Teleseismic P Wave Attenuation in the Upper Mantle Beneath the United States
Using Mw>6, deep earthquakes that occurred between 2006 and 2015 and were recorded by EarthScope™'s Transportable Array, the P wave amplitude spectra were analyzed to examine seismic attenuation patterns in the upper mantle. 2.5 million inter-station P wave spectral ratios were inverted for maps of relative t* variations across the United States in multiple frequency bands between 0.08 and 2 Hz. Broader frequency bands are associated with lower magnitude variations; however, the geographic pattern of attenuation is similar across all frequency bands, and the correlation values were >0.8. The dt* maps from this study were compared with long period surface wave Q models [Dalton et al., 2008] and upper mantle P wave velocity tomography [Schmandt and Lin, 2014]. The magnitude of t* variations inverted from P wave spectra near 1 Hz is ~10x greater than predicted by long-period surface wave studies of upper mantle attenuation. This is caused by the influence of scattering for higher frequency P wave amplitude spectra. Ratios of transverse-to-vertical component spectra are used as a proxy for P wave vii scattering as P wave energy loss due to scattering generally increases with frequency. The geometries between P wave amplitude spectral ratio studies and long period surface wave Q studies are largely similar. I suggest the geographic correlation results from volumes of greater than ~1 Hz scattering intensity preferentially being located in areas of higher mantle temperatures. The active tectonic structures in the western U.S. and presence of partial melts associated with widespread volcanism would cause increased elastic scattering in the same region where thin lithosphere and high mantle temperatures predict greater intrinsic attenuation. The effects of upper mantle temperature on t* estimates can be overwhelmed by upper mantle scattering because similarly high t* values can be found in the cratonic interior and western Cordillera. The results generally indicate that differences in teleseismic body wave spectral decay slopes are more strongly controlled by scattering than intrinsic attenuation.
teleseismic seismology p wave attenuation upper mantle earth united states earthscope scattering intrinsic transportable array usarray t* short period earthquake amplitude spectra Q
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