Science Nugget - By Jeanine Buehler (University of California San Diego) and Peter Shearer (University of California San Diego) - NSF EAR # 1358510 | 2014
We are updating our Pn tomography models for the western United States (Buehler and Shearer [2010, 2014]) to image uppermost mantle velocity structure and anisotropy, as well as crustal thickness constraints, beneath the entire USArray footprint (Buehler and Shearer, [2016, submitted]). The regional Pn phase, which is typically described as a headwave that propagates along the Moho in the uppermost mantle, is sensitive to heterogeneities in the mantle lid. Pn tomography therefore complements other seismic studies with poorer vertical resolution at this depth. The sparser source distribution but generally lower attenuation in the eastern United States, and the resulting longer ray paths, justify the inclusion of additional parameters that account for the velocity gradient in the mantle lid as Pn propagation at long ranges typically deviates from that of a pure headwave (e.g., Phillips et at. [2007], Zhao [1993]). At large scale, Pn velocities are higher in the eastern United States compared to the west, but we observe areas of lower velocities around the New Madrid seismic zone and below the Appalachians (Figure 1). For much of the mantle lid below the central and eastern United States we find a moderate positive velocity gradient, corresponding to the regions of the North American craton. This change in mantle lid gradient structure is also consistent with observed changes in Pn amplitude across different record sections. In terms of anisotropy, we find that the Pn fast axes generally do not agree with SKS splitting orientations, suggesting multiple anisotropic layers in the crust and upper mantle.
Top Figure:Uppermost mantle isotropic velocity structure beneath USArray from Pn tomography.
References
Buehler, J. S., and P. M. Shearer, Pn tomography of the western United States using USArray, J. Geophys. Res., 2010.
Buehler, J. S. and P. M. Shearer, Anisotropy and Vp/Vs in the uppermost mantle beneath the western United States from joint analysis of Pn and Sn phases, J. Geophys. Res., 2014
Buehler, J. S. and P. M. Shearer, Uppermost mantle seismic velocity structure beneath USArray, J. Geophys. Res, submitted.
Phillips, W. S., Begnaud, M. L., Rowe, C. A., Steck, L. K., Myers, S. C., Pasyanos, M. E., & Ballard, S., Accounting for lateral variations of the upper mantle gradient in Pn tomography studies, Geophys. Res. Lett., 2007.
Zhao, L. S., Lateral variations and azimuthal isotropy of Pn velocities beneath Basin and Rangeprovince, J. Geophys. Res., 1993.
