Earth Internal Structure

Important application of seismology

-- remote sensing of Earth interior

most powerful information = seismology

--interaction of seismic waves with material in earth interior

EQ ==> seismic waves, travel outward from source

Waves traveling in earth interior = body waves (P,S)

Info on material properties (e.g., S waves @ core)

Interaction with boundaries (change in composition, phase) inside earth

--examples (Moho, Core-Mantle Boundary, Inner/outer core, ocean floor)

~ effect of different materials on light waves: optics

Seismic travel times:

information on seismic velocities ==> material properties

Reflection: ~ light at mirror

--angle of incidence = angle of reflection

--important application = echo sounding, oil exploration

Refraction: bending of light @ acquarium

change of angle = f (velocities in medium)

Snell's law:

primary info on crustal structure, core velocity, etc.

Conversion: change of phase: P --> S, S --> P, etc.

incident P ==> reflected P,S, refracted P,S

Attenuation: absorption of energy by Earth

Earth = imperfect elastic medium, wave energy --> heat

Adds to loss of energy <== spherical spreading

Applications to volcanology, discovery of asthenosphere

Amplification: resonance of unconsolidated materials

cf. lessons from Mexico City, Loma Prieta

Major boundaries in Earth interior

Crust: Near-surface rocks, rigid outer shell

= sediments + granite + crystalline rx:

-- thickness

oceans = 8-10 km

continents = 30-45 km

mtn. belts = 45-70 km

-- base = Moho

Mantle: change to denser crystalline rocks

-- composition: Si + Fe + Mg

-- thickness ~ 1/2 of earth ==> 2880 km

-- base = sudden change in physical properties: Gutenberg Discontinuity

Core: very high density, metallic

-- composition: Fe + Ni

-- outer core = molten

-- inner core = solid

-- boundary = Lehmann Discontinuity 5140 km

Example of application of seismology:

How do we know about Earth's liquid core?

British seismologist R.D. Oldham (1906) ==> disappearance of P,S waves @ 103°

= shadow zone

--recognized that this could be explained by core of low velocity = lens

Gutenberg (1914) recognized that S waves = stopped completely ==> liquid

--estimated depth @ 2900 km

Boundary indicated by reflection, refraction ==> PcP, ScS, PKP, SKS

Inge Lehmann (1936) ==> weak P waves refracted back to shadow zone

--interp. high velocity inner core

Reflections: PKiKP, Refractions: PKIKP