MPa. At 9 ka BP, the averageorientation of the maximum principal stress (thick lines in Fig.8a) in E. Canada is along the ENEdirection - however stress orientations change by about 30 degrees as one goes from the Great Lakesto northern Quebec and Labrador. Note that for the land grid J8 and in some ocean grids along theBaffin Bay-Labrador Sea, stress orientation is mainly in the NW-SE direction. This is because re-bound stress difference is generally smaller than the tectonic stress difference (5 MPa) except at theland grid J8 and at the ocean grids along the Baffin Bay-Labrador Sea, therefore rebound stress is ableto dominate the stress orientations in these special places. However, by the time at 0 ka BP (Fig.8c),rebound stress difference in all the land grids (including grid J8) have decayed below the 5 MPa level- so that all orientations became dominated by the tectonic stress and uniform stress orientations(within 10 degrees of ENE) are predicted on land. For the ocean grids along the Baffin Bay-LabradorSea, rebound stress difference has also decayed - but not enough for the tectonic stress to be dominant,thus the orientations offshore remain NW-SE to E-W.
It should be noted that the predicted orientation in grid J8 at 9 ka BP is close to the overallstress orientations inferred from the postglacial faults in southeastern Canada, except that the locationof J8 is too far to the north. There are two reasons for this discrepancy: First of all, stress orientationswithin the ice margin are strongly perturbed by the presence of local ice domes which are not resolvedby the spatial resolution of the ICE3G model. Secondly, the dimensions of the finite element grid,which is limited by the spatial resolution of the ice model, result in a coarse grid and a reduction of themagnitude of the rebound stresses near the ice margin (due to spatial averaging in this area of highstress gradient). Therefore, to explain the stress orientations at 9 ka BP, an ice model with finer spatialresolution is required. Due to this limitation, we will not compare the predicted and the observedpaleostress orientations on land, but we will continue to examin if the earth model can explain theobserved rotation of stress orientations in southeastern Canada during the last 9,000 years.
Time = 0 KBP Max. Stress Magnitude = 145.7 MPa
60N
65N
40E
30E
20E
10E
0E
1
2
3
4
5
6
7
8
9
1 0
1 1
1 2
60N
0E10
W
40E20E
0E
Time = 9 KBP Max Stress Magnitude = 157.7 MPa
1
2
3
4
5
6
7
8
9
1 0
1 1
1 2
A B C D E F G H I J K L M N A B C D E F G H I J K L
12
0W
40N
12
0W 6
0W
60N
TIME = 9 ka BP
Max Stress = 190.8 MPa
1
2
3
4
5
6
7
8
9
1 0
1 1
1 2
12
0W
40N
60N
10
0W
80
WTIME = 0 ka BP
Max Stress = 186.8 MPa
1
2
3
4
5
6
7
8
9
1 0
1 1
1 2
Figure 8. Orientation and magnitude of the projected horizontal principal stresses 9000 years ago(a,b) and at present (c,d). This is calculated for model L1 where the tectonic stress has adifferential magnitude of 5 MPa.
(a)
(b)
(c)
(d)
On the other hand, the predicted stress orientations offshore the Canadian east coast do notsuffer from the lack of spatial resolution of the load because the spatial variation of the ocean load ismuch more gradual. Comparing the stress orientations predicted offshore at 9 ka BP with the currentobserved orientations (Fig.2a) shows remarkable agreement between them, suggesting that the anoma-lous stress orientations offshore may be caused by postglacial rebound. However, the predictedorientations offshore at the present (0 ka BP) do not show such good agreement with the observationstoday, indicating that the viscosity in the lower mantle may not be high enough. This disagreementmay also be due to the fact that stress orientations offshore are not well determined [9]. For thisreason, stress orientations offshore will not be considered any further.
In Northern Europe, similar rotations in stress orientations are also predicted during the last9,000 years. At 9 ka BP (Fig. 8b), the orientations of the maximum principal stress is predicted to benon-uniform in Fennoscandia: ranging from E-W and WNW in northern Sweden to NW near thecentre of rebound and again to E-W in northern Germany and then back to NW far from the southernedge of the ice sheet. However, at the present, stress orientations are more uniform and are predictedto lie within 10 degrees of the WNW direction (Fig. 8d). The areas onland that experience most
significant stress rotation are predicted to be the area northeast of Leningrad and in southwesternNorway. The amount of rotation is smallest near the center of rebound around Angermanland, thus,the paleostress orientations inferred from the postglacial faults near Gallivare (Fig.2b) is consistentwith the predicted at 9 ka BP and at the present [11, 42, 43]. The reason is because SHmax due to ridgepush at the North Atlantic is also aligned approximately in the same (NW) direction.
It should be noted that Fig. 8 is computed by assuming a 5 MPa tectonic stress difference. Ingeneral, stress orientations depend on the relative magnitude between the horizontal tectonic stressdifference (SHmax-Shmin) and the rebound stress difference [50]. However, with given ice and earthmodels, the relaxation of rebound stress difference is determined. But, tectonic stress difference ispoorly known, thus a range of values will be used below to study its effect on the stress orientations atdifferent sites in E. Canada and N. Europe. The location of the sites are given by the grid names inFig.8.
Figs. 9a & b show that, in North America, with (SHmax-Shmin) in the range 4-10 MPa, thepredicted stress orientations on land are non-uniform at 9 ka BP but rather uniform (within 15 de-grees) at the present. For the site J8, stress orientation rotates about 90 degrees during the last 9000years if the tectonic stress difference is 4 MPa. However, if the tectonic stress difference is larger than10 MPa, then the temporal stress rotations are smaller (<10 degrees) because the orientation of thetotal stress field becomes dominated by the static tectonic stress. On the other hand, for tectonic stressdifference less than about 4 MPa, large stress rotations will occur, but the predicted stress orientationson land today will be non-uniform - with stress orientation in Labrador (e.g. grid L6) differing fromthat in the Great Lakes (e.g. grid H11) by more than 50 degrees if (SHmax-Shmin) is less than 2 MPa.Thus, in order to explain the contemporary uniform stress orientations and the large stress rotationssince postglacial times, tectonic stress must be in the range 4-10 MPa. These results are in closeagreement with the findings of Adams & Bell [5].
The effect of tectonic stress difference on stress orientations in northern Europe is shown inFigs.9 c & d. Inspection of Fig.9d shows that when the tectonic stress difference is greater than about5 MPa, then it determines the contemporary stress orientations and a uniform stress field is obtained at0 ka BP. However, for tectonic stress difference less than about 4 MPa, then rebound stress will beable to dominate the contemporary orientations resulting in a non-uniform stress field. At 9 ka BP(Fig.9c), the transition between uniform and non-uniform stress orientations occurs around 15 MPa.Thus, if stress orientations had been uniform throughout the last 9,000 years, then the tectonic stressdifference must be at least 15 MPa. However, if the paleostress orientations were non-uniform butbecame uniform at the present, then tectonic stress difference must be about 5-15 MPa. Since we haveno information on whether the paleostress field were uniform or non-uniform, the effect of tectonicstress difference in N. Europe will not be pursued any further.
To conclude this section,let us consider the predicted strain rates over E. Canada and N. Eu-rope since strain rates are also associated with earthquakes [20]. Of the six components of strain, weshall only consider the vertical components: namely vertical strain rate εzz and vertical shear strainrate εzh = εzx + εzy. The former is related to vertical uplift velocity due to the postglacial reboundprocess and the latter is related to the shear stress. The spatial distribution of these strain rates at thepresent time are contoured in Fig.10. Inspection of Figs.10a & c shows that vertical strain rates peaknear the center of rebound where vertical velocity is the highest. The estimated peak value of -7 x
10–17 (1/sec) in E. Canada( or -2.2 x 10–9 /year) is close to what James & Bent [20] found. Inspectionof Figs.10b & d shows that vertical shear strain rate has peaks and troughs closer to the ice margin at
0
2 0
4 0
6 0
8 0
1 0 0
1 2 0
0 1 0 2 0 3 0
b) t = 0 KBP
L6
J 8
J 1 0
H11
For sites in E. Canada
0
2 0
4 0
6 0
8 0
1 0 0
1 2 0O
rie
nta
tio
n
of
Ma
x
Pri
nc
ipa
l S
tre
ss
(de
gre
es
a
nti
clo
ck
wis
e
fro
m
Ea
st)
0 1 0 2 0 3 0
a) t = 9 KBP
- 2 0
0
2 0
4 0
6 0
8 0
1 0 0
1 2 0
Ori
en
tati
on
o
fM
ax
P
rin
cip
al
Str
es
s
(de
gre
es
c
loc
kw
ise
fr
om
E
as
t)
0 5 1 0 1 5 2 0
Difference between HorizontalTectonic Principal Stress (MPa)
c) t = 9 KBP
0
2 0
4 0
6 0
8 0
1 0 0
1 2 0
0 5 1 0 1 5 2 0
d) t = 0 KBP
H7
G10
G3
C6
For sites in N. Europe
Figure 9. The effect of tectonic stress difference on the orientation of SHmax is shown for 4 locationsin North America(a, b) and 4 others in Northern Europe (c,d). Results are computed withModel L1.
the last glacial maxima. It is of interest to note that in E. Canada, large earthquakes are found in thetrough of shear strain-rate along the Baffin Bay-Labrador coast (Figs. 10b & 1a). Clusters of largeearthquakes are also found near the peaks in Southampton Island (north of Hudson Bay) and themouth of the St. Lawrence river (Figs. 10b & 1a). In N. Europe, clusters of earthquakes are also foundin the peak of shear strain-rate in southern Norway around Oslo and Berger (Figs. 10d & 1b). Thesame can be said about the peak in the northwest coast of Norway around Vesteralen. However, otherpeaks in Fig.10b & d are not associated with any clusters of earthquakes and there are clusters ofearthquakes in Fig.1a & b which do not correspond to any peak or trough in Fig.10b & d. Thus, the
2
-1-2-1 -2
-2
-3-1 -1
-3
-2
0
0
1
2
11
0
1
0
0
-1
0
0
1
60N
4 0 N
60
W
4 0 N
60N
12
0W
B) Vertical Shear Strain Rate at t=0 ka BP ( 1/sec) x 1E17
Figure 10. Spatial distribution of current strain rates predicted over E. Canada and N. Europe. Vertical
strain rates ( εzz ) with units of [1/sec] is multiplied by 1017 before being plotted in (a) and
(c). Vertical shear strain rates ( εzh ) with units of [1/sec] is multiplied by 1017 before beingplotted in (b) and (d).
correlation between the location of some peaks or troughs in vertical shear strain rate maps and thespatial distrubution of current earthquakes is likely to be purely fortuitous indicating that intraplateearthquakes involves more than just vertical shear strain rates. Since we have shown earlier in thissection that the spatial distribution of current seismicity can be explained by taking the initial FaultStability Margin to be greater than 1 MPa everywhere except at the pre-weakened zones in E. Canadaand coastal Fennoscandia, where it is set at zero, strain rates will not be discussed any further.
In summary, the observed spatial distribution and mode of failure of earthquakes, their onsettime, the observed current stress orientations in Eastern Canada and Northern Europe and the rotationof stress observed in E. Canada can all be explained by this earth model. The only exceptions are somemodes of failure in the northeast coast of Baffin Island and several sites in eastern United States and
3
3
3
3
-3
-3
-3
-3
-9
-9 -9
0
-6
0
0
00
0
-6
-6
-6
C) Vertical Strain Rate at t=0 ka BP(1/sec) x 1E17
0
0
-2
-2
-2
2 0
00 2
-2-4
-2
-4
4
2
-2
0
0
4
2
0
204
66
D) Vertical Shear Strain Rate at t=0 ka BP(1/sec) x 1E17
A) Vertical Strain Rate at t=0 ka BP (1/sec) x 1E17
111
-4
-2
-4-6-6
-7
-4
-2
-10
1
-1
-2 0
-6
-2
-3
-5
-1
0
-2
-4
-3
0
-1
-1
-2
-3 -4-5
-5
-5
-6
-7
-7
60N
4 0 N
60
W
4 0 N
60N
12
0W
Figure 11. Similar to Fig.6 except for a high viscosity lower mantle (model L2) at two time steps.
8 12
246-2
100
1
0
2
0
-1
-3 -2
1
0
-1-2
-3
-5
-5-4
-4
-3-2
-1
1 0-1 -2
-3
-4
60N
4 0 N
60
W
4 0 N
60N
12
0W
dFSM at 0 ka BP
18
10
6 4
2
2
42
8
0
2
4
4
4
46
6
6
6
66
6
0
2
4
8
8
8
8
8
8
8
10
10
10
10
12
12
12
12
14
14
14
16
1616
60N
4 0 N
60
W
4 0 N
60N
12
0W
dFSM at 18 ka BP
Fennoscandia, where local tectonic stress may have played a slightly more dominant role than thecontemporary rebound stress.
IV.b. Effect of High viscosity Lower MantleIn the last subsection, we saw that changes in the magnitude of the rebound stress trigger
postglacial faulting/earthquake and cause rotation of stress orientations. However, the rate of relaxa-tion of rebound stress is determined by the viscosity structure of the mantle, thus, it is important toinvestigate how the results of the last section will be affected if the earth has a different viscosityprofile. This is particularly important because recent investigations of postglacial sea-levels and Earthrotation point to an increase in viscosity across 670 km and 1200 km depth in the lower mantle [14,31, 32, 33, 34]. Furthermore, Spada et al. [40] claimed that only a high viscosity lower mantle is ableto trigger earthquakes in E. Canada and N. Europe. However, in their treatment of fault potential, onlyrebound stress difference was considered, and the effects of mean stress and tectonic stress have beenneglected (see equation 1). In this section, dFSM as defined in equation (1) will be used to study the
1
67
4
5
2
2
3
345
6
6
7
6
53
2
1
0
1
0
0
1
1
2
3
4
67
7
6
8
7 8
8
9
dFSM at 18 ka BP
- 1 . 0
- 0 . 5- 1 . 5
0 .0
- 0 . 5
- 2 . 5
- 3 . 0
- 3 . 5
- 3 . 0
- 2 . 0
- 1 . 0
- 1 . 5
- 1 . 0
- 0 . 50 .0
0 .5
0 .0
- 1 . 0
- 2 . 0
- 2 . 5
- 2 . 0
- 3 . 5
- 3 . 0
- 4 . 0
- 4 . 0
dFSM at 0 ka BP
0
5
- 1 5 - 1 0 - 5 0
Time (ka BP)
- 505
1 0
- 1 5 - 1 0 - 5 0
Time (ka BP)
4 0 0
4 5 0
5 0 0
5 5 0
6 0 0
Indiana
4 0 0
4 5 0
5 0 0
5 5 0
6 0 0
NW NewFoundland
0
1
2
- 50
51 0
dFSM
4 0 0
4 5 0
5 0 0
5 5 0
6 0 0
Ungava Pen.
4 0 0
4 5 0
5 0 0
5 5 0
6 0 0
Melville Pen.
HmaxHmin
PrinZ
- 505
1 0
dFSM Thrust
HmaxHmin
PrinZ
Thrust
4 0 0
4 5 0
5 0 0
5 5 0
6 0 0
&
S
tres
s (M
Pa)
Labrador Seanear Cartwright
Hmax
Hmin
PrinZ
- 1012
dF
SM
dFSM Thrust
HmaxHmin
PrinZ
Th
rust
dFSM
4 0 0
4 5 0
5 0 0
5 5 0
6 0 0
Charlevoix, Que.
Hmax
Hmin
PrinZ
dFSM Thrust
Hmax
Hmin
PrinZ
dFSM Thrust
Figure 12a. Similar to Fig.7a except for a high viscosity lower mantle (model L2).
effects of a high viscosity (1022 Pa-s) lower mantle on the onset time of instability, mode of failure,magnitude of instability, stress orientations and rotation.
Fig.11 shows that a 1022 Pa-s lower mantle gives larger range of values for dFSM - with higherpeaks but lower troughs both in E. Canada and N. Europe when compared with Fig.6. In particular, themagnitude of the troughs at 0 ka BP go from -1 MPa to about -4 MPa in both Laurentia and Fennoscandia.Fig.12 shows more clearly the temporal variation of dFSM, the stresses and the mode of failure.Comparing Fig.12 with Fig.7 shows that the mode of failure remains the same, while the increase inthe magnitude of dFSM generally results in an insignificant shift in the onset time of instability. Thelargest change can be found for sites such as Newfoundland where the small changes in the values of
dFSM result in a much delayed onset of fault instability. This suggest that accurate determination ofthe timing of earthquakes near the ice margin may be more useful for constraining mantle viscositiesthan for sites near the center of rebound.
Fig.12b also shows that, starting from the last deglaciation, maximum instability in Fennoscandiais attained around 8 ka BP with a value of about -6 MPa. However, for Laurentia, fault instability hasbeen increasing since deglaciation, reaching the value of -3 MPa at present - although the rate ofincrease is much slower now. The difference in maximum instability attained between Laurentia andFennoscandia again implies that larger rebound stress is available to reactivate the faults and, as dis-cussed in the last section, one can argue for larger throw for the postglacial faults in Fennoscandia
3 0 0
3 5 0
4 0 0
4 5 0
5 0 0
5 5 0
Orebro, S. Sweden
0
5
- 1 5 - 1 0 - 5 0
Time from the present (KBP)
- 505
1 0
- 1 5 - 1 0 - 5 0
Time from the present (KBP)
3 0 0
3 5 0
4 0 0
4 5 0
5 0 0
5 5 0
He ls ink iFinnland
3 0 0
3 5 0
4 0 0
4 5 0
5 0 0
5 5 0
AngermanlandSweden
- 5
0
5
- 50
51 0dFSM
3 0 0
3 5 0
4 0 0
4 5 0
5 0 0
5 5 0
Oslo, Norway
Hmax
Hmin
PrinZ
- 4048 dFSM
Hmax
Hmin
PrinZ
Thrust
3 0 0
3 5 0
4 0 0
4 5 0
5 0 0
5 5 0
&
S
tres
s (M
Pa)
Ves te ra lenNorway
Hmax
Hmin
PrinZ
- 20246
dF
SM
dFSM Thrust
Hmax
PrinZ
Thrust
dFSM
3 0 0
3 5 0
4 0 0
4 5 0
5 0 0
5 5 0
Ga l l i va reSweden
Hmax
Hmin
PrinZ
dFSM Thrust
Hmax
Hmin
PrinZ
dFSM Thrust
Thrust
Figure 12b. Similar to Fig.7b except for a high viscosity lower mantle (model L2).
0.06
0.00
0.18
0.180.12
0.06
- 0 . 0 6
- 0 . 0 6 0.00
0.00
0.00
0.06 0.12
0.18
0.12 0.24
0.24
- 0 . 0 9
- 0 . 0 6
- 0 . 0 0- 0 . 0 3
- 0 . 0 0
- 0 . 0 3
0.09
0.06
0.06
0.03
- 0 . 0 0
- 0 . 0 3
- 0 . 0 9
- 0 . 0 6
- 0 . 0 6
- 0 . 0 9
- 0 . 1 2
- 0 . 0 6
- 0 . 0 9
- 0 . 0 3
- 0 . 0 9
- 0 . 0 0
- 0 . 0 3- 0 . 0 6
- 0 . 0 9
- 0 . 1 2
0.03
- 0 . 0 6- 0 . 0 0
0.09
0.06
6 0 N
4 0 N
60
W
4 0 N
6 0 N
12
0W
Figure 13. Spatial distribution of the present rate of change for dFSM in Laurentia and Fennoscandiafor Model L2 at a depth of 12.5 km. Contours are in MPa/ka. Solid contour lines indicatedecreasing fault instability while dashed contours indicate increasing fault instability.
Figure 14. Similar to Fig.8 except for a high viscosity lower mantle (model L2).
TIME = 9 ka BP
Max Stress = 186.5 MPa
12
0W
40N
12
0W 6
0W
60N
1
2
3
4
5
6
7
8
9
1 0
1 1
1 2
12
0W
40N
60N
10
0W
80
W
TIME = 0 ka BP
Max Stress = 180 MPa
1
2
3
4
5
6
7
8
9
1 0
1 1
1 2
A B C D E F G H I J K L M N A B C D E F G H I J K L
60N
60N
65N
65N
40E
30E
20E
10E
0E10
W
TIME = 0 ka BP Max Stress=169 MPa
60N
60N
65N
65N
0E10
W
40E20E
0E
TIME = 9 ka BP Max Stress=180 MPa
than in Laurentia. That fault instability in Laurentia is predicted to increase from the end of deglaciationimplies that earthquake magnitude and frequency in E. Canada will increase in the future, moreover,previously stable area may become more earthquake prone. This is particularly important for theplanning of nuclear waste repositories which need to be located in areas that stay stable for the nextfew thousand years. Fig. 13 shows the spatial distribution of the present rate of change for dFSM.Areas with solid contours are sites where the magnitude of rebound stress available to trigger earth-quakes is decreasing. However, areas with dashed contours are areas where fault instability is increas-ing and thus not suitable sites for the safe storage of nuclear waste in the next few thousand years if theviscosity of the lower mantle is 1022 Pa-s. However, if the lower mantle viscosity is 1021 Pa-s (Model1), then fault instability will be decreasing in the deglaciated areas and this would not be a problem.
0
2 0
4 0
6 0
8 0
1 0 0
1 2 0
Ori
en
tati
on
o
f M
ax
P
rin
cip
al
Str
es
s(d
eg
ree
s
an
tic
loc
kw
ise
fr
om
E
as
t)
0 1 0 2 0 3 0
a) t = 9 KBP
0
2 0
4 0
6 0
8 0
1 0 0
1 2 0
0 1 0 2 0 3 0
b) t = 0 KBP
L6
J 8
J 1 0
H11
For sites in E. Canada
0
2 0
4 0
6 0
8 0
1 0 0
1 2 0
Ori
en
tati
on
o
fM
ax
P
rin
cip
al
Str
es
s
(de
gre
es
c
loc
kw
ise
fr
om
E
as
t)
0 5 1 0 1 5 2 0
Difference between HorizontalTectonic Principal Stress (MPa)
t = 9 KBP
0
2 0
4 0
6 0
8 0
1 0 0
1 2 0
0 5 1 0 1 5 2 0
t = 0 KBP
H7
G10
G3
C6
For sites in N. Europe
Figure 15. Similar to Fig. 9 except for a high viscosity lower mantle (model L2).
The increase in lower mantle viscosity also has important effects on the contemporary stressorientation. Due to the longer relaxation times, rebound stress difference remains large even at thepresent and for a 5 MPa tectonic stress difference, rebound stress will dominate the stress orientationseven up to now, resulting in little stress rotation. Thus the predicted stress orientations remain non-uniform in Laurentia and Fennoscandia throughout the last 9,000 years (Fig.14). Since non-uniformstress orientations are not consistent with the observations today, the tectonic stress difference must beat least 18 MPa in Laurentia and Fennoscandia (Fig.15) so that a uniform stress orientation field isobtained for earth model L2. However, with such high tectonic stress difference, the observed rotationof stress orientations in Laurentia cannot be explained.
In summary, a high viscosity lower mantle will have no effect on the mode of failure. Theeffect on the onset time of instability is insignificant except for sites near the ocean margin, however,even there, more accurate age determination is needed before it can be used to infer lower mantleviscosity. On the other hand, a high viscosity lower mantle can result in areas in E. Canada where faultinstability will increase in the next few thousand years and thus not suitable for the safe storage ofnuclear waste. Finally, stress rotation is very sensitive to the viscosity of the lower mantle. Withviscosity as high as 1022 Pa-s in the lower mantle, any significant rotation of stress orientation duringthe last 9,000 year is prevented, thus it is not possible to explain the observed stress rotation andcontemporary stress orientations in E. Canada simultaneously.
IV. ConclusionsIn conclusion, we have seen that although glacial unloading gives fault instability of the order
of a few MPa which is probably not large enough to dictate fracture, nevertheless intraplate earth-quakes in E. Canada and N. Europe can still be triggered by postglacial rebound stress through reacti-vation of optimally oriented pre-existing faults in tectonically pre-weakened zones. This resolves theissue as to whether tectonic stress or postglacial rebound stress is more important in the generation ofintraplate seismicity in Laurentia and Fennoscandia. We have also shown that the spatial distributionof earthquakes, their onset time, the observed current stress orientations in Eastern Canada and NorthernEurope, the rotation of stress observed in E. Canada and most of the observed modes of failure can allbe explained simultaneously if the mantle has a uniform 1x1021 Pa-s mantle.
The effect of a 1022 Pa-s lower mantle on the ability to explain the observations has also beeninvestigated. It is found that the magnitude of stress, dFSM and the timing of the onset of fault insta-bility are sensitive to viscosity structure. Unfortunately, direct stress measurements are few and dFSMcannot be observed. Also, changes in dFSM is also small, thus, the role of rebound stress is stilllimited to the reactivation of pre-existing faults generated by previous tectonic events - i.e. both tec-tonic forces and rebound stress are still needed to explain current earthquakes in E. Canada (evidencein the distribution of contemporary earthquakes). As for the timing of the onset of earthquakes, theiraccuracy has not been determined well enough for the inference of mantle viscosity. Thus, until higheraccuracy in the determination of the onset of earthquakes/faulting events is available, the proposal ofSpada et al. [40] to constrain mantle rheology from seismicity cannot be realized.
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