1. Earth’s Structure

2. What's in the center of the earth?
The first thing to remember is that NOBODY has ever been there, so what you are about to hear is barely past the Wild and Crazy Idea stage.
What we think we know comes from a study of how earthquake (seismic) waves travel through the earth, and how long it takes for them to get from where the earthquake happens to a recording station.
The basic idea is that different materials transmit seismic waves at different speeds.
With a lot of earthquakes and a lot of recording stations, geophysicists are beginning to get a pretty detailed picture of what is probably down there.
What's in the center of the earth?

3. What's in the center of the earth?

4. What's in the center of the earth?
One of the most distinctive features of the earth's interior is how it seems to be layered by density, with the heaviest stuff in the center, and the lightest material at the surface.
In fact, the earth probably looks a lot like a hard boiled egg if you could cut it open. The yellow stuff in the center (the yolk) relates to what we call the core.
Most geophysicists think that the core is composed of high density materials like iron and nickel.
The egg's shell is like the earth's crust - a thin veneer of rigid, low density material at the surface.
And all the white stuff in between is like the earth's mantle the largest layer which, in the case of the earth, is of medium density, and, in the case of an egg, tastes best with a bit of salt and pepper.

5. What's in the center of the earth?
The core seems to be in two parts - a "solid" inner core with a "liquid" outer layer - and is the final resting place for as much of the high density material as can get there.
The crust is REAL thin relative to the size of the earth - much, much thinner than an eggshell, and is of much lower density than the core.
It is probable that the mantle represents the vast majority of the earth's mass which is still trying to figure out if it is heavy enough to be accepted at the core, or is lower in density and therefore has to float about on the surface with the rest of the scum.

6. What is an Earthquake?
Instrumentally recorded (or felt) ground shaking, normally a result of underground movement on a fault
An earthquake is caused by sudden movement on a sub-surface fault. The energy which is released (which was stored as strain in the rock) is converted to seismic waves which radiate from the earthquake focus. These seismic waves cause ground shaking and can be measured using seismometers.
Seismogram of the 1906 earthquake recorded in Germany
San Francisco 1906 (USGS)

7. Faulting Seismic waves USGS
This session focuses on the seismic waves that result from an earthquake.
Seismic waves
USGS

9. Seismic Waves
The source of an earthquake is called the focus and the epicenter is the point on Earth’s surface directly above the focus. Seismic waves originate at the focus and travel outward in all directions These energy waves are classified as;
1) Body Waves:
2) Surface Waves:
Primary Wave
Love Wave
Secondary Wave
Rayleigh Wave
Two categories of seismic wave exist – body waves (which travel through the entire volume of the Earth) and surface waves (which are restricted to the near-surface). Each category consists of several types of seismic wave which all propagate through the earth with different velocity, amplitude, and particle motion.
Reference: Pages

10. Types of Seismic Wave
Two categories of seismic wave exist – body waves (which travel through the entire volume of the Earth) and surface waves (which are restricted to the near-surface). Each category consists of several types of seismic wave which all propagate through the earth with different velocity, amplitude, and particle motion.
Three-components of a seismometer record proportional to ground velocity of the P and S waves from a local aftershock of the Killari-Latur EQ, India (1993), at a hypocentral distance of 5.3 km
P. Bormann New Manual of Seismological Observatory Practice (NMSOP)

11. Body Waves Two types of body waves:
Originate from the focus and travel in all directions through the body of the Earth.
Two types of body waves:

12. Body Waves 1) Primary Wave (P-Wave)
P-waves move by compressing and expanding (push-pull motion) the material as it travels. Much like sound waves.
These waves can pass through solids, liquids, and gases.
Vibrate in the same direction as wave motion.
These waves have the greatest velocity (6 km/sec) and are the first to reach the seismograph stations.
Compression
Rarefaction
Vibration Direction

13. Body Waves 2) Secondary Wave (S-Wave)
S-waves travel through material by shearing it or changing its shape in the direction perpendicular to the direction of travel.
Because liquids and gases have no shape, these waves do not pass through liquids only trough solids.
These waves are much like the waves on the ocean.
These waves travel through Earth slower (3.5 km/sec.) and are the second to reach seismograph stations.
Vibration Direction

14. Two types of surface waves:
Surface waves differ from body waves in that they do not travel through Earth, but instead travel along paths nearly parallel to the surface of Earth.
Surface waves behave like S-waves in that they cause up and down and side to side movement as they pass, but they travel slower than S-waves.
Two types of surface waves:

15. Surface Waves 1) Love Wave
Surface waves that cause horizontal shearing of the ground. They move in much the same way as a snake slithering across the ground.
Vibrate in a perpendicular direction compared to that of wave motion.
Surface waves are the most destructive and cause the most damage.
Direction of Motion

16. Surface Waves 2) Rayleigh Wave
Surface waves that cause both horizontal (side-to-side) and vertical (up and down) movement within the ground.
Vibrate in a rolling motion in the same direction as wave motion.
Most of the shaking felt from an earthquake is due to these waves and these waves are the most destructive and cause the most damage.
Direction of Motion

17. Seismograph
An instrument used to record seismic waves and the resulting graph that shows the vibrations is called a seismogram.
The seismograph must be able to move with the vibrations, yet part of it must remain nearly stationary.
Heavy Mass
This is accomplished by isolating the recording device (like a pen) from the rest of the Earth using the principal of inertia.
Pen
Earth Motion due to Earthquake
For example, if the pen is attached to a large mass suspended by a spring, the spring and the large mass move less than the paper which is attached to the Earth, and on which the record of the vibrations is made.
Paper Record

18. Seismograph and Seismogram
P-waves, S-waves, and Surface waves are all recorded on the seismogram as seen below:
These paper records are important when seismologist wants to locate the position of the epicenter of an earthquake.
Seismologist can determine the difference in arrival times between the P-wave and the S-wave.

19. Sample Problem Answer:
Contrast the characteristics of Primary and Secondary waves.
Answer:
P - wave
Push - pull waves which vibrate in the same direction in which they move. Fastest earthquake wave and is the first to arrive at seismograph stations. Pass through all states of matter, solids, liquids, and gases.
S - wave
Shake the particles which cause them to vibrate in a perpendicular direction to their motion. Slower than P - wave and is the second earthquake wave to arrive at seismic stations. Pass only through solids.

20. The Moho Andrija Mohorovicic (1857-1936)
Found seismic discontinuity at 30 km depth in the Kupa Valley (Croatia).
Mohorovicic discontinuity or ‘Moho’　
Boundary between crust and mantle
The Moho is a good example of a seismic discontinuity across which seismic velocity is different. Andrija Mohorovicic found a very sharp change in velocity within the earth in Croatia, which was later shown to occur everywhere at varying depths (average of about 40 km under the continents and ~7 km below the seabed under the oceans). The velocity change is interpreted to happen at the boundary between crustal material and mantle material, and is known as the Moho after Mohorovicic.
The Moho
The Moho
Copywrite Tasa Graphic Arts

21. Structure in the Earth results in complicated paths
Lowrie, 1997, fig 3.69
The Earth has a complicated structure. The changes in velocity with depth result in refraction and reflection of the seismic energy. So the ray paths through the Earth are complicated.
USGS
Bolt, 2004, fig 6.3

22. Propagation of Seismic Waves In the Earth; M. Wysession

23. Courtesy R. Mereu Courtesy J. Mori
Ray paths through the mantle are relatively simple. There are a few changes associated with the 410 and 660 km discontinuities, but nothing compared to the deeper earth.
Courtesy R. Mereu
Courtesy J. Mori

24. Because the outer core is a liquid, S waves do not propagate through it (this is because a liquid cannot support shearing) and P waves are slower. So the ray that enters the inner core bends away from the surface, deeper into the earth.
Courtesy J. Mori

25. Forward Branch Backward Branch Courtesy J. Mori
Steeper incident rays are less affected and so we end up with deeper ray paths arriving nearer the source.
Courtesy J. Mori

26. Forward Branch Backward Branch Courtesy J. Mori Shadow Zone
We then have another short forward branch. However, no direct P energy arrives in a zone ~105-~143 degrees from the source – the shadow zone because of the refractive properties of the mantle-core boundary.
Courtesy J. Mori

27. ・ 1912 Gutenberg observed shadow zone 105o to 143o
PcP
・ 1912 Gutenberg observed shadow zone 105o to 143o
・ 1939 Jeffreys fixed depth of core at 2898 km
(using PcP)
Backward
Branch
Forward
Branch
PKP
Forward
Branch
Shadow
Zone
PcP
Shadow
Zone P
Forward Branch
Backward Branch
Forward
Branch
A time-distance plot through the mantle shows this complexity.
Courtesy J. Mori