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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113, B12409, doi:10.1029/2007JB005101, 2008
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Earthquake distribution in southern Tibet and its tectonic
implications
Xiaofeng Liang,1 Shiyong Zhou,1 Y. John Chen,1 Ge Jin,1 Liang Xiao,1 Pingjiang Liu,1
Yuanyuan Fu,1 Youcai Tang,1 Xiaoting Lou,1,2 and Jieyuan Ning1
Received 6 April 2008; revised 2 July 2008; accepted 31 July 2008; published 23 December 2008.
[1] A temporary 37-station seismic array was operated in southern Tibet from June 2004
to August 2005 close to Xigaze along a traverse from Tangra Yum Co rift in the west to
Yadong-Gulu rift in the east. This lateral array of the international Hi-CLIMB project
recorded a total of 885 local earthquakes during the 14-month deployment. Hypocenter
locations of these events were obtained using HYPOINVERSE2000, and about half of
them were relocated by the joint hypocenter determination method. Lateral variations in
crustal seismic P and S wave velocities beneath the array were obtained simultaneously
as part of the station corrections during the relocation procedure. While earthquakes
were scattered in the region, more than 250 earthquakes were clustered within a small area
at about 50 km north of the Indus-Yalu Suture and west of the Pumqu-Xianza rift.
Crustal and uppermost mantle earthquakes are concentrated beneath the Himalayan crest.
Projection of all earthquakes to a north–south profile shows that the subducting Indian
plate passes the Indus-Yalu Suture below the Lhasa Terrane. While earthquakes are
concentrated in the upper crust and upper mantle beneath the Himalayas, seismicity is
mostly restricted to the upper crust to the north, which is consistent with a ductile
middle/lower crust beneath southern Tibet. About 20 deep crustal and uppermost mantle
events (at depths of 50–70 km) were observed to be associated with the subducting
Indian lithosphere. Our relocation results of these local earthquakes in southern Tibet
support the popular ‘‘jelly-sandwich’’ model for the rheology of continental lithosphere in
which the strong seismogenic layers of upper curst and uppermost mantle are separated by
a weak lower crust.
Citation: Liang, X., S. Zhou, Y. J. Chen, G. Jin, L. Xiao, P. Liu, Y. Fu, Y. Tang, X. Lou, and J. Ning (2008), Earthquake distribution
in southern Tibet and its tectonic implications, J. Geophys. Res., 113, B12409, doi:10.1029/2007JB005101.
1. Introduction
[2] The Himalayan Range and the Tibetan Plateau, built
by the continental collision between the Indian and the
Eurasian continents, are the two of the most active tectonic
regions in the world. While significant progress has been
obtained by previous international studies many key questions about this mountain-building process have yet to be
answered, such as the age of the onset of the collision with
estimates ranging from 65 Ma to 43 Ma [Yin and
Harrison, 2000; Yin, 2006], and what is the fate of the
Indian lithosphere beneath the Tibet.
[3] One important aspect for understanding the mountainbuilding process is the rheology structure of the Tibetan
lithosphere. The conventional ‘‘sandwich model’’ proposed
originally by Chen and Molnar [1983] divides the continental lithosphere into two seismogenic layers: the upper
1
Institute of Theoretical and Applied Geophysics, School of Earth and
Space Sciences, Peking University, Beijing, China.
2
Now at Department of Earth and Planetary Sciences, Northwestern
University, Evanston, Illinois, USA.
Copyright 2008 by the American Geophysical Union.
0148-0227/08/2007JB005101$09.00
crust and the uppermost mantle, which are separated by a
ductile/weaker lower crust. This concept has been used to
construct numerical models of Tibetan Plateau [e.g., Zhao
and Morgan, 1985, 1987] and to interpret seismic results
from the INDEPTH projects at southern Tibet. For example
the ‘‘bright spots’’ in seismic reflection profiles and a low
electromagnetic resistance layer in the middle crust were
used as evidence for the existence of a ductile lower crust in
Tibet [Brown et al., 1996; Chen et al., 1996]. Recently, this
simple conceptual model was challenged by Jackson [2002]
and Jackson et al. [2004], who argued that there is probably
just one seismogenic layer (within the crust) for the continental lithosphere. The key issue of this debate is whether
there exists a ductile lower crust. Precise determination of
earthquake distribution in depth could provide important
evidence to discriminate between these two models.
[4] Seismicity across the Himalaya Range has been
investigated by using several local permanent seismic networks in Nepal [Pandey et al., 1999], in Sikkim [De and
Kayal, 2003], and in Bhutan [Drukpa et al., 2006]. Hypocenter depths of central and western Nepal are well
constrained at depths shallower than 30 km and in particular,
intense and continuous seismic activity along the Pumqu-
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Xianza rift was reported by Pandey et al. [1999], who
interpreted that the deep events in the southern Tibet could
be related to compressional bending stresses in the uppermost mantle of the Indian lithosphere, induced by the
lithospheric flexure at the front of the mountain range. In
contrast, earthquakes in Sikkim are continuous from the
surface to the lower crust at 45 km depth [De and Kayal,
2003] and events at midcrustal and deep crustal depths were
also reported by Drukpa et al. [2006] in Bhutan.
[5] Several temporary seismic arrays were operated from
southern Himalaya in Nepal to Lhasa Terrane of southern
Tibet in the past two decades. As a Sino-U.S. collaboration
a dozen of portable broadband seismic stations were
deployed along the Lhasa-Golmud highway in the early
1990s [Owens et al., 1993]. Since then several major field
programs have been conducted in southern Tibet such as the
INDEPTH I, II, and III [Zhao et al., 1993; Nelson et al.,
1996; Huang et al., 2000], the Sino-French projects
[Herquel et al., 1995; Hirn et al., 1995; Wittlinger et al.,
2004], the HIMNT project [de la Torre and Sheehan, 2005],
and the Namcha Barwa Tibet project [Sol et al., 2007] to
image the Indian lithosphere beneath the Tibet. Significant
understanding about the structure beneath Himalayas and
Tibet has been obtained by extensive analyses of the data
collected by these field experiments including local seismicity from Nepal to Lhasa Terrane. For example, a total of
267 earthquakes were located by a 1-D array as a part of the
INDEPTH III studies. All of them were located in the upper
crust from 0 to 25 km depth and several earthquake clusters
were found near the Pumqu-Xianza rift zone in the central
Tibet [Langin et al., 2003]. Thousands of local earthquakes
were recorded by the HIMNT stations in Nepal and southern
Tibet [Monsalve et al., 2006]. The depth distribution of
these earthquakes indicated the existence of double seismogenic layers beneath the Himalayan Range: one layer from
surface to 25 km depth, and the deeper layer from 50 km to
100 km depth.
[6] In this study we used the joint hypocenter determination (JHD) method to relocate earthquakes in southern Tibet
that were recorded by 37 portable seismic stations which
was the lateral 2-D array of the international Hi-CLIMB
project (Himalayan-Tibetan continental lithosphere during
mountain building [Nabelek et al., 2005]). These stations
were deployed for a period of 14 months between July 2004
and August 2005. The main 1-D array of Hi-CLIMB was
composed of closely spaced (4 – 5 km) broadband seismic
stations from Lower Himalaya in Nepal extending into
the Qiangtang Terrane in Tibet (Figure 1a). The roughly
700-km-long north – south profile crosses the Main
Boundary Thrust (MBT), the Main Central Thrust (MCT),
the Indus-Yalu Suture (IYS) and the Bangong-Nujiang
Suture (BNS) [Nabelek et al., 2005].
[7] The 37 portable stations were deployed along a traverse from Tingri to Xigaze in southern Tibet (Figure 1b).
The main objectives of this 2-D array are to better locate
the local earthquakes and to image lateral lithospheric
heterogeneity beneath southern Tibet. Mantle earthquakes
(events below the Moho) had occurred in this area according to previous studies [e.g., Chen and Molnar, 1983;
Chen and Kao, 1996; Zhu and Helmberger, 1996; Chen
and Yang, 2004; Monsalve et al., 2006]. Therefore, an
accurate determination of the focal depth of intermediate-
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depth earthquakes within our seismic array will provide a
unique test for the occurrence of mantle earthquakes here,
which could provide observational constraint to the ongoing
debate of the rheology of continental lithosphere [e.g.,
Jackson, 2002; Jackson et al., 2004; Chen and Yang, 2004].
[8] A Total of 885 local earthquakes were located by
HYPOINVERSE2000 and about half of them were relocated by the JHD method. Built on previous studies, our
results will illuminate the earthquake (depth) distribution
from the Himalayas to the southern Lhasa Terrane and will
provide observational evidence for the debate of seismogenic layer(s) in continental lithosphere. It could also help us
to understand the rheological structure of the underthrusting
Indian lithosphere as well as the Tibetan lithosphere.
2. Tectonics Setting
[9] The main Cenozoic structures in the southern Tibet
include both north trending rifts and NWW trending rightslip faults (Figure 1b) [Armijo et al., 1986, 1989] The active
Main Frontal Thrust (MFT) along the Himalaya marks the
southern edge of the Himalayan Range. The north trending
rifts in the study area are the Yadong Gulu rift, the Pumqu
Xianza rift, and the Tangra Yum Co rift, from east to west.
The origin of these rifts has been debated in the past three
decades. Several mechanisms such as a gravitational collapse model [Molnar and Tapponnier, 1978; Tapponnier et
al., 1981], a lower crust flow model with east – west
extension, a convective removal model of the upper mantle
[England and Houseman, 1989] and an oblique convergence model of Indian subduction [McCaffrey and Nabelek,
1998] have all been proposed as the origin for these rifts.
Whether these rifts are restricted to the upper crust or are
involving the mantle lithosphere has also been under debate
[Masek et al., 1994; Yin, 2000]. The northern ends of the
Tangra Yum Co rift and the Pumqu Xianza rift are
connected by a NWW trending right-slip fault, the Gyaring
Co fault, which belongs to the central Tibet conjugate fault
zone [Taylor et al., 2003]. This strike-slip fault zone has
been suggested as a transfer zone linking north trending
Cenozoic extensional structures in the Lhasa and Qiangtang
terranes [Yin and Harrison, 2000].
[10] In this paper, we adopt Yin’s [2006] definition that
the Indus-Yalu Suture (IYS) divides the Himalayan orogen
in the south and the Tibetan Plateau in the north. We further
refer to the crestal area of the Himalayan orogen as the
Higher Himalaya, its south slope as the South Himalaya
consisting of the Lower Himalaya in the north and SubHimalaya in the south, and its north slope as the North
Himalaya (Figure 3). Thus our array transverses two tectonic units: the Himalayan orogen and the Tibet Plateau. For
convenience, southern Tibet is used in this paper to describe
the location of our array, including the southern Tibetan
Plateau and the North Himalaya.
3. Data and Methods
[11] The lateral 2-D array of Hi-CLIMB includes 32
broadband seismic stations from Peking University (PKU)
and the Institute of Earth Sciences (IES), Academia Sinica,
Taiwan and five short-period seismic stations from China
Academy of Geological Sciences (CAGS). The 16 PKU
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Figure 1. (a) Overview of Tibetan Plateau with the research area outlined by the box. All of the stations
of Hi-CLIMB were shown as light red circles on the map. The directions of plate movements are from
Zhang et al. [2004]. (b) Active structures and station locations of the lateral seismic array of Hi-CLIMB.
Stations used in this research are shown as red triangles (the stations with little data owning to equipment
problems and vandalism are show as purple triangles). The rifts are Tangra Yum Co rift, Pumqu-Xianza
rift, and Yadong-Gulu rift from left to right.
seismographs were equipped with Guralp CMG-3ESP
three-component sensors, with a bandwidth from 0.033 to
50 Hz, and a REF TEK 130– 01 DAS recorder. The IES
seismographs include 12 that equipped with Streckeisen
STS-2 three-component sensors, with a bandwidth from
0.0086 to 50 Hz, and a REF TEK RT72-A DAS recorder,
and 4 that equipped with Nanometrics Trillium 40 threecomponent sensors, with a bandwidth from 0.025 to 40 Hz,
and a Quanterra Q330 recorder. All the five short-period
seismographs were equipped with CDJ-S1 1 Hz threecomponent sensors and a LEAS Hathor 3 recorder. These
37 stations were operated with a sampling rate of 40 or
50 sps from July 2004 to August 2005 for a period of
14 months. Average station spacing was approximately
40 km (Figure 1). Because of instrument problems or
vandalism, recordings from four broadband stations and
all the five short-period stations are not used in this study.
[12] Because of the absence of permanent seismic stations
in Tibet except the Lhasa station, global seismicity catalogs
could not provide a reliable list of local events. Thus we
used an event detection algorithm based on the ratio of the
average short-term amplitude (STA) to the average longterm amplitude (LTA) to identify candidates for local events
[Langin et al., 2003; Turkelli et al., 2003]. Then the P
phases and possible S phases of the selected events were
picked by SAC2000 [Goldstein et al., 2003]. In general, a
band pass of 0.8- to 2-Hz filter was applied before the
picking to eliminate the background noise. If the data were
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too noisy different band-pass filters were tried such as 1 to 5
Hz before picking more phases.
[13] As a first step, we used the Hypoinverse2000
[Klein, 2002] (code recompiled by F. Aldersons, http://
faldersons.net/Software/Hypoinverse/Hypoinverse.html) for
the analysis of all the events from STA/LTA ratio checking
to obtain a single event location (SEL) list. Here, Zhu and
Helmberger’s [1996] model was selected as the reference
1-D velocity model since it has been used in their
waveform simulation analysis for the events close to our
study area [Zhu and Helmberger, 1996; Zhu et al., 2006].
Data used for the SEL analysis were selected by the
following criteria to obtain as many events as possible:
(1) the events have a clear P phase, (2) the number of P
wave phases exceed four, and (3) at least one S phase is
picked. As a consequence, a total of 885 local events were
located by the SEL method.
[14] Next, these 885 events were relocated by the joint
hypocenter determination (JHD) method, which was originally developed by Douglas [1967] and later modified by
Pujol [1988]. Because the mathematical basis and description of this algorithm had been discussed in great details in
previous reports [e.g., Pujol, 1988, 1992, 1995, 1996;
Ratchkovsky et al., 1997], it will not be repeated here.
The JHD method can, in general, simultaneously solve for
both the hypocenter parameters and station corrections.
Here the P and S station corrections represent the accumulated deviations of the seismic velocity distribution from the
simple 1-D layered model along the raypaths. In other
words, the positive and negative corrections correspond to
low- and high-velocity anomalies beneath the stations
relative to the original 1-D velocity model, respectively.
While S wave station corrections are calculated separately
from the P wave station corrections they are not independent of each other because the S wave arrival times are
calculated using a constant Vp/Vs ratio during the JHD
inversion.
4. Results
[15] A total of 437 local earthquakes were relocated by
the JHD inversion. The distribution of these events seems to
be related to the local geological structures (Figure 2). One
obvious feature of all the JHD inversions is that more than
250 events were clustered within a small area, at a distance
of 50 km north of IYS and west of the Pumqu-Xianza rift.
About 40 earthquakes are also concentrated along the
Higher Himalaya, which is parallel to the Main Frontal
Thrust fault (MFT) and could be related to the Main
Boundary Thrust, similar to the events that have been
relocated at Bhutan [Drukpa et al., 2006]. A small cluster
of events was also noticeable between the Pumqu-Xianza
rift and the Yadong-Gulu rift, north of IYS.
[16] Most significantly, about 20 intermediate-depth
(deeper than 50 km) events were located at the southern
ends of both the Pumqu-Xianza rift and the Tangra Yum Co
rift (Figure 2), where intermediate depth events have been
reported [Monsalve et al., 2006]. There was also a deep
earthquake close to the IYS near 89°E, where three deep
earthquakes (shown with their focal mechanisms in
Figure 2) have been studied in great details [Zhu and
Helmberger, 1996]. We noticed that almost all of the
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reported intermediate-depth earthquakes at southern Tibet
are from these two areas, suggesting the undergoing of a
distinct dynamic process here.
[17] All relocated earthquakes are projected to a roughly
north – south cross section along the receiver function profile of Schulte-Pelkum et al. [2005] with a strike of N18°E
(Figure 2). The cross section shows that the Indian plate,
which is subducting beneath the Higher Himalaya, has past
the IYS into the Lhasa Terrane (Figure 3). From the MFT to
the Higher Himalaya, seismicity seems to be continuously
distributed within the whole crust of the subducting Indian
lithosphere, consistent with previous studies [Maggi et al.,
2000a, 2000b; Jackson 2002; Mitra et al., 2005], indicating
a strong/cold Indian lithosphere. However, earthquakes are
divided into two layers beneath southern part of the North
Himalaya. There are no earthquakes between these two
seismogenic layers from our relocation results (Figure 3).
Interestingly, most of the seismicity is limited only to the
upper crust (less than 20 km) in the Lhasa Terrane, except
the one event close to the IYS. Our result is also consistent
with the location result of INDEPTH III studies [Langin et
al., 2003].
[18] An apparent gap in the upper crustal seismicity is
observed at the North Himalaya near the Southern Tibet
Detachment System (STDS) (Figure 3), indicating an inactive STDS. Interestingly, this seismic gap in the upper
crustal seismicity is located above the area with a cluster
of the intermediate-depth earthquakes. These deeper events
are near the concave upward section of the subducted Indian
lithosphere and could be related to the bending stresses
although this explanation requires further study of the
mechanisms of these intermediate-depth events.
[19] As shown in Figure 3, seven earthquakes (from
Harvard CMT catalog) beneath the Lower and Higher
Himalaya are clearly correlated with the subduction of the
Indian lithosphere because of their downdip faulting mechanisms [Baranowski et al., 1984; Molnar and Chen, 1983;
Chen et al., 1981; Ni and Barazangi, 1984; Dziewonski et
al., 1987; Chen and Kao, 1996; Zhu and Helmberger, 1996]
(see also the Harvard CMT catalog). On the other hand,
focal mechanism solutions of both the intermediate depth
events beneath the North Himalaya and the shallow upper
crustal events beneath the Lhasa Terrane are consistent with
a regional stress field of east –west extension [Chen and
Yang, 2004]. The maximum compression axes of these
moderate-sized, intermediate-depth events are oriented
along the downdip direction of the underthrusting Indian
plate. Finally, the location of these intermediate-depth
events indicates that they are associated with the subduction
of the Indian lithosphere (Figure 3).
[20] During the JHD inversion both P and S wave station
corrections were calculated simultaneously in addition to
the hypocenter location and the origin time of each event
[e.g., Pujol, 1996; Ratchkovsky et al., 1997]. Station corrections in general can be attributed to crustal velocity
variations beneath these stations since positive and negative
corrections correspond to lower- and higher-velocity
anomalies relative to the original 1-D velocity model,
respectively. For comparison, six different 1-D velocity
models were used for the JHD inversion in this study and
the results are shown in Figure 4. All the JHD inversion
results from the six different 1-D velocity models show a
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Figure 2. Results of JHD relocation plus focal mechanisms. Hypocenters and focal mechanisms are
color coded according to the depth. The stations used in JHD inversions are shown as dark triangles. The
green line shows the profile position in Figures 3 and 5, which is from 26.873°N, 86.517°E to 30.939°N,
88.059°E. The red line shows the receiver function profile of Jin et al. [2006] used in Figure 3, and the
red ellipse shows the region of strong converted phases from Jin et al. [2006] mentioned in section 5.
Velocity model from Zhu and Helmberger [1996] is used in JHD relocation. Focal mechanisms are from
Chen and Yang [2004], which were collected from Baranowski et al. [1984], Molnar and Chen [1983],
Chen et al. [1981], Ni and Barazangi [1984], Dziewonski et al. [1987], Chen and Kao [1996], and Zhu
and Helmberger [1996], and Harvard CMT catalog. The compressional quadrants of focal mechanisms
are color coded by depth. The extensive quadrants of focal mechanism from Chen and Yang [2004] are
white and the ones from Harvard CMT catalog are gray.
Figure 3. Cross section of seismicity across southern Tibet. Hypocenters are shown as circles, stations
are shown as squares, and focal mechanisms within 150 km width are projected onto this profile.
Location of this profile is shown in Figure 2. Hypocenters are relocated by JHD using the velocity model
from Zhu and Helmberger [1996]. The focal mechanisms with dark compressional quadrants are from
Chen and Yang [2004], and the small gray ones are from Harvard CMT catalog. The Moho and MHT,
both are shown as dashed lines, are from receiver function results of Schulte-Pelkum et al. [2005] and Jin
et al. [2006].
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similar variation in both P and S wave station corrections.
There is a general trend that the S wave velocity decreases
toward the east –northeast direction. More importantly, P
wave station corrections show a negative value (early
arrival) along the Pumqu-Xianza rift (except models 1 and
5), corresponding to a higher P velocity zone at the rift,
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which will be further elaborated in section 5. However, this
feature is not apparent in the S wave station corrections.
5. Discussion
[21] In order to verify the robustness of our JHD inversion results we have applied different 1-D velocity models
and also added random noises to the arrival time picks for
the JHD inversion (Figure 5). Inversion with different
velocity models could examine the dependence of the
inversion to the reference 1-D velocity models and inversion using data with random noise provides us with an
estimate of the sensitivity of inversion results to the phase
pick errors. Several different 1-D velocity models of southern Tibet have been reported using different geophysical
methods, including the receiver function inversion and
surface wave inversions [Cotte et al., 1999; Rapine et al.,
2003; Zhu and Helmberger, 1996; Bassin et al., 2000;
Monsalve et al., 2006]. It is noticed that the stations used
in the previous receiver function studies to obtain the 1-D
velocity models were mostly deployed along the YardongGulu rift. Therefore, velocity models derived from these
receiver function studies might not be a good representative
for the entire southern Tibet because of the possible east –
west variation in seismic velocities, particularly across the
rifts. On the other hand, significant difference does exist
among the velocity models obtained from surface wave
group velocity inversion for the Himalayas, the Lhasa
Terrane and the Qiangtang Terrane [Cotte et al., 1999;
Rapine et al., 2003]. From the depth distribution of relocated hypocenters as shown in Figure 5 it is clear that the
general pattern of the event distribution from all inversions
of the 6 different velocity models is quite similar. We had
also added a ±0.2 s random average distributed noise to the
arrival time picks for a stability check and found that the
added noise had little effect on the inversion results. These
tests show that our relocation results are quite stable to the
uncertainties from the simple 1-D velocity model and phase
picking errors.
[22] We have also relocated the events from the Preliminary Determination of Epicenter (PDE, from National
Earthquake Information Center of USGS) catalog during
the operation of our temporary seismic array, which are
Figure 4. P and S wave station corrections from six
different velocity models. (a) P wave station correction.
(b) S wave station correction. From the P wave station
correction, especially models 2, 3, 4, and 6 in Figure 4a,
there is a negative anomaly beneath Pumqu-Xianza rift.
From the S wave station corrections there is no significantly
pattern correlated with active structure, but northeast strike
increasing trend is found from all of them except model 1 in
Figure 4b. The different velocity model 1 in Figures 4a and
4b is the velocity model of Zhu and Helmberger [1996];
model 2 in Figures 4a and 4b is the Crust 2.0 model [Bassin
et al., 2000]; model 3 in Figures 4a and 4b is the velocity
model 1 of Cotte et al. [1999]; model 4 in Figures 4a and 4b
is velocity model 2 of Cotte et al. [1999]; model 5 in
Figures 4a and 4b is velocity model of Jin et al. [2006];
and model 6 in Figures 4a and 4b is velocity model of
Monsalve et al. [2006].
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Figure 5. Relocation results from six different velocity models. The distribution pattern is as same as
the result using the velocity model from Zhu and Helmberger [1996] shown in Figure 3, so it is clear that
the influence from different velocity models is not so significant. (a) Velocity model of Zhu and
Helmberger [1996]. (b) Crust 2.0 model [Bassin et al., 2000]. (c) Velocity model 1 of Cotte et al. [1999].
(d) Velocity model 2 of Cotte et al. [1999]. (e) Velocity model of Jin et al. [2006]. (f) Velocity model of
Monsalve et al. [2006].
listed in Table 1. In general, our results from the JHD
inversion using data from the local seismic network have
produced a better location for these events compared to the
PDE results. Combination of a good station azimuth coverage with a high-quality record at the nearest station could
provide a good constraint on the depth determination. When
there are enough effective P and S picks, even inversion of
one particular event with an azimuthal gap and a larger
distance from the nearest station, such as the event
20050208 and 20050530, could still produce a reasonable
location result. The location errors depend on the station
distribution for each event (Table 1). We noticed that the
residuals of JHD results are in general larger than the
residuals of HYPOINVERSE2000 results because of the
trade off among different events during the JHD inversions,
which locate all events at the same time with the same
station corrections.
[23] The most striking feature of our relocation results
(Figures 3 and 5) is that there are almost no earthquakes at
depths of 30 to 50 km in the Tibetan crust north of the IYS,
a good indication for the presence of a midcrustal or lower
crustal ductile layer as suggested by many previous studies
[e.g., Zhao and Morgan, 1985, 1987]. The aseismic middle
crust is also consistent with the crustal channel flow models
for the expansion of the Tibetan Plateau [Royden et al.,
1997; Clark and Royden, 2000]. Our result is consistent
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4.1
32.5
3.9
3.8
62.7
29.5
8.0
30.030
27.900
30.010
30.018
28.023
27.756
31.564
88.173
85.906
88.174
88.166
87.838
86.159
88.241
15
16
10
11
9
15
16
20
114
17
20
13
177
175
Effective P and S picks is the number of P and S times with final weights greater than 0.1 during HYPOINVERSE2000 inversion.
215000.52
033551.08
012532.60
090711.11
042111.12
041356.39
105729.93
87.997
85.793
88.118
88.086
87.778
86.072
88.180
29.844
27.938
30.175
29.944
27.929
27.760
31.568
32
13
11
26
45
14
10
0.94
1.07
1.01
1.07
0.83
1.20
1.25
88.1658
85.9105
88.1992
88.1622
87.8107
86.1132
88.2087
30.0273
27.9413
30.0277
30.0310
28.0165
27.7967
31.5708
6.91
6.98
4.41
6.44
59.58
34.26
6.36
0.64
0.63
0.15
0.71
0.20
0.35
0.25
1.73
2.87
0.42
4.47
1.14
2.78
1.15
21.27
42.21
12.04
40.78
0.90
1.54
1.47
181
292
187
182
201
294
279
a
Date
8 Jul 2004
20 Jul 2004
23 Jul 2004
21 Aug 2004
10 Nov 2004
8 Feb 2005
30 May 2005
Horizontal Vertical Maximum Nearest Effective
Error
Error Azimuthal Station P and S
Depth Residual
(km)
(km)
Gap (deg)
(km)
Picksa
Longitude Latitude (km)
(s)
PDE Origin
Depth Residual
Depth Residual
Time (UT) Longitude Latitude (km)
(s)
Longitude Latitude (km)
(s)
JHD
HYPOINVERSE2000
Table 1. Comparison of the Different Locations
0.49
0.92
0.68
0.84
0.68
0.77
0.50
LIANG ET AL.: EARTHQUAKES IN SOUTHERN TIBET
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with previous studies of major earthquakes in this region
using stations outside of Tibet, which had argued for
two seismogenic layers beneath southern Tibet [Chen and
Molnar, 1983; Chen and Kao, 1996; Chen and Yang, 2004].
In addition, our relocated events show a strong cluster near
the Moho, both above and beneath the Moho (Figures 3 and
5), but whether some of these events are truly mantle events
would have to require further study, such as the waveform
fitting analysis [Zhu and Helmberger, 1996], given the
uncertainties in both the JHD depths and the Moho depth
derived from the receiver function study [Schulte-Pelkum et
al., 2005; Jin et al., 2006]. Another feature of our inversions
is that there are no intermediate-depth earthquakes north of
IYS where all of our relocated events were within the upper
crust (Figures 3 and 5), indicating one seismogenic layer in
the Tibetan crust [e.g., Maggi et al., 2000a].
[24] In summary the distribution of local seismicity from
this study seems to suggest that two seismogeneic layers
exist at the collision front, associated with the subduction of
the Indian lithosphere. There are different explanations for
these double seismogenic layers at the collision front. One
is that the base of the southern Tibetan crust comprises a
mafic layer generated during basaltic ponding at the base of
the Gangdese arc [Yin and Harrison, 2000]. The problem
with this explanation is that the basement of the Gangdese
arc may not be likely to be present below the North
Himalaya where earthquakes were present at both the lower
crust and uppermost mantle depths (Figure 3). The second
explanation is due to the initiation of the transformation
from granulite to eclogite when hydrous fluid is present
[Jackson et al., 2004]. This appears to be the most satisfactory model as it is most consistent with our observations.
Further north of IYS there is only one seismogenic layer at
the upper crust, that is, local earthquakes are limited to the
upper crust (0 – 20 km) similar to observations at other
continental active regions such as the southern California
in the North America [Wong and Chapman, 1990] or the
Iran Plateau of Middle East [Maggi et al., 2000b].
[25] Dip-slip fault plane solutions of those intermediatedepth earthquakes shown in Figures 2 and 3 have been used
to suggest a regional stress field of east – west extension
within the subducting Indian lithosphere [e.g., Chen and
Kao, 1996], which supports a model for rifts within the
subducting Indian lithosphere [Yin, 2000]. Drukpa et al.
[2006] had also reported the midcrustal to deep crustal
strike-slip events in Bhutan, interpreted as the result of
oblique convergence. If the subducting Indian lithosphere
was torn apart laterally in the east – west direction, then the
lithosphere on either side of the rift could be subducting
independently from each other. As a consequence strike-slip
displacement could take place along the conjugation faults,
as suggested by the strike-slip character of the fault plane
solutions of these intermediate-depth earthquakes that are
clustered at the southern end of the rifts (Figure 2). On the
other hand, the water content required to facilitate the phase
change from granulite to eclogite phase change [Jackson et
al., 2004] could be introduced by rifting of the subducting
Indian lithosphere. Stresses associated with the eclogite
phase transformation could be at least partially responsible
for these intermediate-depth earthquakes.
[26] Over 60% of our relocated events (250 of 400 from
June 2004 to August 2005) are shallow and clustered near
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LIANG ET AL.: EARTHQUAKES IN SOUTHERN TIBET
the Pumqu-Xianza rift at 30°N (Figure 2). Interestingly,
more than 40% (219 of 500 from 1995 to 2004) of the
events reported in the ISC catalog within our local network
were also from this small area. The active seismicity at this
area was also noticed by Langin et al. [2003] and Yang et
al. [2002], who had suggested that the tidal stresses could
be the cause for these microearthquakes. Guo and Chen
[2008] found thousands of tiny events in this area from the
data recorded by nearby stations of our array and had
speculated that these little events could have been dynamically triggered by the great 2004 Sumatra earthquake,
which was at several thousands kilometers to the south
of our stations. These events could reflect the activities of
geofluids such as magma or hot springs in response to the
strong seismic waves from such a large event. This
phenomenon has been reported at other continents such
as the activities recorded at northern California after the
1992 Landers earthquakes [Hill et al., 1993] and at the
Yellow Stone National Park after the 2002 Alaska Denali
earthquake [Husen et al., 2004]. Boiling springs and other
geothermal activities have been reported at the PumquXianza rift by Hou et al. [2004]. On the other hand, Zhou
and Murphy’s [2005] tomography results showed a wedgeshaped slow velocity anomaly within the lower crust from
85°E to 93°E and north of IYS, which is consistent with
the presence of partial melting in the middle crust as had
been suggested by INDEPTH II deep seismic reflection
experiment along the Yadong-Gulu rift [Nelson et al.,
1996].
[27] Jin et al. [2006] also reported a northeast trending
strong converted phases in their east – west profiles (shown
as the red ellipse in Figure 2), which seems to be correlated
with the northeast increasing gradient of our S wave
corrections. The negative P wave station correction along
the Pumqu-Xianza rift, corresponding to a high P velocity
zone, derived from our JHD inversion is a little surprise
since low P velocity is usually expected at a rift zone. Since
most of the intermediate-depth earthquakes were concentrated at the southern end of the Pumqu-Xianza rift one
possibility could be a shallow Moho beneath the PumquXianza rift, as a consequence of a thinner crust from the
east – west rifting, which could induce a localized mantle
upwelling. Receiver function study of the same seismic
array does indeed reveal a shallower Moho (at least 5 km
shallower) at the rift [Jin et al., 2006]. On the other hand,
there is a low seismic velocity anomaly extending to about
300 km depth beneath the Coma rift to the east of the
Yadong Gulu rift (Figure 2) from both finite frequency P
and S wave tomography inversions, which has been interpreted as localized mantle upwelling due to continental
delamination [Ren and Shen, 2008]. All of the above
discussion points to the case that mantle process plays an
important role during the formation of these north – south
trending rifts in southern Tibet. Whether the subducting
Indian lithosphere was torn apart and lithospheric rupture
zone was established that had induced localized mantle
upwelling or the process of continental delamination had
triggered the mantle upwelling requires further study. Both
mechanisms would have produced a shallower Moho beneath rifts and the localized mantle upwelling could provide
the heat/melts to the rifts and neighbor region, which could
B12409
explain the existence of geothermal activities and hot
springs in the southern Tibet.
6. Conclusions
[28] A total of 885 local earthquakes were located by
HYPOINVERSE2000 and 437 of them were relocated by
joint hypocenter determination (JHD) method. While most
of these earthquakes are scattered within the seismic array,
more than 250 earthquakes were clustered within a small
area near the Pumqu-Xianza rift, which could be related to
activities of geofluids there. Earthquakes are also concentrated along the Higher Himalaya, which is in parallel to the
north of the MFT fault.
[29] Projection of all earthquakes to a north – south profile
shows that the subducted India plate has extended into the
Lhasa Terrane. While earthquakes are divided into two
layers beneath the Himalayas most of the seismicity is
limited only to the upper crust north of IYS. About 20
intermediate depth events (50 – 70 km) were observed and
they could be associated with the combination of separated
subduction of the Indian lithosphere and petrological phase
transformation. There are no earthquakes at depths between
2050 km beneath the array, which is consistent with a
ductile middle/lower crust beneath southern Tibet. The lack
of deeper events north of the IYS is probably related to the
ductile and partially molten middle crust of the southern
Tibet where water could play a major role. Relocation
results of these local earthquakes in southern Tibet support
the popular ‘‘jelly-sandwich’’ model for the rheology of
continental lithosphere in which the strong seismogenic
layers of upper curst and upper mantle are separated by a
weak lower crust.
[30] Finally, P wave station corrections show a highvelocity zone along the Pumqu-Xianza rift zone, which
could be interpreted as a thinner crust from an east – west
extension at the rift. Whether east – west variations in the
Moho depth exist across these rifts in southern Tibet is an
important question in understanding the N – S continental
collision and the E – W extension seen at the surface
tectonics and deserves serious attention from the seismotectonics community.
[31] Acknowledgments. We would like first to thank John Nabelek
for his leadership in the fieldwork of Hi-CLIMB and all the members of the
Hi-CLIMB team who collected the data used in this study. We would also
like to thank the colleagues from CAGS and IES for their instruments and
data sharing. An Yin, James Ni, Rodolfo Console, and an anonymous
reviewer provided constructive comments that improved the manuscript.
Zhuqi Zhang at Peking University helped in making the figures and the
Generic Mapping Tools (GMT) is used in producing all the figures. The
fieldwork is supported by a NSFC grant for international collaboration
(40520120222) and data processing and analyses were also supported by
additional NSFC grants of (40125011, 40521002).
References
Armijo, R., P. Tapponnier, J. P. Mercier, and T. Han (1986), Quaternary
extension in southern Tibet, J. Geophys. Res., 91, 13,803 – 13,872,
doi:10.1029/JB091iB14p13803.
Armijo, R., P. Tapponnier, and T. Han (1989), Late Cenozoic right-lateral
strike-slip faulting across southern Tibet, J. Geophys. Res., 94,
2787 – 2838, doi:10.1029/JB094iB03p02787.
Baranowski, J., J. Armbruster, L. Seeber, and P. Molnar (1984), Focal
depths and fault plane solutions of earthquakes and active tectonics
of the Himalaya, J. Geophys. Res., 89, 6918 – 6928, doi:10.1029/
JB089iB08p06918.
9 of 11
B12409
LIANG ET AL.: EARTHQUAKES IN SOUTHERN TIBET
Bassin, C., G. Laske, and G. Masters (2000), The current limits of resolution for surface wave tomography in North America, Eos Trans. AGU,
81(48), Fall Meet. Suppl., Abstract S12A-03.
Brown, L. D., W. Zhao, K. D. Nelson, M. Hauck, D. Alsdorf, A. Ross, M.
Cogan, M. Clark, X. Liu, and J. Che (1996), Bright spots, structure, and
magmatism in southern Tibet from INDEPTH seismic reflection profiling, Science, 274, 1688 – 1690, doi:10.1126/science.274.5293.1688.
Chen, W., and H. Kao (1996), Seismotectonics of Asia: Some recent
progress, in The Tectonic Evolution of Asia, edited by A. Yin and T. M.
Harrison, pp. 37 – 62, Cambridge Univ. Press, Cambridge, U.K.
Chen, W., and P. Molnar (1983), Focal depths of intracontinental and intraplate earthquakes and their implications for the thermal and mechanical
properties of the lithosphere, J. Geophys. Res., 88, 4183 – 4214,
doi:10.1029/JB088iB05p04183.
Chen, W., and Z. Yang (2004), Earthquakes beneath the Himalayas and
Tibet: Evidence for strong lithospheric mantle, Science, 304, 1949 – 1952,
doi:10.1126/science.1097324.
Chen, W., J. Nabelek, T. Fitch, and P. Molnar (1981), An intermediate
depth earthquake beneath Tibet: Source characteristics of the event of
September 14, 1976, J. Geophys. Res., 86, 2863 – 2876, doi:10.1029/
JB086iB04p02863.
Chen, L., J. R. Booker, A. G. Jones, N. Wu, M. J. Unsworth, W. Wei, and
H. Tan (1996), Electrically conductive crust in southern Tibet from
INDEPTH magnetotelluric surveying, Science, 274(5293), 1694 – 1696,
doi:10.1126/science.274.5293.1694.
Clark, M. K., and L. H. Royden (2000), Topographic ooze: Building the
eastern margin of Tibet by lower crustal flow, Geology, 28(8), 703 – 706,
doi:10.1130/0091-7613(2000)28<703:TOBTEM>2.0.CO;2.
Cotte, N., H. Pedersen, M. Campillo, J. Mars, J. Ni, R. Kind, E. Sandvol,
and W. Zhao (1999), Determination of the crustal structure in southern
Tibet by dispersion and amplitude analysis of Rayleigh waves, Geophys.
J. Int., 138, 809 – 819, doi:10.1046/j.1365-246x.1999.00927.x.
De, R., and J. R. Kayal (2003), Seismotectonic model of the Sikkim Himalaya: Constraint from microearthquake surveys, Bull. Seismol. Soc.
Am., 93(3), 1395 – 1400, doi:10.1785/0120020211.
de la Torre, T., and A. Sheehan (2005), Broadband seismic noise analysis of
the Himalayan Nepal Tibet seismic experiment, Bull. Seismol. Soc. Am.,
95(3), 1202 – 1208, doi:10.1785/0120040098.
Douglas, A. (1967), Joint epicenter determination, Nature, 215, 47 – 48,
doi:10.1038/215047a0.
Drukpa, D., A. A. Velasco, and D. I. Doser (2006), Seismicity in the
Kingdom of Bhutan region (1937 – 2003), Evidence for crustal transcurrent deformation, J. Geophys. Res., 111, B06301, doi:10.1029/
2004JB003087.
Dziewonski, A. M., G. Ekstrom, J. E. Franzen, and J. H. Woodhouse
(1987), Global seismicity of 1979: Centroid-moment tensor solution for
524 earthquakes, Phys. Earth Planet. Inter., 48(1 – 2), 18 – 46,
doi:10.1016/0031-9201(87)90109-9.
England, P., and G. Houseman (1989), Extension during continental convergence, with application to the Tibetan Plateau, J. Geophys. Res., 94,
17,561 – 17,579, doi:10.1029/JB094iB12p17561.
Goldstein, P., D. Dodge, M. Firpo, and L. Minner (2003), SAC2000: Signal
processing and analysis tools for seismologists and engineers, in The
IASPEI International Handbook of Earthquake and Engineering Seismology, edited by W. H. K. Lee et al., pp. 1613 – 1614, Academic, London.
Guo, Y., and Y. J. Chen (2008), Local seismicity in south Tibet remotely
triggered by the Great 2004 Mw 9.0 Sumatra earthquake, Acta Seismol.
Sin., in press.
Herquel, G., G. Wittlinger, and J. Guilbert (1995), Anisotropy and crustal
thickness of northern-Tibet: New constraints for tectonic modelling,
Geophys. Res. Lett., 22, 1925 – 1928.
Hill, D. P., et al. (1993), Seismicity remotely triggered by the magnitude 7.3
Landers, California earthquake, Science, 260, 1617 – 1623, doi:10.1126/
science.260.5114.1617.
Hirn, A., et al. (1995), Seismic anisotropy as an indicator of mantle flow
beneath the Himalayas and Tibet, Nature, 375, 571 – 574, doi:10.1038/
375571a0.
Hou, Z., Z. Li, X. Qu, Y. Gao, L. Hua, M. Zheng, S. Li, and W. Yuan
(2004), The uplifting process of the Tibetan Plateau since 0.5 Ma B.P,
Sci. China, Ser. D, 44, Suppl., 35 – 44.
Huang, W.-C., et al. (2000), Seismic polarization anisotropy beneath
the central Tibetan Plateau, J. Geophys. Res., 105, 27,979 – 27,989,
doi:10.1029/2000JB900339.
Husen, S., R. Taylor, R. B. Smith, and H. Healser (2004), Changes in
geyser eruption behavior and remotely triggered seismicity in Yellowstone National Park produced by the 2002 M 7.9 Denali fault earthquake,
Alaska, Geology, 32(6), 537 – 540, doi:10.1130/G20381.1.
Jackson, J. (2002), Strength of the continental lithosphere: Time to
abandon the jelly sandwich?, GSA Today, 12, 4 – 10, doi:10.1130/
1052-5173(2002)012<0004:SOTCLT>2.0.CO;2.
B12409
Jackson, J. A., H. Austrheim, D. McKenzie, and K. Priestley (2004),
Metastability, mechanical strength, and the support of mountain belts,
Geology, 32(7), 625 – 628, doi:10.1130/G20397.1.
Jin, G., Y. J. Chen, E. Sandvol, S. Zhou, Y. Tang, X. Liang, G. Ye, D.
Wilson, and J. Nabelek (2006), Receiver Function Images of the Crust
and Upper Mantle Structure of Southern Tibet, Eos Trans. AGU, 87(52),
Fall Meet. Suppl., Abstract T53F – 04.
Klein, F. W. (2002), User’s guide to HYPOINVERSE-2000, a FORTRAN
program to solve for earthquake locations and magnitudes, U.S. Geol.
Surv. Open File Rep., 02-171.
Langin, W., L. Brown, and E. Sandvol (2003), Seismicity in central Tibet
from project INDEPTH III seismic recordings, Bull. Seismol. Soc. Am.,
93(5), 2146 – 2159, doi:10.1785/0120030004.
Maggi, A., J. Jackson, D. McKenzie, and K. Priestley (2000a), Earthquake focal depths, Effective elastic thickness and the strength of the
continental lithosphere, Geology, 28(6), 495 – 498, doi:10.1130/00917613(2000)28<495:EFDEET>2.0.CO;2.
Maggi, A., J. Jackson, K. Priestley, and C. Baker (2000b), A reassessment
of focal depth distributions in southern Iran, the Tien Shan and northern
India: Do earthquakes really occur in the continental mantle?, Geophys. J.
Int., 143, 629 – 661, doi:10.1046/j.1365-246X.2000.00254.x.
Masek, J. G., B. L. Isacks, and E. J. Fielding (1994), Rift flank uplift in
Tibet: Evidence for a viscous lower crust, Tectonics, 13(3), 659 – 667,
doi:10.1029/94TC00452.
McCaffrey, R., and J. Nabelek (1998), Role of oblique convergence in the
active deformation of the Himalayas and southern Tibet plateau, Geology,
26(8), 691 – 694, doi:10.1130/0091-7613(1998)026<0691:ROOCIT>
2.3.CO;2.
Mitra, S., K. Priestley, A. Bhattacharyya, and V. Gaur (2005), Crustal
structure and earthquake focal depths beneath northeastern India and
southern Tibet, Geophys. J. Int., 160, 227 – 248, doi:10.1111/j.1365246X.2004.02470.x.
Molnar, P., and W. Chen (1983), Focal depths and fault plane solutions of
earthquakes under the Tibetan Plateau, J. Geophys. Res., 88, 1180 – 1196,
doi:10.1029/JB088iB02p01180.
Molnar, P., and P. Tapponnier (1978), Active tectonics of Tibet, J. Geophys.
Res., 83, 5361 – 5375, doi:10.1029/JB083iB11p05361.
Monsalve, G., A. Sheehan, V. Schulte-Pelkum, S. Rajaure, M. R. Pandey,
and F. Wu (2006), Seismicity and one-dimensional velocity structure of
the Himalayan collision zone: Earthquakes in the crust and upper mantle,
J. Geophys. Res., 111, B10301, doi:10.1029/2005JB004062.
Nabelek, J., J. Vergne, G. Hetenyi, and the Hi-CLIMB Team (2005), A
synoptic view of the Himalayan collision zone and southern Tibet, Eos
Trans. AGU, 86(52), Fall Meet. Suppl., Abstract T52A – 02.
Nelson, K., et al. (1996), Partially molten middle crust beneath southern
Tibet: Synthesis of project INDEPTH results, Science, 274, 1684 – 1688,
doi:10.1126/science.274.5293.1684.
Ni, J., and M. Barazangi (1984), Seismotectonics of the Himalayan collision
zone D Geometry of the underthrusting Indian Plate beneath the Himalaya, J. Geophys. Res., 89, 1147 – 1163, doi:10.1029/JB089iB02p01147.
Owens, T. J., G. E. Randall, F. T. Wu, and R. Zeng (1993), PASSCAL
instrument performance during the Tibetan Plateau passive seismic experiment, Bull. Seismol. Soc. Am., 83(6), 1959 – 1970.
Pandey, M. R., R. P. Randukar, J. P. Avouac, J. Vergne, and T. Heritier (1999),
Seismotectonics of the Nepal Himalaya from a local seismic network,
J. Asian Earth Sci., 17, 703 – 712, doi:10.1016/S1367-9120(99)00034-6.
Pujol, J. (1988), Comments of the joint determination of hypocenters and
station corrections, Bull. Seismol. Soc. Am., 78(3), 1179 – 1189.
Pujol, J. (1992), Joint hypocentral location in media with lateral velocity
variations and interpretation of the station corrections, Phys. Earth
Planet. Inter., 75, 7 – 24, doi:10.1016/0031-9201(92)90114-B.
Pujol, J. (1995), Application of the JHD technique to the Loma Prieta,
California, mainshock-aftershock sequence and implications for earthquake location, Bull. Seismol. Soc. Am., 85(1), 129 – 150.
Pujol, J. (1996), An integrated 3D velocity inversion – joint hypocentral
determination relocation analysis of events in the Northridge area, Bull.
Seismol. Soc. Am., 86(1B), S138 – S155.
Rapine, R., F. Tilmann, M. West, J. Ni, and A. Rodgers (2003), Crustal
structure of northern and southern Tibet from surface wave dispersion
analysis, J. Geophys. Res., 108(B2), 2120, doi:10.1029/2001JB000445.
Ratchkovsky, N. A., J. Pujol, and N. N. Biswas (1997), Relocation of earthquakes in the Cook Inlet area, south central Alaska, using the joint hypocenter determination method, Bull. Seismol. Soc. Am., 87(3), 620 – 636.
Ren, Y., and Y. Shen (2008), Finite frequency tomography in southeastern
Tibet: Evidence for the causal relationship between mantle lithosphere
delamination and the north – south trending rifts, J. Geophys. Res., 113,
B10316, doi:10.1029/2008JB005615.
Royden, L. H., B. C. Burchfiel, R. W. King, E. Wang, Z. Chen, F. Shen, and
Y. Liu (1997), Surface deformation and lower crustal flow in eastern
Tibet, Science, 276, 788 – 790, doi:10.1126/science.276.5313.788.
10 of 11
B12409
LIANG ET AL.: EARTHQUAKES IN SOUTHERN TIBET
Schulte-Pelkum, V., G. Monsalve, A. Sheehan, M. Pandey, S. Sapkota, and
R. Bilham (2005), Imaging the Indian subcontinent beneath the Himalaya, Nature, 435, 1222 – 1225, doi:10.1038/nature03678.
Sol, S., et al. (2007), Geodynamics of the southeastern Tibetan Plateau from
seismic anisotropy and geodesy, Geology, 35(6), 563 – 566, doi:10.1130/
G23408A.1.
Tapponnier, P., J. L. Mercier, R. Armijo, T. Han, and J. Zhao (1981), Field
evidence for active normal faulting in Tibet, Nature, 294, 410 – 414,
doi:10.1038/294410a0.
Taylor, M., A. Yin, F. J. Ryerson, P. Kapp, and L. Ding (2003), Conjugate
strike-slip faulting along the Bangong-Nujiang suture zone accommodates coeval east – west extension and north – south shortening in the
interior of the Tibetan Plateau, Tectonics, 22(4), 1044, doi:10.1029/
2002TC001361.
Turkelli, N., et al. (2003), Seismogenic zones in Eastern Turkey, Geophys.
Res. Lett., 30(24), 8039, doi:10.1029/2003GL018023.
Wittlinger, G., J. Vergne, P. Tapponnier, V. Farra, G. Poupinet, M. Jiang, H.
Su, G. Herquel, and A. Paul (2004), Teleseismic imaging of subducting
lithosphere and Moho offsets beneath western Tibet, Earth Planet. Sci.
Lett., 221, 117 – 130, doi:10.1016/S0012-821X(03)00723-4.
Wong, I. G., and D. S. Chapman (1990), Deep intraplate earthquakes in the
western United States and their relationship to lithospheric temperatures,
Bull. Seismol. Soc. Am., 80(3), 589 – 599.
Yang, M., P. Cai, and W. Hua (2002), Research on current seismic activity
of Xietongmen-Shenzha area of Tibet (in Chinese), South China
J. Seismol., 22(2), 24 – 31.
Yin, A. (2000), Mode of Cenozoic east – west extension in Tibet suggesting
a common origin of rifts in Asia during the Indo-Asian collision,
J. Geophys. Res., 105, 21,745 – 21,759, doi:10.1029/2000JB900168.
Yin, A. (2006), Cenozoic tectonic evolution of the Himalayan orogen as
constrained by along-strike variation of structural geometry, exhumation
history, and foreland sedimentation, Earth Sci. Rev., 76, 1 – 131,
doi:10.1016/j.earscirev.2005.05.004.
B12409
Yin, A., and T. M. Harrison (2000), Geologic evolution of the HimalayanTibetan Orogen, Annu. Rev. Earth Planet. Sci., 28, 211 – 280,
doi:10.1146/annurev.earth.28.1.211.
Zhang, P. Z., et al. (2004), Continuous deformation of the Tibetan Plateau
from Global Positioning System data, Geology, 32(9), 809 – 812,
doi:10.1130/G20554.1.
Zhao, W., and W. J. Morgan (1985), Uplift of Tibetan Plateau, Tectonics, 4,
359 – 369, doi:10.1029/TC004i004p00359.
Zhao, W., and W. J. Morgan (1987), Injection of Indian crust into Tibetan
lower crust: A two-dimensional finite element model study, Tectonics, 6,
489 – 504, doi:10.1029/TC006i004p00489.
Zhao, W., K. D. Nelson, and Project INDEPTH Team (1993), Deep seismic
reflection evidence for continental underthrusting beneath southern Tibet,
Nature, 366, 557 – 559, doi:10.1038/366557a0.
Zhou, H., and M. A. Murphy (2005), Tomographic evidence for wholesale
underthrusting of India beneath the entire Tibetan Plateau, J. Asian Earth
Sci., 25, 445 – 457, doi:10.1016/j.jseaes.2004.04.007.
Zhu, L., and D. V. Helmberger (1996), Intermediate depth earthquakes beneath the India-Tibet collision zone, Geophys. Res. Lett., 23,
435 – 438, doi:10.1029/96GL00385.
Zhu, L., Y. Tan, D. V. Helmbereger, and C. K. Saikia (2006), Calibration
of the Tibetan Plateau using regional seismic waveforms, Pure Appl.
Geophys., 163, 1193 – 1213, doi:10.1007/s00024-006-0073-7.
Y. J. Chen, Y. Fu, G. Jin, X. Liang, P. Liu, J. Ning, Y. Tang, L. Xiao, and
S. Zhou, Institute of Theoretical and Applied Geophysics, School of Earth
and Space Sciences, Peking University, Beijing 100871, China. (johnyc@
pku.edu.c; [email protected]; [email protected]; liangxf@pku.
edu.cn)
X. Lou, Department of Earth and Planetary Sciences, Northwestern
University, 1850 Campus Drive, Evanston, IL 60208-0000, USA.
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