What is the contribution of seismology to Earth structure? Seismology can be used to measure a
physical property of a material, the velocity of elastic waves.
A. Elastic waves are oscillations of
particle displayment in a solid about an equilibrium postion.
True elastic waves involve do not permanently deform the solid or
move particles away from their permanent positions. The deformation or strain created by an elastic wave is
proportional to the stress (force/area) applied to any surface of the solid.
The proportionality constant in this relationship is called an elastic
constant. Elastic constants are
fundamental physical properties of a solid. Elastic waves in air and water are called sound waves or acoustic
the oscillations of particles due to heat, elastic waves propagate in a very
waves propagate outward from a source along three-dimensional surfaces
called wavefronts, similar to the rings of waves seen propagating outward in a
pond centered where a pebble has splashed. The perpendiculars to wavefronts are called rays. Body wave propagation, including
reflection and refraction is often summarized by drawing ray paths. Ray paths follow a principle of least
time, from which Snell's Law can be proved. Two types of body waves exist in a solid -- P waves with
particle motion in the direction of propagation and S waves with particle
motion perpendicular to the direction of motion.
The figure below shows the propagating P and S wavefronts from a seismic
source in a cross section of the earth.
Surface waves are elastic waves whose energy is confined close to the
surface of the earth. The energy of a surface wave exponenentially decays with
depth. In a solid whose properties
vary with depth, surface waves are dispersed (different frequencies travel at different
velocities). Two types of surface
waves exist on solids -- Rayleigh and Love. Rayleigh waves are slower than Love
waves about 0.9 times the S wave velocity at the surface. The particle motion in Rayleigh waves
lies in the plane containing the source, seismometer and center of the
earth. The particle motion of Love
waves is perpendicular to this plane.
The velocities of different frequency components are a function of the
elastic and density sturcture of the upper surface of the Earth.
Click on the link below for examples body wave polarization, surface wave
particle motion, and dispersion:
3. Free oscillations are a
discription of elastic vibrations as fundamental modes of oscillation of the
Earth, having very long wavelength.
Example, the breathing mode, the football mode etc. The frequencies of free oscillations
are fuctions of the velocity and density structure of the Earth. For the lowest frequency oscillations,
it is important to consider the effect of the earth's gravity and rotation on
the restoring force of particles from tbeir equilibrium position.
B. What has been learned about the
structure of the Earth from the study of body wave velocities, surface wave
dispersion, and free oscillation frequencies? A best fitting average earth model has been determined. This model has the following features.
crust -- Si rich, chemically different from mantle Bouyant materal that cannot be subducted
far into the mantle. Continental
crust 20-60 km thick. Oceanic crust 5-10 km thick. Oceanic crust is basalt. (50% Si) Granitic and other Si
richer rocks (Si greater than 50%) occur in upper continental crust. The crust
is thus strongly laterally varying -- different properties in different areas.
upper mantle -- lateral variations are weaker than the crust. Significant lateral variations (up to
10% in S velocity), however, do persist to at least 200 km depth and perhaps up
to 400 km depth on a global scale.
On more local scales (horizontal distances of 100 km), strong lateral
variations occur in and around subduction zones, where descending lithospheric
slabs penetrate up to 650 km in depth and perhaps deeper. Important average radial structure is:
Low velocity zone. LVZ -- zone of decreased velocity or smaller vertical
gradient in P and S veloctiy at 100 - 200 km depth. Best developed beneath
tectonically active regions. Weak
or absent beneath old continental cratons. Likely zone decreased viscosity, plate decoupling and zone
of strong shear deformation. It martks the lithosphere/asthenosphere boundary.
400 km discontinuity -- transition of crystalline lattice structure to
closer packed form -- olivine type lattice to spinel type lattice
650 km discontinuity ---
transistion of crystalline lattice structure to closer packed form -- spinel
type to post -spinel type or metal oxide type, e.g., MgO.
Lower mantle (deeper than 650 km).
Weak lateral heterogeneity (at a constant radius P and S wave velocity
vary less than 1%), except in the lower most mantle.
Lower mantle near the core-mantle boundary. D" region near CMB --
zone of increased lateral hetrogeneity in the lowermost 200-400 km above the
core-mantle boundary. Global
average models have P and S velocity gradients reduced. May be zone of return convective flow,
chemical reaction zone between silicate mantle and liquid iron alloyed outer
core, and contain slab remnants.
Strong rise in temperature near core-mantle boundary reduces velocity
5. outer core -- Liquid iron
alloyed with light element (probably oxygen). Vigorously convecting with material velocities on the order
of km's/year (compare to mantle conveciton velocities of cm's/year). Convection of conducting liquid induces
inner core -- solid, nearly pure iron. P waves travel through the inner core faster in polar
directions than equatorial directions.
This elastic "anisotropy" is not yet well understood and has
been proposed to be related to the orientation crystalline lattice axies in a
high pressure form iron. Why the lattice would asume a preferred orientation is
not yet known.
C. Laterally heterogeneous earth models --
P and S waves recorded by earthquakes have been timed and reported for more
than 105 to 106 earthquakes and
catalogued in databases that can be processed to retrieve earth structure. This processing is called
"tomography". Tomography prodcues a three-dimensional image of the
1. Magnitude of variations -- in
deltaVp/Vs and delta Vs/Vs: 10% or greater lateral variations in the crust and
upper mantle and D". 1% in
the rest of the mantle. No detectable
variations in the outer core. 1-2%
variations in the inner core.
2. Sense and correlations of variations --
Hot and young regions in the upper mantle are slow; Cold and old regions are
fast. Examples, mid-ocean ridge
regions are slow; continental cratons are fast, descending slabs are fast. Wedge
of mantle material beneath island arcs and in front of subducting slabs is
3. Fundamental problems for the
future: Do slabs reach the
core-mantle boundary? Can plume
hot-spots be traced to the core mantle boundary? How do mid-ocean ridge structures connect (if at all) to the
mid and lower mantle.