Spatial variations of P wave attenuation in the mantle beneath North America

We estimate the spatial variation of the seismic parameter t* using teleseismic (epicentral distance = 30 degrees-85 degrees) P wave spectra of about 200 deep (focal depths >200 km) earthquakes recorded by 378 broadband seismometers in the United States and Canada. Relative P wave spectral ratios up to 1 Hz for about 63,000 station pairs with high signal-to-noise ratio and impulsive P waveforms are inverted for t(P)* by least squares inversion. The continental-scale t(P)* pattern correlates to the age of geological terrains and the seismic, heat flow, gravity, and magnetic variations across North America. Predominantly low values of t(P)* are obtained in stable central North America (SNA), and high t(P)* values are obtained for stations in the tectonically active western part of the continent (TNA). This variation is similar to that observed previously in short-period amplitude anomalies, spectral ratio variations, and ScS reverberations. On average, we resolve a contrast in t(P)* between SNA and TNA of about 0.2 s. We resolve regional variations in t(P)*, which correlate with tectonics. Relatively low t(P)* is associated with currently active subduction below Alaska. Relatively high t(P)* is found in SNA below the Appalachians and the Gulf Coast. The consistency between t(P)* and tectonics suggests that the observed variations in t(P)* are, on the scale of around 200-500 km, predominantly due to intrinsic attenuation. The similar patterns in t(P)* and predicted values for a recent global attenuation model confirm this further. The compatibility with the t(P)* computed for attenuation estimated via a thermal interpretation of shear wave velocity anomalies illustrates that variations in seismic velocity are predominantly due to physical effects with a strong attenuation signature, most likely temperature or a combination of temperature and water content.