Sujania Talavera-Soza, Utrecht / UCSD 

Seismic tomographic models based solely on wave velocities have limited ability to distinguish between a thermal or compositional origin for Earth’s 3D structure. Complementing wave velocities with attenuation observations can make that distinction, which is fundamental for understanding mantle convection evolution. For example, a thermal origin for the lower mantle large low-seismic-velocity provinces (LLSVPs) will point to them being short-lived anomalies, whereas a compositional origin will point to them being long-lived, forming stable ‘anchors’ and influencing the pattern of mantle convection. However, current global 3D attenuation models are only available for the upper mantle. Here, we present two 3D global model of attenuation for the whole mantle made using whole Earth oscillations and compare two methods of normal mode inversion. In the upper mantle, we find high attenuation in low velocity regions, suggesting a thermal origin for spreading ridges, agreeing with previous studies. In the lower mantle, we find the opposite, and observe the highest attenuation in the ‘ring around the Pacific’, which is seismically fast, and the lowest attenuation in the LLSVPs. Comparing our model to wave-speeds and attenuation predicted by a laboratory-based viscoelastic model suggests that the circum-Pacific is a colder and small grain-size region, surrounding the warmer and large grain-size LLSVPs. Grain-size is proportional to viscosity in diffusion creep, implying that the LLSVPs are long-lived stable features.