A Global Study of Pn Anisotropy Beneath Continents

Gideon P. Smith1, and Göran Ekström

Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA,  USA.
1Now at Department of Earth and Planetary Sciences, Washington Univ., St. Louis, MO, USA
Journ. Geophys. Res., , in press, 1998.

Abstract

We have applied a two-station method of velocity determination to Pn arrival times reported in the ISC catalog during the period 1964-93. The azimuthal variation of these velocities provides evidence of Pn anisotropy as large as 9.6%. Anisotropic regions are identified by examination of the azimuthal variation of the Pn velocities within circular caps of radius 1.5o and 3o , spaced at 1o intervals across the Earth. This procedure leads to 255 geographically distributed estimates of anisotropy. The length--scale over which the anisotropy varies indicates that uppermost mantle anisotropy, beneath continents, is controlled by regional orogenic strains and is inconsistent with a general interpretation in terms of plate-scale mantle flow. In addition, the correlation between Pn anisotropy and current surface tectonics indicates that the anisotropy is primarily the result of ongoing deformation. We compare our Pn anisotropy results (fast directions and magnitudes) with those of SKS splitting studies to attempt to constrain the depth extent of the anisotropy. This comparison suggests that in some areas, where the fast directions agree, SKS splitting observations may arise from a layer beginning just below the Moho, whereas in others it must have a deeper origin. Where the fast directions agree, the relative magnitudes of the measured anisotropies are consistent with a thin (100--150 km) lithospheric layer of anisotropy.

These 2 figures show Pn velocity vs. azimuth for 1.5o caps centered on the locations shown above each plot. Solid lines are the data, dashed lines are the fit to the data.

Pn anisotropy results for Europe. The center of each symmetric arrow pair is plotted at the center of the cap. Arrows point in the direction of fast Pn propagation and are proportional to the strength of anisotropy. Black and white center points indicate 1.5o and 3.0o radius caps respectively. Triangles show the location of null results. The absolute plate motion vector is also shown. There appears to be a strong correlation between the fast axes and the strike of mountain chains.

Fast directions of Pn for the U.S.