Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - ATUNE (Attenuation Tomography Using Novel observations of Earth\'s free oscillations)

Teaser

Tectonic phenomena at the Earth’s surface, like volcanic eruptions and earthquakes, are driven by convection deep in the mantle. Seismic tomography has been very successful in using earthquakes to elucidate the Earth’s internal structure by making pictures which are very...

Summary

Tectonic phenomena at the Earth’s surface, like volcanic eruptions and earthquakes, are driven by convection deep in the mantle. Seismic tomography has been very successful in using earthquakes to elucidate the Earth’s internal structure by making pictures which are very similar to an MRI scan of our brain. The majority of these pictures image seismic velocity, which tells us where seismic waves travel faster or slower. However, seismic velocity is insufficient to obtain robust estimates of temperature and composition, and make direct links with mantle convection. Thus, fundamental questions remain unanswered: Do subducting slabs bring water into the transition zone or lower mantle? Are the large low-shear velocity provinces under the Pacific and Africa mainly thermal or compositional? Is there any melt or water near the transition zone or core mantle boundary?

Seismic attenuation, or loss of energy, is key to mapping partial melt, water and temperature variations, and answering these questions. Unfortunately, attenuation has only been imaged using short- and intermediate-period seismic data, showing little similarity even for the upper mantle and no reliable lower mantle models exist. The aim of ATUNE is to develop novel full-spectrum techniques and apply them to Earth’s long period free oscillations to observe global-scale regional variations in seismic attenuation from the lithosphere to the core mantle boundary. Whole Earth oscillations occur after large earthquakes and make the Earth `ring like a bell\'. Studying Earth\'s free oscillations is similar to listening to the tones of a musical instrument. By investigating how much the tones of the Earth are `out of tune\', we are able to determine the 3D variations in velocity, density and attenuation in our planet. Scattering and focussing - problematic for shorter period techniques - are easily included using cross-coupling (or resonance) between free oscillations not requiring approximations. The recent occurrence of large earthquakes and increase in computer power have made it possible to increase the frequency dependence of attenuation to a much wider band, allowing us to distinguish between scattering (redistribution of energy) versus intrinsic attenuation. ATUNE\'s overall objective is to deliver the first ever full-waveform global tomographic model of 3D attenuation variations in the lower mantle, providing essential constraints on melt, water and temperature for understanding the complex dynamics of our planet.

Work performed

We have developed the new methods to measure attenuation and anisotropy and have just started making the actual measurements. We find that radial modes provide us with new constraints in 1D attenuation but also, quite unexpectedly, on the radial anisotropy structure of the inner core. The resonance between spheroidal and toroidal modes shows evidence for the existence of strong azimuthal anisotropy in the upper mantle, which will help us in the future to constrain mantle flow.

Final results

We are making normal mode measurements that have not been made before, and expect new constraints on 3D variations in mantle attenuation especially for the lower mantle.