J. S. Buehler

Anisotropy and Vp/Vs in the uppermost mantle beneath the western United States from joint analysis of Pn and Sn phases

Pn and Sn phases are valuable for resolving velocity structure in the mantle lid, as they propagate horizontally right below the Moho. Relatively few Sn tomography attempts have been made compared to Pn, because Sn is often highly attenuated or buried in P wave coda. USArray has greatly increased data coverage for regional phases, and both Pn and Sn are routinely picked by network analysts. Here we jointly invert Pn and Sn arrival time residuals with a modified time-term analysis and a regularized tomography method and present new maps of crustal thickness, uppermost mantle P velocity perturbations, Vp/Vs ratios, and azimuthal anisotropy strength and orientation beneath the western United States. The results indicate partially molten mantle below the Snake River Plain and the Colorado Plateau. The seismic structure of the top ∼40 km of the mantle below the Colorado Plateau differs from that seen at greater depths in other studies, such as surface wave or teleseismic body wave tomography, whereas the Snake River Plain anomaly just below the Moho is comparable to structures seen at about ∼200 km depth. Pn fast axes provide complementary information to SKS shear wave splitting observations, and our analysis indicates that in several regions in the western United States the orientation of azimuthal anisotropy changes with depth in the upper mantle. However, we have so far been unable to resolve shear wave splitting directly in Sn waveforms, which seem to be dominated by Sn-SV energy.

Uppermost mantle seismic velocity structure beneath USArray

We apply Pn tomography beneath the entire USArray footprint to image uppermost mantle velocity structure and anisotropy, as well as crustal thickness constraints, beneath the United States. The sparse source distribution in the eastern United States and the resulting longer raypaths provide new challenges and justify the inclusion of additional parameters that account for the velocity gradient in the mantle lid. At large scale, Pn velocities are higher in the eastern United States compared to the west, but we observe patches of lower velocities around the New Madrid seismic zone and below the eastern Appalachians. For much of the mantle lid below the central and eastern United States we find a moderate positive velocity gradient. In the western United States, we observe a moderate gradient in the region of the Juan de Fuca subduction zone, but no significant gradient to the south and east of this region. In terms of anisotropy, we find that the Pn fast axes generally do not agree with SKS splitting orientations, suggesting significant vertical changes in anisotropy in the upper mantle. In particular the circular pattern of the fast polarization direction of SKS in the western United States is much less pronounced in the Pn results, and in the eastern US the dominant Pn fast direction is approximately north-south, whereas the SKS fast polarizations are oriented roughly parallel to the absolute plate motion direction.

Moho temperature and mobility of lower crust in the western United States

We use measurements of mantle P-wave velocity from the Moho refracted phase, Pn, to estimate temperature within the uppermost few km of the western U.S. mantle. Relative to other approaches to modeling the deep geotherm, using Pn velocities requires few assumptions and provides a less uncertain temperature at a tightly constrained depth. Assuming a homogeneous mantle composition, Moho temperatures are lowest in an arc that extends from the High Lava Plains through western Montana and the high-plains region of Wyoming and western Kansas/Nebraska. Highest temperatures are observed under recent (<10 Ma) volcanic provinces and are consistent with melting. Estimates of lower crustal viscosity suggest that the western U.S. west of the Laramide deformation front likely has regions of mobile lower crust that decouple upper crustal and upper mantle tractions.

Decomposing Leftovers: Event, Path, and Site Residuals for a Small‐Magnitude Anza Region GMPEDecomposing Leftovers: Event, Path, and Site Residuals for a Small‐Magnitude Anza Region GMPE

Ground-motion prediction equations (GMPEs) are critical elements of probabilistic seismic hazard analysis (PSHA), as well as for other applications of ground motions. To isolate the path component for the purpose of building nonergodic GMPEs, we compute a regional GMPE using a large dataset of peak ground accelerations (PGAs) from small-magnitude earthquakes (0:5 ≤ M ≤ 4:5 with >10; 000 events, yielding ∼120; 000 recordings) that occurred in 2013 centered around the ANZA seismic network (hypocentral distances ≤180 km) in southern California. We examine two separate methods of obtaining residuals from the observed and predicted ground motions: a pooled ordinary least-squares model and a mixed-effects maximum-likelihood model. Whereas the former is often used by the broader seismological community, the latter is widely used by the ground-motion and engineering seismology community. We confirm that mixed-effects models are the preferred and most statistically robust method to obtain event, path, and site residuals and discuss the reasoning behind this. Our results show that these methods yield different consequences for the uncertainty of the residuals, particularly for the event residuals. Finally, our results show no correlation (correlation coefficient [CC] <0:03) between site residuals and the classic site-characterization term VS30, the time-averaged shearwave velocity in the top 30 m at a site. We propose that this is due to the relative homogeneity of the site response in the region and perhaps due to shortcomings in the formulation of VS30 and suggest applying the provided PGA site correction terms to future ground-motion studies for increased accuracy. Electronic Supplement: Peak ground acceleration (PGA) dataset.