University:
Widener University
Course:
PHYS-313 | Classical Mechanics
Academic year:

2023
Views:

450

Pages:

12
Author:

Devin Fry

Seismic Velocity and Density Jumps across the 410- and 660-kilometer Discontinuities What did they do? ► Used modeling to determine velocity and density changes across 410 and 660 ► Based models on reflected long-period seismic waves ► 410 results agree with PREM ► 660 results do not closely agree with PREM Introduction ► Reflected waves give good agreement (within 1%) for discontinuity depth ► P and S wave velocities are not as precisely determined ► Velocity jumps disagree by up to a factor of 2 ► Density is difficult and often based on simple scaling Separate velocity and ρ ► Studied the behavior of reflection coefficients as a function of ray angle ► Used observed amplitudes of waves reflected off of the bottoms of each discontinuity ► These waves arrive as precursors to SS & PP ► Stacked 19 years worth of data What was measured? ► Measured relative amplitude between reflected waves and PP and SS ► Made some corrections for time shift of each phase A note on discontinuities ► This study used long period waves ► This precluded resolving gradients of v and ρ finite distances ► In other words, this may have made the discontinuities appear sharper than they actually are. Created a model by . . . ∆α, ∆β, ∆ρ ► Compared synthetic amplitudes with observations to define confidence ellipsoids ► Best fit model 410: higher v, lower ρ than PREM ► Best fit model 660 in close agreement with EK1 model: ∆α=2.0%,∆β=4.8%, ∆ρ=5.2% ► Combining Discussion ► Modeling results assume a simple first-order change in velocity and density ► Cannot distinguish between a gradual and a sharp change Discussion of the 410 ► Density and S-velocity jumps at 410 agree with Pyrolite model of 55 % olivine and only marginally agree with piclogite model of 35% olivine ► This study proposes ~55% mantle olivine Discussion of the 660 ► Observed absence of large change in Bulk Modulus consistent with piclogite models ► Still unsure “more complete forward modeling will be required” ► Shortfall in density increase must be accompanied by increase elsewhere in the transition zone Discussion of the 660 ► Suggest 520-km (absent in PREM) ► 2 to 3% increase possible here ► Density jump is important in mantle convection ► If Clapeyron slope is negative, the 660 will resist thermally driven flow. ► Given the disagreement with PREM regarding density . . . Discussion of the 660 ► Know that buoyancy scales with density ► A 5% density contrast across the 660 boundary could be enough to preclude layered convection