influence of crustal layering and thickness on co-seismic ... · no.1 tan hongbo, et al. influence...
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Geodesy and Geodynamics 2011 ,2(1) :47-54
http://www. jgg09. com
Doi:10.3724/SP.J.1246.2011.00047
Influence of crustal layering and thickness on co-seismic effects of Wenchuan earthquake
Tan Hongbo1'2 ,Shen Chongyang1
'2
, Xuan Songhai1'2 and Yang Guangliang1
'2
1 lnstirute of Seis11Wlogy, CEA, Wuhan 430071, Chma
'CrustaJal MovemenJ.laborator, Wuhan 430071, Chma
Abstract: Using the PSGRN/PSCMP software and the fault model offered by USGS and on the basis of finite
rectangular dislocation theory and the local layered wave velocity structures of the crust-upper-mantle, the in
fluences of crustal layering and thickness on co-seismic gravity changes and deformation of Wenchuan earth
quake have been simulated. The results indicate that: the influences have a relationship with the attitude of
faults and the relative position between calculated points and fault. The difference distribution form of simula
ted results between the two models is similar to that of co-seismic effect. For the per centum distribution, it'
s restricted by the zero line of the co-seismic effects obviously. Its positive is far away from the zero line. For
the crustal thickness, the effect is about 10% - 20%. The negative and the effect over 30% focus around the
zero line. The average influences of crustal layering and thickness for the E-W displacement, N -S displace
ment, vertical displacement and gravity changes are 18.4% ,18. 0% ,15. 8% and 16. 2% respectively, When
the crustal thickness is 40 km, they are 4. 6% ,5. 3% ,3. 8% and 3. 8%. Then the crustal thickness is 70
km, the average influences are 3. 5%, 4. 6% ,3. 0% and 2. 5% respectively.
Key words: Wenchuan earthquake ; co-seismic effects ; crustal layering; crustal thickness ; fmite rectangular
dislocation theory
1 Introduction
Wenchuan Ms8. 0 earthquake took place at Longmen
shan fault zone , which is the secondary fault zone in
the middle of the most active N-S seismic belt in Chi
na [ ll . The Longmenshan fault zone is located at the
joint of Songpan -Ganzi orogenic zone and Y angzi
block, which is the composition of the east borderline
of the Qinghai-Tibet plateau together with Minshan up
lift. It 's quite different in the structure of crustal
Received ,2010·12-25; Acoepted ,2011.01-24
Corresponding author: Tel. 13476091954; E-mail: thbhong@ 163. com
This study was supported by the National Natural Science Foundation of
China ( 40574012 ) and the Earthquake Science Joint Foundation
(A07030)
layering and thickness of different areas because of the
nappe structure environment[2 -s]. In the northwest,
the crustal density of the Qinghai-Tibet plateau is smal
ler and the crustal thickness can reach to 70 km,
which is thinning eastwardly. While in the southeast,
crustal thickness of Sichuan basin is about 40 km. The
local crustal structure of Longmenshan fault zone pro
vides the basis for simulation study by using dislocation
theory.
Many scientists have researches on co-seismic effects
by using dislocation theory at present[•-•l. Their stud
ies show that the influence of layered structure is larger
than other factors of medium parameters in dislocation
simulation, but their conclusions are not identical, and
don ' t have specific description about the influence dis
tribution. They also don' t consider the factor of crustal
48 Geodesy and Geodynamics Vol. 2
thickness and the influence on co-seismic gravity chan
ges caused by every factor. In layered elastic-viscoe
lastic half space, the surface deformation and gravity
changes caused by VVenchuan earthquake have been
simulated by Tan Hongbo[ 10J. The results between
measuring and simulating are different , which may be
related to the crust medium parameter selecting. Based
on that, the influence of VV enchuan co-seismic deform
ation and gravity changes caused by crustal layering
and thickness will be simulated by using the PSGRN/
PSCMP software[ 11 J. And the distribution of percentage
will be elaborated too. It may offer some gists for ex
plaining the mechanism of VV enchuan earthquake prep
aration.
2 Fault model and layered medium model
This model is inversed by Chen Ji ( USGS ) , of which
the fault plane is inconsistent with the slip direction
( Fig. 1 ) . Red star represents the epicenter, the color
represents slippage value, and the arrowhead repre
sents the moving direction of top-wall opposite to the
footwall. Isoline is the rupture start time. The optimum
fault parameters are as follows: strike is 229°, dip an
gle is 33 °, 21 X 8 blocks along strike and dip, every
sub-fault is 15 km X 5 km.
There are lots of results about deep seismic sounding
tomography , natural and artificial earthquake in the ar-
f S. h d y . [5,13-15] Th . ea o 1c uan an unnan provmce . eu re-
sults indicate that the crust-upper mantle appears to be
layered. But the parameters of the wave velocity layered
]10
! 20
-240
I
0 180
Strike=229 deg • km
-180 -120 -60
I I
360 540 I
720
IIIII! em 900
0
Figure 1 Cross section of the fault including the
movement distribution (from USGS)
model are different from each other by dissimilar way
and data. Overall consideration, we choose the model
as table 1 , where the density is gained from the Nafe
Drake density-velocity experience transform formular
y[16l , and the viscosity modulus is from the result of
Tan Kai[l?J.
For better analyzing the influence of crustal layering,
we design a homogeneous crust model (Tab. 2) based
on the weighted average of crust parameters in each
layer thickness. Obviously, the difference between
simulant results of the two models can reflect the influ
ence of crustal layering[JsJ. VVe choose two models as
examples, one is 70 km crustal thickness of the north
west Qinghai-Tibet plateau and the other is 40 km crus
tal thickness of the southeast Sichuan basin. Compa
ring with the homogeneous crust model of which the
crustal thickness is 57 km, co-seismic influence caused
by different crustal thickness will be quantitatively sim
ulated and analyzed.
Table 1 Layered structure of the crust-upper-mantle based on the average velocity
h(km) VP ( km/s)
0-20 5.98
20-40 6.55
40-57 6.83
57-00 7.75
Table 2
h(km) VP ( krnls)
0.0-57.0 6.4335
57.0-00 7.7500
V, (km/s) p(kglm3) 71(Pas)
3.45 2679.0 00
3.78 2835.0 00
3.84 2977.0 4.0E + 19
4.35 3175.0 1. OE + 19
Homogeneous structure of the crust-upper-mantle
V,(krnls) p(kglm3) 7J(Pas)
3.6821 2822.6 O.OE +00
4.3500 3175.0 1. OE + 19
a
1.0
1.0
0.0
0.0
a
1.000
0.000
notes: Where h is the depth, VP is the P wave velocity, V, is the S wave velocity, p is density, 7J is the viscosity modulus, a is the relax ratio, a = 0 means
complete elastic medium, a = 1 means Maxwell viscoelastic medium.
No.1 Tan Hongbo, et al. Influence of crustal layering and thickness on co-seismic effects of Wenchuan earthquake 49
3 The influence of surface deformation and gravity changes caused by crustal layering
The pictures of the co-seismic deformation and gravity
changes are similar between the homogeneous crust
model and layered crust model. For the homogeneous
model , the radiation range of the pictures is larger. As
shown in Figure 2 ( a) , we can see that : the outline is
similar to the picture of co-seismic E-W horizontal dis
placement, which are divided into northwestern and
southeastern parts against the rupture zone. The iso
lines are antisymmetric : maximum positive difference
can reach to 55 mm; maximum negative difference can
35
®
Xining ® Lanzhou
60mm 50 40 30 20 10
-hnm -2 -3
reach to - 85 mm. Most of the difference focus on the
fault around, and decays in the far field. The differ
ence of N -S horizontal displacement ( Fig. 2 ( c ) ) is
similar to the N -S horizontal displacement in outline ,
but present to be negative in the southwest. So, the
picture comes to be four quadrants positive and nega
tive distribution: the maximum positive difference can
reach to 25 mm; the maximum negative difference can
reach to - 45 mm. The difference of vertical displace
ment ( Fig. 2 ( e) ) and gravity changes ( Fig. 2 ( g) ) fo
cus on the fault around. It's not obvious in four quad
rants distribution comparing with vertical displacement
and gravity changes, but presents a positive and nega
tive distribution along fault, and decays fast in the near
field.
35"N
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Xining ., Lanzhou
80%
70
60
50
40
30
20
15%
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31 -4 31 "N
103"E 107"E (a) Difference ofE-W displacement
Xian
31 "N
27"N'--------'--------'---------'---'--------'--------' 99"E 103"E 107"E
(c) Difference ofN-S displacement
-5 -10 -20
® Chongqing
-30 -40 -50 -60 -70 -80
-85mm 27"N'------'-..,.,.,_--'------'-----'----'---------'
30mm 25 20 15
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-10 -15 -20 -25 -30 -35 -40
99"E 103"E 107"E (b) Difference percentage of E-W displacement
-45~7 N L __ __._ _ __,__.___._-'Sl"-'-----__JJ_-----'
99"E 103"E 107"E (d) Difference percentage ofN-S displacement
·10
-20
-30
-40
-50
-60%
80%
70
60
50
40
30
20
15
10
0%
-10
-20
-30
-40
-50
-60
-70
-80
-90%
50 Geodesy and Geodynamics Vol. 2
35"N
31 "N
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Xining ., Lanzhou
Xian®
@
Chongqing
60mm
50
40
30
20
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1mm
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-5
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70
60
50
40
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-10
-20
-30
-40
-50
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27"N '-----------'--------'-------'-----'-----_J__-__j -35mmZ7"N c__ __ _J._ __ _j_ __ ___~_-=__lf.!i~L--__j -70%
99"E 103"E 107"E
(e) Difference of vertical displacement
@
Lanzhou
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22
20
99"E 103"E
(f) Difference percentage of vertical displacement
@
Lanzhou
80%
35"N 18 35"N
70
60
50
40
30
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15
10"/o
31"N
@
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27"NL_ ____ L_ ____ L_ ____ ~~--~--~
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(g) Difference of gravity changes
16
14
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8
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-1
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-3
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31 "N -10
-20
-30
-40
-50
-60
27 "N L__ ____ _L_ ____ ~----__L_L----'-',.___,.--'-__ __j -70%
99"E 103"E 107"E
(h) Difference percentage of gravity changes
Figure 2 Co-seismic effect difference and its percentage between the layered crust model and the
homogenous crust model( gravity unit:10- 8 ms -z; deformation unit: mm; percentage unit: %)
Percentage of co-seismic effect difference between
homogeneous crust model and layered crust model is
the ratio of the co-seismic effect difference and the sim
ulation results of homogeneous crust model (wipe off a
few ratio which are bigger than 1 ) . The right in Figure
2 shows that co-seismic effect difference and its per
centage are closely related to each other. Most of the
percentage is positive and the value is between 10%
and 20% . While negative and big one are near the
positive and negative transforming area. The More
close to the zero line , the larger the difference percent
age is. Based on the close relationship between co
seismic effect and fault attitude , or the relative position
between the simulated points and fault[ 19'20 l , we can
infer that the influence of co-seismic effect caused by
crustal layering has a relationship with fault attitude,
and the relative position between the simulate points
and fault.
By comprehensive consideration, using absolute val
ue and taking an average of the difference percentage ,
we can get that: the influence of the E-W horizontal
displacement caused by crustal layering is 18. 4% ; for
the N -S horizontal displacement, E-W and N -S vertical
displacement and gravity changes, they are 18. 0% ,
15.8% and 16.2% respectively.
No.1 Tan Hongho, et al. Influence of crustal layering and thickness on co-seismic effects of Wenchuan earthquake 51
4 The influence of surface deformation and gravity changes caused by crustal thickness
Difference distribution of surface deformation and gravi
ty changes among the 40 km, 70 km, and 57 km crus
tal thickness is showed in Figure 3. For co-seismic
effect difference of 40 -57 km crust model ( left in Fig.
3) , the pictures of E-W horizontal displacement are
similar to the difference of E-W displacement: the iso
lines are left-right two quadrants antisymmetric distri
bution. The maximum positive change is 26 mm, while
the maximum negative change is - 20 mm. The extre
mum appears 0. 2 o beside the fault , and decays against
the fault. The difference of N-S horizontal displacement
is four quadrants distribution: on the left, it's positive
difference, and the extremum is 12 mm · on the right ' '
it' s negative difference, and the extremum is - 11
mm. The difference of vertical displacement and gravi
ty changes focus on the rupture area , decaying against
the fault, and the maximum positive difference can
reach to 18 mm and 3. 5 X 10 - 8 ms - 2; while the maxi
mum negative difference can reach to - 30 mm and
-7 X 10 - 8 ms - 2• For co-seismic effect difference of 70
-57 km crust model ( right in Fig. 3 ) , it ' s similar in
outline , but smaller in the radiation range , and reverse
in the change value.
For better understanding the influence of surface de
formation and gravity changes caused by crustal thick
ness, difference per centum has been calculated by u-
" 26mm
Xining u 22
18
35•N 14
10
6
4
3
2
1mm
0
31 ON -1
-2
-3
-4
-5
-8
-12
40km-57km -16
sing the similar way which Figure 2 has used. The dis
tribution has similar characteristics with Figure 2 :
difference per centum is restricted by the co-seismic
zero line, where negative and large value focus on. U
sing absolute value and taking an average of the differ
ence percentage , we can get that : for the difference
pictures of 40 - 57 km crustal thickness model differ-'
ence per centum of the E-W displacement , N -S dis
placement, vertical displacement and gravity changes
are 4. 6% , 5. 3% , 3. 8% and 3. 8% respectively;
while for 70 -57 km crustal thickness model, they are
3. 5% ,4. 6% ,3. 0% and 2. 5% respectively.
Conclusively, co-seismic effect difference of the 40
-57 km model is bigger than that of the 70 - 57 km
model. The radiation range of the E-W and N-S dis
placement difference is wider, and decays slowly a
gainst the fault. The radiation range of the vertical dis
placement difference and gravity changes difference is
smaller , and decays fast. Surface deformation and
gravity changes caused by W enchuan earthquake are
increasing when the crustal thickness increases. The
influence caused by crustal thickness is biggest on the
N -S displacement , then the vertical displacement and
E-W displacement, smallest on the gravity changes.
5 Discussion and conclusions
1 ) Crustal layering has great influences to the co
seismic deformation and gravity changes: for W enchuan
earthquake , the influences to E-W displacement , N -S
displacement, vertical displacement and gravity changes
" 8mm
Xining ®- 6 J4lzll u
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70km-57km
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(a) Difference ofE-W displacement
52
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40km-57km
Geodesy and Geodynamics
12mm
10
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-3
4
-5
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103°E
99oE
(b) Difference ofN-S displacement
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-5 -6 -10 -14 -18 -22 -26
31 °N
-30mm 27°N
99°E
(c) Difference of vertical displacement
3.5Gal 3.0 2.5
2.0 1.5 35°N
1.0
0.5 0.0 -0.5 -1.0
-1.5
-2.011Gal -2.5 -3.0 31 °N
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-7.0 11Ga!z7oN
lOTE 99oE
(d) Difference of gravity changes
®
Lanzhou
70km-57km
103°E
<!' Lanzhou
70km-57km
103°E
107oE
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107°E
® Chongqing
107oE
Figure 3 Co-seismic effect difference among crustal thickness of 40 km , 70 km and 57 km
(gravity unit: 10 - 8 ms - 2; deformation unit: mm)
Xian"
Vol. 2
4.5mm
3.5
2.5
1.5
1.0
0.5
0.0
-O.Smm
-1.0
-2.0
-3.0
-4.0
-5.0
-5.5mm
13mm
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4
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-1
-2
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4
-5
-7mm
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2.6
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0.2
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-0.2
-0.4
-0.8
-10
-1.411 Gal
No.I Tan Hongbo, et al. Influence of crustal layering and thickness on co-seismic effects of W enchuan earthquake 53
are 18. 4% , 18. 0% , 15. 8% and 16. 2% respective
ly, which are bigger than the results analyzed by Deng
MinglP1• She got the per centum by comparing the
maximum co-seismic difference of the two models with
the maximum co-seismic effects. That is not compre
hensive , because the maximum influence per centum is
no around the fault. Our results are more similar to
Pollitz1' 1 , Sun and Okubo' s171 , but with some value
over 100% , because the position of the co-seismic
effect zero lines simulated by different models are no
the same. So , the difference per centum is singular
when closing to the zero line. Generally speaking, the
inlluence distribution caused by crustal layering is reli
able.
2 ) The inlluences of co-seismic deformation and
gravity changes caused by crustal layering have a rela
tionship to the attitude of fault and the relative position
between calculated points and fault. Results of homo
geneous crust model and crustal layering model have
the biggest difference in the near field, which decays
against the fault with similar outline to co-seismic
effects. The per centum distribution is restricted by the
zero line of the co-seismic effects obviously : most of
the percentage is positive and the value is between
10% and 20%; while negative and big one are near
the positive and negative transforming area.
3 ) Crustal thickness has a little effect on co-seismic
deformation and gravity changes : in the case of 40 km
crustal thickness , the inlluences of E-W displacement,
N -S displacement, vertical displacement and gravity
changes caused by Wenchuan earthquake are 4. 6% ,
5. 3% , 3. 8% and 3. 8% respectively; while in the
case of 70 km crustal thickness, they are 3. 5% ,
4. 6% ,3. 0% and 2. 5% respectively.
4) Comparison between simulated and measured re
sults
As an example of comparison between simulated and
measured results, we get the results of 3 GPS continu-
ous observation stations which are Pixian( 103. 76°E,
31.91° N), Mianyang ( 104.73° E, 31.44° N) and
Qionglai(103.31°E,30.35°N) (Tab. 3) 1211 • Mian
yang and Pixian stations are in the east of the surface
rapture zone. The shortest distances between these two
stations and Yingxiou-Beichuan fault are 47 km and 28
km respectively. Qionglai station is in the southeast of
the surface rapture zone. Only judging on vertical di
rection, Qionglai simulated results is consistent with
Pixian ' s, but reverse for Mianyang station, because
the precision of GPS vertical displacement is too low.
To horizontal displacement, Pixian simulated and
measured results are consistent in terms of direction ,
but great different on magnitude. The differences be
tween Mianyang simulated and measured results are
7% and 49% on E-W and N-S displacement. Taking
account of crustal layering and thickness inlluence , the
E-W displacement simulated result is credible, but the
difference is biggest on the N-S displacement. For
Qionglai station, the direction is reverse on N -S dis
placement, and consistent on E-W displacement with
34% difference which is over the range of crustal laye
ring and thickness inlluence.
Generally speaking, some station 's simulated and
measured results can be inosculated in error range of
analyzed crustal layering and thickness. But in most
cases, it' s not reasonable. Back to the original ques
tion, there are two factors which cause the difference
between simulated and measured value : medium model
difference and fault model difference. Since two main
factors( crustal layering and crustal thickness) in medi
um model can not explain the difference , the reason
might be the fault mode difference. The fault model in
versed by far field earthquake wave is too simple, and
has big difference with the actual situation. Joint inver
sing Wenchuan earthquake fault model by using gravi
ty, leveling and GPS data will be the best way to solve
the difference between simulated and measured data.
Table3 Comparison between simulated and measured results( unit: mm)
Pixian Mianyang Q;onglai displacement
measured simulated measured simulated measured .unulated
E-W -563 -189.5 -305 -285 -15 -26
N-S 426 64.5 66 131 -3 10.4
vertical 81 43.9 14 -9.4 28 27
54 Geodesy and Geodynamics Vol. 2
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