THE DYNAMIC EARTH (continued) Structure of the Earth from Seismic Studies
Seismic energy waves can provide a picture of the interior of the earth
> it is like an "x-ray" of the earth's interior
We need to look at the behaviour of seismic waves
> like any energy wave, seismic waves travel at different velocities through different
materials
>> look back at the factors that affect seismic velocity: the velocity is inversely
proportional to the density of the material, and directly proportional to the
elasticity of the material
> when seismic energy waves pass from one material into another, in which there is a
change in seismic velocity, there is REFLECTION and REFRACTION at the boundary
between the two materials
Consider REFLECTION for a minute:
> the depth to a boundary can be determined if we know the velocity of the energy
transmission in the medium and the time for the energy to go down and bounce back up
>> for example, to determine the depth of the ocean, we can bounce sound waves
(SONAR) off the ocean floor. We can measure the velocity of sound in water, and
we can measure the time for the sound to go down and back up again. This gives
us the depth of the ocean water (i.e. the distance down to the boundary between
water and rock).
BUT!! Consider the problem of determining the depth to a boundary inside the earth using
seismic waves:
> the problem there is that we DON'T KNOW ahead of time the velocity of the seismic
waves at various depths in the earth
>> after an earthquake, all we can measure are the TRAVEL TIMES (how long it
took for the energy to reach us); we don't know what the velocities are
>>> (remember: all the seismograph record at a seismic station provides
for you are the times of the P, S and Surface Wave arrivals)
SO!!!!!!
A critical point is that, to use seismic energy waves to figure out what the earth is like inside, we
make a MODEL of the earth in which we model two things: (1) the distribution of material in the
earth, and (2) the seismic velocities in those materials. Then we receive the seismic energy
waves after an earthquake (or a nuclear test blast), and see if the ARRIVAL TIMES of the
energy at all of our seismic stations matches what we predict from our model. (In other words,
we enter all of the parameters of our model into a computer, and then create a "computer"
earthquake to see if the arrival times in our computer simulation match what we found in nature
for the real earthquake
(Note: for this to be successful, we do have to know where and when the earthquake took place,
as precisely as possible. That is why nuclear test blasts (time and place well known) are great
ways of testing our computer model.)
This process of creating a MODEL and then testing the ARRIVAL TIMES against is is called
INVERSE THEORY MODELLING.
>to be successful at INVERSE THEORY MODELLING, we need to have:
1. A LOT of seismic stations to reduce our uncertainty in the correctness of our model;
2. Good knowledge of the TIME of the earthquake or person-made blast;
3. Prodigious COMPUTER POWER to carry out the matching of the model
The model can try to match both the reflection AND the refraction of seismic energy waves . So,
let us think a bit about REFRACTION:
> when seismic energy waves pass from a medium with a higher seismic velocity into
one with a lower seismic velocity, the waves are refracted TOWARDS the normal
> when seismic energy waves pass from a medium with a lower seismic velocity into one
with a higher seismic velocity, the waves are refracted AWAY from the normal
Let's try a few EARTH MODELS, where each successive model is thought to be a better
representation of the true earth:
Model 1: an earth with a uniform density and elasticity throughout.
> in such a case, we can readily predict how long it is after a quake until the first P or S
energy reaches any seismometer, since the velocity of the waves don't change as they go
through the earth
>> BUT the model isn't very good, because we found that P and S energy after an
earthquake was arriving at most stations sooner than expected. This led to Model 2.
Model 2: an earth that has a steady increase in seismic velocity from the surface to the
center
>> this would result in outward-curved paths for the waves due to the refraction away
from the normal as the energy passes from material with lower velocity into material with
higher velocity, until it starts coming back up, at which point the refraction is towards the
normal (see diagram in the book). This was a better model than Model 1, but it turned out
not to explain everything, as there were seismic stations that, after an earthquake, didn't
receive any P- or S-wave arrivals. That led to Model 3.
Model 3: An earth with a LIQUID CORE starting at a depth of about 3000 km.
>> this model was needed, because no S-waves were being received between 105o , on
one side, and 105o , on the other side, right opposite the earthquake on the far side of the
earth (see diagram in textbook). This region of no S-wave arrivals is known as the SWAVE SHADOW ZONE.
>>> since S-waves can not pass through a liquid, it seemed reasonable to model the
CORE as a liquid that blocked the S-waves
>> opposite the earthquakes on the other side of the earth, there was found a girdle (
between 105o and 140o ) where no P-waves arrived (P-WAVE SHADOW ZONE)
>>> this was best explained by a CORE acting to refract the P-waves as they came
through. Furthermore, the P-waves that DID arrive exactly opposite the earthquake came
a little later than expected; this indicated that the seismic velocity in the CORE was lower
than that in the outer part of the earth (called the MANTLE), which would be reasonable
if the CORE were a liquid (i.e. low elasticity and high density).
Model 4: however, it was found that the P-wave arrivals opposite an earthquake,
although a little late in coming, didn't come as late as they would have for a uniform
CORE; also, there were reflections off of a boundary inside the core.
>> this led to Model 4 that argued for a solid INNER CORE at a depth of about 5000 km
So, by Model 4 we had a pretty good picture of the deep interior of the earth, consisting
of a solid inner core with a liquid outer core, surrounded by a solid mantle.
BUT WHAT ABOUT THE OUTER PART (the upper 700 km) OF THE EARTH?
Model 5: Mohorovicic determined that a better model for the earth required there to be a
thin outer part of the earth (called the CRUST) with seismic velocities markedly less than
in the MANTLE
> this became clear when it was found that, at a few hundred kilometers from a
quake, P waves were arriving sooner than expected for an outer earth with crustal
velocities
>> waves were taking a fast route by going down into the mantle and
zipping along at higher velocities, and then coming back up
>> by studying the arrival times, it is possible to determine the depth to
the CRUST-MANTLE boundary
>>> there were two types of crust found:
1. The CONTINENTAL CRUST with average thickness of ca. 30 km (but
varying from 20 up to 70 km thick, with the thickest part being where the
continental mountain ranges are)
2. The OCEANIC CRUST with a uniform thickness of ca. 5 km (and
covered by seawater to an average depth of 5 km)
>>>> the P-wave seismic velocities are:
Continental crust: 6 km/sec
Oceanic crust: 6.5 to 7 km/sec
Mantle just below the crust: 8 km/sec
Model 6: Finally, there was a refinement of Model 5 to explain the fact that seismic
waves going through the upper mantle were arriving a little bit later than expected
> thus, Model 6 argued that there was a region of the upper mantle between ca.
100 km depth and 400 km depth where the seismic waves were slowed up a little
bit (dropping from 8 km/sec down to ca. 7.7 km/sec)
>> this became known as the LOW VELOCITY ZONE
>>> it was realized that the explanation for this was that the rocks
in the region of the low velocity zone were partially melted (90%
solid, with 10% liquid trapped between the mineral grains), which
lowered the rock's elasticity and hence lowered the seismic
velocity
>>>> the reason for this partial melting was that the
temperature gradient in the earth passed into the
temperature range needed to partially melt rocks at that
depth (see diagram in the textbook)
> thus, in Model 6 a new boundary was defined: the boundary between the outer 100 km
of the earth that is brittle (called the LITHOSPHERE), and the more plastic part of the
mantle below 100 km (extending from 100 km down to about 400 km) called the
ASTHENOSPHERE (see the diagram in the textbook)
>>> the LITHOSPHERE includes the outer part of the mantle (above 100
km deep) AND the crust (both continental and oceanic)
>>> the difference between the mantle in the lithosphere and the mantle in
the asthenosphere is just one of physical state (the former is brittle; the
latter is plastic); the mantle in both places is the same composition
(NOTE: the LITHOSPHERE and the ASTHENOSPHERE become very important when
we discuss the theory of PLATE TECTONICS)
Last revision: 4 October 2000
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