Constraining Jumps in Density and Elastic Properties at the 660 km Discontinuity Using Normal Mode Data via the Backus-Gilbert Method

Abstract

We apply the Backus-Gilbert approach to normal mode center frequency data, to constrain jumps in P, S, bulk-sound speed and density at the “660” discontinuity in the earth’s mantle (∼650–670 km depth). Different 1-D models are considered to compute sensitivity kernels. When using model PREM (Dziewonski & Anderson, 1981, Physics of the Earth and Planetary Interiors, 25, 297–356. doi:10.1016/0031-9201(81)90046-7) as reference, with a “660” at 670 km depth, the best-fitting jumps in density, P- and S-wave speeds range from (5.1–8.2)%, (5.3–8.0)%, (5.0–7.0)%, respectively, so the PREM values lie outside the ranges of acceptable density and P wave speed jumps. When shifting the depth of “660” to 660 km, the density and S wave speed jumps increase, while the P-wave speed jump decreases. Normal mode data do not support a global transition at 650 km depth. The density jumps are closer to those of pyrolite than PREM, while our bulk-sound wave speed jumps suggest a larger garnet proportion at “660.”.