Crustal thickness and average Vp/Vs ratio variationsin southwest Yunnan, China, from teleseismic receiver functions

Pan Wang,1 Liangshu Wang,1 Ning Mi,1 Jianhua Liu,2 Hua Li,1 Dayong Yu,1

Mingjie Xu,1 Xingchen Wang,1 and Zhiwei Guo1

Received 27 May 2009; revised 16 June 2010; accepted 3 August 2010; published 23 November 2010.

[1] Recently, many portable broadband three‐component seismic stations have beendeployed in Yunnan, China. We use teleseismic receiver functions to determine the crustalthickness (H) and the crustal average compressional to shear wave velocity (Vp /Vs) ratio (k)variations beneath in southwest Yunnan. By comparing the synthetics generated fromdippingMohomodels with those from a flatMohomodel, it reveals that for the rays travelingalong or around the updip direction, the differential arrival times of Ps and multiplesgenerated at the dipping Moho are comparable with those generated at the flat Moho and theresult from stacking with these synthetics is a better estimate of model structure. For thesynthetics traveling along or around the downdip direction, the differential arrival times ofPs and multiples generated at dipping Moho are smaller than those generated at flat Mohoand make the H‐k stacking results deviate from the model structure. Thus, the ambiguityinduced by dipping Moho can be reduced significantly by only stacking with the raystraveling along or around the updip direction. Applying this technique to 19 stations, weobtained thatH ranges from ∼35–42 km and the k ranges from ∼1.66–1.78. From the centralSimao basin, H increases northward and northeastward; k increases significantlynortheastward across the Ailao Shan–Red River shear zone.

Citation: Wang, P., L. Wang, N. Mi, J. Liu, H. Li, D. Yu, M. Xu, X. Wang, and Z. Guo (2010), Crustal thickness and averageVp /Vs ratio variations in southwest Yunnan, China, from teleseismic receiver functions, J. Geophys. Res., 115, B11308,doi:10.1029/2009JB006651.

1. Introduction

[2] In southwest Yunnan China, the Ailao Shan–Red River(ASRR) shear zone and the Simao basin situate to thesoutheast of the Tibet plateau and to the northwest of SouthChina Sea (Figure 1). The topography varies greatly, and therecent studies imply that lower‐crust materials flow from theTibet plateau toward this region [Clark and Royden, 2000].The ASRR shear zone is a large strike‐slip suture in Asia[Leloup et al., 1995; Tapponnier et al., 1982; Tapponnieret al., 2001].The crustal structure is important to understandthe dynamic process in this region. Relatively refined crustalstructure in Yunnan is revealed by active source wide‐anglereflections profiles and teleseismic receiver functions anal-ysis [Hu et al., 2005; Langston, 1977b; Owens et al., 1984;J. Wu et al., 2001; Xu et al., 2006]. The crust thickness variesfrom ∼30 km at Simao to ∼60 km at Zhongdian, and theMohodips toward the north generally [Zhang and Wang, 2009].[3] However, previous seismic researches on the crustal

structure are limited in this region, because the wide‐angle

reflection profiles and permanent seismic stations do not wellcover this region. And the average crustal Poisson’s ratiobeneath the Simao station is ∼0.315 obtained from H‐kstacking [Hu et al., 2005], which is larger than the Poisson’sratio of oceanic crust (∼0.302–0.306) [Christensen, 1996].According to the researches of Hayes and Furlong [2007]and Lombardi et al. [2008], we propose that such a largePoisson’s ratio in the continent (corresponding to a very highVp /Vs ratio) might deviate from the real Poisson’s ratio dueto a dipping Moho. Here, we aim to analyze how a dippingMoho disturbs the results of H‐k stacking, and provide moredetails about the average crustal thickness (H) and the aver-age Vp /Vs ratio (k) variations, based on portable broadbandthree‐component seismic stations.

2. Regional Setting

[4] The geology background is generalized as follows[Bureau of Geology and Mineral Resources of YunnanProvince, 1990; Leloup et al., 1995; Tapponnier et al.,1982; Tapponnier et al., 2001]. Our study region is locatedbetween the Yangtze block and the Indochina block(Figure 1). The Simao basin is a part of the Indochina block,and the Chuxiong basin belongs to the Yangtze block. TheASRR shear zone is a large‐scale strike‐slip suture in Asia.Themajor faults are the Lancang River Fault (LRF), the AilaoShan Fault (ASF), and the Red River Fault (RRF). The LRF is

1School of Earth Sciences and Engineering, Nanjing University,Nanjing, China.

2Institute of Geology and Geophysics, Chinese Academy of Sciences,Beijing, China.

Copyright 2010 by the American Geophysical Union.0148‐0227/10/2009JB006651

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the west margin of the Simao basin. The ASF and the RRF aremargins of the ASRR shear zone. There are various rocks inthis region. The Jurassic and Cretaceous sandstone distributein both the Simao basin and Chuxiong basin. The Triassiclimestone covers the basement along the southwestern mar-gin of the Chuxiong basin. And both high‐grade gneisses andlow‐grade gneisses are exposed in the ASRR shear zone. Alarge area of granite outcrops on the west side of the LRF, andultramafic rock outcrops along the ASF.[5] In the 1980s, several active source seismic projects

were carried out in Yunnan [Kan et al., 1986; Lin et al., 1993;Yan et al., 1985]. The upper lithosphere P velocity structuresin Yunnan obtained from tomography analysis recently [Baiand Wang, 2004; Zhang et al., 2006; Zhang et al., 2007],reveal the crustal structure as follows: the crust in Yunnan iscomposed of upper crust, middle crust and lower crust, andthe upper crust includes sedimentary cover and basement.The average P wave velocity of the crust is ∼6.25 km/s in theSimao basin and ∼6.45 km/s in the Chuxiong basin. Two ofthese active source wide‐angle reflection profiles cross ourstudy region, referred to as Simao‐Zhongdian profile (AA’)and Menglian‐Malong profile (BB’) (Figure 1b). Thetomography results of Bai and Wang [2004] and Zhang et al.

[2006], indicate that the Moho dips toward the north beneaththe Simao basin, and toward the northeast beneath the ASRRshear zone (Figure 2).

3. Data, Method, and Results

[6] Seismic observation in Yunnan was carried out fromthe beginning of 2002 to the end of 2003 in cooperationwith Institute of Geology and Geophysics Chinese Academyof Sciences, which was supposed by then National NatureScience Foundation of China. We use then 19 stations of theseismic array were located in the study region (Figure 1).Total 1253 pairs of radial Pwave receiver functions (R‐PRF)and tangential P wave receiver functions (T‐PRF) are com-puted from time domain maximum entropy deconvolution[Q. Wu et al., 2003]. The back‐azimuth (BAZ) of the eventsnearly span the whole BAZ range, with even concentratedwithin the range of ∼30–80° and ∼110–180° (Figure 1c).

3.1. H‐k Stacking

[7] Zhu and Kanamori [2000] introduced a techniqueof grid search over crustal thickness (H) and average crustalVp /Vs ratio (k), the so‐called H‐k stacking technique. The

Figure 1. (a) Tectonic sketch map of Southeast Asia after [Peltzer and Tapponnier, 1988; Tapponnieret al., 2001]; the study region is in the box. (b) Geologic sketch map of the west Yunnan, China, modi-fied from Bureau of Geology and Mineral Resources of Yunnan Province [1990]; triangles are the seismicarray. Major faults: LRF, Lincang River Fault; ASF, Ailao Shan Fault; RRF, Red River Fault; F1, Jinggu‐Zhenyuan Fault; F2, Wuliang Shan Fault; F3, Babian River Fault. (c) Epicenters of teleseismic for receiverfunctions used in this study.

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method sums the weighted receiver function conversionphase (Ps) and multiples (PpPs and PpSs + PsPs) at thepredicted differential arrival times respecting to direct P, fordifferent pairs ofH and k. The summation is over all recordedevents for a single station. The “best” estimate for the crustalmodel is the maximum summation.

S H ; kð Þ ¼X

w1A tPsð Þ þ w2A tPpPs� �� w3A tPsPsþPpSs

� �� � ð1Þ

where tPs, tPpPs and tPsPs+PpSs are the differential arrivaltimes of Ps, PpPs and PsPs + PpSs, respectively; A(tPs),A(tPpPs) and A(tPsPs+PpSs) are the amplitude at the pre-dicted differential arrival times of Ps, PpPs and PsPs + PpSs,respectively; w1, w2 and w3 are the weights, and w1 + w2 +w3 = 1.[8] In our research, we have chosen a value of 6.25 km/s for

the average crustal Vp in Simao basin and the ASRR shearzone, and a value of 6.45 km/s in the Chuxiong basin. Wegive the same weights as Zhu and Kanamori [2000], for thelow signal‐to‐noise ratio of multiples (w1 = 0.7, w2 = 0.2 andw3 = 0.1). The error estimation is given by boot‐strapping[Chevrot and van der Hilst, 2000; Hetland et al., 2004]. Thesummation S(H,k) is normalized as S’(H,k). TheH‐k stackingresults are presented in Table 1. The crustal structure beneathstations S4, S7, S11 and S16 (Figure 3) greatly deviates fromthe nearby stations, or differs from the reflection seismicresults [Bai and Wang, 2004; Zhang et al., 2006]. There areevidentially great H and k variations in a small region,according to these stacking results. And multiple peaksappear in the amplitude maps of the stacking results

(Figure 3), the summations of which are larger than 0.9. Boththe emergence of multiple peaks and the large H and k valuesare unexplainable. We believe that these problems are due tothe complicated crustal structure beneath the stations. A flatMoho is assumed in the H‐k stacking technique [Zhu andKanamori, 2000], however, the Moho in this region dipstoward north or northeast (Figure 2); the results stacking withthe synthetics, from a dipping Moho earth model, deviatefrom the model structure [Lombardi et al., 2008]. We focuson the effects of rays propagating along different directionson the H‐k stacking result, to approach the real crustalstructure, based on synthetics experiments.

3.2. Synthetics

[9] Assuming a planar Moho, we consider five models,which are one flat Moho model and four dipping Mohomodels with Moho dipping toward the north (Figure 4a). Inthese models, the Moho depth beneath the receiver is 30 km,and the average crustal Vp /Vs ratio is 1.720. The Moho dipangles (a) are 0°, 5°, 10°, 15° and 20°, respectively. Syn-thetics are computed using the RAYSUM code [Frederiksenand Bostock, 2000]. The ray parameter of the syntheticsequals 0.08 s/km, corresponding to an epicentral distance of30°, and we give a relatively shorter source pulse (0.5 s) toincrease the resolution.[10] The 8 is the relative azimuth between the Moho dip

direction and events (8 = BAZ – dip direction). The raypathsof direct P and multiples propagating along both updipdirection and downdip direction are presented in Figure 4b.And we call rays propagating along or around the updipdirection (downdip direction) as rays in the updip zone

Figure 2. Simplified crustal P velocity structure from active source wide‐angle reflection profile (a) AA’and (b) BB’ [Bai and Wang, 2004; Zhang et al., 2006]. The reflection phases Pm, Pg, P2, and P3 are fromMoho, the top of the basement, the middle crust and the lower crust, respectively; the black bars are theMoho depth from the updip stacking, and the gray bars are from the all events stacking.

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(downdip zone) for short. The synthetic R‐PRF and T‐PRFare sorted by 8 (Figures 4c and 4d).We set direct P arriving at0 s for each trace, thus, the arrival times of Ps and multiplesequal to their differential arrival times respecting to direct P.3.2.1. Effects of Dipping Moho on Receiver Functions[11] Previous studies [Cassidy, 1992; Langston, 1977a]

indicate that, for a dipping Moho, the amplitude and arrivaltimes of the Ps and the multiples vary depending on BAZ andray parameter; the phases propagating along the downdipdirection (8 = 180°) arrive earlier than those along the updipdirection (8 = 0°). The directP, conversional Ps andmultiplesappear in T‐PRF, as dipping Moho deflects P and S waves;the amplitude in T‐PRF component is zero for arrivals trav-eling along both updip and downdip directions, and becomeslarge for arrivals traveling along strike direction of theinterface; The phase PpPp can be observed in R‐PRF.[12] Furthermore, in Figure 4, both R‐PRF and T‐PRF vary

periodically with 8, and symmetrically about both the updipand downdip directions. As 8 increases, the polarities of thedirect P in T‐PRF varies from positive to negative in theupdip direction (8 = 0°), and from negative to positive inthe downdip direction (8 = 180°). Hence, the dip directionof the interface can be identified from the polarity variation ofdirect P in T‐PRF. Additionally, compared with the worksof Cassidy [1992], the polarity in T‐PRF are reverse becausethe receiver functions in our work are computed in RZTcoordinate.[13] In Figure 5, the arrival times of all paths and earth

models are sorted by 8. Only the 8 ranging of 0–180° areshown since the symmetric variation of R‐PRF, and the rayswith 8 in the range of 0 ∼ 90° are in updip zone and the othersare in downdip zone. It is shown that the arrival times of Psand multiples are not very sensitive to 8 and a in the updipzone, but significantly depend on both 8 anda in the downdipzone. In the updip zone, the arrival times of Ps and multiplesgenerated at dipping Moho are relatively close to the arrivaltimes of those generated at the flat Moho (solid line in

Figure 5); in the downdip zone, the larger the 8 or the a is,the shorter the arrival times of the Ps and multiples are.[14] We use � to quantify the relative arrival times shift

induced by dipping Moho. �i = [ti(a0, 8) − ti(a, 8)]/ti(a0, 8)100%, where i = Ps, PpPs and PsPs + PpSs; a0 = 0. Forexample, tPs(10, 180) is the arrival time of Ps generated at thedipping Moho with a equaling 10° and traveling alongdowndip direction (8 = 180°), and tPs(0, 180) is the arrivaltime of Ps with a equaling 0° and 8 equaling 180°. The rel-ative arrival time shift �Ps(10, 180) = [tPs(0, 180) − tPs(10,180)]/tPs(0, 180) × 100%. The relative time shift of Ps andmultiples generated at dipping Moho are shown in Table 2.[15] When a is relatively small (a ≤ 5°), ∣�∣ is very small in

both updip zone and downdip zone; as a increases, ∣�∣ arestill very small in the updip zone, and become very large inthe downdip zone. The smallest ∣�∣ is in a 8 range of 30–60°and a a range of 5–15°. Obviously, for these rays, the arrival

Table 1. All Events H‐k Stacking Results

Station Latitude (deg) Longitude (deg) Elevation (km) na Vpb (km/s) Hc (km) kd

S1 24.00 101.11 1.3 98 6.25 41.2 ± 2.8 1.740 ± 0.064S2 23.86 100.89 1.2 53 6.25 41.6 ± 0.3 1.694 ± 0.007S3 23.80 100.26 1.4 110 6.25 36.2 ± 5.0 1.724 ± 0.078S4 23.74 100.62 1.1 79 6.25 43.6 ± 2.2 1.618 ± 0.038S5 23.71 101.15 1.0 92 6.25 39.2 ± 1.9 1.716 ± 0.058S6 23.68 101.64 1.9 41 6.25 40.8 ± 1.0 1.700 ± 0.017S7 23.66 101.85 0.9 12 6.25 42.6 ± 5.4 1.636 ± 0.121S8 23.59 100.07 1.8 38 6.25 35.6 ± 0.5 1.712 ± 0.014S9 23.50 100.74 1.1 55 6.25 36.6 ± 0.4 1.696 ± 0.014S10 23.38 100.94 0.9 87 6.25 36.0 ± 0.7 1.672 ± 0.013S11 23.32 101.15 1.3 66 6.25 36.8 ± 2.4 1.660 ± 0.059S12 23.31 101.39 1.5 74 6.25 39.0 ± 1.5 1.694 ± 0.044S13 23.50 101.68 1.4 29 6.25 35.8 ± 0.4 1.758 ± 0.015S14 23.22 100.60 0.8 59 6.25 35.2 ± 0.4 1.686 ± 0.019S15 23.00 101.23 1.2 111 6.25 37.0 ± 0.1 1.628 ± 0.006S16 23.24 101.74 0.8 87 6.25 33.8 ± 0.2 1.806 ± 0.008Y1 24.06 101.95 1.6 59 6.45 41.0 ± 1.4 1.830 ± 0.070Y2 24.18 102.39 1.4 76 6.45 43.6 ± 2.7 1.684 ± 0.062Y3 23.77 102.01 1.7 27 6.45 41.8 ± 2.7 1.746 ± 0.061

aThe number of stacked events.bThe crustal average P velocity.cThe estimated crustal thickness.dThe estimated crustal average Vp /Vs ratio.

Figure 3. All events H‐k stacking S’(H, k) of stations S4,S7, S11, and S16; the crosses mark the maximum.

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times of Ps and multiples are almost as the same as thosegenerated by the flat Moho. We call these rays as best fittingrays.[16] Additionally, while a is very large (a = 20°;

Figure 4d), the situation becomes more complicated due tothe emergence of PpPp. For the ray propagating along theupdip direction (8 = 0°), the PpPp arrives close to the PpPsand the amplitude of PpPp is about three times of PpPs inR‐PRF; For the ray propagating along the downdipdirection (8 = 180°), the PpPp arrives very close to Ps(Figures 4d and 5), and receiver functions are very strange.We present the radial and vertical component of this ray andfurther experiments in Figure 6 to make this point clear.[17] For this ray (8 = 180°), the incidence of PpPp at Moho

is already larger than the critical angle of the total reflection.Thus, the amplitude of PpPp is very large in both vertical andradial components. And the large incident angle of PpPs atthe receiver makes the polarization almost in the verticaldirection (Figure 6a). Therefore, the vertical component doesnot satisfy the implicit assumption of the theories of receiverfunctions, which the vertical component behaves mostlylike one Dirac delta function [Langston, 1979]. So, some

Figure 5. The Ps and multiples’ arrival time of the syn-thetics from different crustal models (Figure 4a), sorted bythe 8; the ray parameter of synthetics is 0.080 s/km.

Figure 4. (a) Single‐layer crustal structure models. The Moho dips toward the north; the dip angle aequals 0°, 5°, 10°, 15°, and 20°, respectively. (b) The raypaths of the direct P and multiples propagatingalong the updip direction and the downdip direction. (c) R‐PRF and T‐PRF sorted by the relative azimuth8. The ray parameters (p) of the synthetics is 0.08 s/km; the Moho dip angle a is 10°. (d) R‐PRF and T‐PRFsorted by 8; p = 0.080 s/km, and a = 20°.

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strange features appear in this “receiver function” (Figures 4dand 6a).[18] We removed the PpPp and PpPs in the vertical com-

ponent, then deconvoluted the modified vertical componentform the radial component (Figures 6b and 6c). Those strangefeatures disappear. Therefore, the effects of PpPp should beaware while applying H‐k stacking.[19] In this section, we notice that, although the arrival

times of Ps and multiples shift by the effects of dippingMoho, the arrival times of some rays in the updip zone arealmost as same as those generated by a relative flat Moho.And the PpPp makes the deconvolusion results no longer asreceiver functions, for some rays in the downdip zone, ifMoho dips heavily. Evidentially, the model structure will bebetter approached by applying H‐k stacking to the syntheticsin the updip zone with small absolute value of �, excludingthose in the downdip zone.3.2.2. Updip and Downdip Stacking[20] Setting Vp = 6.00 km/s, w1 = 0.7, w2 = 0.2, and w3 =

0.1, we applyH‐k stacking to the synthetics with 8 ranging of0 ∼ 30°, 30 ∼ 60° and 150 ∼ 180° separately. And we call thestacking with the synthetics in the updip (downdip) zone asupdip (downdip) stacking for short. The stacking results are

shown in Table 3. Without considering the uncertainty, theresults of updip stacking and downdip stacking are almostthe same, if a is relatively small (a = 5°); as a increases, theresults of updip stacking (columns 0° ≤ 8 ≤ 30° and 30° ≤ 8 ≤60°) correspond with the models, but the results of downdipstacking (column 150° ≤ 8 ≤ 180°) deviate from the modelwith a decreasing of H and a increasing of k. The resultstacking with 8 in range of 0 ∼ 30° and a equaling 20° pre-sented two peaks in the summation map (Figure 7b). The onewith large amplitude deviated from the Model, and the onewith smaller amplitude was relatively close to the Model.Here, we isolate the SA(tPs), SA(tPpPs) and SA(tPsPs+PpSs)from S(H,k), which are normalized as SPs(H,k), SPpPs(H,k)and SPsPs+PpSs(H,k), to analyze the deviation of the down-dip stacking from the model structure and the appearanceof multiple peaks (Figure 7).[21] As a is relatively large (a = 10°, Figure 7a), the high‐

value belts for PpPs and PsPs + PpSs in the amplitude map ofSPpPs(H,k) and SPsPs+PpSs(H,k) shift leftward in the down-dip stacking results, compared with those in the updip zonestacking results. These shifts are induced by the early arrivalsof multiples, essentially by the dipping Moho, and make theestimate low in H and high in k.

Figure 6. (a) The synthetic propagating along the downdip direction (8 = 180°) in the crustal model withaequaling 20°; the deconvolution results frommodified vertical components. (b) Only PpPp is removed fromthe vertical component. (c) Both PpPp and PpPs are removed.

Table 2. The Relative Arrival Time Shift of Each Phase in Different Crust Models, �

aa (deg)

Updip Zone (0° ≤ 8 < 90°)b Downdip Zone (90° ≤ 8 ≤ 180°)b

Ps PpPs PsPs + PpSs Ps PpPs PsPs + PpSs

5 −1.4 ∼ 0.3 −2.6 ∼ 0.7 −2.0 ∼ 0.9 0.3 ∼ 2.0 0.7 ∼ 3.9 0.9 ∼ 3.810 −2.2 ∼ 1.1 −3.7 ∼ 2.9 −2.2 ∼ 3.5 1.1 ∼ 4.6 2.9 ∼ 9.0 3.5 ∼ 9.215 −2.2 ∼ 2.6 −3.2 ∼ 6.4 −0.5 ∼ 7.9 2.6 ∼ 7.6 6.4 ∼ 15.0 7.9 ∼ 15.920 −1.4 ∼ 4.6 −1.0 ∼ 11.1 3.2 ∼ 13.7 4.6 ∼ 11.0 11.1 ∼ 21.7 13.7 ∼ 23.9

aHere a is the Moho dip angle.bHere 8 is the relative BAZ from subtracting the Moho dip direction from the BAZ of events.

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[22] As a is very large (a = 20°), the highest‐value beltin the SPpPs(H,k) amplitude map of updip stacking is not forPpPs but PpPp. Thus the largest summation peak is consistedof the amplitude of Ps and PpPp. It makes the estimate very

low in H and very high in k. This problem is introducedby that the H‐k stacking only gives the predicted differentialarrival times of Ps and multiples, but without the phaseidentification. Usually, the PpPp is very weak or invisible in

Figure 7. S’(H,k), SPs(H,k), SPpPs(H,k), and SPsPs+PpSs(H,k) of the updip stacking with the synthetics in the8 range of 0 ∼ 30° and the downdip stacking with the synthetics in the 8 range of 150 ∼ 180°; the dashedcross marks the model structure, the solid cross marks the maximum summation in S’(H,k), and the smallersolid cross marks the second large peak. (a) The stacked synthetics are generated by a crustal model witha = 10°. (b) The synthetics are generated by a crustal model with a = 20°.

Table 3. H‐k Stacking Results of Synthetics From Dipping and Flat Moho Models

0° ≤ 8 ≤ 30° 30° ≤ 8 ≤ 60° 150° ≤ 8 ≤ 180°

a(deg) H (km) K H (km) K H (km) k

0 30 1.725 30.6 ± 0.1 1.710 ± 0.004 30.6 ± 0.1 1.716 ± 0.003 29.0 ± 0.4 1.740 ± 0.00910 31.2 ± 0.2 1.704 ± 0.004 30.6 ± 0.2 1.714 ± 0.004 27.2 ± 0.5 1.774 ± 0.01115 30.6 ± 0.2 1.714 ± 0.005 30.0 ± 0.3 1.724 ± 0.006 25.4 ± 0.7 1.808 ± 0.01220 18.2 ± 0.1 2.224 ± 0.008 28.2 ± 0.3 1.762 ± 0.006 34.0 ± 0.6 1.926 ± 0.01020 31.2 ± 0.6 1.700 ± 0.014a

aThe H and k respected to the second large summation peak of S’(H, k).

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receiver functions. But in this case (a = 20°, 8 = 0°), the PpPpis very strong in R‐PRF with the amplitude about three timesof PpPs, and the predicted arrival times of PpPs, given bycertain pairs of H and k, are just around the arrival time ofPpPp. Hence, in this pattern of amplitude map, PpPp wasmistaken for PpPs and the best estimate is the second largepeak in S(H,k) (Figure 7b).[23] As a is very large (a = 20°) the result of the downdip

stacking is meaningless, because the stacked “receiver func-tions” are meaningless.[24] The contrasts between the updip and the downdip

stacking results indicate that the updip stacking results is amuch better estimate of the crustal structure than the resultsof downdip stacking. The more the best fitting rays involved,the more reliable estimate obtained. The results of updip zoneand downdip zone are almost the same if Moho dips slightly(a ≤ 5°), and the situation becomes complicated if Mohodips very much (a ≥ 20°).3.2.3. Effects of Ray Parameter[25] Another important factor affecting the arrival times of

Ps and multiples is ray parameter (or epicentral distance).Based on the earth models in Figure 4a, synthetics with pequaling 0.070 s/km and 0.049 s/km are generated, and sortedby of 8 (Figure 8). The best fitting rays are in the 8 rangesof 15–60°and 0–45°, for the synthetics with p equaling0.07 s/km and 0.49 s/km, respectively.[26] These ray parameters chosen in the synthetic experi-

ments correspond to the epicentral distance range of 30–80°,which are usually for P wave receiver functions. Thus, whileapplying updip stacking to the real data, we suggest that the 8of the rays should be in a range of −60–60°. It will ensure thatmost of best fitting rays involved in the stacking, even if theMoho dip direction is imprecisely identified.

3.3. Real Data

[27] We learn from the synthetics experiments that, ifMoho dips heavily, the crustal structure obtained from updipstacking will better approached the actal crustal structure.Therefore, we search the Moho dip direction according to the

variation of the direct P polarity in T‐PRF, and apply updipstacking to approach the crustal structure.3.3.1. Moho Dip Direction[28] The T‐PRF of each station are sorted by BAZ, and

several of them are shown in Figure 9. According to therelationship between the direct P polarity variation in T‐PRFand Moho dip direction, we infer that the Moho beneath S4,S6, S11, S15 and Y2 dips toward the NWW, N, NNW, SEEand NE, respectively. The predicated Moho dip directionsof all stations are shown in Table 4.[29] Here, the direct P polarity variations of several sta-

tions are not as expected, with the polarity either positive ornegative for all BAZ. Note that these stations with uncertainMoho dip direction are situated nearby (or on) faults. Itimplies that the uncertainty might be induced by more com-plicated structure. For these stations, we assess the Mohodip direction from nearby stations and the reflection profilesinstead [Bai and Wang, 2004; Zhang et al., 2006].3.3.2. Updip Stacking Results[30] Setting w1 = 0.7, w2 = 0.2, w3 = 0.1, Vp = 6.25 km/s in

the ASRR shear zone and Simao basin, and Vp = 6.45 km/s inthe Chuxiong basin. As the Moho beneath S7 dips towardNE, the rays in the updip zone are traveling in the Chuxiongbasin, and we set Vp = 6.45 km/s for S7. According to thepredicted Moho dip direction, the updip staking results arepresented in Table 4. We also apply downdip stacking forcomparison with the synthetic experiments. As being lackof events traveling in the downdip directions, there are onlyseveral downdip stacking results obtained.[31] We present the SPs(H,k), SPpPs(H,k) and SPsPs+PpSs(H,k)

amplitude maps of S4 and S11 in Figure 10 to compare updipzone and downdip stacking results with the synthetic exper-iments. The solid cross marks the maximum, the dash crossmarks the crustal structure estimated from the updip stacking.In the results of downdip stacking, there is a significantleftward shift of the high‐value belt for PpPs in SPpPs(H,k),and the amplitude maps are similar to the synthetic experi-ments in Figure 7a. Especially, the amplitude maps of updip

Figure 8. The arrival times of Ps and multiples with different ray parameters in different crustal models(Figure 4a).

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stacking for S4, almost match the pattern of updip stacking inFigure 7b with strong effects of PpPp.[32] The Moho depth beneath each station is obtained by

subtracting the elevation from the crustal thickness estimatedfrom updip stacking result. By interpolating the Moho depthand the k of each station, the Moho depth and the averagecrustal Vp /Vs ratio variations in this region are presented inFigure 11.

4. Discussion and Conclusion

4.1. Crustal Thickness H

[33] According to the Table 4 and Figure 11a, the crustalthickness increase from ∼35 km in the central Simao basin to

∼42 km northward in the north Simao basin, and to ∼42 kmnortheastward in the south Chuxiong basin. In the ASRRshear zone the crustal thickness is ∼37–38 km. The Mohodepth variation (Figure 11a) corresponds with the Moho dipdirections identified from T‐PRF, and both of them agreewith the results of the reflection profiles [Bai and Wang,2004; Zhang et al., 2006].[34] Stations S2, S7, S15, S16, Y2 and Y3 are located on

the two reflection profiles, and Moho depths beneath thesestations are marked on the profiles (Figures 1, 2, and 11a),which agree with the results of the reflection profiles(Figure 2). We also mark the all events stacked results inFigure 2. Obviously, the updip stacking results are compa-rable with the all events results where the Moho is almost flat

Table 4. H‐k Stacking Results in Updip Zone and Downdip Zone, Respectively

Station Vp (km/s) Moho Dip Direction

Updip Zone Stacking Downdip Zone Stacking

BAZa H k n BAZ H k n

S1 6.25 N −60 ∼ 60 41.6 ± 4.9 1.754 ± 0.069 43 150 ∼ 240 34.0 ± 7.2 1.800 ± 0.180 3S2 6.25 NNW 0 ∼ 80 41.6 ± 0.3 1.692 ± 0.004 28S3 6.25 NE 80 ∼ 150 36.2 ± 4.3 1.714 ± 0.099 46 260 ∼ 330 32.8 ± 5.9 1.820 ± 0.140 10S4 6.25 NWW −60 ∼ 0 40.8 ± 3.4 1.696 ± 0.064 11 150 ∼ 240 36.2 ± 1.2 1.782 ± 0.018 6S5 6.25 N −30 ∼ 90 40.6 ± 1.1 1.730 ± 0.020 37S6 6.25 N −30 ∼ 50 38.0 ± 3.8 1.748 ± 0.04 7 120 ∼ 240 35.4 ± 4.3 1.958 ± 0.090 9S7 6.45 NE −60 ∼ 60 39.6 ± 2.3 1.734 ± 0.046 3S8 6.25 NEb 0 ∼ 120 35.8 ± 0.9 1.706 ± 0.015 31S9 6.25 NEb 0 ∼ 90 37.0 ± 1.9 1.704 ± 0.168 21S10 6.25 NW −80 ∼ 40 35.0 ± 0.9 1.702 ± 0.023 11 100 ∼ 220 34.8 ± 3.6 1.730 ± 0.107 38S11 6.25 NW −60 ∼ 0 35.6 ± 2.2 1.710 ± 0.041 30 120 ∼ 180 29.0 ± 3.9 1.768 ± 0.083 17S12 6.25 NWb −60 ∼ 0 37.4 ± 2.1 1.712 ± 0.014 9S13 6.25 NE 60 ∼ 120 37.0 ± 3.6 1.768 ± 0.053 14 240 ∼ 330 35.2 ± 1.0 1.792 ± 0.044 6S14 6.25 NEb 0 ∼ 90 35.0 ± 0.3 1.696 ± 0.016 26S15 6.25 SEE 90 ∼ 180 36.4 ± 0.8 1.654 ± 0.011 65S16 6.25 Nb −60 ∼ 40 37.8 ± 1.5 1.664 ± 0.017 11 120 ∼ 180 33.8 ± 0.7 1.838 ± 0.020 20Y1 6.45 NE −60 ∼ 60 41.8 ± 3.1 1.786 ± 0.073 28Y2 6.45 NE 0 ∼ 90 42.2 ± 0.8 1.716 ± 0.010 31Y3 6.45 NEb 0 ∼ 100 41.2 ± 3.3 1.764 ± 0.103 9

aThe BAZ of stacked events.bThe Moho dip directions are estimated from seismic reflection profiles [Bai and Wang, 2004; Zhang et al., 2006] and nearby stations.

Figure 9. The T‐PRF of several stations sorted by BAZ; the polarity variation of direct P, which impliesthe Moho dip direction, is marked on the vertical axis.

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beneath the receiver, but the Moho depth obtained from allevents stacked results is about 4 km shallow than the oneobtained from updip stacking beneath S16, where theMoho issharply dipping. Though the Moho beneath S9 dips sharply,the Moho depths obtained from all events stacked results andupdip stacking are almost the same, since the recorded raysare nearly in the updip zone.[35] Note that the Moho beneath S15 dips toward SEE. It

differs from the others. We think that the S15 is just on theSEE side of a Moho uplift caused by isostasy. It is becausethat the thickness of the sedimentary cover is over 10 km andthe Moho obviously uplift there [Bai and Wang, 2004]. In thestudies of gravity anomaly [Lou and Wang, 2005], the centerof a negative anomaly in the shallow‐layer apparent densitymap is just to the NWWof S15. This implies that the center ofthe Moho uplift is to the NWW of S15.

4.2. Average Crustal Vp /Vs Ratio k

[36] The average crustal Vp /Vs ratio is the least constrainedparameters from both laboratory and field measurements.Here, we established the crustal composition model com-bining the exposed rocks at outcrops and the P wave velocitystructure to estimate this parameter. Comparison between theVp /Vs ratios obtained from the updip stacking and the crustalcomposition model will allow us to examine the reliability ofour results.[37] For the compositional layered crust, the average

crustal Vp /Vs ratio can be defined approximately as:

k 0 ¼ VP

VS¼ H

tP� tSH

¼P hi

VSiP hiVPi

¼P hiki

VPiP hiVPi

ð2Þ

Figure 10. The R‐PRF of stations S4 and S11. The S’(H,k), SPs(H,k), SPpPs(H,k), and SPsPs+PpSs(H,k) ofthe updip stacking and the downdip stacking. The dashed cross marks the crustal structure estimated fromupdip H‐k stacking; the solid cross marks the maximum summation of S’(H,k). (a) Station S4: the PpPpemerges in the SPs(H,k) of updip stacking, and thePpPs in the SPpPs(H,k) of downdip stacking shifts leftwardcompared with the one in the SPpPs(H,k) of updip stacking. (b) Station S11: thePpPs in the SPpPs(H,k) down-dip stacking shifts leftward.

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where tP and tS are the vertical traveling time in crust, forP wave and S wave, respectively; hi, Vpi, Vsi and ki are thethickness,P velocity, S velocity, andVp /Vs ratio of each layer,respectively.[38] The composition of continental lower crust in conti-

nent is thought to be intermediate granulites [Christensen,1965; Christensen and Mooney, 1995; Ringwood andGreen, 1966; Smithson et al., 1981]. The widely exposedgranite in Yunnan implies that the middle crust and thebasement are composited of granite and/or granite gneiss. Theleucogranulitite, granulites and acidic gneiss of the AilaoShan group are thought to be the basement of Yangtze blockand the major rocks in the middle crust [Zhou et al., 1998].According to the crustal P velocity structure in Figure 2 [Baiand Wang, 2004; Zhang et al., 2006], we have simplifiedcrustal composition and Pwave structure beneath stations S2,S7, S15 and Y3 (Table 5). These four stations are located inthe north of Simao basin, the ASRR shear zone, the centralSimao basin and the limestone belt, respectively. The Vp /Vs

ratio of rocks under relative pressure and temperature areobtained from laboratorial measurements [Christensen, 1996;Wang et al., 2005]. Assuming that the P and S velocity ofrocks are only constrained by pressure, temperature and thecomposition of rocks, the estimated average crustal Vp /Vs

ratio are shown in Table 6 for comparison.[39] The values of k′ correspond with the updip stacking

results, but, differ from the k values of downdip stacking andall events stacked results in the sedimentary basin (S16) andthe ASRR shear zone (S6, S13), where Moho sharply dipsnortheastward (Table 6).[40] In the Figure 11b, k increases from the central Simao

basin to the limestone belt of the Chuxiong basin. We inferthat this k variation is controlled by two main factors. One isthat the major composition of upper crust varies from sand-stones of low Vp /Vs ratio to limestone of high Vp /Vs ratio, andthe other is that the thickness of lower crust, which consists of

high Vp /Vs ratio rocks, increases from several kilometers to∼15 km (Figure 2b).

4.3. Updip H‐k Stacking

[41] In our works, a series of synthetic experiments weremade to analyze the effects of dipping Moho on the results ofH‐k stacking focusing on rays traveling in different direc-tions. The analysis indicate that, if the dip angle is small, theupdip stacking and the downdip stacking results are compa-rable to the Model; if the dip angle is relatively large, theupdip stacking result is a good estimate of the model struc-

Figure 11. (a) The Moho depth variation in southwest Yunnan. (b) The crustal average Vp /Vs ratio vari-ation in southwest Yunnan.

Table 5. Simplified Crustal Compositional Model

Layer hia (km) Major Rocks Vpi

a (km/s) kib

Y3; Limestone Belt in Yangtze BlockSedimentary cover 5 limestone 5.52 1.800Basement 14 granite and granite gneiss 6.20 1.710Middle crust 7 granite and granite gneiss 6.30 1.732Lower crust 16 felsic granulite 7.20 1.795

S7; ASRR Shear ZoneBasement 5 granite gneiss 5.52 1.710

10 granite and granite gneiss 6.20 1.710Middle crust 15 granite and granite gneiss 6.30 1.732Lower crust 10 felsic granulite 7.20 1.795

S2; in the North of Simao BasinSedimentary cover 10 sandstone 5.52 1.570Basement 8 granite and granite gneiss 6.20 1.710Middle crust 8 granite and granite gneiss 6.30 1.732Lower crust 15 felsic granulite 7.10 1.795

S15; Sedimentary Basin in the South of SimaoSedimentary cover 12 sandstone 5.52 1.570Basement 9 granite and granite gneiss 6.20 1.710Middle crust 9 granite and granite gneiss 6.30 1.732Lower crust 5 felsic granulite 7.10 1.795

aHere hi and Vpi are thickness and average P velocity of each layergeneralized from the tomography of the wild‐angle reflection profile [Baiand Wang, 2004; Zhang et al., 2006], respectively.

bThe Vp /Vs ratio of rocks in column “major rocks,” from the laboratorialmeasurement results [Christensen, 1996; Wang et al., 2005].

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ture, and the downdip stacking result deviates from the modelstructure with a result low in crustal thickness and high inaverage crustal Vp /Vs ratio; the updip stacking also can get ridof the disturbance of PpPp, if the dip angle is very large.[42] According to the reflection profiles (Figure 2) [Bai and

Wang, 2004; Zhang et al., 2006], the Moho beneath S4, S7,S11 and S16 dips significantly. While applying both updipstacking and downdip stacking to real data, the downdipstacking results are lower in crustal thickness and higher inaverage crustal Vp /Vs ratio, compared to the updip stackingresults. This situation corresponds with the pattern of thesynthetic experiments. The crustal thickness obtained fromupdip stacking agrees with the wide‐angle reflection results[Bai and Wang, 2004; Zhang et al., 2006], and the k valuescorrespond with the those estimated from the simplifiedcrustal composition model, where the Moho dips sharply. So,for a dipping Moho, it is better to obtain the crustal structureby applying updip stacking, which only stacks rays travelingalong or around the updip direction.[43] Now, it is easy to understand how a dipping Moho

causing the deviated H‐k stacking results and the multiplepeaks solution, when all events were stacked (Figure 3). Therecorded events relatively concentrate within several direc-tions. The dipping Moho deflects the rays. The degrees of thearrival time shifting of the multiples for these rays are dif-ferent in different directions, and, there are more than onehigh‐value belt in the amplitude maps of SPpPs(H,k) and SPsPs+PpSs(H,k). The belt with the highest amplitude in SPpPs(H,k)and SPsPs+PpSs(H,k) dominates the maximum, and the othersmake the amplitude map of S’(H,k) multiple peaks.[44] However, there are some factors restricting the appli-

cation of updip stacking. The assumption is that Moho isplanar, and actually Mohomay be curved. The stacked eventsshould be in the updip zone, and thus all the record eventscannot be completely used. The accuracy of the determinedMoho dip direction depends on the distribution of events andthe quantity of the recorded events, and is affected by crustalinterfaces with different dip directions [Cassidy, 1992; Chenand Liu, 2000].

[45] Acknowledgments. Eric Hetland, an anonymous reviewer, andthe associate editor are thanked for their helpful comments and suggestionson the paper. Comments from Qin Wang and Zhouchuan Huang improvedthis paper. This research is supported by the National Science Foundationof China (grants 40634021, 40034010, and 40574015).

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Table 6. Comparison Between k′ and k

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k

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North Simao S2 1.70 1.692 ± 0.004 1.694 ± 0.007Sedimentary basin S15 1.67 1.654 ± 0.011 1.628 ± 0.006

S16 1.664 ± 0.017 1.838 ± 0.020 1.806 ± 0.008ASRR shear zone S7 1.74 1.734 ± 0.046 1.636 ± 0.121

S6 1.748 ± 0.040 1.958 ± 0.090 1.700 ± 0.017S13 1.768 ± 0.053 1.792 ± 0.044 1.758 ± 0.015

Limestone belt Y3 1.76 1.764 ± 0.103 1.746 ± 0.061

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