Les failles actives Identification - images satellitaires - terrain Fonctionnement - vitesse moyenne - cycle sismique
Les failles actives
Identification - images satellitaires - terrain
Fonctionnement - vitesse moyenne - cycle sismique
124!W
124!W
122!W
122!W
120!W
120!W
118!W
118!W
116!W
116!W
32!N 32!N
34!N 34!N
36!N 36!N
38!N 38!N
40!N 40!N
Sismicité de la Californie (1973-2004)
La faille de San Andreas n'est pas active ???
Ruptures de surface du séisme de Kokoxili (2001, Mw = 7.9)
Une faille active modifie le paysage
Combinaison d’images SPOT (10 m) et IKONOS (! 1 m)
(Documents Y. Klinger, Tectonique IPGP)
Faille du Kunlun
Formation des facettes triangulaires
Versant ouest contrôlé par une faille Versant est contrôlé par l'érosion
Tanghenanshan
Topographienumérique
Image Landsat
Tanghenanshan
Topographie + Image satellitaire
SW NE
© Yves Gaudemer, 2005
SW NE
Van der Woerd et al. 2003
Plis actifs
Meyer 1991
Shibaocheng
Photo P. Tapponnier
120 km
Décalage du Fleuve Jaune (Huanhe) par la
faille de Haiyuan
Comment fonctionne une faille ?
Si la faille fonctionne régulièrement, avec un séisme caractéristique, le temps de
récurrence T de celui-ci est simplement :T = U/V
où U est le déplacement cosismiqueet V est la vitesse de glissement sur la faille.
Exemple :M = 8, U = 10 m, V = 1 cm/an —> T = 1000 ans
Déterminer U et V
At six different sites we dated as many as 93 quartz pebblesand 22 organic samples to constrain the ages of the cumulativeoffsets of 11 distinct morphological markers. The large numberof samples is paramount in obtaining statistically meaningfulcosmogenic ages, and in identifying clearly and discardingoutliers on any given terrace. Several independent values of
both the marker ages and slip-rates, based either on cosmo-genic or 14C dating, are consistent with one another at each siteand from site-to-site (Fig. 31), giving us confidence in individualrate determinations. Overall, our estimate of the slip-rate isdetermined within an uncertainty of t2.0 mm yrx1, and theaverage value of 11.5 mm yrx1 is robust (Fig. 31).For offset risers, the ages we take, and hence the slip-rates,
depend on field interpretation regarding the terraces (strathin general, fill in one case), the nature of which may vary fromsite-to-site or at a given site. An end-member scenario, con-sidered highly unlikely, is that the ages of all riser offsets arethose of upper terraces. Slip-rates derived in this scenarioare y7 mm yrx1. In view of geological evidence, we believe7 mm yrx1 to be implausibly small. Nonetheless, this value isgreater than the rate inferred from two epoch remeasurementsof the few extant GPS stations north and south of the KunlunFault (Chen et al. 2000). Until a denser and longer GPS surveyis undertaken, slip-rates based on this technique should beregarded as preliminary.Our average rate value of 11.6t0.8 mm yrx1 in Xidatan–
Dongdatan (Fig. 15), which is based only on cosmogenicdating of offset terrace risers, is consistent with those derivedfrom other geological studies with different dating tech-niques. Zhao et al. (1994) and Zhao (1996), in particular,documented 10 offsets of gullies and terrace risers in Xidatan,ranging from 10 to 152 m, and retrieved seven TL or 14Cages, ranging from 2.14 to 12.0 kyr BP, with uncertainties of6–9 per cent. From this, they derived a Holocene slip-rateof about 11.5 mm yrx1. Unfortunately, without error estimateson the offsets, the uncertainty on that rate cannot be assessed.The average Quaternary rate estimated by Kidd & Molnar(1988) (10–20 mm yrx1), loosely brackets both our values andthose of Zhao et al. In a strict sense, however, it cannot besimply compared with either, because it corresponds to a muchlonger time-span. Recall that the long-term rate of Kidd &Molnar (1988) is deduced from the separation between LowerPleistocene moraines containing boulders of pyroxenite, southand east of the Kunlun Pass, and their presumed source area,30 km to the west, in the mountains north of the fault. Thefactor of two in the estimate comes chiefly from the largeuncertainty in the age of the lake-beds that overlie the moraine(2.8–1.5 Ma) (Kidd & Molnar 1988; Qian et al. 1982; Wu et al.1982). To this uncertainty on the age, one should also probablyadd a y30 per cent error on the offset, due to poor definitionof the eastern piercing point (Kidd & Molnar 1988: Fig. 6).Hence, while it is interesting to note that there is no first-order discrepancy between the loosely constrained, averageQuaternary rate (Kidd & Molnar 1988) and the Holocene slip-rates derived from 14C, TL or cosmogenic dating, kinematicmodels of the current tectonics of northeast Tibet making useof the latter rates will be more precise.
5.2 Climatic origin of the morphology
Along the Xidatan–Dongdatan valleys, the relatively smallnumber of terrace levels (about seven, Fig. 5) found along mostof the streams implies a relatively uniform morphologicalresponse on a regional scale. It seems that most of the streamsemplaced terraces at discrete, comparable times. Not all thestreams deposited all the terrace levels identified, however. Thevariable distribution of terraces along the different streamsis thus probably due to local conditions in each watershed.
Figure 28. View, looking west, along fault strike of the shutter ridgewest of Maqen (see Fig. 27b for location). The stream incision acrossthe ridge reveals several tens of metres of thick, vertical gouge.
Figure 29. View of the offset LGMmoraine ridge and T3 terrace level,abandoned after 11 156t157 yr BP.
Figure 30. View, looking south, of a small gully offset by the KunlunFault. The offset, probably resulting from only one large earthquake, isabout 12 m.
382 J. Van Der Woerd et al.
# 2002 RAS, GJI 148, 356–388
Mesure du déplacement cosismique U
<— Faille du Kunlun (Tibet)
12 mSéisme de Manyi (M = 7.6, Tibet)
—>
(Van der Woerd et al., 2002)
(Peltzer et al., 1999)
(Peltzer et al., 1999)
<—
Pour calculer V, on mesure le décalage d'objets (moraines, cônes alluviaux, etc) que l'on peut dater (14C, 10Be, 26Al, 37Cl)
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WG EG
WV
CO
EOWO
M2
2 km
M1 EC
63±11ka
63±9 ka
61±9 ka
63±7 ka 61±12 ka
77±15 ka
72±13 ka
51±9 ka
53±5 ka
38±4 ka
40±6 ka
46±8 ka
35±5 ka
32±6 ka
84±16 ka
22±5 ka
41±9 ka
32±5 ka
52±7 ka
6±2 ka
43±4 ka
47±4 ka
40±5 ka
49±7 ka
44±5ka
105±13 ka
107±11 ka
53±6 ka
53±6 ka
85±11 ka 12±2 ka
30±3 ka
22±3 ka
35±5 ka
14±4 ka
17±3ka
4±1 ka
70±10 ka
41±6 ka
73±8 ka
42±8 ka
(Mériaux et al., 2000)
26.3 ± 1.4 mm/an
(Mériaux et al., 2000)
At six different sites we dated as many as 93 quartz pebblesand 22 organic samples to constrain the ages of the cumulativeoffsets of 11 distinct morphological markers. The large numberof samples is paramount in obtaining statistically meaningfulcosmogenic ages, and in identifying clearly and discardingoutliers on any given terrace. Several independent values of
both the marker ages and slip-rates, based either on cosmo-genic or 14C dating, are consistent with one another at each siteand from site-to-site (Fig. 31), giving us confidence in individualrate determinations. Overall, our estimate of the slip-rate isdetermined within an uncertainty of t2.0 mm yrx1, and theaverage value of 11.5 mm yrx1 is robust (Fig. 31).For offset risers, the ages we take, and hence the slip-rates,
depend on field interpretation regarding the terraces (strathin general, fill in one case), the nature of which may vary fromsite-to-site or at a given site. An end-member scenario, con-sidered highly unlikely, is that the ages of all riser offsets arethose of upper terraces. Slip-rates derived in this scenarioare y7 mm yrx1. In view of geological evidence, we believe7 mm yrx1 to be implausibly small. Nonetheless, this value isgreater than the rate inferred from two epoch remeasurementsof the few extant GPS stations north and south of the KunlunFault (Chen et al. 2000). Until a denser and longer GPS surveyis undertaken, slip-rates based on this technique should beregarded as preliminary.Our average rate value of 11.6t0.8 mm yrx1 in Xidatan–
Dongdatan (Fig. 15), which is based only on cosmogenicdating of offset terrace risers, is consistent with those derivedfrom other geological studies with different dating tech-niques. Zhao et al. (1994) and Zhao (1996), in particular,documented 10 offsets of gullies and terrace risers in Xidatan,ranging from 10 to 152 m, and retrieved seven TL or 14Cages, ranging from 2.14 to 12.0 kyr BP, with uncertainties of6–9 per cent. From this, they derived a Holocene slip-rateof about 11.5 mm yrx1. Unfortunately, without error estimateson the offsets, the uncertainty on that rate cannot be assessed.The average Quaternary rate estimated by Kidd & Molnar(1988) (10–20 mm yrx1), loosely brackets both our values andthose of Zhao et al. In a strict sense, however, it cannot besimply compared with either, because it corresponds to a muchlonger time-span. Recall that the long-term rate of Kidd &Molnar (1988) is deduced from the separation between LowerPleistocene moraines containing boulders of pyroxenite, southand east of the Kunlun Pass, and their presumed source area,30 km to the west, in the mountains north of the fault. Thefactor of two in the estimate comes chiefly from the largeuncertainty in the age of the lake-beds that overlie the moraine(2.8–1.5 Ma) (Kidd & Molnar 1988; Qian et al. 1982; Wu et al.1982). To this uncertainty on the age, one should also probablyadd a y30 per cent error on the offset, due to poor definitionof the eastern piercing point (Kidd & Molnar 1988: Fig. 6).Hence, while it is interesting to note that there is no first-order discrepancy between the loosely constrained, averageQuaternary rate (Kidd & Molnar 1988) and the Holocene slip-rates derived from 14C, TL or cosmogenic dating, kinematicmodels of the current tectonics of northeast Tibet making useof the latter rates will be more precise.
5.2 Climatic origin of the morphology
Along the Xidatan–Dongdatan valleys, the relatively smallnumber of terrace levels (about seven, Fig. 5) found along mostof the streams implies a relatively uniform morphologicalresponse on a regional scale. It seems that most of the streamsemplaced terraces at discrete, comparable times. Not all thestreams deposited all the terrace levels identified, however. Thevariable distribution of terraces along the different streamsis thus probably due to local conditions in each watershed.
Figure 28. View, looking west, along fault strike of the shutter ridgewest of Maqen (see Fig. 27b for location). The stream incision acrossthe ridge reveals several tens of metres of thick, vertical gouge.
Figure 29. View of the offset LGMmoraine ridge and T3 terrace level,abandoned after 11 156t157 yr BP.
Figure 30. View, looking south, of a small gully offset by the KunlunFault. The offset, probably resulting from only one large earthquake, isabout 12 m.
382 J. Van Der Woerd et al.
# 2002 RAS, GJI 148, 356–388
Paléo-sismologie
LichenométrieDendrochronologie
GéomorphologieTranchée de l'Evêque (Yammoûneh, 2002 – CNRSL & IPGP)
Photo J. Vanderwoerd
Quand a eu lieu le dernier séisme ?
Remblai
02
03a
03c
03d
03f03e
03c
05
04d
04a04b 04b05
03b
01
06a 07a 07a
07a
07a
07b
04a
04c
03d/f
06b08
08
08
05
04+05
06c
10
10
0909
1011
11
1113
15
15
1412
14
14
12
15
12
13
14
15
16
1617a
17a
(17/18)b 19b
20
23
23
24-25
21
2219/21?18a
27 28
27
29
26
27
28
29
30
3132
34
35
37
39a
3840a4142
4546
4849
50
4344
36
33
30
28
29
30
23
22
23
27
23
28
29
30
27
28
29
30 31
32
33
34
35
3637
38 39b40b
41
39b-40b
42+43
44
4546
47
47
{
54
55
(56+57)?
{
50{
54
55
56
57
58
59
60
61
{
48
49
5152
53
53
51
52
F1
F2
F4
F5
F6
F7
F3
F10F9
F8
C3C2
C1
C4
C5
Kazzâb Trench
(Yammoûneh basinIPGP / CNRSL, 2001)
1 m
E W
Tranchée de Kazzab (IPGP/CNRSL, 2001)
Daëron 2004
Temps Temps Temps
Glis
sem
ent
!max
!min
Parfaitement périodique T prévisibleTn = f(Un-1)
U prévisibleUn = f(Tn-1)
U = VT
U = VT U =
VT
Trois modèles de séismes pour une failleà vitesse moyenne constante
4000
10
20
30
40
50
800 1200 1600 2000
Année
Dép
lace
men
t cu
mul
é (m
)
8.9 cm/an
2.4 cm/an3.1 cm/an
Les failles fonctionnent-elles vraiment à vitesse constante ?Sur quelle échelle de temps ?
(d'après Weldon et al. 2004)
Faille de San Andreas (Wrightwood, Californie)
Question :les failles fonctionnent-elles régulièrement ?
Certains physiciens et sismologues pensent que le fonctionnement des failles est complètement aléatoire
1
0 2 4 6 8
10
100
1000
10000
100000
b ! 1
Magnitude
Nom
bre
de s
éis
mes
Loi de Gutenberg-Richter
Log N(M) = a - bM
Modèle de patins et ressorts
Dessin : Y. Gaudemer
Toute tentative de prédiction/prévision serait vaine...
Critère de Mohr-CoulombÀ la rupture, la contrainte cisaillante " et la contrainte
normale !n sur les surfaces potentielles de rupture sont liées par une relation linéaire :
" = co + #!n
l'enveloppe de Mohr est une droite
"
!n
co
$ # = tan($)
" = c o + #
! n
co est la cohésion# est le coefficient de friction interne
$ est l'angle de friction interne(analogie avec le frottement)
Contrainte de Coulomb
!n
"
" = c o
+ #! n
#!n
"max
" = c o
+ #! n
#!n
!C = " - #!n
Il est facile de voir que la quantité !C = " - #!n
(ou !C = " - #(!n - pf) s'il y a des fluides)ne peut jamais excéder une valeur critique. Cette quantité !C est appelée contrainte de Coulomb.
Quelle est la relation entre co et la valeur critique ?
!C = "max - #!n
contrainte normale contrainte normale
cont
rain
te c
isai
llant
e
cont
rain
te c
isai
llant
e
La notion de cycle sismique
! Juste après le séisme précédent
" !C augmente mais la faille reste
bloquée
# !C atteint la valeur critique :
c'est le séisme
$ Les contraintes sont relâchées, un nouveau cycle commence...
Il est très difficile de mesurer la contrainte de Coulomb et, donc, de savoir si une faille est proche de la rupture ou non.
Mais on peut calculer facilement les variations de !C engendrées par des séismes proches.
+ =
Rise DropA. Coulomb stress change for right-lateral faults parallel to master fault Stress
B. Coulomb stress change for faults optimally oriented for failure
N27°E regional compression (!r) of 100 bars; µ’ = 0.75
!r
right-lateral shearstress change
effective friction x
normal stress change
right-lateral Coulombstress change
+ =
µ' (-!n)"s !f+ =
+
shear stresschange
effective friction x
normal stress change
Coulomb stresschange
+ =
µ' (-!n)"s !f+ =
+
left-lateral
right-lateralOptimum
Slip Planes
R R
opt
+ =
%!C = %" + #%!n
%" #%!n %!C
Exemple d'un séisme sur une faille dextre :
augmentation diminutionsans changement
0 20 40 600
20
40
60
80
100
Optimum right-lateral planes
JoshuaTree
Homestead
Galway Lake
Landers
Valley
Coulomb Stress Change (bars)
-.50-.250
.50
.75
-.75
.25
SpringsNorth Palm
km
1992 Landersrupture trace
Exemple : le séisme de Landers (Californie, 1992, M = 7.3)
Les séismes de Galway Lake (1975), Homestead Valley (1979), North Palm Spring
(1986) et Joshua Tree (1992) ont modifié !C dans la
région du futur séisme de Landers.
Conséquences sur la faille de San Andreas
33°30'
116°
117°3
0'
35°30'
Coulomb Stress
Change (bars)
25 km
ML!1 during 25 days
after Landers main shock
-0.2-0.10.0
0.20.3
-0.3
0.1
S A
N
J A C
I N T O
S A N
A N
D R
E A
S
27 Nov-4 Dec
Mojave SegmentCoachella Valley
SegmentSan Bernardino Mtn. Segment
35 mm/yr, last event in 1857 24±3 mm/yr, 1812 25-30 mm/yr, 1680
Co
ulo
mb
S
tres
s C
ha
ng
e (
ba
rs)
Distance East of Palmdale (km)
Pre
dic
ted
Sli
p
(cm
)
200180160140120100806040200-20-400
10
20
30
40
50
60
Removes 6.2<M<6.6
Load
Adds Load Equivalent to
5.7<M<6.4
San B
ernard
ino
Indio
Bom
bay
Bea
ch
Slip Needed to RelieveShear Stress Changes
Adds 6.2<M<6.5
Load
Palm
Spri
ngs
Thre
e P
oin
ts
ImmediateStress Changes Adds load
Removes load
Adds load
Removes loadLong-TermStress Changes
200180160140120100806040200-20-40
Coulomb Stress Changeon the San Andreasat 6.25 km depth
-4
0
4
8
µ = 0.75
µ = 0.0
EW
Coulomb Stress Changeon the San Andreas
Fault (for µ = 0.4)
-4
0
4
8
Long-Term(plate)
Immediate(halfspace)
EW
12
Fig. 14 ( ! UP)
Les séismes de Joshua Tree, Landers et Big Bear ont
augmenté !C sur certaines parties de la faille de SA et l'ont diminué sur d'autres
1906
1857Où est une faille à l'intérieur
de son cycle sismique ?
Pas d’accélération visible de la production d’énergie
Pas de séisme proche ?
Accélération de la production d’énergie
Séisme proche ?
1970 1980 1990 20001960
1.0 108
5.0 107
Californie du Nord
aléatoire
non aléatoire
1960 1970 1980 1990 2000
6.0 108
4.0 108
2.0 108
Californie du Sud
aléatoire
non aléatoire
(Bowman et King, 2001)
Comment varie la production d'énergie sismique avant un séisme ?
(Bowman et King, 2001)
Outil de prévision opérationnel ? Pas encore...
La production d’énergie sismique régionale a accéléré avant le séisme
(mais on l'a mesurée après...)
1992 1994 1996 1998 2000 2002 2004
Cum
ula
tiv
e B
enio
ff s
train
0
1.0 106
2.0 106
3.0 106
4.0 106
5.0 106
Year
(Feuillet et King, 2004)