LECTURE 16
SEISMOGRAPHS and THE EARTH'S INTERIOR
As stated in the last lecture, seismographs are based on the theory of pendulums where the shaking of the Earth recorded has to have a lower periodicity than the pendulum or spring of the seismograph. The truly long period seismographs are all based on the slow swinging of the horizontal (not quite, of course) pendulum.
It is interesting to think a little about the limitations of a seismograph. For example, imagine a vertical; seismograph with a horizontal P wave approaching it. Such a wave would be vibrating back and forth horizontally in the direction of propagation (movement) and therefore, would not be recordable on a vertical seismograph because such a seismograph only records vertical motion. It would partly record a horizontal S wave because such waves partly (i.e., not counting the horizontal components) vibrate vertically. It would not record anything from a vertical S wave (which, as you will see, does not exist on Earth) because such waves vibrate horizontally. It would, of course, record a vertical P wave (which does exist).
A horizontal seismograph's recordings would depend on its orientation. Such a seismograph pick up no motion from a vertical P wave and only some of the waves from S waves. Summarizing, a seismic station generally has three seismographs, one vertical and two horizontal at right angles to each other. That way, any earthquake wave from any direction can be recorded.
Experiments in labs reveal than two waves (P and S waves) penetrate through materials. What is more such experiments have determined the factors about material that determined the speeds of travel of these waves. It has been shown that only two factors determine the velocity of waves -- rigidity (a complex term, but strength or ease of deformation is good enough for us) and density. The mathematics isn't simple because (partly) the lines are curved. However, we can make simple generalizations (but see the illustration). Denser materials slow down the transmission of P and S waves. Increased rigidity, in contrast, speeds up the waves. We have the record underground that through the mantle P and S waves speed up (not linearly) all the way to the core. This is determined by the arrival times of waves as speeding up as the seismograph is farther and farther away from the earthquake. At about 104o from the earthquake (about 7200 miles) this increase suddenly (if 2 or 3 degrees is sudden) stops and the P wave velocity sharply decreases from about 8 miles per second to less than 5 miles per second. No S wave is reported more than 104o from an earthquake. This has an important meaning. S waves penetrate only solids which means the Mantle is a solid and (at least over the time of an earthquake vibration) while the deeper part of earth is a liquid. This core of the earth is about 1800 miles down from Earth's surface.
The core is clearly a liquid because it does not carry S waves. It is also dense; the P wave sharply slows down and this is partly explained by the loss of strength (liquid compared with solid) and by its increased density. Presumably, the core was made of an iron nickel mix similar to some meteorites. This would give the core a density of 10-12 times that of water. Recall we have an Earth with an average density of 5.5 and a light crust. The mantle and core have to make up this discrepancy.
The mantle has a density of about 3.5 near the crust and this increases downward to about 5.5 near the core. This density increase is caused by the increased pressure causing a series of changes to denser and denser minerals of the same composition. There may, however be chemical differences through the mantle.
Anyway the P wave that penetrates the core slows down enormously and this refracts the waves deep into the core so from about 104o (see above) to about 143o no P wave is found. For any earthquake this makes a belt around the Earth at the appropriate distances called the shadow zone. In this zone there would be no P or S waves but there would be L waves and SS or PP and other complexly reflected waves.
Inge Lehman was a Danish woman of the 1920's to 30's who discovered the 'inner core". This is inside the core and has a diameter of about 1560 miles and is at a depth of about 3180 miles. This core is solid and it is interesting to see how she discovered this fact.
First of all, recognize the problem. The way one determines if a substance below ground is a solid is that only solids transmit S waves and if an S wave penetrates, then the substance is solid. The inner core is surrounded by liquid, so how can one get an S wave there to make the test? Well Ms. Lehman was clever and persistent. She examined thousands of records (and this was before computers) and she discovered that there was a persistent late arriving P wave for earthquake waves that passed all the way through the Earth. After the first P wave the seismograph shakes a little more then ordinary microseisms and this second P wave was often masked by this noise, but her persistence paid off because she eventually saw the pattern. Her interpretation follows.
The P wave penetrates the outer core (unlike the S wave) and eventually hits the inner core. It then goes through the inner core a little faster then through the outer core because the inner core is solid and stronger (not yet known by her or anyone else). As a first act in hitting the inner core the P wave is sort of like a hammer and so it creates an S wave (albeit weak) there. This S wave persists through the inner core because the inner core is solid. It, being an S wave, falls behind it creating P wave. As it leaves the inner core it dies because it is entering a liquid. However as a last act it behaves as a hammer (just like its creating P wave) and makes a P wave (very weak) in the liquid. This P wave now travels just as fast as its stronger original creator and comes to surface at a seismograph and makes the weak second P wave.
Since Lehman's original discovery, other supporting things have come to light. For example, the P wave is faster there and that suggests greater strength (specifally rigidity). In addition. there are recognizable reflections from the inner core identifying its size.
Reflections from the base of the crust identified a surface seperating the crust and the mantle. This is called the Mohorovicic discontinuity after its Yugoslav discoverer (1909). By the way I add, it wasn't Yugoslavia in 1909 and isn't now. This used to be thought to be an important contact, but now in plate tectonics we talk about the crustal lithosphere the moves on materials in the upper (but not top) mantle.
This layer, the Aesthenosphere, was named by Barrel in 1914. Its importance, however, was not recognized by Wegener or others until late in the 1960's. The Aesthenosphere (50 - 150 miles below Earth's surface) is recognized by the fact that seismic wave velocities are diminished there. In effect the general increase in velocities through the mantle is diminsihed there. Wave velocities diminish because of decreased strength (rigidity) and consequently this slow zone is explained by diminishing strength. Generally this depth is thought to be partially melted. This weak zone is the zone upon which the crustal lithosphere (crust and adjoined uppermost mantle) drift during palte tectonics.
Last modified July 17,1997