Abstract Using seismic data recorded by YunnanTelemetry Seismic Network from January 1, 2000,to May 31, 2005, the polarization directions of fastshear waves are obtained at 15 seismic stationsby SAM technique, which is a systematic analy-sis method on shear-wave splitting. The resultsshow that predominant directions of polarizationsof fast shear waves at most stations are mainlynearly in the N–S or NNW directions in Yunnan.The predominant polarization directions of fastshear waves at stations located on the active faultsare consistent with the strike of active faults, di-rections of regional principal compressive strainsfrom GPS measurement, and directions of re-gional principal compressive stress. A few of thestations show that polarization patterns of fastshear waves are more complicated or inconsistentwith the strike of active faults and the directions ofprincipal GPS compressive strains; these stationsare always located at the junction of several faults.
Y.-t. Shi · Y. Gao (B) · J. WuInstitute of Earthquake Science,China Earthquake Administration,Beijing 100036, Chinae-mail: [email protected]
Y.-j. SuEarthquake Administration of Yunnan Province,Kunming 650041, China
We conclude that the predominant polarizationdirection of fast shear waves indicates that thedirection of the in situ maximum principal com-pressive stress is controlled by multiple tectonicaspects, such as the regional stress field and faults.
Keywords Shear-wave splitting · Polarizationof fast shear wave · Principal compressive stress ·Active faults
1 Introduction
Seismic anisotropy is a universal phenomenonin the crust (Crampin and Atkinson 1985). Theshear-wave splits when it travels through ananisotropic medium. Shear-wave splitting canshow the anisotropic characteristics in crust andit can be used to analyze crustal stress field con-dition and describe the static and the dynamicstate of the related anisotropic parameters (Gaoet al. 1999). The polarization of shear-wave split-ting reflects the direction of the in situ principalcompressive stress in the crust under the seismicstation (Crampin 1978).
Yunnan area is at east of the Tibetan Plateauand is in the south end of the North–South Earth-quake Belt in China. The seismic tectonics inYunnan area are very complicated, and it is one ofthe strongest seismic active areas in China. From
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1 January 2000 to 31 December 2006, there weremore than 24 earthquakes with M ≥ 5.0, includ-ing three M ≥ 6 earthquakes. This paper willadopt the seismic waveform data of the YunnanTelemetry Seismic Network (YTSN) to study theanisotropy in the crust in Yunnan area and to ana-lyze the regional stress distribution characteristicsand the relation with the faults.
2 Tectonic background
There are mainly three groups of strikes of activefaults in Yunnan area: N–S, NW, and NE. Thedistribution of faults has obvious regional char-acteristics (Fig. 1). Taking the Red River faultas a boundary, there are mainly the N–S strikefaults (N–S faults, same as below) with some NE
strike faults (NE faults, same as below) in the eastand mainly NW faults with locally intersectedfaults consisting of NE faults and NW faults inthe west. The big fault zones in this Yunnan areaare Red River fault, Nujiang fault, Xiaojiang fault,and so on (Deng et al. 2002). Red River faultcrosses central Yunnan area and is rather seismi-cally active. Nujiang fault is a big substantial faultzone of lithospheric scale, which controls the westboundary of Baoshan (BS) area. It is currentlyseismically active. The Sichuan–Yunnan rhombicblock is one of the most seismically active zonein the Chinese mainland. Within Yunnan area,the east and west sides of this rhombic block are,respectively, Xiaojiang–Anning River strike-slipfault with near N–S striking and Red River strike-slip fault with near N–S or NW striking (Yangand Ma 2003). In this zone, including boundary,
Fig. 1 Distribution ofseismic stations, fault, andearthquake activity inYunnan. Black trianglesare stations of YTSN. Thesolid circles areearthquakes with M = 5from January 1, 2000, toNovember 30, 2005.1 Nujiang fault,2 Lancangjiang fault, 3Red River fault, 4CX–TH fault, 5Xiangjiang fault,6 LJ-Jianchuan fault,7 YM fault, 8Wuliangshan fault, 9Zhongdian fault, 10Nandinghe fault,11 Longling fault, and12 Chenghai fault
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earthquakes are mainly in strike-slip type (Wuet al. 2004; Zhang et al. 2005a, b).
The axis of principal compressive stress inYunnan area is near N–S (Kan et al. 1983). Theresult of GPS measurement indicates that Yunnanarea could be divided into four parts according tothe strain and stress characteristics (Fig. 1). Thedirections of local principal compressive strainsare approximate N–S in the west part and thecentral Yunnan part. The principal compressivestrain rate of the eastern part is nearly zero (Yanget al. 2003). According to the focal mechanismsof small earthquakes, the direction of compressivestress field in Yunnan area is the NNW directionturning to the near N–S direction (Wu et al. 2004).
In the west of Yunnan, earthquakes mainlyconcentrate in the Dali–Chuxiong (CX) area andBS–Lincang–Simao area (Su 2002) because thereare several quite large active fault zones in thisarea. The stations of Lijiang (LJ), Heqing (HQ),and Tuanshan (TS) et al. distribute on or nearHonghe fault zone. CX locates at the CX–Tonghai(TH) fault zone. BS is at the edge of Nujiangfault zone. The quite active Xiaojiang fault zoneis located in the eastern area. Dongchuan (DC) isat the Xiaojiang fault zone. TH station is locatedat the junction of Xiaojiang fault and the CX–TH fault (Fig. 1). It is useful to analyze shear-wave polarizations at these stations, to investigatethe predominant alignment direction of the crustalextensive-dilatancy anisotropy (EDA) cracks, aswell as to realize the anisotropic characteristics,the stress characteristics in the crust, and the faultcharacteristics.
3 Techniques and data
When seismic shear waves travel through aniso-tropic media, they split into faster and slower splitshear waves with different velocity, with nearlyorthogonal polarizations. In the crustal rock be-low critical depth, cracks are vertically paral-lel, namely, EDA cracks (Crampin and Atkinson1985). The EDA cracks alignment is an aniso-tropic structure. The polarization directions of thefast shear waves are consistent with the direc-tion of aligned cracks. In another words, the pre-dominant polarization direction of the fast shear
waves is in accordance with the direction of in situmaximum horizontal principal compressive stress.The time delay of slow shear wave reflects theanisotropic degree of crack crust, which is influ-enced by the physical features of the cracks andthe saturated fluid.
When shear wave encounters free surface,shear waveform will be distorted if the incidenceangle is larger than the critical angle. The criticalangle is about 35◦ in Poisson medium, which is alsothe limited range of the shear-wave window. How-ever, the shear-wave window may be enlarged to40◦–45◦ for curved wave front at surface deposi-tion with low velocity.
There are many analysis methods for shearwave splitting. However, seismic signals are gener-ally disturbed by various noises, especially in smalltime scale. When the magnitude of one earth-quake is small, stationary signal cannot reflect wellthe time-variance characteristics of the seismic sig-nal. Most methods used currently are of scanningtechnique. The result is seriously dependent onthe selection of the window length and S/N ratio.Therefore, it is hard to get the result in accordancewith the real signal when single method is used toidentify slow and fast shear wave. Although manyused analysis methods, the polarization analysis(Crampin 1978) is one of the most usual methods.
Because most of the difficulty in analysis ofshear wave splitting is in the accurate recognitionof fast and slow shear wave, the system analysismethod (SAM) was proposed, which is based onthe correlation function. It includes three parts,the calculation of cross-correlation function, elim-ination of time-delay, and analysis of polarization,and it has a function of self-examination (Gao andZheng 1995; Gao et al. 2004b). The theory of SAMmethod is briefly introduced as follows since it isused in this research.
1. Calculation of cross-correlation function: Ac-cording to the arrival time of the shear wave,two horizontal directions of shear waveformsare intercepted. By modifying the rotationangle α and the time delay �t, the cross-correlation function is calculated. The begin-ning direction of the angle α is the north, and itchanges clockwise within [0, 180◦]. Supposingthat the time difference between the fast and
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slow shear waves is not more than 2T, the timedelay �t will change within [−T, T]. If the po-larization direction of the fast shear wave is α,that of the slow shear wave is approximately avertical component. The correlation functionis calculated for two horizontal componentsx(t) and y(t + �t). With the maximum func-tion, the rotation angle α and time difference�t will be the polarization direction of the fastshear wave and the time delay of the slowshear wave, respectively.
Because many factors affect records, the resultcalculated from the cross-correlation function isusually a little inconsistent with the real observa-tion. However, the calculation of cross-correlationfunction gives us some effective information forthe polarization test. The best case is the Dong-fang isolated earthquake swarm in Hainan, China,with an original SAM method (Gao et al. 1998).
2. Elimination of time delay: The waveform ofthe slow shear wave is moved forward by thetime-delay �t to keep the time consistency of
the first motion with the fast shear wave so asto eliminate the effect of the time delay.
3. Test analysis of polarization: The polarizationdiagrams before and after the adjustment ofthe time delay are compared to test the ac-curacy of the results. If polarization is morelinear after eliminating the effect of the timedelay, the result is reliable; otherwise, the re-sult is unreliable and should be recalculated oranalyzed by another technique.
TS station is a station of YTSN; the preliminar-ily processing of a waveform data at TS is shownin Fig. 2. The recorded earthquake occurred onFebruary 19, 2003, with a depth of 17 km and amagnitude of 1.8. The epicenter was located at25.47◦N, 100.19◦E, and the epicentral distance was16.66 km. According to the velocity structure (Wuet al. 2001); the incident angle was 28.0◦, which iswithin the shear-wave window. According to thecalculation results, the direction of polarizationwas about 135◦, and the time delay of the slowshear wave was between 0.06 and 0.08 s (Fig. 2).
Fig. 2 Calculation ofcross-correlationfunction. The waveform isintercepted by 200sampling points. a Theisoline ofcross-correlation functionwith different delay timeand different polarizationangle, the black solidellipse is the biggest valueof cross-correlationfunction. b The maximumvalue and the minimumvalue distribution of thecross-correlationfunction. c Threecomponents of waveform:UD, NS, and EWseparately representvertical, north–south, andeast–west components,and two erect dashed linesframe the length of shearwaveform that is used tocompute thecross-correlation function
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Fig. 3 Polarization pattern of two horizontal shear wavesand waveforms. a Polarization of two horizontal shearwaves. S1 and S2 indicate the start positions of fast andslow shear waves. b, c Shear-wave waveform of north–south (NS) and east–west (EW) components. The erecteddotted line indicates the arrival of shear wave
The polarization diagram of two horizontalshear waves is shown in Fig. 3. Because of theshear-wave splitting, the polarization is nonlinearfor the two horizontal components. Based on theresults in Fig. 2, the waveform in Fig. 3 is rotatedto obtain the fast and slow shear wave (Fig. 4band c). The slow shear wave is moved forward toeliminate the effect of the time delay and then newcorrected polarization plot is obtained (Fig. 4a).If there is some mistake in the direction of po-larization of the fast shear wave, it is impossibleto obtain fast and slow shear wave because twowaveforms are mixed together. If the time delayis not correctly calculated, there will be a non-linear feature in the polarization. The result inFig. 4 shows that the parameters of the shear-wavesplitting are reliable with SAM analysis. In thiscase, the preliminary result from the calculation
Fig. 4 Analysis and test of polarization of shear waves.a Polarization of fast and slow shear waves, which has beeneliminated by the effect of time delay. b, c Fast and slowshear-wave waveform (F and S). The dotted line indicates,respectively, the arrivals of fast shear wave and slow shearwave
of cross-correlation function is at 135◦ and 0.07 s,respectively, for the polarization direction of thefast shear wave and the time delay of the slowshear wave (Fig. 2). According to test of elimina-tion of time-delay and analysis of polarization, theresult adjusted to 135◦, 0.04 s, as shown in Fig. 4.Since the result of the fast shear wave is relativelyaccurate, it assists the analysis of the time delay ofthe slow shear wave. The above results show thatthe SAM is one of reliable effective methods toanalyze shear-wave splitting at present.
4 Results
The data used in this study are from January 1,2000, to May 31, 2005. According to the velocitystructure of the shear wave of Yunnan area (Wu
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et al. 2001; Zhang et al. 2005a, b), shear-waveincident angles of seismic waves are calculatedby all stations of YTSN. The earthquakes werechosen by a very strict restriction with the shear-wave window with the angle of 37◦ in this study.
The depths of many earthquakes were notgiven in YTSN because of all kinds of reasons.However, these data are very valuable in thestudy of local seismic anisotropy. Following withthe understanding of the shear-wave window andthat the depths of most earthquakes are at about10 km or above in the Yunnan area, the data
with the epicenter distance less than 10 km werealso included as the supplement data in this study,but only available in analysis of polarizations.Figure 5 compares the polarizations of fast shearwaves from the data obtained the earthquake fo-cal depths with those from the supplement data.It can be seen that the data at 15 stations havebeen increased much, and the directions of the po-larizations of fast shear waves of the supplementdata are fundamentally consistent with the resultsof the selected earthquakes calculated within theshear-wave window but show more consistency
Fig. 5 Comparison ofpolarizations of the fastshear waves. Thehomolographic projectionrose plots are shown inthe figure. The outer largecircle is the shear-wavewindow of 37◦. Lines 1, 3,and 5 are calculated fromthe data within theshear-wave window.Lines 2, 4, and 6 arecalculated from moredata including thesupplement data withoutfocal depth determinationbut epicenter distance lessthan 10 km. The numbersin brackets behind stationcodes are numbers ofevent records
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Table 1 Parameters of shear-wave splitting from YTSN stations
Station Polarization of fast shear wave Polarization of fast shear wave Average time(within shear wave window) (with supplement data more) delay
Name Code Average Number Average Numberpolarization polarization
(Fig. 5). Especially at the stations DC, LQ, MI,YL, and YX, with very few original data of fo-cal depths, the results improved much. The pre-dominant polarization of the fast shear waves ismuch clearer after increasing supplement data.According to Table 1, there is a small differenceof calculated results between original data andsupplemented data at most stations, except theWD station. The main reason is that the datawithin the shear-wave window are far less than thesupplemented data at WD. The results indicatethat this kind of data processing technique usingthe supplement data could increase the reliabilityof the results under the specific data condition,especially for those with very limited data.
The polarizations of the fast shear waves of 15stations in the Yunnan area are shown in Figs. 6and 7. The predominant polarizations of the fastshear waves are consistent with the direction ofthe local principal compressive stress. They arecontrolled by the local stress field and relatedto fault distribution and geological structure. Itcan be seen that the predominant polarizationdirections of fast shear waves at six stations [LJ,BS, Cangyuan (CY), MI, TH, WD] are near N–S,the predominant polarization directions of fastshear waves at six other stations (TS, CX, DC, LQ,YL, WS) are in the NW or NNW direction, thepredominant polarization directions at one station
[Yimen (YM)] is in the NE direction, and thepredominant polarization directions at one station(HQ) is complex and predominant polarization isnot very obvious (Figs. 6 and 7).
5 Discussions
Shear-wave velocity structure shows strong lateralheterogeneity in the Yunnan area. The shear-wave velocity in the north is obviously slowerthan in the south down to 10 km in depth, whileit is faster in depths from 10 to 20 km. The crustalthickness in the northwest is approximately 62 km,and that in the southeast is approximately 32 km(Wu et al. 2001; Zhang et al. 2005a, b). The com-plex distribution of faults (Fig. 1) reflects the localcomplex tectonics. In addition, because TibetanPlateau moves eastwards (Zeng et al. 1992; Zhangand Klemperer 2005), the western part of Yunnanis intensively affected by the Europe–Asian Plateand the Indian Plate. The surface movementpattern becomes more complex (Yang et al. 2003;Liu et al. 2001). By comparison, the influence onthe eastern part of Yunnan is relative weaker.This special tectonic background of Yunnanresults in complex local characteristics of crustalstress field in Yunnan. This study also shows somecomplexity in stress field.
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Fig. 6 Distribution of polarizations of fast shear waves at YTSN in Yunnan, SW China. Only the result from data withinshear-wave window are shown in the homolographic projection rose plots
LJ station is in the southwest of the Sichuan–Yunnan rhombic block, and it is a seismicallyactive zone. According to optical axis determina-tion of calcite crystals, the direction of principalcompressive stress is not consistent in the LJ area,and there is a quite large stress adjustment in thisregion, which causes the direction of the regionalstress field to turn from NW to NE or N–S (Yuet al. 2002). The local principal compressive stressdirection is at N–S by research on the LJ Ms7.0earthquake in February 3, 1996 (Han et al. 2004).The directions of polarizations of fast shear wavesat station LJ are near the N–S direction (Fig. 6),which indicates the direction of in situ principalcompressive stress is near N–S. This direction isnearly consistent with compression strain of NNWby the GPS measurement (Yang et al. 2003).
HQ station located a complex geological zone,which is a basin mainly controlled by NE andE–W fault (Xing et al. 1986). Red River fault
striking near the N–S direction goes through thisbasin, which has very strong seismic activity. Thepredominant polarization of fast shear waves isnot obvious in this study, and the result only showstwo unobvious relative predominant directions ofnear N–S and near WNW (Fig. 6). Accordingto focal mechanisms of two earthquakes largerthan Ms5.0 in 1986, the direction of P axis isnear N–S in this zone (Dong 1990). The directionof compressive strain is at NNW by GPS (Yanget al. 2003). The difference between the above tworesults is very small. The polarization of fast shearwave at HQ indicates extremely complicated insitu principal compressive stress around HQ sta-tion. However, this result is limited to a smallzone, different from the obviously predominantN–S direction at LJ station, 50 km away northto station HQ. HQ is located at the juncture oftwo local faults (Fig. 6). The stress is variableover crossing faults, as well as at end point of a
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Fig. 7 Averagepolarizations of fast shearwaves in Yunnan and thecomparison with GPSmeasurement, focalmechanisms, and regionalprincipal compressivestress. The thick straightlines indicate the averagepolarizations of fast shearwaves at stations, and thelengths of lines indicatethe scales of the timedelays
fault (Li 1992). The research on Jianchuan area,20 km west to HQ, indicates that the polarizationsof fast shear waves are very locally complex andpolarizations at eastern stations are basically con-sistent in N55◦W, although at one station no clearsingle predominant polarization (Lei et al. 1997).Gao et al. (1995, 1999) and Lei et al. (1997) alsoconfirmed the locally complex polarizations of fastshear waves in the crust.
TS station is in the Dali area at an intersectionarea of Red River fault zone and other fault zones.According to mechanical simulation on Dali seis-mic tectonic environment by the finite elementmethod, the direction of principal compressivestress in the Dali area is N15◦W to N20◦W, that is,the NNW direction (Xing and Ma 1985). It is con-sistent with the polarizations of fast shear wavesat station TS (Fig. 6). The result is also consistentwith principal compressive strain direction fromGPS in the TS area (Fig. 6). Station TS is located
at the Red River fault, a Late Pleistocene activefault. According to the study of a large-scale activefault, polarizations of fast shear waves at stationson active faults are different from those at sta-tions away from faults and are always consistentwith strikes of faults (Crampin et al. 2003; Wuet al. 2007). The result at station TS supports thisconclusion.
BS station is located in the east of Nujiangfault (Fig. 1). Considering the width of the faultzone, station BS could be located on the Nujiangfault zone. According to analysis of 36 recordswith the focal depth and 642 records of epicentraldistance within 10 km in this study, the directionsof polarizations of shear waves at BS are at N–Sand slightly turning to NNE (Fig. 6). This resultis consistent with the principal compressive strainof NNE direction obtained from GPS (Yang et al.2003). According to eight earthquake records, thedirections of polarizations of fast shear waves
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obtained are near E–W at BS station (Gao et al.2004a), it is different but consistent with the pos-sible second predominant direction in this study.The possible reason is that the data used by Gaoet al. (2004a) are just from the edge of the shear-wave window, at about a 45◦ incidence angle. Be-cause of the influence of the crack aspect ratio, thedirection of polarization of fast shear wave couldbe different from that of local principal compres-sive stress (Gao and Crampin 2006). Accordingthe velocity structure (Wu et al. 2001), a morestrict shear-wave window of 37◦ was adopted;therefore, most of data in Gao et al. (2004a) arenot included in this study.
CY station is located at the boundary zone.Although there are no geological data obtainedfrom another side of the international boundary,NE-striking and NW-striking faults can be seennear station CY within China. These two faultsrespectively distribute northwest and northeast tostation CY (Fig. 6). Polarizations of fast shearwaves at CY are obviously at N–S, basically con-sistent with the principal compressive strain of theNNE direction from GPS (Yang et al. 2003).
CX station is nearly on the CX–TH fault, whichis a Late Pleistocene active fault (Fig. 1). Thepolarizations of fast shear waves at station CX arein the NW direction, consistent with the directionof principal compressive strain from GPS (Yanget al. 2003), as well as the strike of the active fault(Fig. 6). Using data from a small local seismicnetwork, which is over 70 km northwest to CXand also near to the CX–TH fault zone, the po-larizations of fast shear waves were obtained fromNE140◦ to NE164◦, and the average is at NE152.4◦(Qian et al. 2002). The polarizations of fast shearwaves in this study are consistent with Qian et al.(2002), parallel to the strike of the active fault(Figs. 6 and 7).
YM station is located in the Sichuan–Yunnanrhombic block, at the juncture of YM fault andCX–TH fault. Predominant polarization of fastshear waves is at NEE near the E–W direction,which is approximately perpendicular to the di-rection of local principal compressive strain fromGPS measurements, the direction of local prin-cipal compressive stress and the strike of mainactive fault in this zone. It is inconsistent withpolarizations at station CX located on an identical
fault zone. The possible reason is that station YMis located at a juncture of several faults. It is notonly located at the joint of CX–TH fault and YMfault, but also at the joint of two secondary faultsof CX–TH fault and an Early Quaternary activefault striking nearly N–S. In the joint of faults, thestress field also is very complicated. It results in aninconsistency of polarizations of fast shear wavesbetween stations YM and CX.
DC station is located at the east part of Yunnan.The tectonic line is near the N–S direction inthe DC zone, where it is also seismically active.Xiaojiang fault is the major active fault in theYunnan area (Fig. 1). Station DC is located on theXiaojiang fault zone of striking N–S. Accordingto Liu et al. (2002), when the Sichuan–Yunnanrhombic block moves toward the SSE direction,the DC zone is in a tectonic turning point and thefault is hard to wriggle, where stress in the crustis easily concentrated and strain energy is easilyaccumulated. Few earthquakes occurred in thiszone; therefore, the strain energy accumulatedin the crust is not easily released. According tothe focal mechanisms for several years, the axisof principal compressive stress in the DC zoneis at NW–SE. The predominant polarization offast shear waves at DC obtained in this study isin the NW (data within shear-wave window) orNNW direction (with supplement data more). Be-cause there are only four records within the shear-wave window, the reliability of results obtainedfrom 130 records (supplement data) is higher bycomparison. The predominant polarization of fastshear waves at DC is consistent with the directionof principal compressive strain from GPS, thestrike of active fault, and the direction of principalcompressive stress in Yunnan (Fig. 6).
Station MI is also located in the eastern partof Yunnan, east to the Xiaojiang fault zone. Al-though there are not a lot of data at MI, the pre-dominant polarization of fast shear waves is nearN–S. According to focal mechanisms, the direc-tion of principal compressive stress is at NE60.3◦in the eastern Yunnan area (Liu et al. 2002). Thereis an early Quaternary active fault less than 5 kmto the northwest of MI. It can be seen in Fig. 6 that,although MI is 20–25 km away from active faultzone striking N–S and is located in a relativelystable tectonic area, its local principal compressive
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stress direction is still controlled by the Xiaojiangfault zone in the west, at about N–S, consistentwith the direction of principal compressive strainfrom GPS, as well as regional principal compres-sive stress.
TH station is located at the southeast end of theSichuan–Yunnan rhombic block. The Xiaojiangfault approximately striking N–S is cut by the RedRiver fault striking NW, forming an acute anglezone. Both Xiaojiang fault and Red River faultare the boundary of the Sichuan–Yunnan rhombicblock. Both of them endure the movement towardSSE of the Sichuan–Yunnan rhombic block, whichresults in the concentration of stress in this acuteangle zone. Two earthquakes, MS 6.5 in 1965 andMS 7.7 in 1970, occurred in this zone. The direc-tion of principal compressive stress axis is at theNNE from focal mechanisms of both earthquakes(Kan et al. 1977; Liu et al. 1999). The predominantpolarization of fast shear waves is at about N–S,consistent with principal compressive strain direc-tion from GPS and strike of active fault, as wellas regional principal compressive stress (Figs. 6and 7), and there is little difference with directionof principal compressive stress axis of NNE ob-tained from focal mechanism. The result suggeststhat the Xiaojiang fault zone has a larger influenceon local principal compressive stress field.
Station YX is near the Nandinghe fault, whichis divided into two faults by the Lancangjiangfault. The big Lancangiang fault is located in theeast of YX and influences this zone. With fivedata in the shear wave window and four moresupplement data, the predominant polarization offast shear waves is nearly in the N–S direction,consistent with the direction of stations BS, WD,and CY. Furthermore, the second predominantpolarization of fast shear waves is nearly in the NEdirection at YX (Figs. 5 and 6), which suggests apossible influence from the fault striking the NEdirection.
Station LQ in Luquan is located in the Sichuan–Yunnan rhombic block, with the YM fault in thewest and the extremely active Xiaojiang fault inthe east. There was an Ms6.1 earthquake in theLuquan area in April 18, 1985, at a depth of 9 km.Li (1996) obtained the polarization in the NWdirection according to the aftershock sequenceof the Luquan earthquake. It is consistent with
the predominant polarization of fast shear wavesin this study. However, LQ also shows a secondpredominant polarization in the NE direction,specially seen from the more supplemental data(Fig. 5).
Station WS is located in the east part ofYunnan, where there is a relatively stable SouthChina block (Song et al. 1998). The biggest earth-quake in this zone since 1970 was the FuningMs5.8 earthquake in 1982. The predominant po-larization of fast shear waves at WS is nearly in theNNW direction (Figs. 6 and 7). It is consistent withresults of most stations in east Yunnan. The axisof the principal compressive stress in east Yunnanis at NE60.3◦ from focal mechanisms (Liu et al.1999), inconsistent with this study. However, theresult at WS in this study is consistent with thatneighboring stations and the tectonic background,although the data at WS are very few.
6 Conclusions
The Yunnan area is strongly influenced by theEuropean–Asian and Indian plates. The tec-tonic characteristics are complicated. Using thewaveform data recorded from YTSN, crustalanisotropy in Yunnan could be obtained fromshear-wave splitting in this study.
The predominant polarization of fast shearwaves suggests the directions of in situ principalcompressive stress. This study shows that polar-izations of fast shear waves are consistent withthe direction of principal compressive strain fromGPS and the direction of in situ principal compres-sive stress, it is strongly related to the direction ofregional principal compressive stress in Yunnan.The directions of polarizations at stations locatedon active faults are consistent with the strikes ofactive faults, for example, stations TS, BS, CX,DC, and TH. Researches on the characteristics ofpolarizations of fast shear waves in Yunnan showthat the distribution of the polarizations couldsuggest whether the faults are active or not. Thisresult is consistent with the research of Wu et al.(2007) and Gao et al. (2008).
Complicated local tectonics could control or in-fluence the direction of predominant polarizationof fast shear waves of stations, which result in the
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inconsistent direction with the strike of the majoractive fault or much difference between neigh-boring stations. It can also lead the polarizationsof fast shear waves to be more scattered or tohave multiple predominant directions, such as atstations HQ and YM.
According to researches on tectonic back-ground and seismic activity, when no precise fo-cal depth information of earthquakes is available,waveform data of earthquakes that occurred nearto the station can be used as a supplement data.This kind of data supplement technique is helpfulto enhance the reliability of the polarizations offast shear-wave analysis in case of very limiteddata. The tectonics in Yunnan is very complicatedwith many active faults. More data are neededto obtain more detailed seismic anisotropy in thisarea.
Acknowledgements This study was supported by an IES-CEA Project 2008. We thank colleagues from YTSN forhelping with collecting the data in this study.
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