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Produced by the NASA Center for Aerospace Information (CASI)
SURFACE WAVE TOMOGRAPHY

Don L. Anderson

Seismological Laboratory

California Institute of Technology

Pasadena, California 91125

Surface waves are now being used by several groups to map lateral

heterogeneity (Nakanishi and Anderson, 1982, 1983, 1984a,b, Woodhouse and

Dziewonski, 1984) and anisotropy (Tanimoto and Anderson, 1984a,b, Nataf,

Nakanishi and Anderson, 1984) of the upper mantle on a global basis. The

method involves measuring the phase and/or group velocity over hundreds of

small arcs and long arcs connecting earthquakes and seismic stations. These

averages are then converted to three-dimensional images of the seismic velocity

structure and, hence, this is a form of tomography. The large amount of data 

processing required is made feasible by the existance of long-period digital

seismic networks including IDA (international Deployment of Accelerome

SRO (Seismic Research Observatories) and GDSN (Global Digital Seismic Network).

These instruments are operated by a variety of university and government groups

including U.C. San Diego, U.S.G.S., D.A.R.P.A. a-..id D.O.E. with the cooperation

of many countries. The global coverage is still very sparse compared to the

analog W.W.S.S.N. (World Wide Standardized Seismic Network) but preliminary

results are very encouraging. The possibility of an expanded global digital 

network of broad-band seismic stations is now being pursued actively by the

United States and several other countries. Because of the sparseness of the

present network, mantle structure can only be mapped with fairly low resolving

power. Only features with half wavelength of the order of 2,000 km can be

detected.

IMASA-CM- 175373) SURFACE hAYE TOdOGRAPHY MU4-17728

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The maps on the cover show the results from one recent study (Nataf,

Nakanishi and Anderson, 1984). Shown are the parameters VSV and XI at two

depths, 250 and 350 km. VSV is the velocity of vertically polarized shear

waves, determined primarily from fundamental mode Rayleigh waves, and XI is

related to the difference between VSH, the velocity of horizontally polarized

shear waves, and VSV. The parameter XI is therefore a measure of anisotropy.

Positive XI means VSH > VSV. Aggregates composed of a-axis horizontal olivine

crystals, for example, are expected to have XI > 0. This situation is probably

diagnostic of horizontal flow. XI < 0 is expected for vertical a-axis

ozientation of olivine and implies vertical flow. Blue areas in the maps

represent fast regions or VSH> VSV. Orange areas are slower than average or

VSH < VSV. The maps are spherical harmonic representations including

coefficients up to and including order and degree six.

In spite of the lack of short wavelength information, there is much

important information in these maps. Midoceanic ridges and regions of recent

volcanic activity are generally slow at 250 km. The regions near the Tasman

Sea - New Zealand, Red Sea - African rift and western North America are slow.

The central Pacific, between Hawaii and Tahiti is also slow at this depth.

Fast regions include the Canadian and Fennoscandian shields and the Siberian

platform, as expected, but also the North Pacific and the eastern Indian Ocean.

Many hotspots are on the edges of low-velocity regions rather than centrally

located.

The parameter XI is negative over regions of upwelling (East Pacific Rise,

Antarctic-Pacific Rise, South Indian Rises, Mid-Atlantic Rise and the Red Sea

region and areas of presumed downwelling (Japan, Philippines, Mariannas,

Sumatra). Plate interiors are generally positive XI, suggesting horizontal

flow at 250 km.
38, p. 10,267-10,283, 1983.

correlation between 220 km and 400 km was assumed in the inversion. The

differences are therefore particularly instructive. The central Pacific, Red

Sea and Mid-Atlantic ridge slow anomalies persist, suggesting that these are

relatively deep seated. Most shields are no longer evident. The thermal

anomaly associated with the East Pacific Rise appears to be displaced. At 450

km depth, not shown, most ridges are fast and most subduction regions are also

fast. The velocity and ZI parameter at 350 km are consistent with upwelling

flow along the East Pacific Rise, the central and southern Mid-Atlantic Rise

and the Red Sea area. These parameters are consistent with downwelling in the

western Pacific. An upwelling is implied in the south central Pacific.

Acknowledgements

This research was supported by National Science Foundation grant EAR81-

15236 and National Aeronautics and Space Administration contract NSG-7610.

Contribution number 4039, Division of Geological an-1 Planetary Sciences,

California Institute of Technology, Pasadena, California 91125.

References

Nakanishi, I., and D. L. Anderson, World-wide distribution of group velocity

of mantle Rayleigh waves as determined by spherical harmonic inversion, Bull.

Seismol. Soc. Am., 72, p. 1185-1194, 1982.

Nakanishi, I., and D. L. Anderson, Measurement of mantle wave velocities and

inversion for.lateral heterogeneity and anisotropy, Part I., J. Geophys. Res.,
-4-

Nakanishi, t., and D. L. Anderson, Aspherical heterogeneity of the mantle from

phase velocities of mantle waves, Nature, 307, 117-121, 1984a.

Nakanishi, I., and D. L. Anderson, Measurements of mantle wave velocities and

inversion for lateral heterogeneity and anisotropy, Part II. Analysis by the

single-station method, Geophys. J. R. astr. Soc., in press, 1984b.

Nataf, H.-C., I. Nakanishi, and D. L. Anderson, Anisotropy and shear velocity

heterogeneities in the upper mantle, Geophys. Res. Lett., in press, 1984.

Tanimoto, T., and D. L. Anderson, Lateral heterogeneity and azimuthal

anisotropy of the upper mantle: Love and Rayleigh waves 100-250 sec_. J.

Geophys. Res., 1984a (submitted)

Tanimoto, T., and D. L. Anderson, Mapping convection in the mantle, Geop),ys.

Res. Lett., 1984b (submitted).

Woodhouse, J. H., and A. M. Dziewonski, Napping the upper mantle: three

dimensional modelling of earth structure by inversion of seismic waveforms, J. Geophys. Res. 1984 (submitted).
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