AZIMUTHAL ANISOTROPY IN THE EARTH FROM OBSERVATIONS OF SKS AT GEOSCOPE AND NARS BROADBAND STATIONS

Transcription

1 Bulletin of the Seismological Society of America, Vol. 79, No. 5, pp , October 1989 AIMUTHAL ANISOTROPY IN THE EARTH FROM OBSERVATIONS OF SKS AT GEOSCOPE AND NARS BROADBAND STATIONS BY LEV P. VINNIK, VERONIQUE FARRA, AND BARBARA ROMANOWIC ABSTRACT We have analyzed observations of low-frequency SKS waves recorded on horizontal components at several broadband stations of the GEOSCOPE and NARS arrays. Splitting of SKS is consistently observed and in good agreement with a hypothesis of azimuthal anisotropy due to transverse anisotropy with a horizontal axis, in the upper mantle beneath the receivers. We have designed an inversion method, which takes advantage of observations made simultaneously at different azimuths and which permits us to obtain estimates of the direction of fast axis of anisotropy as well as the time delay between the orthogonal quasi-shear waves. In general, the directions of fast velocity obtained, both for stations around the Atlantic and around the Pacific, appear to be related to directions of past and present plate motions. INTRODUCTION Information on azimuthal anisotropy in the mantle is of interest to geophysicists since it can be related to dynamic processes inside the earth. Though the presence of anisotropy in the upper part of the earth is well established, not enough is known about it as a 3-D phenomenon. Spectacular results were recently obtained from the observations of long-period surface waves (e.g., Montagner and Tanimoto, 1988). Data of this kind are very useful in studies of large-scale structure, whose characteristic lateral dimension is of the order of a few thousand kilometers. To resolve smaller scale structure, body-wave data are required. The most powerful techniques for retrieving the parameters of anisotropy from body-wave data are based on the phenomenon of shear-wave splitting. A straightforward way to observe this phenomenon is to resolve the split waves on highfrequency records of explosions or local earthquakes (e.g., Hirn, 1977). However, the results of such analysis are often ambiguous due to multipathing, scattering, and source complexity, effects particularly strong at high frequency. An alternative approach is to study this phenomenon in a low-frequency range where the period of oscillation is much longer than the time delay between the quasi-shear waves (Vinnik et al., 1984). In this range, the indirect effect of splitting can be very strong, whereas the effects of scattering and multipathing are relatively unimportant. The problem of source complexity can be avoided by observing SKS and similar phases whose polarization is independent of the source properties. The merits of this approach were clearly demonstrated by Kind et al. (1985) and further documented recently (Silver and Chan, 1988; Ansel and Nataf, 1989). In this paper, we adopt the same approach to study azimuthal anisotropy at a set of globally distributed stations. We present results of the analysis of SKS and similar phases at several broadband stations of the GEOSCOPE network, and of the NARS array. In section 2, we briefly describe the procedure used for inversion of the observed data. The data and results of inversion are presented in section 3. Some observations of short-period ScS and SKS phases are presented in section 4. Finally, in section 5, we discuss our results. 1542

2 AIMUTHAL ANISOTROPY IN THE EARTH 1543 INVERSION METHOD This method has been described in detail elsewhere (Vinnik et al., 1988a). We will therefore only briefly summarize it here. We assume that, in the upper part of the earth beneath the receiver, SKS is propagated through a transversely isotropic layer with a horizontal axis of symmetry (axis of fast velocity). Then the primary, SV-polarized wave splits into two quasi-shear waves, $1 and $2. In the horizontal plane, the vectors of displacement in $1 and $2 are nearly orthogonal; the direction of displacement in $1 is nearly coincident with the direction of the axis of symmetry and $2 is delayed with respect to S~ by 5t sec, after travelling through the anisotropic medium. The adopted sign convention is as follows. Axis R (radial) points to the direction from the source to the receiver, in the horizontal plane. Axis T (transverse) can be obtained by rotating R counter-clockwise by 90. The angle f~ between the axis of symmetry and the axis R is obtained by a clockwise rotation of R with respect to the axis of symmetry. The projections of displacements in S~ and $2 on the axes R and T, for a unit amplitude in the incoming radial component, can be expressed approximately as (Vinnik et al., 1988a): A1,R ~- COS2~ A1.T = sin ~ cos A2,R -~ sin2/~ A2,T ~- --sin ~ cos (1) Then if the incoming SV is a harmonic wave of the form cos ~t where o~ is angular frequency and t is time, we can write: R(t) --cos2~ cos ~t + sin2~ cos(wt - w6t) T(t) ~- 0.5 sin 2~ cos wt sin 2~ cos(~t - ~o~t) (2) Assuming that w6t << 1, equations (2) can be written as: or R(t ) ~- cos wt T(t) ~- -0.5w6t sin 2~ sin wt T(t) ~- 0.56t sin 2fiR'(t) (3) Thus, in the long-period approximation, there are two diagnostic properties of azimuthal anisotropy: 1. The T component of SKS behaves in time like the derivative of the R component. 2. The amplitude ratio T/R depends on the sine function of 2~ (angular period is 180 ). To test the diagnostic properties of azimuthal anisotropy in real data, a special signal-processing procedure was devised (Vinnik et al., 1984). It is basically a procedure of harmonic angular analysis. A comprehensive test of the diagnostic properties of azimuthal anisotropy requires numerous events, well distributed in azimuth. Unfortunately, the operational time of the networks at our disposal is too short to meet these requirements. However, the number of records at some stations

3 1544 L. P. VINNIK, V. FARRA, AND B. ROMANOWIC is sufficient to evaluate the parameters of assumed anisotropy (direction of the fast velocity and St). In order to do this, we have developed a technique fairly similar to that applied by Kosarev et al. (1987) for evaluating crustal parameters from teleseismic P-wave recordings. In the study of the crust by Kosarev et al., the R-component records were synthesized from the observed -components for a number of values of the unknown model parameters; the optimum values minimize the difference between the synthetic and observed R components. In our study, we synthesize the T component of SKS and similar phases from the observed R component. Apparently, a different technique was used by Silver and Chan (1988). For the model considered, the radial and transverse components of SKS are simply related by: T= f * R (4) where [ is a linear filter. In agreement with equations (2), the Fourier transform of this filter can be written as: We calculate the function: 1 - exp(-i~ost) f(co) = 0.5 sin 2fl cos2 B + sin2 fl exp(-icost) (5) E(a, 6t) =[N ~ events (T(t) - T*(t, ~, St)) 2 dt] 1/2 -f R2(t) dt ] (6) where a is a trial azimuth for the axis of fast velocity counted clockwise from North, N is the number of events, and T* (t, a, St) is the theoretical T component obtained via equation (4). The normalization by energy of the radial component guarantees against bias due to the strongest events. E is calculated for many values of a and 5t in a search for the pair (a, St) which minimizes E. A quantification of accuracy of the resulting values of a and 5t is difficult since we do not know the properties of noise which are present in the records. Instead, we present the plots of E(a, St) from which the reader may judge the accuracy of the resulting estimates. DATA AND RESULTS The data used in this study come primarily from the GEOSCOPE network. A description of this network can be found elsewhere (Romanowicz et al., 1984). Some additional data for Europe come from the NARS array (Nolet and Vlaar, 1982). Coordinates of all the stations considered are given in Table 1. The three-component records were obtained from the broadband velocity output with a digitizing frequency of 5 Hz (GEOSCOPE) or 8 Hz (NARS). In order to improve the signal-to-noise ratio, many records were additionally filtered by a second-order Butterworth filter. Only such records were kept for which the SKS phase was clearly identifiable on the raw records. Out of 21/2 years of GEOSCOPE broadband data, 30 records were selected for 24 events, as listed in Table 2. Data for four events of 1986 were also analyzed on the NARS array, as obtained from the GDSN event tapes. Examples of records are displayed in Figures 1 and 3 through 9. The resulting plots of E(a, St) are shown in Figure 10, and in Figure 13 we give a map which summarizes the results and compares them to results obtained at other stations by Vinnik et al.

4 AIMUTHAL ANISOTROPY IN THE EARTH 1545 (1988a) and Silver and Chan (1988). In what follows, we describe the data and results for every station. For station SSB, we have a relatively large quantity of data. Their backazimuths are in the first and third quadrants (12 to 85 and 198 to 245 ). Although this distribution does not allow us to perform a harmonic analysis of the azimuthal variations, we can show that the available data do not contradict the assumption of 180 periodicity. This is demonstrated by the records shown in Figure 1, which are quite representative of the data selected in this study. Backazimuths of the two events differ by approximately 180. In both cases, the particle motion is clockwise and nearly elliptical. Both major axes coincide in direction with the axis R. The elliptical form of the particle motion means that the T component is shifted with TABLE 1 COORDINATES OF GEOSCOPE AND NARS STATIONS Station Latitude Longitude Elevation Network SSB N 4.54E 700. GEOSCOPE WFM N 71.49W 87.5 GEOSCOPE CAY 4.95 N 52.32W 25. GEOSCOPE AGD N 42.82E 450. GEOSCOPE SC N W 261. GEOSCOPE KIP N W 70. GEOSCOPE INU N E GEOSCOPE CAN S E GEOSCOPE NE N 9.17E NARS NE N 5.17E NARS NE N 4.09W NARS NE N 5.79E NARS NE N 4.05W NARS T T r L R R I0(> IO- ~00-5O(> It> - 50(> O O O0 r r SSB E~enl 2 boz, 12 SSB [~eqt 22 baz ~ 198 FIG. 1. Examples of SKS observations for two events recorded at station SSB. a) event 2; b) event 22 (see Table 2). The horizontal component records have been rotated to radial (R) and transverse (T) and the particle motion plot is given with respect to the orientation for the T and R axes, as indicated, in the time window corresponding to SKS and indicated by vertical bars below the R seismogram. Time marks indicate minutes. Note that scales can be different on T and R components.

5 1546 L.P. VINNIK, V. FARRA, AND B. ROMANOWIC respect to R by approximately a quarter period. The clockwise direction of motion indicates, in agreement with equation (2), that the events are in the first and third quadrants with respect to the axis of fast velocity. The amplitude ratio between the lengths of the minor and major axes is notably higher in Figure la than in lb. This can be understood, in agreement with equation (3), as a result of higher frequency content in Figure la. The preferred values of a and 5t for SSB are 140 and 1 sec (see Fig. 10). These estimates provide a good fit between the theoretical and observed T components. In order to assess the accuracy of the resulting estimates, we present the individual plots of E(a, St) for every event (Fig. 2). They indicate that, for individual records, the errors in both parameters can be very large when the backazimuth is close to the azimuth of the axis of symmetry or differs from it by approximately 90 (events 7, 8, 15). In other cases, the estimates of the azimuth deviate from the final value by not more than 20, and those of 5t by about 0.5 sec (events 2, 9, 22). When analyzing a number of records simultaneously, provided they are distributed in azimuth, data whose arrival direction is parallel or at right angles to the axis of symmetry provide valuable constraints on the directions of fast and slow axes. In O~ 2o 2o O O0 2o W J ~9 W J (_0 LU J (_ Q 180. ~~ SSB EvenL 2 SSB Evenb 7 SSB EvenL 9 baz = 12oS baz = 42o4 baz = 85 2 O0 2 Qo 2 2o OJ J iqcl ~ 180 SSB EvenL 8 SSB EvenL 15 18C SSB EvenL 22 baz = 80o3 baz = FIG. 2. Plots of contours of residuals versus angle a and delay 6t individual events. baz - 198o 1 at station SSB obtained using

6 AIMUTHAL ANISOTROPY IN THE EARTH 1547 the case of SSB, we see from Figure 2 that all three such data are consistent, to within 15, with a direction 90 away from the preferred value of a, and, in general, all six observations are consistent with the final solution. We believe that random errors in the final estimates based on a number of recordings are of the order of 10 for the azimuth and 0.3 for ~t. Examples of records at station WFM are shown in Figure 3. Figure 3a presents a record of SKS with a strong T component which is shifted with respect to R by approximately a quarter period. Figure 3b shows an example of SKS without any evidence of energy in the T component; this can be explained by a coincidence between the event azimuth and the direction of fast velocity. The estimates of a and 5t for WFM from seven records are 80 and 0.8 sec, respectively (Fig. 10). Accuracy of these estimates is comparable to that for SSB. Our results for SC are based on three records; all of them are displayed in Figure 4. The presence of a T component which is shifted with respect to R by T T ~0 O0 45 r P 986 [ 67 r I I J 4 I EO P3 WFM E~'enf 5 boz, WFM E~'er~l 5 baz " FIG. 3. Same as Figure 1 for GEOSCOPE station WFM. a) event 5; b) event 3. IT T O0 [ lfi 50 O0 198{} , 33 (}-00 SC Event ~6 baz ~ SC Event 16 baz bcz Event E1 Daz = ~58-~ FIG. 4. Same as Figure 1 for GEOSCOPE station SC. a) event 16; b) event 21; c) event 26.

7 1548 L.P. VINNIK, V. FARRA~ AND B. ROMANOWIC approximately a quarter period is evident in every record. The estimates of a and 5t are 100 and 1.3 sec (Fig. 10). Accuracy of these estimates is probably somewhat lower than for SSB and WFM, because fewer records were available. The estimates for AGD are, like those for SC, based on three records which are shown in Figure 5. Again, the effects of presumed azimuthal anisotropy are reasonably clear. We note for example that the particle motion is anticlockwise for events 17 and 18, whose backazimuths differ by only 25, and clockwise for event 19, whose backazimuth is almost at a right angle with the preceding events. We verify that the final solution puts these two groups of events in different (adjoint) quadrants. We obtain 45 for ~ and 1.2 sec for 5t (Fig. 10). Accuracy of these estimates is in the same range as for SC. The estimates for KIP are also based on recordings of three events (Fig. 6); but, unlike the other stations, one of the recordings belongs to ScS (Fig. 6c). For ScS, we follow the procedure recommended by Vinnik et al. (1988a). We rotate the horizontal components so that one of them coincides with the principal direction T T T i i I I I I 56 l ~0 ~ O0 1988, ,00 AGD Event 17 baz = ~8 6 AGD Event [8 baz = 3 3 AGD Event 19 baz = ~83.1 FIG. 5. Same as Figure 1 for GEOSCOPE station AGD. a) event 17; b) event 18; c) event 19. T A/ 5o 1 T 4 "1 I I I I I I t O I~ g t986 t kip Event 6 baz : KIP Event g6 baz " KIP Event 27 baz : FIG. 6. Same as Figure 1, for GEOSCOPE station KIP. a) event 6; b) event 26; c) event 2 (for this event the plot is for ScS exceptionally).

8 AIMUTHAL ANISOTROPY IN THE EARTH 1549 of the particle motion, whereas the second is orthogonal to the first. For convenience, these components in Figure 6 are labeled R and T since further processing of them is exactly the same as for the R and T components of SKS. The records of SKS (Figs. 6a and b) are from events in practically opposite azimuths. This gives the opportunity to check the 180 periodicity of the particle motion of SKS. The result of this test is favorable to the hypothesis of anisotropy: the directions of the particle motion in both cases are the same (clockwise); the amplitude ratios T/R are different, but this is in agreement with the difference in dominant periods of oscillation. Interesting, the apparent period of oscillation in the T component of SKS in Figure 6a is notably shorter than in the R component (15 sec versus 22 sec). This can be explained by the fact that the T component is the derivative of the R component (equation 3). Estimates for the parameters of anisotropy for KIP are 45 and 1.5 sec (Fig. 10). Accuracy of these estimates is probably comparable with those for SC and AGD. For CAN, we have six recordings of SKS. Figure 7 shows the two most important ones for our conclusions. In both cases, the T component of SKS is not visible, though the level of noise is fairly low; the amplitude ratio T/R for SKS in these records cannot be much higher than about This means that either 5t is very small or the azimuths of the events coincide with the direction of the axis of symmetry and the perpendicular direction. The latter is unlikely since the difference between the azimuths is 110. Using equation (3), we obtain for the former an estimate that 5t < 0.4 sec; the other data at our disposal do not violate this limit. Thus 5t at CAN is at least two times lower than at other stations. This anisotropy is too weak for measuring the direction of fast velocity using our data. For INU we have only one record of SKS, shown in Figure 8a. The resulting plot of E(a,5t) (Fig. 10) shows that the estimate of 5t is highly uncertain. The estimate of a is in the range 180 _ 30. Later we shall present additional evidence of plausibility of this direction. For CAY we have two recordings from events in the same region; one of them is shown in Figure 8b. The T component of SKS is well visible. As in the case of INU, the value of 5t is highly uncertain (Fig. 10). The value of a is in the range of 100 _4-40. Our analysis of anisotropy at NARS is based on the records from the GDSN tape for We found that the level of noise in the horizontal components of this network is often high, which complicates the analysis. Figure 9 shows examples of - 25(~ 4C R 8 R 6 ~2~ I I ] I I 1988 i 9? 55 0 O0 i O0 CAN Even~ 13 boz : 142 CAN Evenl 18 boz = 32 3 FIG. 7. Same as Figure 1, for GEOSCOPE station CAN. a) event 13; b) event 18.

9 T 1- R R 25~ F988 FTO 23 I2 40 O0 P O O0 [NO Evenl ~4 boz : 557 CAY Evenl 12 boz = FIG. 8. Same as Figure 1: a) for station INU (event 24); b) for station CAY (event 12). T 3 ~ IT T R~ R~ " 7 2 " t t986 t O NEO5 Event l baz = 2954 NEI3 Event l baz : z [ t O0 NEI5 Event 5 baz = 377 rt T L_R as~ R ~ 2~ I J O0 NEI5 Event,3 baz = 12 ~ R &. 2 ~ -2 i I 3f~ 36 t NEI7 Event 2 baz 352 = - t FIO. 9. Same as Figure 1 for observations on the NARS array, a) event 1 at NE05; b) event 1 at NEI3; c) event 5 at NE15; e) event 2 at NE15; d) event 2 at NE

10 AIMUTHAL ANISOTROPY IN THE EARTH o 2 2 I I/ O 2, IJJ J <: : I ' 180:! 180of LQNi i SSB ( ~G ) WFM ( ~7 ) SC ( ~3 ) KIP (~3) O= 2o " I / ( /-.:~2 ~ 2o d> ISO: IS( NU Event 2~ CAY ( ~2 ) AGD ( ~3 ) baz = 55=7 O 2o 2o go \ x \'~. x "~--.q O. 2o o. 2o LII J (f) 180, NE13 + NE17 NE15 (~3) NED5 (~3) NEO2 (~1) FIG. 10. Plots of contours of residuals versus angle ~ and delay 5t when the inversion is performed simultaneously for several events distributed in azimuth. The number of data is indicated next to the station name. The minimum residual defines the solution kept.

11 1552 L.P. VINNIK, V. FARRA, AND B. ROMANOWIC the records which were found suitable for further processing. The resulting plots of E(~, St) are shown in Figure 10. For station NE02, the estimates of the parameters are based on one recording and, for this reason, are rather uncertain. However, the plot of E(a, 50 for this station is very much the same as for station NE05 for the same event. This implies consistent values of ~ and St. We also combined data for stations NE13 and NE17, since they are located very close to each other. Figure 10 shows that the resulting values of a change from approximately 90 to 110 in the Iberian Peninsula (stations 13 and 17) to about 60 to 80 at other stations further North. There is a pronounced decrease of the values of 5t from about 1.3 sec in the southwestern end of the profile to about 0.5 sec in the central and north-eastern parts. OBSERVATIONS OF SHORT-PERIOD SKS AND ScS So far, we have considered records whose dominant period is much longer than 5t (4-1 sec). On several seismograms, we have found clear SKS and ScS arrivals at periods comparable to St. We discuss these data, since there is at least a theoretical possibility to constrain the parameters of anisotropy by observations in the shortperiod range. Figure lla shows the SKS record and the corresponding particle motion plot for event 2 of Table 2, observed at station WFM. The periods (around 2 sec) are very short for a teleseismic S wave; the polarization of this phase is nearly linear and deviates by about 40 from the radial direction. One could think that this phase belongs to a local event and arrives at the time of SKS by coincidence. However, a similar short-period arrival is recorded by one of the NARS stations (Fig. llb), which makes the coincidence unlikely. In both cases, the deviations from radial polarization are too large to be easily explained by lateral heterogeneity (Cormier, 1984). Nevertheless, we do not see any other possibility to explain these observations. ~ T 4OO- 4O ~ 4(}0 I I II t O0 ~g ~4 0 O0 35 ~ baz - 67 NEI5 Eve~l boz = 12 5 FIG. 11. Examples of short-period SKS observations, a) event 2 observed at station WFM; b) event 2 observed at NARS station NE15. For explanation of the plots see caption for Figure 1.

12 TABLE 2 EPICENTRAL DATA AND CORRESPONDING STATIONS FOR SS OBSERVATIONS Event Date Time Latitude Longitude Depth Station Distance Backazimuth no ( ) ( ) (kin) ( ) N W 27. NE NE NE E 538. SSB WFM NE NE W 547. WFM N E 33. NE NE N E 48. WFM NE NE W 62. KIP N E 54. SSB N E 107. SSB N 98.87E 11. SSB W 16. WFM N W 10. CAY N W 10. CAN CAY W 33. CAN S 70.37W 33. CAN W 71. SSB N E 21. SC N E 33. AGD N W 10. AGD CAN N 60.58W 56. AGD S 72.40W 54. CAN E 66. SC W 114. SSB E 33. WFM N W 10. INU N E 44. WFM N 95.12E 92. WFM SC KIP N E S z ~ O SO 1987o!27o 9~ 19o 40~ ~ 138o 3o 2Io 20~ o I95o ]NU EVENT 28 INU EVENT 29 INU EVENT 30 FIG. 12. Short-period recordings of ScS at station INU. 1553

13 1554 L.P. VINNIK, V. FARRA, AND B. ROMANOWIC TABLE 3 EPICENTRAL AND STATION DATA FOR ScS OBSERVATIONS Event Date Time Latitude Longitude Depth Station Distance Backazimuth no. ( ) ( ) (km) ( ) S W 583. KIP N E 430. INU N E 542. INU N E 576. INU In Figure 12, we present short-period recordings of ScS at station INU. The parameters of the events are given in Table 3. In the record of event 29, the first arrival of ScS on the N-S component is 3 sec earlier than on the E-W one. Qualitatively, this is consistent with the fast velocity direction for INU found from SKS data (Fig. 8). The other records do not show this difference between the horizontal components, although the location of event 30 is very close to event 29. We conclude that, just like for SKS, short-period recordings of ScS are too much affected by heterogeneity in the earth's medium, or perhaps source effects, to be useful in our study. DISCUSSION AND CONCLUSIONS We have measured SKS splitting parameters at a number of high-quality broadband stations distributed worldwide (Fig. 13). First, we have shown that, at all of these stations, the observed effect is in very good agreement with the diagnostic effects of azimuthal anisotropy, thus confirming once more the observations and interpretation made earlier by Vinnik et ai. (1984) and further documented by Silver and Chan (1988). As seen from Figure 13, our results are generally consistent with those of previous authors (Vinnik et al., 1988a; Silver and Chan, 1988) over large areas. Further interpretation of this effect is, however, complicated by the fact that the depth interval responsible for the observed effects is rather uncertain. Most likely, azimuthal anisotropy is the result of a preferred orientation of crystals in the upper mantle. This orientation could be caused by various factors. It could be related to passive motion of the lithosphere over the stationary asthenosphere. In this case, the most likely location for anisotropy would be in the transition zone between the two shells, and the fast direction of anisotropy would be close to the direction of plate motion (e.g. Leven et al., 1981). Alternatively, the preferred orientation could be induced by flow in the asthenosphere. In this case, the fast direction of anisotropy would coincide with the direction of flow, but might differ from the direction of plate motion (e.g. Tanimoto and Anderson, 1984). Azimuthal anisotropy in the uppermost mantle can also be related to past and present deformation of the lithosphere (e.g. Fuchs, 1983). Various sources of anisotropy can be present in the same region but, perhaps, at different depths. In what follows, we will try to identify the possible sources of anisotropy whose effects are dominant in our data. The data shown in this paper can be classified in two major groups: an Atlantic group (stations SSB, WFM, CAY, NARS) and a Pacific group (SC, KIP, INU, CAN). Station AGD in eastern Africa will be treated separately. The stations of the Atlantic group exhibit a pronounced east-west trend in the direction of fast velocity. The same trend is obvious in the data of the GRF array (Vinnik et al., 1988a) and some mid-continental stations in North America (Silver and Chan, 1988), where the direction becomes closer to NE-SW. The directions of fast velocity in North

14 AIMUTHAL ANISOTROPYIN THE EARTH 1555 A) ~'~-~'COL ~ RSNT L95 ~05 gl5 a25 a35 a45 a a75 a LONGITUDE B ) KEV ~ -- 4tll J,5 SEC IOSEC 0 5 SEC r~ b- I0 0 lo go LONGITUDE FIG. 13. Summary of the results obtained in this study (squares), compared to results obtained by previous authors (circles) for other stations, a) stations in North America and the Pacific (except station INU for which the direction of maximum velocity found in this study is 180 ). b) stations in western Europe and North Africa. Determinations at STU and GRF are from Vinnik et al. (1988a). All other circles are from Silver and Chan (1988). The scale indicates that 5t is greater than the indicated value. America are close to those of greatest horizontal principal stress (oback and oback, 1980). This state of stress is explained by these authors as being due to the ridge push and viscous drag at the bottom of the lithosphere during opening of the Atlantic ocean. We could explain the principal component of azimuthal anisotropy

15 1556 L.P. VINNIK, V. FARRA, AND B. ROMANOWIC beneath central and eastern North America by such an effect. A similar process could be responsible, at least partly, for anisotropy beneath western Europe, though the dominant direction of fast velocity in that region deviates from the direction of absolute plate motion (NW-SE) as given by Minster and Jordan (1974). The deviation could be explained, for example, by an interaction between the European and African plates. A difference between the directions of anisotropy at the top of the mantle and in its deeper parts is evident in available seismic data from west Germany (Vinnik et al., 1988a). This discrepancy implies the presence of different sources of anisotropy at different depths. Interesting, the direction of fast velocity at a depth of 640 km in Europe is close to the above mentioned direction of plate motion (Vinnik et al., 1989). There is a notable deviation from E-W direction of fast velocity in Europe at station SSB. This direction (140 clockwise from North) is found from high-quality digital data, consistent with one another, and can be rated as very reliable. The source of this anomaly is probably shallower, in the lithosphere of the Massif Central, where this station is located. The direction obtained previously for station STU by Vinnik et al. (1988a) also deviates from the east-west trend, pointing to a possible perturbing effect related to the proximity of the western end of the Alpine belt. Proceeding to the Pacific group of stations, we note that SC is located in a part of North America which is marked by extension in the east-west direction (e.g. oback and oback, 1980), whereas the motion of the Pacific plate is NW-SE. The data for SC are clearly consistent with the former direction, although it could also be related to the direction of subduction of the former Farallon plate (Cockerham and Ellsworth, 1982). It is consistent with the direction obtained at WWSSN station LON by Silver and Chan (1988). The fast direction at KIP (45 ) is close, not to the direction of present-day plate motion, but to the fossil direction of spreading in the lithosphere, as given by Nishimura and Forsyth (1988). The direction of fast velocity found in the same region from Rayleigh-wave data by Nishimura and Forsyth (1988) is consistent with ours. At INU, the fast direction is also close not to the present-day direction of plate motion, but to the fossil spreading direction in the oceanic lithosphere. Qualitatively, similar results for the western part of the Pacific were obtained by Shimamura (1984) from deep seismic sounding data and by Nishimura and Forsyth (1988) from Rayleigh-wave data. Fukao (1984) observed splitting of short-period ScS waves from a deep seismic event at 20 stations in Japan. His value of the direction of fast velocity beneath Japan is , which is practically coincident with our estimate for INU; on the other hand, Ando (1984) and Ando et al. (1983) presented somewhat different estimates for the same region using short-period ScS and direct S waves from sources beneath Japan. The weakness of anisotropy at CAN is surprising since a strong anisotropy was found in this region from long-period surface-wave data (Montagner and Tanimoto, 1988). We cannot completely rule out the possibility that our observations are in unfavorable azimuths, the anisotropy from surface wave studies showing a fast NE-SW direction. This example emphasizes the importance of looking at data in various azimuths before drawing any definite conclusion. In general, the data for the Atlantic and Pacific groups suggest that the directions of fast velocity are related to the past and present plate motion directions. This conclusion is further strengthened by the data for AGD, which indicate that the fast direction of anisotropy is close to the direction of spreading of the Red Sea

16 AIMUTHAL ANISOTROPY IN THE EARTH 1557 Rift. A similar trend has been found in surface-wave data by Hadiouche et al. (1989). A rough estimate of the anisotropic layer thickness can be obtained from the following considerations. For the delay 5t we can write: 5t/t ~ -~v/v where t is the average time of S-wave propagation in the anisotropic layer, 5v is the difference between the velocities of the quasi-shear waves propagating in the near vertical direction and v is the average S velocity in the layer. For a composite material containing 30 per cent of the transversely isotropic olivine and 70 per cent of anisotropic upper mantle rock, 5v/v is around 0.02 (e.g., Crampin and King, 1977). Then ~t -- 1 sec implies t -~ 50 sec, corresponding to a layer 250 km thick. Thus the observed 6t require a significant part of the upper mantle to be anisotropic. We already mentioned a few examples of qualitative agreement between our results and the results obtained by others. In fact, however, the agreement is not always so good. For example, there is a discrepancy between the dominant E-W direction of fast velocity in North America from SKS data and the dominant N-S direction from long-period surface waves (Montagner and Tanimoto, 1988). The discrepancy in the region of CAN, in Australia, which we already mentioned, is another example, though not so well established. We do not understand the reasons for these disagreements, but the difference in scales to which the different kind of waves are sensitive, could be one of them. Another reason could be the different way in which depth is sampled by the different kinds of waves. In conclusion, it is clear that our data are very consistent with an interpretation in terms of azimuthal anisotropy. The causes of this well-observed phenomenon are clearly complex and many additional data of different kinds will be needed to determine, for each station where it is observed, its local or large scale character, its relation to past or present plate motions, and the depth range at which it occurs. ACKNOWLEDGMENTS We are thankful to the GEOSCOPE team and the NARS project for providing the high-quality digital data used in this study. This paper benefited from discussions with J. P. Montagner and O. Hadiouche. It was realized under the sponsorsphip of Institut National des Sciences de l'univers, grant "ASP GEOSCOPE 1988". It is I.P.G.P. Contribution no REFERENCES Ando, M. (1984). ScS polarization anisotropy around the Pacific Ocean, J. Phys. Earth Ando, M., Y. Ishikawa, and F. Yamazaki (1983). Shear wave polarization anisotropy in the upper mantle beneath Honshu, Japan, J. Geophys. Res. 88, Cockerham, R. S. and W. L. Ellsworth (1980). Remnants of the subduction process in the mantle beneath Central California, Seism. Soc. Am. Abstract 80. Cormier, V. F. (1984).The polarization of S waves in a heterogeneous isotropic earth model, J. Geophys. Res. 56, Crampin, S. and D. W. King (1977). Evidence for anisotropy in the upper mantle beneath Eurasia from the polarization of higher mode seismic surface waves, Geophys. J. R. Astr. Soc. 49, Fuchs, K. (1983). Recently formed elastic anisotropy and petrological models for the continental subcrustal lithosphere in southern Germany, Phys. Earth Planet. Interiors 31, Fukao, Y. (1984). Evidence from core reflected phases for anisotropy in the earth's mantle, Nature 309, Hadiouche, O., N. Jobert, and J. P. Montagner (1989). Anisotropy of the African Continent inferred from surface waves (in press).

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