Earth Structure

We can use arrival times and amplitudes of phases to determine the interior structure of the Earth. In fact, most of what we know about the interior of the Earth comes from this kind of information. Seismic waves are like the X rays of the Earth. We can CAT-scan the Earth.

Here are the basic steps:

  1. Identification of a phase as a coherent arrival.

  2. Explanation of this phase in the context of an Earth model.

There are two basic ways to do this:

  1. Active source seismology, in which you create your own energy souce (an explosion or a weight drop, for example).

    Accelerated Weight Drop Source

  2. Passive source seismology, in which you rely on natural sources of energy (like earthquakes).

There are real advantages to using active sources, such as high resolution image production, but the energies involved only allow us to investigate crustal depth stucture (at most).

Details:

Changes in structure cause waves to bend and refract. Smooth variations cause bending, sharp variations cause new kinds of phases to be generated, including some reflections.

Reflected Wave

Critically Refracted Wave

Refracted Wave

Curved Ray in a Gradually increasing Velocity Medium

We start by constructing a one-dimensional model of the Earth. Main features are:

Crust, Mantle, Outer Core, Inner Core

Secondary Features:

Lithosphere, Low Velocity Zone (asthenosphere), Phase Transitions in Upper Mantle, Variations in Crustal thickness.

Details of the crust:

  1. The crust is thicker beneath continents (10-70 km) than it is beneath oceans (5-8 km).
  2. Vp in oceans is about 7.0 km/s and in continents is about 6.0 km/s. Composition: basalt vs. granite.
  3. Continental crust is complicated, oceanic crust is simple.

The boundary between the crust and mantle is called the Mohorovicic Discontinuity after the seismologist who first discovered it.

Details of the mantle:

  1. Vp in upper mantle about 8 km/s; should be ultramafic (no feldspar; lots of olivene and pyroxene).
  2. Velocity increases until about 125 -200 km depth, then reverses in the asthenosphere, where rocks flow easily and may be partially melted. The asthenosphere extends to depths of about 200 km, and may or may not exist beneath all continents.
  3. Phase changes at 400 and 670 km depth. Olivene -> Spinel -> Perovskite. Same composition; different structure.
  4. Composition is more or less homogeneous due to convection, but some lateral variations exist.
  5. We can determine anistropy from shear wave splitting.

Details of the core:

  1. We know the outer core is liquid because of the shadow zone for shear waves and strong refraction for P waves.
  2. Other phases show that the inner core is solid. Looking at changes in anistropy over time show that the inner core is rotating.
  3. The core is dense ( 10-13 gr/cc) and appears to be composed of Fe and Ni, along with some sulpher. Fe and Ni are consistent with wave velocities and with meteorites. Existence of magnetic field provides more evidence.
As we become more sophisticated in imaging, we produce 3D structures (laterally varying). Main features: