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Click Here JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113, B12409, doi:10.1029/2007JB005101, 2008 for Full Article 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- B12409 1 of 11 B12409 LIANG ET AL.: EARTHQUAKES IN SOUTHERN TIBET 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- B12409 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 2 of 11 B12409 LIANG ET AL.: EARTHQUAKES IN SOUTHERN TIBET B12409 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 3 of 11 B12409 LIANG ET AL.: EARTHQUAKES IN SOUTHERN TIBET 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 B12409 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 4 of 11 B12409 LIANG ET AL.: EARTHQUAKES IN SOUTHERN TIBET 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]. 5 of 11 B12409 B12409 LIANG ET AL.: EARTHQUAKES IN SOUTHERN TIBET 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, B12409 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]. 6 of 11 B12409 LIANG ET AL.: EARTHQUAKES IN SOUTHERN TIBET B12409 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 7 of 11 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 B12409 B12409 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 8 of 11 B12409 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. 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