Geodesy and Geodynamics 2012,3 ( 1) :1 -7

http://www. jgg09. com

Doi:10.3724/SP.J.l246.2012.00001

Co-seismic deformation and gravity changes of the 2011

India-Nepal and Myanmar earthquakes

Liu Chengli 1 ' 2 , Zheng Y ong1 , Shan Bin 1' 2 and Xiong Xiong1

1 State Key Laboratory of Geodesy and Eanh ' s Dynamic, Instituu of Geodesy and Geophysics, ChWse Academy of Sciences, Wuhan 430077, China

2 Graduate University of Chinese Academy of Sciences, Beijing 100049 , China

Abstract: Co-seismic deformation and gravity field changes caused by the 2011 Mw6. 8 Myanmar and Mw6. 9

India-Nepal earthquakes are calculated with a finite-element model and an average-slip model, respectively,

based on the multi-layered elastic half-space dislocation theory. The calculated maximum horizontal displace-

ment of the Myanmar earthquake is 36 em, which is larger than the value of 9. 5 em for the India-Nepal earth-

quake. This difference is attributed to their different focal depths and our use of different models. Except cer-

tain differences in the near field, both models give similar deformation and gravity results for the Myanmar

event.

Key words: India-Nepal earthquake; Myanmar earthquake; average-slip model; finite-element rupture mod-

el; gravity

1 Introduction

Two strong earthquakes occurred near the southwestern

border of China in 2011 , and caused serious damages

in the neighboring Yunnan and Tibet regions, respec-

tively. One is an Mw6. 8 earthquake occurring in Moog

Hpayak, Myanmar on March 24. The epicenter is loca-

ted at 20. 698 °N, 99. 889 °E, and the focal depth is a-

bout 10 km. The focal mechanism shows a left-lateral

strike slip. Myanmar and the southern part of Yunnan

province belong to Yunnan-Myanmar seismotectonic

block[I,z], which is located to the south of the eastern

structure knot of Tibetan plateau , which includes

southern Yunnan, Myanmar, Vietnam and other re-

gions. Its western boundary is the " arc" subduction

Received :2011-11-27; Accepted :2011-12-{)7

Corresponding authar:Tel: +86-13971417171; E-mail : lcl8669@ 126. com

This work is supported by grant 201008007 from China Earthquake

Administmtion ,National Natural Science Foundation of China ( 40974034,

41174086)

zone, where the Indian plate subducts beneath the My-

anmar plate, resulting in some right-lateral strike-slip

faults and large earthquakes ( Fig. 1 ( b) ) . Its eastern

and southern boundaries are, respectively, Jinshajiang-

Honghe fault, and the Sumatra-Andaman sea. It is di-

vided into western and southern tectonic blocks by Nu-

jiang-Lancangjiang fault.

The other earthquake has a magnitude of Mw6. 9 ,

occurring on September 18 in India-Nepal; it is located

at 27. 43°N, 88. 33°E, and has a focal depth of about

19. 7 km (USGS) and a focal mechanism showing left-

lateral strike slip with an oblique thrust component.

The location is near the boundary between the India

and Eurasia plates, in the mountainous region of north-

eastern India near the Nepalese boarder, where many

earthquakes had occurred ( Fig. 1 ( a) ) . According to

USGS , the earthquake source is located within either

the upper Eurasian plate or the underlying India plate,

rather than at the plate boundary.

In this paper , we present an analysis of the co-seis-

mic surface deformation and gravity changes caused

2 Geodesy and Geodynamics Vol. 3

30•N

22•N

28•N

20•N

18•N

90•E 92 OE 96 OE 98 OE 100 OE 102 OE 104 OE

(a) Location of the India-Nepal Mw6.9 earthquake (b) Location of the Myanmar Mw6.8 earthquake

Figure 1 Location of the India-Nepal Mw6. 9 earthquake and the Myanmar Mw6. 8 earthquake

( Yellow stars indicate epicenters ; white circles , historical earthquakes ( Mw > 4. 5) ; green circles , main cities ; yellow circles , aftershocks)

by these two events. The results may provide some use-

ful information about the boundary between the India

and Eurasia plates, the eastern structural belt of Tibet-

an plateau, and the long-term tectonic motion and in-

ter-seismic deformation field [5 -9l.

2 Theory and models

For the Myanmar earthquake, we used a finite-element

method to invert its co-seismic rupture parameters.

However, because of the uncertainties of the source in-

formation about the India-Nepal earthquake, it is diffi-

cult to use this approach to obtain a co-seismic rupture

model. Thus, we used an average-slip model based on

the empirical formula provided by Donald L, et al[ 10J.

We also used the PSGRN/PSCMP[llJ software and a

multi-layered crust model in our calculation. Wang[ 12l

proposed an orthogonal normalization method to calcu-

late the Green function of seismic stress field. Based

on this method and a viscoelastic multi-layered model,

he established a co- and post-seismic deformation mod-

el and produced a corresponding numerical meth-

od[13'14l. By using this method we calculated the theo-

retical deformation and gravity changes caused by these

two earthquakes.

2. 1 Average-slip model

We used some teleseismic data and the inversion proce-

dure developed by Ji , et al [ 15] to generate a rupture

model for the Myanmar earthquake, and an empirical

equation to generate an average-slip model for the Indi-

a-Nepal earthquake, with the following regressions be-

tween average displacement (AD ) , subsurface rupture

length ( RLD ) , rupture width ( RW) , and moment

magnitude (M) [1o]

Log (AD) = a 1 + b1M

Log (RLD) = a2 + b2M Log (RW) = a 3 + b3M

the empirical coefficients of a 1 , a2 , a 3 , b1 , b2 , b3 were

found by Donald L, et al [ JOJ to be -4. 80, - 2. 44,

- 1. 01 , 0. 69, 0. 59 and 0. 32, respectively, for a

strike-slip earthquakes. By applying this average-slip

model to the India-Nepal earthquake, we obtained a

length along the strike direction of 55 km and a width

along down-dip direction of 16 km. The distribution of

aftershocks shows a dip of 74 degrees and a strike of

217 degrees. Global CMT (http://www. globalcmt.

org/CMTsearch. html) gave a rake of -18 degrees;

USGS, a focal depth of 19. 7 km.

2. 2 Finite-element rupture model

Using broadband seismograms recorded by Global Seis-

mic Network ( GSN) (Fig. 2( b)), and based on the fi-

nite-element inversion method [ 15 '16] , we derived a rup-

ture model for the Myanmar earthquake ( Fig. 2 ( a) ) .

No.1

Liu Chengli,et al. Co-seismic deformation and gravity changes of the 2011

India-Nepal and Myanmar earthquakes 3

4

l 8 l 12 Cl

16

a- 4

"' 2 -18 -12 16 0 6

~~~~~~ .... ~~~em 0 50 100 150 200 250 300 350 400 450 500

(a) Co-seismic slip model (White arrows

represent amplitude and direction of the slips)

12

(B) Distribution of used seismic

Stations( triangles; The red star indicates the hypo/epicenter)

Figure 2 Co-seismic slip model of the Myanmar earthquake

Mter comparing with the distribution of aftershocks, we

determined a fault plan with the strike of 250° and dip

of 86 °. In order to get a high resolution image of the rupture process, we divided the fault plane into 171 el-

ements with a spatial dimension of 2. 0 km by 2. 0 km.

2. 3 Layered crustal model

According to the velocity model of eastern Nepal[!?]

and based on the model of Crust 2. 0 , we established

the multi-layered crustal models in India-Nepal and

Myanmar areas, respectively. In this paper, we only

discuss co-seismic effect caused by these two events ,

and hence assume the crust and upper mantle to be

purely elastic layers, and the model parameters are lis-

ted in table 1 and table 2.

Table 1 A multi-layered earth model for the

India-Nepal earthquake

Serial No.

2

3

4

Serial No.

2

3

4

5

Depth Vp Vs Density (km) (km·s- 1 ) (km·s- 1 ) (km·s- 3 )

0.0-3.00 5.5 3.2 2 100

3.00-23.0 5.7 3.2 2 400

23.0-55.0 6.3 3. 7 2 700

55.0 8.0 4.5 3 450

Table 2 The multi-layered model for the

Myanmar earthquake

Depth Vp Vs Density (km) (km·s- 1 ) (km·s- 1 ) (km·s- 3 )

0.0-1.00 2.5 1.2 2 100

1.00-19.0 6.1 3.5 2 750

19.0-36.0 6.3 3.6 2 800

36.0-45.0 7.1 4.0 3 100

45.0 8.0 4.6 3 350

3 Modeling co-seismic deformation and gravity changes

Here we present the result of our calculation for the

surface co-seismic deformation and gravity changes of

the Myanmar and India-Nepal earthquakes according to

the above-mentioned models.

3.1 Co-seismic horizontal displacement

Figure 3 shows the calculated co-seismic horizontal

surface displacements caused by the India-Nepal and

Myanmar earthquakes. It may be seen that their maxi-

mum displacements are quite different, about 9. 6 em

and 36 em, respectively, although their magnitudes are

comparable. The difference may be due to either differ-

ent models used or different focal depths ( Tab. 3 ) .

The depth of the India-Nepal earthquake we used is

19. 7 km (from USGS) , which is almost twice as large

as the depth of Myanmar earthquake ( 10 km) ; hence,

the smaller displacement.

3. 2 Co-seismic vertical displacement and gravity

changes

Figures 4 and 5 show , respectively, the calculated co-

seismic vertical deformation and gravity changes

caused by the India-Nepal and the Myanmar earth-

quakes. The vertical displacements of the India-Nepal

earthquake show an irregular pattern in four quadrants

( Fig. 4 ( a) ) , which is consistent with its focal mech-

anism, which shows a left-lateral strike slip with an

oblique thrust component. This pattern indicates up-

ward displacements in northern and southern near-

field, and western and eastern far-field (due to com-

4 Geodesy and Geodynamics Vol. 3

30"N

I r

Mw6.9e

5cm

CHINA

~ asa

(a) Horizontal co-seimic surface displacements of the India-Nepal earthquake

22"N

~---Chiang Khong

. ·L~2o·N

oe.D~ ;: 100 "E 101 "E 102 "E

(b) Horizontal co-seimic surface displacements of the Myanmar earthquake

Figure 3 Horizontal co-seimic surface displacements ( Stars indicate epicenters ; circles , main cities)

Table 3 Estimated depth of the India-Nepal

earthquake by different institutes

Institutes Depth (km)

IGP-CEA I 20

USGS 11 19.7

Harvardm 47.4

I http :I /www. csi. ac. en; ll http :I I earthquake. usgs. gov; m http :I /www. globalcmt. org

pression) , with a maximal uplift of 4. 65 em. It also

indicates downward displacements in the northern and

southern far-field, and western and eastern near-field

( due to tension ) , with a maximal subsidence of

- 1. 26 em. The gravity change shows a similar pattern

( Fig. 4 ( b ) ) . The gravity changes are large in the

near-field , with extreme values of 7. 68 f.LGal and

- 1. 49 f.LGal, respectively, but relatively small in the

far field.

On the other hand , the co-seismic vertical displace-

ment and gravity changes caused by the Myanmar

earthquake show a clear symmetrical pattern in four

quadrants ( Fig. 5 ) : compressive uplift of as much as

3. 89 em in the two N-W and E-S uplifting lobes and

tensile subsidence of as much as - 1. 28 em in the two

N-E and S-W lobes. The gravity changes are consist-

ently similar, with extreme changes of 10. 99 f.LGal and

- 8. 02 f.LGal, respectively ( Fig. 5) .

4 Sensitivity of parameters

4.1 Focal-depth uncertainty

In the inversion of the India-Nepal earthquake the focal

depth is quite uncertain for lack of effective con-

straints , and this uncertainty affects our simulation re-

sults. In order to asses this effect, we select four near-

field points around the hypocenter to examine the sur-

face deformation and gravity changes with different fo-

cal depths ( Fig. 6) . The results show that the effect is

relatively small on vertical displacement and gravity

changes ( Fig. 6 ( b) and ( c) ) , but relatively large

on horizontal displacement (Fig. 6 (d) ) , at near-field

locations.

4. 2 Differences between the average-slip and

rmite-element rupture models

As shown in figures 7 and 3 (b) , for the Myanmar

earthquake the maximum co-seismic near-field surface

horizontal displacements calculated on the basis of the

average-slip model are smaller than those of the finite-

element model. As to the corresponding vertical dis-

placements ( Fig. 8 ) , both models give symmetric dis-

tributions of comparable size in the far field with

change of sign in near field , where the maximum uplift

and decline are 1. 87 em and -4. 39 em, respective-

ly, according to the average-slip model; these are

slightly larger than the values of the finite-element

No.1

Liu Chengli,et al. Co-seismic deformation and gravity changes of the 2011

India-Nepal and Myanmar earthquakes

em 6.00 3.00 2.00 1.00 0.10 0.03 0.01

-0.10 -0.03 -0.10 -0.30 -0.50 -1.00

24'NL-----~----~~---L----~--~ -3.00

84'E 86'E 88'E 90'E 92'E 84'E 86'E 88'E 90'E (a) Co-seimic surface vertical displacement (b) gravity changes

92'E

Jl Gal 12.00 6.00 1.50 1.10 0.03 O.Ql

-0.01 -0.10 -0.30 -0.50 -1.00 -3.00

Figure 4 Co-seimic surface vertical displacement and gravity changes caused by the India-Nepal earthquake

( Positive is downward ; yellow stars indicate the epicenter; green circles , main cities)

96'E 100 'E 102 'E

(a) Co-seimic surface vertical displacement

em 4.00 2.00 0.50 0.30 0.10 0.03

-0.01 -0.03 0.10 0.30 0.50 1.50 3.00

96'E 100 'E 102 'E

(b) Gravity changes

J1Gal 12.00 6.00 3.00 1.00 0.10 0.00 0.01

-0.01 -0.03 -1.00 -2.50

-10.00

Figure 5 Co-seimic surface vertical displacement and gravity changes caused by the Myanmar earthquake

( Positive is downward ; yellow stars indicate the epicenter; green circles , main cities)

Figure 6 Variation of vertical displacement ( positive downwards in b) , gravity ( c) ,

and horizontal displacement (d) with focal depth for the India-Nepal earthquake at

four locations shown as stars in corresponding colors. Dash line

indicates the focal depth shown in table 3.

5

6 Geodesy and Geodynamics Vol. 3

98 'E 99 'E 100 'E 101 'E 102 'E

Figure 7 Horizontal co-seismic surface displacements of the Myanmar earthquake obtained

by using a finite-element rupture model ( red arrows) and an average-slip model.

(green arrows) . Star indicates the epicenter; circles, main cities

20

96'E 100 'E

(a)

102'E 96'E 100'E

(b)

102'E

4.00 2.00 0.50 0.30 0.10 0.03

-0.01 -0.03 -0.10 -0.30 -0.50 -1.50 -3.00

Figure 8 Vertical co-seismic surface displacement of the Myanmar earthquake (positive is downward)

obtained by using ( a) a finite-element rupture model, and ( b) an average-slip model

rupture model. shown in figure 8 ( a) . This discrepancy

is due to the simplification of the slip parameter in

the average-slip model. However, without a reliable

co-seismic rupture model, the simulation result of the

average-slip model is still acceptable.

5 Conclusions and discussion

1 ) Although both earthquakes are strike-slip events

with comparable magnitudes, they have different calcu-

lated maximum co-seismic horizontal displacements

(India-Nepal, 9. 5 em; Myanmar, 36 em). This dis-

crepancy may be due to their different focal depths and

the use of different slip models.

2) The vertical displacement and gravity changes of

both earthquakes exhibit a pattern of four quadrants.

Different from Myanmar earthquake, the signs of the

gravity changes in the near-field for the India-Nepal

earthquake are almost opposite to those in the far field,

and the near-field displacement changes show asym-

metric distribution. The gravity changes are consistent

with the vertical deformation, but in the near-field, it

is only at the epicenter that gravity has increased; with

increasing epicentral distance, the gravity shows a de-

crease and then an increase.

3 ) The results based on the two different models are

only slightly different in the near-field due mainly to

the simplified parameters of average-slip model. Thus,

No.I

Liu Chengli,et al. Co-seismic deformation and gravity changes of the 2011

lodia-Nepal and Myanmar earthquakes 7

without a reliable finite-element rupture model , the use

of the average-slip model is acceptable.

Our analysis of co-seismic surface displacement and

gravity field changes for the India-Nepal earthquake

and Myanmar earthquake may provide useful informa-

tion for the research of seismic activity, long-term tec-

tonic activity , and deformation field in the houndary

zone between the India and Eurasia plates, the east-

em structural belt of Tibetan plateau , and surrounding

areas.

It should be noted that the complexity nf rupture

have a significant effect on the model computation , es-

pecially in the near field. In this study, we used the

average-slip and finite-fault models. H more data

( such as near-field GPS , InSAR and teleseismic ) are

available in the combined inversion to obtain a more

accurate co-seismic rupture model, we should be able

to obtain a better result.

Acknowledgement:

We would like to thank Prof. Wang Rongjiang for

providing calculation software. Seismic data used in

this paper is provided by the United States IRIS seismic

data management center. The figures were made using

free GMT (Generic Mapping Tools) software.

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Co-seismic deformation and gravity changes of the 2011 India-Nepal and Myanmar earthquakes1 Introduction2 Theory and models3 Modeling co-seismic deformationand gravity changes4 Sensitivity of parameters5 Conclusions and discussion