Anisotropy and large-scale lateral inhomogeneity of the upper mantle

Physics of the Earth and Planetary Interiors, 26 (1981) 171—178

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Elsevier Scientific Publishing Company, Amsterdam — Printed in The Netherlands

Anisotropy and large-scale lateral inhomogeneity of the upper mantle S.D. Kogan Institute ofPhysics of the Earth, Academy of Sciences of the U.S.S.R., Moscow (U.S.S.R.)

(Received January 22, 1979; revised and accepted January 20, 1981)

Kogan S.D., 1981. Anisotropy and large-scale lateral inhomogeneity of the upper mantle. Phys. Earth Planet. Inter., 26: 171— 178. An investigation of travel-time residuals of P waves as compared to an average global travel-time curve with a base-line determined shows that about 400—500 km of the upper mantle at the boundary with the crust and a similar thickness of the lower mantle at the boundary with the core are laterally inhomogeneous. The upper mantle both under continents and under oceans consists of crust—mantle blocks distinguished by longitudinal wave velocity, surface tectonics and intensity of the heat flow. It has been revealed that within individual crust—mantle blocks there is an azimuthal relationship of P-wave travel-time indicative of possible anisotropyof upper mantle elastic properties down to depths of the order of several hundred kilometers.

1. IntroductIon The investigation of the detailed structure of different shells of the Earth is an important feature of modern seismology. In late 1950s and early l960s, the world network of seismic stations was considerably cxtended and re-equipped. The improved accuracy and detail of seismic observations have revealed a great number of facts inconsistent with conventional ideas of the lateral structural homogeneity of the Earth’s crust and upper mantle. Some tendency for an increase of seismic-wave travel-times to stations in tectomcally-active regions relative to those on ancient shields has been shown by a number of authors (Kondorskaya, 1957; Cleary and Hales, 1966; Starovoyt et al., 1970; Kuzin, 1973; Sipkin and Jordan, 1976; Pupinet, 1977, etc.). Several papers (Anikanov et al., 1974; Aki et al., 1976; Vinnik, 1976; Antonova et al., 1978, etc.) have been devoted to small-scale lateral inhomogeneities of the crust and upper

mantle structure of individual regions. However, no one has yet successfully studied the lateral inhomogeneity of the upper mantle as a whole. An attempt to investigate the entire upper mantie was made by Dziewonski et a!. (1977), though according to the authors themselves (p. 239), “The results for depths less than 1100 km are unreliable...” When investigating inhomogeneities of the Earth’s structure, the requirements for accuracy of the initial travel-time curves significantly increase since it is the residuals of observed travel times with reference to those predicted by theoretical travel-time curves that usually serve as criteria for the existence of velocity anomalies in certain regions. For construction of the average global P-wave travel-time curve (Kogan, 1980), use was made of observations from 120 surface sources in different regions with exactly known co-ordinates and origin

003l-9201/81/0000.-0000/$02.50 © 1981 Elsevier Scientific Publishing Company

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curves by other authors. Causes of the differences

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in the residuals are discussed in detail by Kogan (1980). The procedure for constructing a travel-

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time curve and its transformation to a base line is

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described Kogan (1976). 2. Lateral by inhomogeneity of the upper mantle

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An analysis of travel-tjme residuals r~( A) observed at stations I with reference to the average global travel-time curve of P-waves l( A) (Kogan

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Fig. I. Comparison of residual curves to the Jeffreys—BuIlen travel-time curves of 1940. Data from: I, the author; 2, Seismological Tables (1968); 3, Jordan and Anderson (1974); 4, Julian and Sengupta (1973).

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times. In all, about 4500 P-wave arrivals to 700 stations were employed.

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1980) revealed a distinct regularity m the distnbution of travel-time anomalies over the Earth’s surface. The system of observations used (Fig. 2) a!lowed us to distinguish, using the station residuals 8t1(A) = 1(A) t(A), about 40 regions differentiated by travel-times of P waves (Fig. 3). The significance of variations of the regional corrections was checked at the 90% confidence level against a comparison criterion of mean values

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Fig. 2. Locations of seismic stations used in the study of lateral inhomogeneity in the mantle.

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Fig. 3. Global map of large-scale lateral inhomogeneity of the upper mantle indicated by anomalies of P-wave travel-times (81,.) with respect to the average global travel-time curve (Kogan, 1980). 1, 8:, values: minus indicates t,(~)t(~). 2, Direction of P-wave approach for regions with evidence for the azimuthal dependence of 8:,. 3, Number of stations used for calculation of 8:,.

(Pagurova, 1968). For each region, r, the region corrections 6tr were calculated, with an estimation of their accuracy, as weighted-average corrections from data of all stations in the region independent of the direction of wave approach. The regional corrections obtained account for differences of the real velocity of longitudinal waves in a layer of the crust and upper mantle approximately 400—500 km thick, from a mean velocity corresponding to the travel-time curve applied. Positive corrections indicate low-velocity regions in the upper mantle, and negative corrections high-velocity regions. A mean depth of about 500 km in which the lateral inhomogeneity of the upper mantle is cvident was found by distinctions in the shape of travel-time curves constructed by observations from different regions at epicentral distances up to

25°.Similar conclusions have~also been reached by Hales and Herrin (1972). It was noticed that for some regions there is a relationship between travel times and azimuths of the wave approach. This observation will be discussed later. Firstly, we shall consider the close correlation of the regions of mantle heterogeneity with surface tectonics. Comparing our map of regionalization of the Earth (Fig. 3) with that of the Earth’s tectonics (Beloussov, 1975), it can clearly be seen that each of our regions is situated within a certain tectonic zone. In so doing, it was also found that there is a close interrelation between the regional correction 6t,. and the degree of tectonic activity in a region of observation. This can be seen, for example, in ancient platforms of Precambrian folded basement (Table I). Since the space dis-

174 TABLEI Mean anomalies of P-wave travel times in the crust and the upper mantle of ancient platform regions Platform

6t, (s)

Type of tectonic activity

East European Siberian North American Chinese Indian West Australian Afncan-Arabian

—1.5 — 1.2 — 1.1 —0.6 —0.3 —0.2 +0.6

stable stable stable seismic seismic seismic nfting

flows are low (—0.8—1.0 ~tcalcm2s~), and the regional corrections obtained, ôt, = 1.4 to —0.8 s, are evidence of small longitudinal-wave —

travel-times. Thus, the large-scale lateral inhomogeneity of the upper mantle manifests itself in variations of parameters of different geophysical fields. It is caused by a complex interrelation of thermal, mineralogical, chemical, and other factors (Ringwood, 1969; Herrin, 1972; Magnitsky and Zharkov, 1970). Among these, irregular heating of .

tribution of the travel-time anomalies allows for large-scale lateral inhomogeneity of mantle structure down to average depths of 400—500 km, the close correlation with surface tectonics can be considered as confirmation that it is processes occurring at great depths in the mantle rather that those in the crust that play the dominant role in tectonic movements. The travel-time anomaly distribution was cornpared with global maps of heat-flow distribution (Chapman and Pollack, 1975; Suetnova, 1979). This comparison showed almost complete coincidence of regions of high heat flow with those with high P-wave travel times, travel times in regions of low heat flow being less. For example, in the Great Basin of the western U.S.A., where heat flows are highest (up to 2 Iscal cnf2 s (Simmons and Roy, 1969), we obtained a high positive correction: 6t, = + 1.0 s. For southern California and the Sierra Nevada, where heat flows are less intense, travel thnes are also lower: 61,. = +0.9 to + 0.5 s. For the North American platform, heat ~‘)

the upper mantle is likely to be one of the most important. The influence of temperature on longitudinal wave velocity exceeds that of density by an order of magnitude (Horai and Simmons, 1968). Conventionally, the crust and the uppermost part of the upper mantle is considered to have a block mosaic structure defined by deep faults. Our results indicate that on a larger scale the upper mantle down to 400—500-km depth is also subdivided into blocks. These crust—mantle blocks were combined into five groups by degree of present tectonic activity and by P-wave travel-time anomalies (Table II). For upper-mantle blocks of different types, an approximate estimate of possible velocity variations, ÔV (%), with reference to mean values, was carried out by mean regional corrections for a given group to the average global P-wave traveltime. If the variations of P-wave travel-times observed among blocks of different types is associated with a 200—400-km thick crust—mantle layer, the maximum velocity difference between the blocks in stable ancient platforms and those of the Pacific geosynclines will be about 9 and 4%, respectively. The smallest difference from the mean

TABLE II Classification of the crust—mantle blocks by degree of tectonic activity and travel-time anomalies of P-wave Regions of the crust—mantle blocks

8ir

(s)

Variation of the velocity (%) with thickness of the blocks ____________________________________________

Ancientplatforms Caledonianand Hercyniangeosynclines Alpine geosynclines Active platforms and rift zones Pacific geosyndlines

—0.9 —0.7 . —0.8— +0.6 +0.2 +0.7

200km 4.3 3.4 3.8—2.0 0.5 2.4

400km 2.1 1.6 1.9—1.2 0.4 1.4

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velocity is characteristic of the upper mantle between tectonoactive platforms and rift zones. 3. Anisotropy As stated above, the relationship between the regional corrections, ôt,, and the direction of wave approach was found at the 90% confidence level for 13 regions (Fig. 3). The relationship between travel times and wave-approach azimuths determined by teleseismicobservations when P-waves propagates at 200—400-km depth in the upper mantle almost vertically under a station is obtamed for the first time. Work on the velocity aiusotropy of Pn-wave propagation along the Mohorovi~acdiscontinuity is well-known. Figure 4 shows the author’s results and those of Chesnokov (1977). Directions of Pn120

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wave propagation with the maximum velocity and P-waves with minimum travel-times are seen to coincide well for central Europe (Baniford, 1973) and the Hawaii region (Raitt, 1969). The author’s data for western Australia are consistent with those for the north-eastern Indian Ocean. The amsotropy coefficient

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a

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for Pn-wave velocity is as high as 10%. For P-waves, the difference of travel times according to direction of approach is from 0.3—0.5 (as in central Asia) to 1.5 s (as the Hawaiian islands and western Australia). An approximate estimation of the anisotropy coefficient varies with the thickness of the crust—upper mantle layer where it reveals itself. Assuming that the elastic properties of the upper mantleare anisotropic down to 200—400-km

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176

depth, a is 7—3%, respectively. If only the first 50—100 km of the upper mantle were anisotropic, the anisotropy coefficient would increase up to 20%, which is unlikely, The assumption that the anisotropy of upper mantle elastic properties may be responsible for the azimuthal relationship for P-wave travel times follows primarily from: (1) coincidence with data on Pn waves; and (2) consistency of data on directions in which seismic waves propagate with maximum and minimum velocities, which is retamed over immense areas, contradicting the idea that randomly distributed inhomogeneities might cause the azimuthal relationship. If the effect revealed is actually evidence for amsotropy in the upper mantle, the following facts should be noted: (1) The upper mantle is anisotropic under both

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continents and oceans. (2) In some cases, the directions of anisotropy axes are constant over large territories including both tectomcally active and stable regions (e.g., Europe). In other areas, these directions are different in neighboring regions (e.g., the western U.S.A.). (3) Particular anisotropies are present over the entire depth of crust and upper mantle from crystalline basement down to about 400—500 km (e.g., the Russian platform). (Data on crystalline basement from Nevsky et al., 1974.) 4. Lateral inhomogeneity of the lower mantle Allowance for the effect of lateral inhomogeneity of the upper mantle on travel times of longitu-

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Fig. 5. Lateral inhomogeneity of the lower mantle at depths over 2400—2500 km by observed residuals of P-wave travel-times related to the average global travel-time curve (Kogan, 1980). Positions ofT signs correspond to points of the deepest penetration of the ray projected on the day. Minus signs: travel-time less than the average. Plus: travel-time greater than the average. Circled plus or minus: 5.

177 TABLE III Mean anomalies i of P-wave travel times in the lower mantle at depths over 2400—2500 km Regions of the lower mantle

(s)

Northern Eurasia North America Arctic Ocean Pacific Ocean • Arctic and Pacific Oceans

1.39±0.27 —0.44±0.21 +0.55±0.26 + 1.08 ±0.20 +0.83±0.19 —

dinal waves enabled the identification of traveltime anomalies for these waves, related to lateral inhomogeneities of lower mantle structure. We assume that, at close directions of ray approach, the station correction for an anomaly in upper mantle structure under a station does not vary with epicentral distance, i.e. if azimuths from a station i to epicenters i1 and f2 differ by no more than 30°.Results show that for A1 ~ 83°(i.e. at ray penetration depths exceeding 2400 kin) the station corrections are unequal, i.e. = &t,~( A, 83°) 2) ~ 0 ~‘

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i-values being well-held over the area. In Fig. ~, signs of i are shown at locations corresponding to the points of deepest penetration of the rays projected on the surface. Mean values ~ for various regions of the lower mantle are shown in Table HI. The possible velocity difference between highvelocity regions of the lower mantle under northem Eurasia and low-velocity regions of the lower mantle under the Pacific Ocean is about 4%. The data on large-scale inhomogeneities of lower mantle structure obtained in the present work show good consistency with results of investigations using deep earthquakes (Julian and Sengupta, 1973). The reliability of the results of Julian and Sengupta is a consequence of their use of Pacific earthquake observations and the application of Jeffreys— Bullen travel-time curves for this epicentral region.

84—104 83— 99 83— 96 84— 93 83— 96

Number of observations

Variations of the velocity 6V(%)

12 26 19 9 28

2.2 0.7 0.9 1.8 1.5

5. Conclusions (1) The upper mantle consists of individual blocks, of hundreds to thousands of kilometers horizontal extent, and 400—500-km depth. The crust—mantle blocks are distinguished by longitudinal wave velocities, active surface tectonics and high heat. flows. (2) It has been shown that within these crust— mantle blocks there is relationship between traveltimes of P-waves and the direction of their propagation. If the elastic properties of the upper mantle are amsotropic down to 200—400-km depth, the effective coefficient of anisotropy may be something like 7—3%. (3) Data on large-scale lateral inhomogeneity of lower mantle structure have been obtained for depths greater than 2400—25.00 km. The maximum variation of P-wave travel times (about 2.5 s) has been found for northern Eurasia and Pacific Ocean. (4) The middle mantle at depths from about 800 to 2300 km is likely to be the most homogeneous (with due regard for data on deep earthquakes). About 400—500 km of the upper mantle at the bound~with the crust and much the same thicknessof lower mantle at the boundary with the core show lateral inhomogeneity of about the same order (maximum variations of longitudinal wave velocities are up to 5%). The possible existence of any connection between the nature of the inhomogeneities in the upper and lower mantle requires further investigation.

178

References Aki, K., Christofersson, A. and Husebye, E.S., 1976. Threedimensional seismic structure of the lithosphere under Montana LASA. Bull. Seismol. Soc. Am., 66: 501— 524. Anikanov, Y.E., Pivovarova, NB. and Slavina, L.B., 1974. Trekhmernoe pole skorostei foka]noi zony Kamchatki. In: Matematicheskie Problemy Geofiziki. Issue 5, part I, Nauka SO Akad., Nauk SSSR, Novosibirsk, pp. 92—117. Antonova, L.V., Aptikaev, F.F., Kurochkina, R.I., Nersesov, I.L., Nikolaev, A.V., Ruzaykin, A.I., Sedova, E.N., Sitnikov, A.V., Tregub, F.S., Fedorovskaya, L.D. and Khalturin, V.!., 1978. Eksperimentalnye seismicheskie issledovaniya nedr zemli. Nauka, Moscow, 156 PP~ Bamford, D., 1973. Refraction data in Western Germany— a time-term interpretation. Z. Geophys., 39: 907—927. Beloussov, V.V., 1975. Osnovy geotektoniki. Nedra, Moscow, 262 PP Chapman, D.S. and Pollack, H.N., 1975. Global heat flow: a new look. Earth Planet. Sci. Lett., 28: 23—32. Chesnokov, E.M., 1977. Seismicheskaya anizotropiya verkhnei mantii Zemli. Nauka, Moscow, 144 PP~ Cleary, J. and Hales, A.L., 1966. An analysis of the travel times of P-waves to North American stations in the distance range 32°to 100°.Bull. Seismol. Soc. Am., 56: 467488. Dziewonski, A.M., Hager, B.H. and O’Connel, R.J., 1977. Large-scale heterogeneities in the lower mantle. J. Geophys. Res., 82: 239—255. Hales, A.L. and Herrin, E., 1972. Travel times of seismic waves, In: E.C. Robertson (Editor), The Nature of the Solid Earth. Cambridge University Press, pp. 172—215. Herrin, E., 1972. A comparative study of upper mantle models: Canadian shield and Basin and Range provinces. In: E.C. Robertson (Editor), The Nature of the Solid Earth. Cambridge University Press, pp. 216—231. Horai, K. and Simmons, G., 1968. Seismic travel time anomaly due to anomalous heat flow and density. J. Geophys. Res., 73: 7577—7588. Jordan, T.H. and Anderson, D.L., 1974. Earth structure from free oscillations and travel times. Geophys. J. R. Astron. Soc., 36 (2): 411 60. Julian, B.R. and Sengupta, M.K., 1973. Seismic travel time evidence for lateral inhomogeneity in the deep mantle. Nature (London), 242: 443—447. Kogan, S.D., 1976. Eksperimentalny godograf volny P gonzontalnaya neodnorodnost mann. DokI. Akad. Nauk SSSR, 230 (6): 1318—1321. Kogan, S.D., 1980. 0 vremenakh probega prodolynkh seis-

micheskikh voln v usloviyakh gorizontalno neodnorodnoi verkhnei mantii. Izv. Akad. Nauk. SSSR, Fir. Zemli, No. 6: 3—13. Kondorskaya, N.y., 1957. Po povodu regionalnykh osobennostei vremen probega seismicheskilth voln. Izv. Akad. Nauk SSSR, Ser. Geofiz., No. 7: 895—913. Kuzin, I.P., 1973. Skorosti voln P i S v verkhnei mantii Kamchatki. Izv. Akad. Nauk SSSR, Fir. Zemli, No. 2: 3—16. Magnitsky, V.A. and Zharkov, V.N., 1970. Priroda sloev ponizhennykh skorostei seismicheskilth voln v verkhnei mantii Zemli. In: Problemy stroeniya zemnoi koty i verkhnei mann, Verkhnyaya mantiya, No. 7. Nauka, Moscow, pp. 197—212. Nevsky, MV., Epinatyeva, A.M. and Volosov, S.G., 1974. Issledovanie seismicheskoi anizotropli kristallicheskogo fundamenta. DokI. Akad. Nauk SSSR, 218 (5): 1082—1085. Pagurova, B.!., 1968. Kriteiy sravneniya srednikh znacheny p0 dvum normalnym vyborkam. Soobshcheniya p0 vychislitelnoy matematike, No.5, Computational Centre, Moscow, 33 pp. Pupinet, G., 1977. Heterogeneites du manteau terrestre deduites de la propagation des ondes de volume — iinplication geodynamique. These, Université Scientifique et Médicale de Grenoble, 234 pp. Raitt, R.W., 1969. Anisotropy of the upper mantle. In: J. Hart (Editor), The Earth’s Crust and Upper Mantle. Am. Geophys. Un., Washington, DC, pp. 250—256. R.ingwood, A.E., 1969. Composition and evolution of the upper mantle. In: J. Hart (Editor), The Earth’s Crust and Upper Mantle. Am. Geophys. Un., Washington, DC, pp. 1—17. Seismological Tables for P Phases, 1968. Bull. Seismol. Soc. Am., 58 (4): 1196—1240. Simmons, G. and Roy, R.F., 1969. Heat flow in North America. In: The Earth’s Crust and Upper Mantle. Washington, DC, pp. 78—81. Sipkin, S.A. and Jordan, I.N., 1976. Lateral heterogeneity of the upper mantle determined from the travel times of multiple ScS. J. Geophys. Res., 81: 6307—6320. Starovoyt, O.E., Savarensky, E.F. and Fedorov, S.A., 1970. Stroeme obolochki Zemli po nablyudeniyazn dlinnoperiodnykh poverkhnostnykh voln. In: Problemy stroeniya zemnoy kory i verkhnei mantii, No 7, Nauka, Moscow, pp. 90—97. Suetnova, E.I., 1979. Novye resultaty sfericheskogo garmonicheskogo analiza mirovykh dannykh teplovogo potoka. In: Teoreticheskoe i eksperimentalnoe izuchenie teplovykh potokov. Nauka, Moscow, pp. 123—137. Vinnik, L.P., 1976. Issledovanie mann Zemli seismicheskimi metodami. Nauka, Moscow, 198 pp.