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SCIENCE CHINA Earth Sciences
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*Corresponding author (email: [email protected])
• RESEARCH PAPER • September 2014 Vol.57 No.9: 2036–2044
doi: 10.1007/s11430-014-4827-2
A rupture blank zone in middle south part of Longmenshan Faults:
Effect after Lushan Ms7.0 earthquake of 20 April 2013 in
Sichuan,
China
GAO Yuan1*, WANG Qiong1,3, ZHAO Bo2,3 & SHI YuTao1,3
1 Institute of Earthquake Science, China Earthquake
Administration, Beijing 100036, China; 2 China Earthquake Networks
Center, China Earthquake Administration, Beijing 100045, China;
3 Institute of Geophysics, China Earthquake Administration,
Beijing 100081, China
Received July 19, 2013; accepted November 14, 2013; published
online May 20, 2014
On April 20, 2013, the Lushan Ms7.0 earthquake struck at the
southern part of the Longmenshan fault in the eastern Tibetan
Plateau, China. The shear-wave splitting in the crust indicates a
connection between the direction of the principal crustal
com-pressive stress and the fault orientation in the Longmenshan
fault zone. Our relocation analysis of the aftershocks of the
Lushan earthquake shows a gap between the location of the rupture
zone of the Lushan Ms7.0 earthquake and that of the rup-ture zone
of the Wenchuan Ms8.0 earthquake. We believe that stress levels in
the crust at the rupture gap and its vicinity should be monitored
in the immediate future. We suggest using controlled source
borehole measurements for this purpose.
Lushan earthquake, Longmenshan Fault, rupture gap, crustal
seismic anisotropy, double difference relocation, bore-hole
measurements of stress change
Citation: Gao Y, Wang Q, Zhao B, et al. 2014. A rupture blank
zone in middle south part of Longmenshan Faults: Effect after
Lushan Ms7.0 earthquake of 20 April 2013 in Sichuan, China. Science
China: Earth Sciences, 57: 2036–2044, doi:
10.1007/s11430-014-4827-2
1 The Lushan Ms7.0 earthquake and the back-ground of regional
seismicity
The Ms7.0 earthquake that hit Lushan County, Ya’an, in the
Sichuan province of China on April 20, 2013 at 08:02 (Bei-jing
time) caused serious casualties in Lushan, Baoxing, and the
surrounding area. According to the China Earthquake Network Center
(CENC), the location of the epicenter was at 30.0° N, 103.0° E and
the magnitude and depth of the hypocenter were Ms7.0 and 13 km,
respectively. Here, we refer to this earthquake, called the “4·20”
Ms7.0 Lushan earthquake in Sichuan, as the Lushan earthquake.
Since Cenozoic times, the Indian plate has been pushing
the Eurasian plate, causing the uplift of the Tibetan Plateau
and a shortening and thickening of the crust in that region. The
eastern part of the Tibetan Plateau, from north to south, extrudes
northeastward, eastward, and southwestward, re-spectively. This
large-scale continuous tectonic movement frequently brings about
strong earthquakes in the interior of the Tibetan Plateau and its
vicinity (Gao et al., 2000, 2001). The Lushan earthquake occurred
in the southern part of the Longmenshan (LMS) fault which is
located along the east-ern margin of the Qinghai-Tibetan block
(QTB). Because of the eastward extrusion of the QTB and the
resistance of the rigid crust of the Sichuan basin, crustal
material beneath the eastern margin of the QTB flows eastward,
leading to an upward thrust of the ductile material in the lower
crust which forms the thrust-type LMS fault (Zhang et al., 2013).
The tectonic features and velocity structure of the LMS
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Gao Y, et al. Sci China Earth Sci September (2014) Vol.57 No.9
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zone display velocity variations that are strongly transverse,
and heterogeneity in the crust (Wang et al., 2010; Zhang et al.,
2009a, 2013; Lei et al., 2009; Zhang et al., 2011). Based on the
crustal structure in the LMS zone (Lei et al., 2009), the
hypocenter of the Lushan earthquake lies in the southern part of
the LMS fault in the fast P-velocity variation zone; this is highly
consistent with reports from recent studies of the southeast margin
of the Qinghai-Tibetan Plateau (Gao et al., 2000).
According to the global earthquake catalog of the Pre-liminary
Determination of Epicenters (PDE), the distribu-tion of earthquakes
of M≥5 after the year 2000 in the east-ern margin of the QTB and
its vicinity shows the following characteristics (Figure 1). (1)
Before the Wenchuan Ms8.0 earthquake on May 12, 2008, earthquakes
occurred mainly in the east and southeast margins of the QTB and
extended into the Yunnan Province. Over a period of eight years and
four months a total of 23 earthquakes of M≥5 were rec-orded in the
region, about 2.8 earthquakes per year. (2) In the period after the
Wenchuan Ms8.0 earthquake and before the Lushan Ms7.0 earthquake,
the distribution of earth-quakes mainly in the east and southeast
margin of the QTB was similar and extended to Yunnan. In the
central and northern parts of the LMS fault, however, many strong
af-tershocks of the Wenchuan earthquake were recorded, as well as
one earthquake in the Sichuan basin. In less than five years, 19
earthquakes of M≥5 occurred in this region in addition to the
Wenchuan earthquake and its 73 after-shocks of M≥5, an average of
about 3.9 earthquakes per year. (3) After the Lushan earthquake,
only one earthquake occurred apart from the six Lushan earthquake
aftershocks of M>5; this was the Mb5.3 earthquake on April 25,
2013.
The preliminary report of the CENC shows that this earth-quake
was located at the junction of three counties: Changning, Hongxian,
and Wenxing in Sichuan province and its magnitude was Ms4.8. Thus,
we see an increase in the earthquake frequency in the period
between the Wen-chuan Ms8.0 earthquake and the Lushan Ms7.0
earthquake, compared with the period before the Wenchuan
earthquake, apart from the Wenchuan earthquake and its
aftershocks.
2 Direction of the principal compressive stress indicated by
shear-wave splitting in the crust and its relationship to the LMS
fault
To study the crustal anisotropy around the LMS fault, rec-ords
of the aftershocks of the Wenchuan Ms8.0 earthquake were analyzed.
These were recorded by portable seismic stations around the LMS
fault set up after the Wenchuan earthquake, and by the permanent
Sichuan Seismic Net-works (Zhang Y J et al., 2008). The shear-wave
splitting results beneath the stations were obtained using the
System-atic Analysis Method (SAM) of shear-wave splitting (Gao et
al., 2008b). Shi et al. (2009) found that up to the bounda-ry of
Anxian County, the predominant direction of the po-larizations of
the fast shear-waves was NNE at stations lo-cated in the northeast
part of the LMS fault (zone B). This is consistent with the strike
of the fault. For stations located in the central and southern
central part of the LMS fault (zone A), the predominant direction
of the polarization of the fast shear-waves was NW, nearly
perpendicular to the strike of the fault. From the area southwest
of the epicenter of the Lushan earthquake in the southern part of
the LMS fault,
Figure 1 Seismicity of the Longmenshan fault zone and vicinity.
Distribution of M≥5 earthquakes (a) from January 1, 2000 to May 11,
2008; (b) from May 12, 2008 to April 19, 2013, yellow pentagram
represents Wenchuan earthquake; (c) from April 20, 2013 to April
30, 2013, yellow pentagram represents the Lushan earthquake. Faults
are depicted in blue lines. Earthquake data are from the PDE USA
catalog. Red dots indicate Ms5.0–5.9 earthquakes and brown larger
dots indicate Ms6.0–6.9 earthquakes. Yellow pentagrams represent
the Wenchuan earthquake of May 12, 2008 and the Lushan eartqhake of
April 20, 2013. The magnitudes of these two earthquakes are from
China Earthquake Network Center (CENC).
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extending almost to the Xianshuihe (XSH) fault (zone C), the
polarization directions of the fast shear-waves appear very
scattered; however, the average direction is almost E-W (Figure
2).
Studies have shown that the predominant direction of fast
shear-wave polarization is generally parallel to the direction of
the in situ principal compressive stress (Gao et al., 2008c, 2011,
2012) whose characteristics are related to the regional tectonics
and could indicate a hidden strike-slip fault (Gao et al., 2011).
The varying polarization directions of the fast shear-waves of the
Wenchuan earthquake aftershock se-quence indicate a thrust in the
southwest section of the LMS fault and clear strike-slip motion in
the northeast section of the fault (Figure 2). Geological studies
after the Wenchuan earthquake verified the existence of local
strike-slip faults in the northeast section of the LMS fault.
Comprehensive re-sults at three stations (MDS, GZA, and L5503) near
the intersection of the LMS fault, the XSH fault, and the An-
ninghe (ANH) fault show that the average direction of the fast
shear-wave polarization was close to E-W, although that of station
L5503 was very scattered (Shi et al., 2009). This is related to the
complicated stress distribution induced by the complex local
tectonics around station L5503.
After combining the records of the temporary and per-manent
seismic stations, additional observational infor-mation of crustal
shear-wave splitting in the LMS fault zone was obtained from longer
near-field records of small local earthquakes including aftershock
sequences of the Wen-chuan earthquake (Shi et al., 2013). Using
records of small local earthquakes from January 2000 to April 2010,
the central and western parts of the LMS fault were further
di-vided into two sections, with their boundary at the location of
the Lushan earthquake. Along the LMS fault we identi-fied three
subsections of predominant polarizations of fast shear-waves. The
aftershock sequence of the Wenchuan earthquake also showed that the
boundary line between the
Figure 2 Equal-area projection rose diagram of polarizations of
fast shear-waves in the crust from the Wenchuan aftershock sequence
data.The Longmen-shan Fault is divided into three parts: zones A,
B, and C. The three equal-area rose diagrams of fast shear-wave
polarizations (white circles) are the results of all the available
data in zones A, B, and C. Black lines indicate faults and black
thick arrows indicate the direction of the compressive stress;
arrow with circle represents the principal compressive strain
induced from GPS data. Yellow pentagram in the north represents the
2008 Wenchuan earthquake and that in the south is the 2013 Lushan
earthquake. Blue triangles are regional seismic stations and white
triangles are temporary stations. Straight lines at the station
show the average polarization direction per station. Red lines
represent effective data from more than 11 records and brown lines
represent effective data from only one or two records. Grey small
dots are Wenchuan aftershocks after relocation. QTB:
Qinghai-Tibetan Plateau; LMS: Longmenshan Fault; XSH: Xianshuihe
Fault; ANH: Anninghe Fault.
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Gao Y, et al. Sci China Earth Sci September (2014) Vol.57 No.9
2039
sections of different polarizations was in the same area, near
Anxian County. Our analysis showed results similar to those of a
previous study that was based only on the after-shock sequence of
the Wenchuan earthquake (Shi et al., 2009): the predominant
direction of the fast shear-wave polarizations in zone B is close
to NNE, consistent with the strike of the fault. The predominant
direction in zone A is close to NW, nearly perpendicular to the
strike of the LMS faults but showing an obvious predominant
direction con-sistent with the surface strike of the fault. In zone
C, the polarizations of the fast shear-waves are scattered;
however, the overall predominant polarization is nearly E-W (Figure
3).
In the southern part of the LMS fault and in zone C where
several faults intersect, the shear-wave splitting is related to
the regional stress field influenced by the faults and the deep
tectonics. At stations MDS and SMI, along the Sichuan basin, the
direction of the predominant fast shear-wave polarization was close
to E-W. This is related to
the eastward push of the QTB that is obstructed by the Si-chuan
basin (Figure 3). We used local records (11 records from MDS and 19
from GZA (Shi et al., 2009)) of the Wenchuan earthquake aftershock
sequences and some small earthquakes before and after the
earthquake to obtain effec-tive data of shear-wave splitting. The
predominant fast shear-wave polarization at MDS had two directions;
one was almost N-W and the other was close to ENE. The fast
shear-wave polarization at GZA was scattered, without an obvious
predominant direction, but the average direction was about E-W with
a standard error of 32.8° (Figure 2).
From seismic activity recorded between January 2000 to April
2010 at MDS and GZA we obtained effective shear- wave splitting
data (48 records from MDS and 52 from GZA) (Shi et al., 2013). The
predominant fast shear-wave polarization direction at MDS is
clearly ENE (63.5°). The polarizations of the fast shear-waves at
GZA is scattered, with no obvious predominant polarization, but the
average direction is about in ENE, close to E-W, with a
standard
Figure 3 Equal-area projection rose diagram of polarizations of
fast shear-waves in the crust in the study area. Data are from
January 2000 to April 2010, and the temporary stations recorded
only the Wenchuan aftershock sequences. The Longmenshan Fault is
divided into three parts: zones A, B, and C. The three equal-area
rose diagrams of fast shear-wave polarization (white circles) are
the results of all the available data in zones A, B, and C. The
diagram in zone C includes three permanent stations and a temporary
station L5503 with only one record; red lines show the
polarization. Blue and white triangles rep-resent the regional
permanent and temporary seismic stations, respectively. Yellow
pentagrams represent the 2008 Wenchuan earthquake and the 2013
Lushan earthquake. Other notations are the same as in Figure 2.
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2040 Gao Y, et al. Sci China Earth Sci September (2014) Vol.57
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error of 35.1° (Figure 3). Thus, we found that the predomi-nant
polarization of the fast shear-waves at station MDS changed from
two predominant polarizations to one pre-dominant polarization.
This suggests a change of the stress field in the crust under
station MDS. The predominant fast shear-wave polarization in the
ENE direction indicates an in situ horizontal principal compressive
stress in the ENE di-rection, which leads to a NE-striking rupture
of the thrust fault. This inference needs further evidence. In
addition, the predominant polarization of the fast shear-waves at
SMI is clearly in the WNW direction, close to E-W (95.8°). The
horizontal principal compressive stress in this direction is almost
perpendicular to the ANH fault nearby, which results in some
locking of the ANH fault.
3 Relocation analysis of the Lushan earthquake and its
aftershocks
To monitor the aftershocks of the Lushan earthquake, tem-porary
seismic networks were deployed immediately after the earthquake by
the research institutes of the China Earthquake Administration and
the Earthquake Administra-tion of Sichuan Province. Combining the
data recorded by the permanent and temporary seismic networks, we
per-formed relocation analysis of the aftershock sequence of the
Lushan earthquake.
The double difference relocation algorithm (Waldhauser et al.,
2000), as a relative location technique, has been ap-plied most
widely to relocation problems of earthquakes in recent years. It
utilizes travel-time differences for pairs of earthquakes at each
seismic station to reduce the depend-ence of the velocity model.
The algorithm does not need to define the main shock and does not
rely on the initial loca-tion of the earthquake, thus avoiding
inaccuracy due to lo-cation error of the main shock (Zhao et al.,
2013). It was used for the relocation of the aftershock sequence of
the 2010 Yushu earthquake and the results showed that the
af-tershocks were well distributed along the strike of the Yu-shu
fault, confirming the reliability of this method (Zhao et al.,
2012). In this study we applied the double difference relocation
algorithm to the aftershock data of the Lushan earthquake.
Using observations from the temporary seismic network set up
after the Lushan earthquake by the earthquake emer-gency management
system of the China Earthquake Ad-ministration, the aftershock
sequence of the Lushan earth-quake from April 20 to May 2, 2013
(Figure 4) was used for the relocation. We collected records from a
total of 3813 earthquakes and obtained relocation estimates of 3708
of these earthquakes using the double difference relocation
algorithm. We used records from 75 seismic stations, which included
60 stations of the regional permanent seismic net-works and 15
stations of the temporary seismic networks
Figure 4 Relocation of the April 20, 2013 Lushan earthquake,
Sichuan and its aftershock sequences. Aftershock sequence was
recorded from April 20 to May 2, 2013. Yellow circle is the
mainshock of the Lushan Ms7.0 earthquake and small red circles are
the aftershocks after relocation. Triangles are seismic stations
(blue: permanent stations, white: temporary stations). Other
notations are the same as in Figure 2.
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Gao Y, et al. Sci China Earth Sci September (2014) Vol.57 No.9
2041
(Figure 5). Comparing the histograms before and after the
relocation, the aftershocks of the Lushan earthquake oc- curred
mainly at a depth range of 10–20 km.
The initial rupture depths of the hypocenters of the Lushan
earthquake and the five strong aftershocks of mag-nitude M≥5 range
from 15 to 20 km. The initial rupture depth of the hypocenter of
the main Ms7.0 shock is about 18 km (Table 1). In this depth at the
southern part of the LMS fault, the hypocenter of the Lushan
earthquake is at the transition zone between the high velocity and
low velocity zones and is almost within the high velocity zone,
which is the structure that promotes generation of large
earthquakes (Gao et al., 2000).
The parameters of the focal locations of the Lushan earthquake
and its strong aftershocks of M≥5 (Table 1) show a significant
change in the focal depth parameters. It should be noted that the
focal depth is a seismic parameter that is difficult to determine
since it involves measurements and definitions which vary. For a
strong earthquake with a large range of rupture, different depth
definitions will result in different depth values (Gao et al.,
1997).
4 The “no rupture zone” in the central Long-menshan fault
Based on the relocation of the Wenchuan earthquake after-shock
sequence (Zhao et al., 2011), the Lushan earthquake and its
aftershock sequence occurred in the southern part of the LMS fault,
southwest of the Wenchuan earthquake. From the earthquake location
map (Figure 6) we see no ac-tivity in the middle and southern
regions of the central part of the LMS fault, between the location
of the Lushan earth-quake including its aftershocks and that of the
Wenchuan earthquake including its aftershocks; this region is
called the “no rupture zone”.
Based on the results of the emergency analysis of the fo-cal
source of the Lushan earthquake by the Institute of Earthquake
Science 1) , China Earthquake Administration (Gao et al., 2001),
and the rapid results from other research institutions around the
world, the focal mechanism of the main shock of the Lushan
earthquake shows a reverse-type motion of high dip angle. Combining
this with the distribu-tion of faults and aftershocks, we conclude
that the fault
Figure 5 Histograms before and after relocation of aftershock
sequences of the April 20, 2013 Lushan Ms7.0 earthquake, Sichuan.
(a) Before relocation; (b) After relocation. Aftershock sequence is
recorded from April 20 to May 2, 2013.
Table 1 Relocation results of M≥5 aftershocks of the Lushan
Ms7.0 earthquake a)
Time Before relocation
After relocation
Magnitude Ms Latitude (N) Longitude (E) Depth (km) Latitude (N)
Longitude (E) Depth (km)
2013-04-20 08:02:46 30.3° 103.0° 13 30.29° 102.97° 17.8 7.0
2013-04-20 08:07:29 30.3° 102.9° 10 30.35° 102.95° 18.5 5.1
2013-04-20 11:34:14 30.1° 102.9° 11 30.18° 102.88° 15.1 5.3
2013-04-21 04:53:44 30.3° 103.0° 17 30.35° 103.04° 19.8 5.0
2013-04-21 17:05:22 30.3° 103.0° 17 30.33° 103.03° 16.4 5.0
a) Hypocenter parameters before relocation; magnitudes are from
the rapid official report of CENC
1) Institute of Earthquake Science, China Earthquake
Administration. 2013. Working information, Nos. 31, 33, 34.
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2042 Gao Y, et al. Sci China Earth Sci September (2014) Vol.57
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Figure 6 Aftershock sequences of the 2013 Lushan Ms7.0
earthquake and 2008 Wenchuan Ms8.0 earthquake. Large and small
yellow circles represent the 2008 Wenchuan earthquake and Lushan
earthquake, respectively. White circles represent strong
aftershocks of 5.0≤M
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Several faults intersect in the southern end of the LMS fault.
Unlike the thrust motion in the central part of the LMS fault, the
tectonics in the southern end of the LMS fault varies because of
the intersection of several faults; therefore, the stress is also
dissimilar. However, the Lushan earthquake itself was a thrust
event, as was the Wenchuan earthquake. This highlights the
existence of an unbroken zone between the location of the Lushan
earthquake and that of the Wenchuan earthquake.
The energy released by the Wenchuan earthquake origi-nated from
the slow accumulation of long-term large-scale crustal strain in
the LMS fault in the western margin of Si-chuan Basin. This strain
is caused by the Bayan Har block pushing against the LMS fault from
the west (i.e. the Songpan-Garze block in Figure 6) (Jiang et al.,
2009). The seismic profile across the LMS fault reveals a
discontinuity in the Moho and a sharp increase in the velocity
ratio Vp/Vs (Zhang et al., 2009b, 2010). The velocity structure
also shows a velocity variation under the LMS fault (Lü et al.,
2013), indicating deep structure. Based on the tectonic
characteristics of the region and on past and current earth-quake
activity, we note that for at least 1100 years before 2008 there
were no earthquakes of magnitude ≥Ms7.0 in the LMS fault. Along the
LMS fault, a zone formed with very low seismicity compared with the
seismic activity of the surrounding faults, and in 2008 the
Wenchuan Ms8.0 earthquake occurred in this “blank zone” (Wen et
al., 2009). After the Wenchuan earthquake, because of the
continuous squeezing eastward of the Songpan-Garze block in the
east-ern Tibetan Plateau, studies suggested that a risk of rupture
existed in the unbroken south segment of the LMS faults. The Lushan
earthquake released some of this accumulated strain, thus reducing
the possibility of an earthquake larger than Ms7.0 occurring in the
near future in the southern seg-ment of the LMS faults. However, we
still need to keep a watchful eye on this “no rupture zone” to
monitor the risk of it rupturing again.
5 Discussion and conclusions
The focal locations (Zhao et al., 2011; Zhang R Q et al., 2008)
and stress background (Zhang Y J et al., 2008; Shi et al., 2009,
2013; Ding et al., 2008) of the Wenchuan and Lushan earthquakes on
the LMS fault were obtained by analyzing the activity of
aftershocks and studying shear- wave splitting data. In this study
we examined the orienta-tion of the regional principal compressive
stress in the zone that extends southwest from the epicenter of the
Lushan earthquake along the LMS fault to the intersection of the
LMS, XSH, and ANH faults and the adjacent region. We found that the
orientation changes from NW in the northern part of this zone to
almost E-W further south. The spatial variation of the stress
indicates deep tectonic movement which will require further
investigation.
The Lushan earthquake occurred in the southern part of the LMS
fault. A “no rupture zone” appears after the Lushan earthquake,
which is of about 60 km long between the rupture zone of Lushan
earthquake and the rupture zone of Wenchuan earthquake. Within this
no-rupture zone, an earthquake of magnitude Ms6.2 occurred in 1970;
however, more than 40 years have passed since then. Our relocation
study of the aftershocks of the Lushan earthquake indicates that
the Lushan earthquake did not cut through itself and the rupture
zone of the Wenchuan earthquake. It is important for scientific
research and for the safety of the population in the region to pay
close attention to any changes in the “no rupture zone” between the
locations of the Lushan and Wenchuan earthquakes, by enhancing
seismic monitoring in the southern part of the LMS faults.
Close monitoring of stress variation in the crust can de-tect
signs of tectonic activity. Studies have shown that crus-tal
shear-wave splitting is one of the valid methods for monitoring
stress change caused by earthquake activity (Gao et al., 2004,
2008a; Crampin et al., 2008). However, this technique requires a
large number of small earthquake events in the vicinity of the
seismic stations since the data sources are seismic waves of small
earthquakes. More ef-fective methods are the cross-well techniques
using artifi-cial borehole sources and borehole records, e.g.
stress- monitoring sites (Gao et al., 2008a; Crampin et al., 2008).
A study of the San Andreas Fault, California continuously monitored
artificial borehole signals and measured velocity changes of
seismic waves (Niu et al., 2008). This is another proven effective
borehole method to measure the change of stress by a controllable
source. Both borehole measurement methods require significant
expenditure; therefore, their use is limited to measuring changes
of stress and wave velocity in key zones only.
Earthquakes occur frequently in the eastern and south-eastern
areas of the Tibetan Plateau. Monitoring very small changes in
stress and velocity throughout this region will deepen our
understanding of the seismic structure and seis-mic sources
affecting the tectonic activity in the region.
This work was supported by the National Natural Science
Foundation of China (Grant Nos. 41174042, 41040034) and the China
National Special Fund for Earthquake Scientific Research in Public
Interest (Grant No. 201008001). The relocation data of the
aftershocks of the Lushan earth-quake were obtained from the China
Earthquake Network Center (CENC) and several research institutions
of the China Earthquake Administration. We would like to thanks the
data sharing program of earthquake emergen-cy. We thank reviewers
for their comments and suggestions to improve this manuscript.
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