Tibet surface wave seismology
Structure of the crust and upper mantle of Tibet using seismic surface waves
We use the dispersion of seismic surface waves, Rayleigh and Love waves, across Tibet to refine our image of lateral and vertical variations in structure. Because these waves are sensitive to both the crust and the mantle structure, we are studying both with somewhat different goals. We found that Rayleigh and Love wave dispersion curves (group speed as a function of period of the wave) cannot be fit with the same elastic moduli. Love waves propagate faster than is predicted by any crustal structure that matches the Rayleigh wave group speeds (Figure 1). Thus, the crust must be anisotropic such that shear within horizontal planes is resisted by a larger modulus than is shear on vertical planes. The region where such anisotropy exists, shown in red in Figure 1, is also the region where present-day deformation is dominated by normal faulting and crustal thinning (Figure 2). A simple physical explanation is that layers of mica, for which shear moduli are larger within the sheets of mica than from one sheet to another, have been rotated preferentially into horizontal orientations, as crust extends horizontally and thins [Shapiro et al., 2004]. If this explanation is correct, it implies that the middle crust has been compressed vertically, and therefore extended horizontally. Thus, some flow within the crust, as if within a confined channel, seems to have occurred.
The structure of the mantle that we infer from surface wave dispersion includes features that have been recognized before: a relatively high-speed uppermost mantle beneath much of southern Tibet and northeastern Tibet and a low-speed uppermost mantle in northern Tibet. Perhaps more surprising, however, is the evidence of a high-speed layer at depth beneath Tibet, near 250 km. Resolution of fundamental mode surface waves does not constrain the structure at greater depth. A possible explanation for the high-speed uppermost mantle is than blobs of cold uppermost mantel became unstable and sank into the underlying asthenosphere.
Shapiro, N. M., M. H. Ritzwoller, P. Molnar, and V. Levin (2004), Thinning and flow of Tibetan crust constrained by the seismic anisotropy, Science, 305, 233-
Figure 1. Maps of radial anisotropy in the Tibetan crust. (A) to (D) show measured and calculated surface-wave dispersion inversion for a point in western Tibet (34°N, 84°N) and structures used to calculate dispersion. (A) Fit to the observed dispersion curves with the monotonically increasing, isotropic parameterization of the shear wave speeds in the crust seen in (B). (C) Fit with radial anisotropy in the middle crust seen in (D). In (A) and (C), the predicted (thin black line) group speed dispersion curves correspond to the best-fitting crustal structures in (B) and (D), respectively. (E) Strength of radial anisotropy in the middle crust from the best-fitting radially anisotropic model, represented as the idealized travel time difference between Sv and Sh waves propagating vertically through the middle crust. (F) The minimum strength of radial anisotropy required to explain the surface-wave dispersion data, as in (E). Solid lines in (E) and (F) show selected major active faults, and dashed lines are approximate locations of sutures (From Shapiro et al., 2004).