A Comparative Study of Two Large Low Velocity Structures at the Base of the Earth's Mantle

Abstract:

We recently identified an exceptionally large (~900 km in diameter) ultra-low velocity zone (ULVZ) at the base of the mantle, near the northern border of the Pacific Large Low Shear Velocity Province (LLSVP) and in close vicinity to the projection on the CMB of the Hawaiian hotspot (Cottaar and Romanowicz, 2012). This ULVZ was detected and characterized from Sdiff waveform observations from western Pacific earthquakes, at USArray stations and 3D forward modelling using the spectral element method. These waveforms, filtered in the 10-30 s range, showed the presence of a prominent postcursor, separated from the main phase by up to 50 s in certain azimuth ranges. Reproducing these post-cursors and their behavior in travel time and amplitude required an ULVZ with velocity reduction of ~20% in shear velocity and about 25 km in height above the core-mantle boundary (with some trade-offs). This ULVZ is below the detection of global tomographic approaches, although it is located at the base of the Hawaiian plume, as imaged in our recent mantle tomographic model (SEMUCB-wm1, French and Romanowicz, 2014, in revision).

On the other hand, the low shear velocity PERM anomaly at the base of the mantle (Lekic et al., 2012), which has a similar spatial extent, extends several hundred km into the mantle, with apparently a less reduced velocity contrast (about -7%) and it is detected in most global tomographic models.

Here, we have assembled an augmented dataset of S and Sdiff phases illuminating both anomalies from different directions, and illustrate their contrasting and frequency dependent effects on waveforms, in order to better constrain the difference in the nature of the two velocity bodies.