CHINESE JOURNAL OF GEOPHYSICS Vol.48, No.3, 2005, pp: 692∼700
RELOCATION OF SMALL EARTHQUAKES IN WESTERN SICHUAN,
CHINA AND ITS IMPLICATIONS FOR ACTIVE TECTONICS
ZHU Ai-Lan1 XU Xi-Wei1
ZHOU Yong-Sheng1
YIN Jing-Yuan2 GAN Wei-Jun1 CHEN Gui-Hua1
1 Institute of Geology, China Earthquake Administration, Beijing 100029, China
2 Seismological Bureau of Shanghai, Shanghai 200062, China
Abstract We relocated 13367 small earthquakes that occurred in western Sichuan between 1992 and 2002 using
a double-difference (DD) earthquake location algorithm to improve the relative location precision. The relocated
microseismicity forms highly organized structures that correlate with the surface faulting well: showing the flower
structure across the simple strike-slip fault, and the organized but dispersed structure across the pull-apart
basin and thrust fault. Beneath thrust fault, there exists an aseismic layer. The seismicity shows segmentation
characteristics on active fault zones. Other significant tectonic features that were previously obscured by routine
location errors are also revealed by the relocated seismicity. Some blind faults are delineated by lineages of
seismicity that are suggestive of faulting structures. A series of large voids in seismicity appear with dimensions
of tens of kilometers on the Xianshuihe-Anninghe-Zemuhe fault zone that have been aseismic over the 10-year
time interval, suggesting that these segments may be locked and storing strain energy for release in future large
earthquakes. A 5km thick aseismic layer appears in most places in western Sichuan plateau at depths of 15∼20km.
The crustal strength envelopes are calculated for the western Sichuan plateau based on the results from hightemperature and high-pressure experiments. The result shows that the granite at depths of 14 to 19km appears
to be ductile, which is in good agreement with the thickness and depth range of the aseismic layer, suggesting
that the aseismic layer may be the result of the ductile deformation from the granite in the upper crust.
Key words
Double difference algorithm, Small earthquake relocation, Active tectonics, Western Sichuan.
1 INTRODUCTION
It is generally accepted that seismicity is closely correlated with tectonic activity, and microseismicity
contains rich tectonically active information. However, in most cases, the accuracy of earthquake locations
from routine location is quite limited, which usually obscures the true tectonic features, hence putting great
impediment on utilization of these basic data for active tectonic study and seismic hazard estimation.
The precision of the relative locations among events can be improved dramatically by using relative location
methods which avoid the need for 3-D detailed velocity structure model to accurately predict the travel times
from seismic source to receivers. In this way, the scatter among hypocenters can be reduced, and fine structures
of seismicity get revealed. Therefore these techniques provide a powerful tool for using microseismicity to image
the crustal fault, blind active tectonic structure, plate subduction zone and volcanic tectonics[1∼5] .
Western Sichuan, located in the southeastern margin of Qinghai-Tibetan plateau, is one of the most
seismically and tectonically active regions in continental China. Due to the extrusion of material from the
plateau, the rhombic Sichuan-Yunnan tectonic block rotates clockwise to southeastward, which leads to the
strong faulting activity and high-frequency occurrence of large earthquakes inside and especially along the
boundary of the block[6∼9] . As the northern part of this well-known block, western Sichuan has attracted the
notice of many researchers for a long time to carry out investigations on relationship between large earthquake
activity and active tectonics, and on the dynamic process of block rotation activity[6,7,10,11] . Several researchers
did relocation works using different data and different methods for this region. Yang et al.[12] relocated 6496
events out of 10057 earthquakes in central-western China covering this area from 1992 to 1999 using the double
difference method[2] . Sun et al.[13] relocated 129 events in Sichuan using Hypoinverse and grid search method
E-mail: [email protected]
Zhu A L et al.: Relocation of Small Earthquakes in Western Sichuan · · ·
693
based on a 1-D layered velocity model. However, all the previous authors concentrated on the relocation
methods they used, almost without further analyzing the relocated seismicity for tectonic activity study. In
this study, we relocate 13367 small earthquakes that occurred in western Sichuan between 1992∼2002 using
the double-difference earthquake location algorithm to improve the accuracy of relative locations, and then
investigate the relation between precise microseismicity and surface faulting, and image the tectonic structures
and features that were obscured by the uncertainty from routine location, or not discovered by the surface
tectonic investigations. We also calculate the crustal strength envelopes for western Sichuan region to investigate
the crustal deformation character suggested by distribution of relocated hypocenters, to provide the seismological
and rheological evidence for understanding the dynamic process of block rotation activity.
2 EARTHQUAKE RELOCATION
2.1 Data
The study area is located in western Sichuan
(25.5 ∼ 35◦ N, 98◦ ∼ 106◦ E) covered by the Sichuan
Seismic Network (SSN). The arrival times of Pg and
Sg waves recorded at 70 stations (Fig. 1) of the SSN
between 1992 and 2002 are used for relocating events,
which have been manually picked by the SSN analysts.
To ensure the stability of the solution of DD-equations,
we only chose those events recorded by at least 4 stations. A total of 13367 such events were selected, containing 118819 Pg and 80140 Sg picks. The magnitudes
of these events are from ML 0.1 to 4.9.
◦
2.2 Relocation Method
The double-difference method[2] is used to relocate the selected earthquakes, which is a relative location method having the advantage over the master
event relocation method and station corrections in a
sense for it can relocate the events in a large region
simultaneously without affecting the precision of each
event. The basic equation of this algorithm is expressed
as
∂tj
∂tik
∆mi − k ∆mj = (tik − tjk )o − (tik − tjk )c , (1)
∂m
∂m
Fig. 1 Overview of active tectonics in study area and
distribution of the Sichuan seismic network
or written out in full
∂tj
∂tj
∂tj
∂tik
∂ti
∂ti
∆xi + k ∆y i + k ∆z i + ∆τ i − k ∆xj − k ∆y j − k ∆z j − ∆τ j = drkij ,
∂x
∂y
∂z
∂x
∂y
∂z
(2)
where drkij represents the double difference between the observed difference (tik − tjk )o and calculated difference
(tik − tjk )c for two events i and j recorded by the same station k. ∆mi = (∆xi , ∆y i , ∆z i , ∆τ i )T are the changes
of hypocentral parameters (xi , y i , z i , τ i ) to make the model better fit the data.
A system of linear equations is formed by combining Eq.(2) from all hypocentral pairs for a station, and
for all stations
W Gm = W d,
(3)
where G is a matrix of size M × 4N containing the partial derivatives, M is number of double-difference
observations, N is number of events; d is the data vector containing the double-differences; m is a vector of
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length 4N, (∆xi , ∆y i , ∆z i , ∆τ i )T , containing the changes in hypocentral parameters to be determined, and W
is a diagonal matrix to weight each equation.
The mean shift of all earthquakes during inversion is constrained to zero for each coordinate direction and
origin time
N
X
∆mi = 0.
(4)
i=1
2.3 Relocation Result
A one dimensional layered velocity structure model is required by DD-equation. The study area covers
almost the whole western Sichuan plateau and western part of Sichuan basin. The crustal structures of the two
regions are quite different[14∼16] . To account for the distinct structure of the two regions, we perform two sets
of relocations: one containing 10118 events in western Sichuan plateau, and the other 3179 in Sichuan basin,
which are to be relocated separately using different velocity models[14,16] .
The conjugate gradient algorithm (LSQR) is used to solve the DD equations for both sets of events. In
the inversion, the data need to be weighted based on their individual qualities. The characteristics of the
phases determine their legibility, that is the precision of their readings. Generally, the quality of Pg reading is
better than that of Sg. So we assigned a weight of 1.0 to all Pg and 0.5 to all Sg in a rough way. A certain
search radius is used in the calculation to match the event pairs. Using different search radius may result in
different precisions of the relocation results and different number of relocated events. Taking the large area
investigated into account, we set 10km as the search radius for both groups of events. Out of 10118 earthquakes
in western Sichuan plateau 7588 events could be relocated. The RMS residual decreases from 2.4s before to 0.55s
after relocation. The average standard error estimates are 1.2 and 1.8km for horizontal and vertical direction,
respectively. 2530 events out of 3179 earthquakes in the basin area were relocated. The RMS residual decreases
from 2.33s before to 0.7s after relocation. The average standard error estimates are 2 and 1.7km for horizontal
and vertical direction, respectively.
Fig. 2 Map view of microseismicity before (a) and after (b) relocation
between 1992∼2002 in western Sichuan
The reliability of the error estimation can be evaluated by the singular value decomposition (SVD) or
Bootstrap method[2] . We applied SVD to relocate a small swarm consisting of 47 events in the plateau. The
Zhu A L et al.: Relocation of Small Earthquakes in Western Sichuan · · ·
695
error estimates are consistent with those from LSQR, indicating the above error estimates are reliable. To test
the potential effect of the velocity model to the result, we also used a single model to relocate the events in
whole area. The relocation result shows the entire seismicity pattern unchanged, suggesting that the results are
stable.
Figure 2 shows the seismicity before and after relocation with the same number of events. In map view, the
relocated seismicity becomes highly clustered. In this aspect, it is consistent with the previous result[12] . Only
52% events in the SSN catalog have depth estimates, ranging from 0∼40km. After relocation, most hypocenters
in both plateau and basin are distributed at depths of 0∼15km, amounting to 90% of all the relocated events.
Fig. 3a and 3b show a clear aseismic layer existing in the depth interval between 15 and 20km in most places in
the plateau, and fewer hypocenters at depths of 20∼50km. While in the basin area, no such apparent aseismic
layer is observed (Fig. 3c and 3d).
Fig. 3 Cross sections of relocated seismicity along latitude (a, c) and longitude (b, d)
for the western Sichuan plateau (a, b) and the Sichuan basin (c, d)
3
RELATIONSHIP BETWEEN THE SEISMICITY AND SURFACE ACTIVE TECTONIC
STRUCTURES
In order to accurately display the relationship between seismicity and surface faults, we reinterpreted the
surface traces of main active faults in study area (Fig. 1) using the digital ETM satellite images with resolution
of 15m on the basis of previous field investigation results1)[17] . Fig. 2b shows that the relocated seismicity is
highly organized to align along the active faults, demonstrating their close relationship. Followings are the
revealed seismicity pattern and features that were obscured by the routine location errors, and undiscovered by
the surface active tectonic investigations as well.
3.1 Seismicity Patterns on Different Kind of Faults
Different active tectonics cause different seismicity patterns. There are various typical kinds of active faults
in the study area, including strike-slip fault, thrust fault and pull-apart basin developed in localized extensional
environment, which show different seismicity patterns and structures in both map view and cross section.
1) Seismological Bureau of Sichuan Province. Strip mapping for active faults along the Xianshuihe-Anninghe-Zemuhe fault
zone at a scale of 1:50000, 1995
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(1) Strike-slip fault
In map view, the seismicity shows narrow linear pattern on this kind of fault with simple structure, such
as on the Luhuo-Daofu segment of the Xianshuihe fault zone (Fig. 2b), which is a large left-lateral strike slip
fault zone. Fig. 4a displays the relocated seismicity on the cross section perpendicular to the strike of this fault,
showing a typical flower structure as the hypocenters dispersing upward, while thinning downward.
(2) Pull-apart basin
Normally in pull-apart basins, the seismicity patterns are dispersed from both map view and cross section.
The Garzê pull-apart basin is the largest left-lateral step in the study area, which is developed in the conjunction
part of two large left-lateral strike-slip fault zones, the Garzê-Yushu fault zone and the Xianshuihe fault zone
(Fig. 2b), characterized by the secondary NE trending normal faulting[18] . The cross-section of the relocated
seismicity perpendicular to its structure strike (Fig. 4b) shows dispersed feature.
As shown in the above cross-section views (Fig. 4a and Fig. 4b), the average hypocentral depths in extensional tectonic regime are shallower than those in strike-slip faulting zone.
Fig. 4 Cross section of relocated seismicity perpendicular to the structure strikes
(a) Luhuo-Daofu segment of the Xianshuihe fault zone; (b) Garzê pull-apart basin;
(c) Middle segment of the Longmenshan thrust fault zone.
(3) Thrust fault
The relocated seismicity forms highly organized structure along thrust faults, such as along the Longmenshan fault zone (Fig. 4c), which is a huge thrust fault zone in the study area consisting of a series of low-angle
thrust faults dipping to NW. Fig. 4c is the cross-section of relocated seismicity perpendicular to the middle segment of this fault zone, delineating a series of low-angle faults dipping to NW with the hypocenters distributed
on the upthrown sides of the faults, which correspond to the surface faults well. It also displays that there exists
an aseismic layer beneath the faults between 15∼20km depth, which may be the decollement interface for the
thrust fault zone in mechanic sense.
The above deep structures delineated by the relocated seismicity correspond to the surface faulting well.
Note that these are the behavior patterns of seismicity on typical active faults in the study area. While in
most cases, the fault geometry, faulting feature, and combination relation among the active faults are quite
complicated, which lead to the complex behavior of the seismicity. Where seismicity remains diffused after
relocation may be the highly fractured zone caused by the interaction of several groups of faulting.
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3.2 Segmentation Character
The relocated microseismicity shows the consistent feature of segmentation with surface traces along large
active fault zones in study area, for example, along the Longmenshan fault zone and the Xianshuihe-AnningheZemuhe fault zone, the seismicity patterns vary greatly on different segments. It indicates that any variation in
geometry, faulting property of active faults is also reflected in the change of seismicity, suggesting that active
faulting in the study area controls the seismicity strongly.
3.3 Blind Active Faults Revealed by Seismicity
One of the main purposes of the small earthquake relocation is to reveal unknown active structures.
Through analyzing earthquake epicenter distribution and hypocentral profile, several significant blind active
faults in our study area are revealed (Fig. 2b), including a branch fault of the Xianshuihe fault zone near Moxi,
a NW trending fault in the front basin in the northeastern end of the Longmenshan fault zone and a EW
trending one in Xichang transition part between the Anninghe fault zone and the Zemuhe fault zone. All the
blind active faults may be formed in the same stress mechanism with their surrounding tectonic structures,
and the finding of them is especially of importance for reconsidering the tectonic combination and conversion
relationships at special tectonic locations.
3.4 Relationship Between the Litang Fault Zone and the Lijiang-Xiaojinhe Fault Zone
The Lijiang-Xiaojinhe fault zone is considered to be the southwestern extension part of the LongmenshanJinpingshan-Yulongxueshan thrust fault system, forming a sub block boundary inside the rhombic SichuanYunnan block from previous surface active tectonic investigations[19] . However, the seismicity turns to be an
arc connecting the Litang fault zone and the western part of the Lijiang-Xiaojinhe fault zone as a whole (Fig. 2b).
Their connection may provide new evidence for further partition of the block and extension of these faults.
3.5 Aseismic Segments Along the Xianshuihe-Anninghe-Zemuhe Fault Zone
After relocation, on the huge Xianshuihe-Anninghe-Zemuhe strike-slip fault zone, there appear a series of
obvious aseismic segments with dimensions of tens of kilometers from Daofu to Qianning, Mianning to Xichang,
and Xichang to Puge. Since there are about 11 earthquakes greater than M 7.0 and 29 earthquakes over M 6.0
recorded along this fault zone, these segments appearing to have been aseismic for the time interval of 10 years
(1992∼2002) suggest that they may be locked for accumulation of strain energy and high stress, hence could be
appointed as the future strong earthquake hazard regions.
4 THE ASEISMIC LAYER IN WESTERN SICHUAN PLATEAU AND ITS IMPLICATION
FOR BLOCK ROTATION
Most of the relocated hypocenters are distributed at depths of 0∼15km in the upper crust in the study
area, and there exists an apparent aseismic layer about 5km thick between the depths of 15 and 20km in most
places in western Sichuan plateau, beneath which there are fewer earthquakes at depths from 20 to 50km in
some places. It suggests that the upper crust is brittle, and there is also brittle deformation in the middle and
lower crust in some places, while the deformation in the layer from 15 to 20km in most places in western Sichuan
plateau is ductile.
To study the deformation character of the aseismic layer, we calculated the crustal strength (stress limit)
(Fig. 5) for western Sichuan plateau based on fault friction law and rock rheological experimental data. In the
calculation, the crustal structure is from the result of Xiong et al.[20] . Temperature is calculated with a procedure
by Chapman[21] , using heat flow data from Hu et al.[22] . Heat production rates and thermal conductivities are
from Wang[23] . Flow laws of quartzite and quartz diorite[24] are used for rheology of upper and middle crust,
and those of fine-grained gabbro collected from Panxi, Sichuan[25] are used for the rheology of lower crust and
crust-mantle transition zone. For friction strength, the case of strike-slip fault is considered, assuming vertical
stress (i.e., pressure due to gravity) σv = (σ1 + σ3 )/2. In Fig. 5, the friction line (the straight line) is obtained
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from Byerlee’s friction law, while the curve at depths of 14 to 19km is the rheological strength of quartz which
represents crustal granite, and the curve at depths of 25 to 38km represents the rheological strength of granitic
gneiss, and the curve at depths of 40.2 to 53km represents the semi-brittle and semi-ductile deformation of dry
mafic granulite.
Fig. 5 The crustal strength envelopes
for western Sichuan
As shown in Fig. 5, the deformation character of
granite at depths of 14 to 19km is ductile. The depth
range and thickness of this granitic ductile layer are
consistent with those of the aseismic layer, suggesting
that the aseismic layer may be the result of the ductile
deformation of crustal materials.
Double seismic zones have been observed in many
subduction zones around the world, and the aseismic
layer separating the upper and lower seismic layers
is assumed to be caused by the mechanical strength
changes (or ductile deformation) within plate[4] . While
it is the first time that the aseismic layer is found in
continental crust through earthquake relocation. According to the previous deep seismic soundings made
in western Sichuan, there exists an ubiquitous low velocity layer with a thickness varying from 5 to 15km
in the lower part of upper crust[15] or in middle crust[16,20,26,27] . In the upper crust, there also exists a low
resistivity layer[28] . The phenomenon of both low velocity layer and low resistivity layer appearing in the same
position is considered being caused by some tectonic reasons[15] . The location and thickness of the low velocity
layer and the low resistivity layer observed by geophysical detections are consistent with those of the aseismic
layer and granite ductile layer, suggesting that this aseismic layer should be a tectonic feature rather than
artifact of earthquake relocation process. One possible explanation for this phenomenon is that the deep part
of upper crust undergoes ductile deformation, which may provide basic condition for the eastward extrusion of
the material from Tibetan plateau and block rotation activity in Sichuan-Yunnan region. It needs extensive
study to determine whether it is a decoupling layer between the upper and deeper layers, meaning this aseismic
layer is the bottom boundary of the block, or a detachment interface inside the block. No matter what it would
be, however, we can conclude that it reflects the ductile deformation character of the deep part of upper crust
in the depth range of 15 to 20km in most places in the western Sichuan plateau, whose existence would affect
other tectonic deformation and mechanism.
Most of the relocated hypocenters with depths greater than 30km are mainly distributed along the Longmenshan fault zone and the Anninghe fault zone (Fig. 2b) which construct the eastern margin of the Tibetan
plateau and the southern segment of the central tectonic zone of continental China.
5 CONCLUSIONS
From 13367 small earthquakes that occurred in western Sichuan during 1992 to 2002, 10118 events were
relocated using the double-difference (DD) earthquake location method. The relative locations have been
improved dramatically, which enables us to get the fine seismicity structure. Through analyzing the relocated
microseismicity, a series of significant tectonic structures or features are revealed, which were previously obscured
by routine earthquake location and undiscovered by surface active tectonic investigations yet as well.
The relocated microseismicity becomes highly organized that correlates to surface faulting closely, showing
apparent linear feature on simple strike slip faults in map view and typical flower structure in cross section
oriented perpendicular to the strike, and organized but dispersed structure on thrust fault and in extensional
pull-apart basin. The relocated seismicity also displays segmentation behavior along large active fault zones.
Zhu A L et al.: Relocation of Small Earthquakes in Western Sichuan · · ·
699
There appear a series of aseismic segments with dimensions of tens of kilometers on the Xianshuihe-AnningheZemuhe fault zone which could be presumed as the future earthquake hazard regions. Some blind faults are
revealed by alignment of the seismicity, including a branch fault of the Xianshuihe fault zone near Moxi, a NW
trending fault in the northern end of the front basin of the Longmenshan fault zone, and a near E-W trending
one in the transition part between the Anninghe fault zone and the Zemuhe fault zone around Xichang. A 5km
thick aseismic layer is observed in most places in western Sichuan plateau between the depths 15 and 20km. The
calculated crustal strength envelopes show that granite at depths of 14 to 19km appears to be ductile, whose
thickness and depth range are in good agreement with those of the aseismic layer, indicating that the aseismic
layer may be the result from the ductile deformation of the granite in the deep part of the upper crust. We
suggest that the existence of this aseismic layer may be associated with the eastward extrusion of the material
from the Tibetan plateau and block rotation activity in Sichuan- Yunnan region.
ACKNOWLEDGMENTS
In this study, we used the earthquake relocation procedure from F. Waldhauser and W. Ellsworth. Prof.
Wen Xueze, Xie Jiakang, Xu Jie, and Dr. Zhou Yuanze and Zhan Yan, and the anonymous reviewers provided
helpful comments to improve the paper. This study was financially supported by the National Key Basic Research Program (2004CB418401), the National Key Project of Basic Condition Platform (2003DIA6N005),
the Preliminary Special Project for Key Basic Research of Ministry of Science and Technology of China
(2003CCB00600) and the National Natural Science Foundation (40474067).
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