Seismic Detection of Rigid Zones at the Top of the Core

University:
Widener University
Course:
CHEM-255 | Organic Chemistry I
Academic year:

2023
Views:

33

Pages:

7
Author:

Vibsekty

Seismic detection of rigid zones at the top of the core “Continents CRZ of the core” (Core Rigidity Zone) [Wysession, Nature 1996] [Buffett et al., 2000] Previous studies indicating a CRZ: Buffett et al, Science 2000 Nutation data to be fit by a conducting layer Garnero & Jeanloz, GRL 2000 Similarity in SPdKS waveforms by both ULVZ & CRZ Seismic probe: ScP Data [Lay et al., 1998] ∆=40˚~44˚ [Courtesy of Emily van Ark, originally from Rost & Revenaugh, 2003] Complex waveforms: Large postcursors (Unlikely to be produced by ULVZ) Model space ULVZ CRZ δVp= 0~ -15% δVs= 0~ -30% δρ= 0~ 50% H = ~ < 10 km Vs= up to 5.5 km/s δρ= 0~ -40% H= up to 2 km Intermezzo Gaussian Beam Method (GBM) Ray concept + wave theory; high-frequency seimogram in inhomogeneous media; time-economic & avoids singularity Reflectivity Layered half-space model; reflection/transmission coefficients fk-analysis Standard array-analysis method to detect weak phases by constraining slowness & backazimuth through a grid search in spectral domain to find a power maximum indicative of coherent wave energy [e.g., Rost, 2000] More Considerations: 1-D modeling appropriate? 3-D structure exists, but good matches between synthetic and obs. justify the simplification. Could “Sc*spP” be just a scattered phase? Similar waveforms and travel time difference from different events argue against the possibility. P phase travels further within the slab and would have been more complicated. [Karason & van der Hilst, 2000] Possible CRZ models Chemical reaction zone (MgxFe1-x)SiO3 + 3[(1-x)-s]Fe = xMgSiO3 + sSiO2 + [(1-x)-s]FeSi + [3(1-x)-2s]FeO Unlikely to produce a layer > ~ a few meters Sediments at the top of the core: ------------------------------------Lighter Elements: Saturated due to exchange within the mantle; enriched in outer core by cooling & solidification of inner core Chemical equilibrium maintained by a mushy layer of lighter element-bearing solid phases + liquid Fe, situated within CMB topographic heights (~350 m).