CO-SEISMIC DISPLACEMENT OBSERVATIONIN IN

DECORELATE BELT ALONG SEISMIC FAULT OF 2008

WENCHUAN EARTHQUAKE

Qu Chunyan, Zhang Guifang

State Key Laboratory of Earthquake Dynamics,

Institute of Geology, China Earthquake Administration(CEA)

Beijing 100029, e-mail: dquchy@sohu.com

2011.07

Tectonic Environment of Wenchuan earthquake

SAR Data and Processing Method

Processing Result and Interpretation

Conclusion

Outline

Tectonic Environment of Wenchuan earthquake

Tectonic Environment:

Wenchuan earthquake

occurred on Longmen

Shan thrust fault zone, it

bounds between the

Tibetan plateau and

Sichuan basin, where

the topography is high and

steep, the vegetation is

heavy. This make field

investigations very

difficult. Thus we used D-

InSAR technology to

capture the whole

coseismic deformation

fields.

Long Menshan thrust fault zone

Consist of 3 brunchs

Ms≥6.0After shock Ms=4.0-4.9Ms=5.0-5.9 (5.12 2008 ~ 7.5 2008)

F1

F2

F3

Main shock

Tibetan Plateau

Sichuan Basin

SAR data and processing Method

data processing: We used 7 track pairs of

ALOS/PALSAR data (471-

477). We adopted the two-pass

differential interferometric

mode and software GAMMA

to process these SAR data and

get the whole deformation

field caused by wenchaun

earthquake .

Active fault Lunmenshan thrust fault zone Track coverage

Ms≥6.0After shock Ms=4.0-4.9Ms=5.0-5.9 (5.12 2008 ~ 7.5 2008)

This Fig is the InSAR mosaic interferometric fringes from 7 data tracks, it shows the coverage scope and pattern of the entire deformation field caused by the Wenchuan earthquake. But We can note along the Northeastern direction from Yingxiu to Qingchuan, There is a clear incoherent zone, showed as the belt region encircled by yellow-dash line. We can not obtain the displacement of this area by InSAR technology. But using offset tracking method ,we can get the larger displacement in incoherent zone nearby the rupture trace. Pink big circle denotes epicenter of the main shock.

InSAR mosaic interferometric fringes

As shown in previous Fig, The InSAR technique has encountered the problem of serious incoherence on interferometric phases due to large displacement gradients nearby the fault. We can get the whole picture of the surface displacement field, but we can not know some detail information about fault rupture and deformation, such as how the rupture traces extending along the fault, and how large the displacements nearby the fault. To solve this problem, we attempt to use the pixel offset tracking method, which is not limited by phase coherence and allow the big displacements nearby the fault to be measured at a better precision. Some resulting picture showed as follow Figs.

Get the larger displacement in incoherent zone around the rupture trace by offset tracking

The Fig shows the displacements of 4 data tracks (472,473,474,475), derived from pixel offset tracking in range direction, each of them exhibit a clearly almost straight NE-trending line, which reveal the rupture trace.

Rupture trace and displacement revealed by single data track

This Fig show the entire surface rupture trace mosaicked from 7 data tracks along the Yingxiu-Beichuan fault, it indicate the scope and pattern of the surface rupture zone, it starting from about 26km west to Yingxiu town, extending northeast to 7km south of Qingchuan county. The total length of rupture belt is about 238km long. In addition, Along the Guanxian-Jiangyou fault, there is also a rupture belt, it is about 66km long.

Mosaic Rupture trace and displacement from 7 data tracks

This Fig shows the segmentation of displacement along the Yingxiu-Beichuan fault in range direction .Based on features of the rupture traces, such as their straightness, integration, continuity and deformation width on either side of the fault, The entire rupture zone can be divided into 5 segments. Showed as the Seg 1 To Seg5. Seg6 is for Guanxian-Jiangyou rupture zone.This is very important and useful for fault model construction and fault slip inversion.

Seg.1Seg.2Seg.3

Seg.4

Seg.5

Seg.6

Segmentation of displacements along the Yingxiu-Beichuan fault

Positions and features of Seg1:(105.130/32.425_104.577/31.966)

Segment 1 is from Suhe to

Chengjiaba, about 75km long,

it is straight and narrow, with

bigger deformation amplitude in

north wall, and broad and weak

deformation in south wall.

Seg.1

Positions and features of Segment 1

Seg.2

Positions and features of Segment 2

Positions and features of Seg2:

(105.130/32.425_104.577/31.966)

Segment 2 is from Chengjiaba to

Gaochuan, about 50km long, it is

complex and variable. The

displacement is mainly in 4~6m,

several points reaches 7~9m

Seg.3

Seg.4

Seg.5

Seg.6

Positions and features of Segment 3,4,5&6

Seg3: 61.5km long, straight, NE extendingSeg4: 29.5km long, complex and variable at Xiaoyudong, There is NW rupture, which offsets Yingxiu-Beichuan rupture zone left-laterally.Seg5: 27km long, featured by varied colors, no clear traces on seen.Seg6: Hanwang to Xiang’e, 66km-long rupture along Guanxian-Jiangyou fault.

unclear rupture trace

12 km south to Yingxiu-Beichuan fault, along Hanwang-Hongbai town-Bailu-Xiang’e, clear colored belt, 66km-long rupture along Guanxian-Jiangyou fault, with sense consistent to south wall of Yingxiu-Beichuan fault in

north, no visible rupture trace

Hanwang to Xiang’e

Guanxian-Jiangyou104.186/31.491103.703/31.070

6

unclear rupture trace

27km long, featured by varied colors, no clear traces on seen, smaller displacement less than 1m

Yingxiu to Dajinping

West to Yingxiu103.460/30.993103.344/30.874

5

complex rupture

29.5km long, relief and curve, local deformation 3-5m west of Xiaoyudong, at Hongkou and Yingxiu,

individually up to 6m, broader deformation in upper wall

Xiaoyudong to Yingxiu

Hongkou-Yingxiu103.720/31.213103.460/30.993

4

uniform rupture

61.5km long, straight, NE extending, broader deformation in upper wall up to 15~20km

Gaochuan to Xiaoyudong

Qingping-Jingtang104.159/31.627103.720/31.213

3

complex rupture

50km long, relief, big width (2~8km) and displacement, 4~6m depressions southwest of Chaping, Leigu and

northeast of Beichuan, few reaching 7~9m

Chengjiaba to Gaochuan

Beichuan-Chaping104.577/31.966104.159/31.627

2

Uniform rupture

75km long, straight, narrow, bigger amplitude in north wall, broad and weak deformation in south wall

Suhe to Chengjiaba

Pingtong-Nanba105.130/32.425104.577/31.966

1

NoteFeatures of rupture traceslocationName and positionsNo.

Table 1 Description of positions and features of each rupture segments derived from pixel offset tracking Method

The Fig shows Comparison of rupture traces from this work and rupture sites from field investigation. Red lines is rupture traces. Black dot short lines hint borders between segments. Blue, green and pink little dots are field investigation sites of Xu et al. It is obvious that the surface rupture determined by this work is well in accordance with that from the field investigation. But this result cover the entire fault and reveal a continuous rupture and its exact position, geometric variations, and segmentation along fault strike

Comparison of rupture traces and rupture sites from field investigation

This Fig shows displacements on northern side of rupture traces from pixel offset (Left) and vertical displacement from field investigation(Right: Xu et al, 2008)We can see the displacement distribution is very complex, it varied along the fault strike. Overall, the average displacement in the north of the rupture is 2.95m, individual max value is up to 7~9m. In generally, The result of this paper agree with the field investigation.

Comparison of Displacements between this work and field investigation

-2

0

2

4

6

8

1 10 20 30 40 Distance/km dis

plac

emen

t/m

-4

-2

0

2

4

6

8

1 10 20 30 40 Distance/km d

ispl

acem

ent/

m

This Fig shows displacement profiles in Beichuan and Dujiangyan respectively across the YBF, it indicates the largest displacements for the entire rupture zone is about 6m in up side and 2m in down side of the seismic fault

(a) East of Beichuan.

(b) North of Dujiangyan

Displacement profiles

By pixel offset tracking, we can reveal surface rupture traces and large displacements nearby seismic fault. This is a good supplement to conventional InSAR method. The result from pixel offset tracking shows that the wenchuan earthquake has created a 238km-long surface rupture zone along YBF. The average displacement is about 2.95m and 1.75m , respectively on northern and Southern side, Nearby Beichuan and north of Dujiangyan, the deformation is great and complex. In terms of the rupture pattern and deformation widths on either side, the YB rupture zone can be divided into 5 segments. Along the GJ fault, there is a 66km-long rupture belt, with 2m displacement, but no visible rupture traces . At Xiaoyudong, there is a 8km long NW-directed deformation belt, it offsets the NE-trending Yingxiu-Beichuan rupture zone in left-lateral manner.

Conclusions

Thank yo u ve ry m uc h !