9:45 AM - 10:00 AM
[S14-1-05] Seismic evidence for broad attenuation anomalies in the asthenosphere beneath the Pacific ocean
We present QsADR17, a global model of upper mantle shear wave attenuation based on a massive Rayleigh-wave dataset (372,629 fundamental and higher-mode attenuation curves measured in the period range 50–260 s). We first correct our dataset for focusing-defocusing effects and geometrical spreading, and perform a stringent selection to only keep robust observations. Then, data with close epicenters recorded at the same station are clustered, as they sample the same Earth's structure. After this pre-selection, our dataset consists in a set of about 35,000 fundamental and higher-mode attenuation curves that constrain the Rayleigh wave intrinsic attenuation in the upper mantle. The logarithms of the attenuation along the individual rays are then inverted to obtain global maps of the logarithm of the Rayleigh-wave attenuation for each mode and each period. The set of attenuation maps is finally inverted at depth to obtain a depth-dependent model of attenuation QsADR17.
QsADR17 presents strong agreement with surface tectonics in the uppermost 200 km of the mantle, with low attenuation under continents and high attenuation under oceans. Beneath oceans, the highest attenuations are found beneath the mid-oceanic ridges at 50 and 100 km depths, but also in old oceanic basins generally in the vicinity of hotspots. Although attenuation decreases with increasing crustal ages, the presence of strong attenuating anomalies in the vicinity of hotspots produces departure from a simple half-pace cooling model. In the Pacific ocean, we observe a strong attenuating anomaly in the long wavelength component of our signal in the depth range 50-150 km. We suggest that this anomaly results from the horizontal spreading of several thermal plumes within the asthenosphere. Strong velocity reductions associated with high attenuation anomalies of moderate amplitudes beneath the East Pacific Rise, the Red Sea and the eastern art of Asia may require additional mechanisms, such as partial melting.
QsADR17 presents strong agreement with surface tectonics in the uppermost 200 km of the mantle, with low attenuation under continents and high attenuation under oceans. Beneath oceans, the highest attenuations are found beneath the mid-oceanic ridges at 50 and 100 km depths, but also in old oceanic basins generally in the vicinity of hotspots. Although attenuation decreases with increasing crustal ages, the presence of strong attenuating anomalies in the vicinity of hotspots produces departure from a simple half-pace cooling model. In the Pacific ocean, we observe a strong attenuating anomaly in the long wavelength component of our signal in the depth range 50-150 km. We suggest that this anomaly results from the horizontal spreading of several thermal plumes within the asthenosphere. Strong velocity reductions associated with high attenuation anomalies of moderate amplitudes beneath the East Pacific Rise, the Red Sea and the eastern art of Asia may require additional mechanisms, such as partial melting.