Q M1D: A 1D model of shear attenuation in the mantle from differential body waves

Abstract

We introduce a new 1D model of shear attenuation in the mantle (QM1D) constructed from measurements of differential attenuation of S and ScS body waves. We build a dataset of over 90,000 measurements of $\delta t^*_{\text{ScS-S}}$ from seismograms with high-quality S and ScS waveforms from deep earthquakes between 20–75° epicentral distance from 1990–2024. To ensure that measurements are robust, we require that values of differential attenuation are similar between the instantaneous frequency matching and waveform matching methods. Using this dataset, we perform a Bayesian inversion to construct 1D profiles of mantle shear quality factor Qμ and its uncertainties. Our dataset has a high sensitivity to the mid-mantle (660–2000 km) corresponding to the turning depths of the S phases, which maps into lower uncertainty in the ensemble distribution at these depths. QM1D suggests that the mantle is more attenuating than PREM on average; but, notably, that there is a strongly attenuating layer around 750 km depth and that there are weakly attenuating layers around 900 km and 2000 km depth. Due to the source-receiver distribution for the phases, our dataset has a sampling bias in the mantle around the edge of the Pacific which may not be representative of the whole mantle. The weakly attenuating zones in the mantle may correspond to a viscosity jump around 900 km depth and a further viscosity jump in the deep mantle around 2000 km depth, whilst the thin strongly attenuating layer may correspond to regions of slab stagnation caused by a potential thin layer of low viscosity beneath the mantle transition zone in the circum-Pacific region.