Teleseismic P wave spectra from USArray and implications for upper mantle attenuation and scattering

Abstract

Teleseismic P wave amplitude spectra from deep earthquakes recorded by USArray are inverted for maps of upper mantle Δt∗ for multiple frequency bands within 0.08-2 Hz. All frequency bands show high Δt∗ regions in the southwestern U.S., southern Rocky Mountains, and Appalachian margin. Low Δt∗ is more common across the cratonic interior. Inversions with narrower frequency bands yield similar patterns, but greater Δt∗ magnitudes. Even the two standard deviation Δt∗ magnitude for the widest band is ∼2-7 times greater than predicted by global QS tomography or an anelastic olivine thermal model, suggesting that much of the Δt∗ signal is nonthermal in origin. Nonthermal contributions are further indicated by only a moderate correlation between Δt∗ and P travel times. Some geographic variations, such as high Δt∗ in parts of the cratonic interior with high mantle velocities and low heat flow, demonstrate that the influence of temperature is regionally overwhelmed. Transverse spectra are used to investigate the importance of scattering because they would receive no P energy in the absence of 3-D heterogeneity or anisotropy. Transverse to vertical (T/Z) spectral ratios for stations with high Δt∗ are higher and exhibit steeper increases with frequency compared to T/Z spectra for low Δt∗ stations. The large magnitude of Δt∗ estimates and the T/Z spectra are consistent with major contributions to Δt∗ from scattering. A weak positive correlation between intrinsic attenuation and apparent attenuation due to scattering may contribute to Δt∗ magnitude and the moderate correlation of Δt∗ with travel times.