Seismically determined elastic parameters for Earth's outer core.

Area of Science:

• Geophysics
• Seismology
• Earth Science

Background:

• Earth's magnetic field originates from turbulent convection in the liquid iron outer core.
• Seismic properties of the outer core are crucial for understanding its composition and convection, but discrepancies exist between seismic models.
• Experimental determination of iron alloy properties at extreme conditions is challenging, hindering compositional inferences.

Purpose of the Study:

• To infer equation-of-state (EoS) parameters of Earth's outer core directly from seismic normal mode observations.
• To develop a new seismic model, Elastic Parameters of the Outer Core (EPOC), that is physically realistic and resolves existing discrepancies.
• To refine understanding of the outer core's composition based on its seismic properties.

Main Methods:

• Directly inferred EoS parameters from normal mode center frequency observations.
• Developed the EPOC seismic model with three parameters, ensuring physically realistic behavior with increasing pressure.
• Compared EPOC model predictions with existing seismic models (PREM) and body wave data.

Main Results:

• The EPOC model better predicts normal mode frequencies than the Preliminary Reference Earth Model (PREM).
• EPOC shows improved consistency with body wave-derived seismic models, resolving a long-standing discrepancy.
• EPOC predicts a lower velocity at the top of the outer core, a steeper velocity gradient with depth, and higher density compared to PREM.
• These findings imply the outer core alloy is lighter and more compressible than previously inferred for PREM.

Conclusions:

• The EPOC model provides a more accurate and consistent description of Earth's outer core seismic properties.
• The inferred composition suggests a lighter, more compressible alloy in the outer core.
• EPOC's velocity gradient better explains seismic wave travel times, challenging previous models of the uppermost outer core.