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

2023
Views:

314

Pages:

10
Author:

Payten Sexton

Evidence for a Ubiquitous Seismic Discontinuity at the Base of the Mantle Discontinuity @ CMB • Sharp 2-3% velocity discontinuity ~250 km above the CMB • Scd phase between S and ScS (core reflection) in the 65 – 83° distance range • Variation in relative timing and amplitude – Intermittent 2 Layers = 1 Discontinuity • What is the depth of this discontinuity? • How do we use the shear wave anomaly to make inferences about the depth of the discontinuity? • • • • ǻV: local velocity anomaly Į(h): thermal expansivity (depth) Vprem(h): shear wave velocity (depth) ī: Shear Modulus (temp.) = 6 * wG ( )p wU 'T 2'V D ( h) Vprem(h) (* 1) What if the Discontinuity is a Phase Transition • “Dynamic and seismic models suggest a phase change is more likely than thermal gradients or chemical heterogeneity” • • • • • • h: depth hph: elevation of phase transition a ǻT=0 Ȗph: Clapeyron slope 'T wph: width of phase transition (5 km) rph: ? U ( h) rph(h) h hph Jph Z: 0 – 1 g – 0 = low pressure phase 1 – 1 = high pressure phase Z [1 tanh(rph / wph)] 2 Statistical Analysis of Phase Boundary Parameter Space Using Synthetic Seismograms GTScd S ª1 «N ¬ N 2º (Tsynth Tdata) ¦1 » ¼ Preferred Model • Ȗph = 6 MPa/K • hph = 200 km 1 2 Preferred Model • Ȗph = 6 MPa/K • hph = 200 km • Ȗph = 0 MPa/K • hph = 250 km • Ȗph = -4 MPa/K • hph = 275 km Preferred Model • Ȗph = 6 MPa/K • hph = 200 km Preferred Model • Ȗph = 6 MPa/K • hph = 200 km Southern Caribbean: 2.75% velocity jump @ 250 km above CMB Northern Caribbean: 2.45% velocity jump @ 290 km above CMB Inverse Problem • If local structure can modulate the strength of triplication (assuming ubiquity). • Is it possible to predict the observed geographic pattern using the structure inferred by tomographic inversions? • Central Pacific = Weak triplication • Caribbean = Stong triplication Conclusions • “The agreement of these trends with observations indicate that the D” discontinuity may a ubiquitous feature with the strength of the resulting seismic triplication modulated by larger scale structure” • “The discontinuity must be correlated with large scale structures such that it’s elevated in fast regions (hot) and depressed in slow regions (cold) – implying a positive clapeyron slope.” These guys look pretty smart now! • “No relevant phase transition has yet been observed in the major elements of the lower mantle” Ab-initio calculations for Post Perovskite • 9.56-9.85 MPa/K (Oganov and Ono 2004) • 7.5±0.3 MPa/K (Tsuchiya et al., 2004)