1. Earthquakes
And Earth’s Structure

2. Earth’s Structure
View of Earth from moon. We know what the surface of the Earth looks like. What does the inside look like?

3. Origin of the Earth Meteors and Asteroids bombarded the Earth
Gravitational compression
Our early Earth grew from a barrage of extraterrestrial impacts, increasing in mass over time. In the early stages of planetary accretion, the Earth was much less compact that it is today. The accretionary process led to an even greater gravitational attraction. Gravitational energy converted into heat. Radioactive elements are unstable, and over time become more stable. Radioactive decay processes releases heat. U-238 to Pb Initial accretion led to a homogeneous sphere that eventually became density stratified with the heavier iron and nickel sinking to the center of the earth, and the lighter silicates (rocky) floated to the top.
Density Stratified planet

4. Earth’s Interior Core Mantle Crust dense Iron and Nickel
Inner Core - solid
Outer Core - liquid
Mantle
Less dense than core
Iron and Magnesium silicates
Mostly solid
Upper mantle is partially molten
Core – composed of two parts. The core is very dense and composed mostly of Iron and Nickel. The inner core is solid. This is because there is so much pressure exerted that it can only exist in the solid form. The outer core is liquid iron and nickel. Again, it is very dense but because it is in its liquid form it is less dense than solid and forms the outer core.
Mantle is composed of Iron and Magnesium silicates (Si + O). It is mostly solid, but the uppermost part of the mantle is partially molten (slush – where you do have liquid and solid mixed). Now, this is very important in plate tectonics.
Crust –This is the outermost layer. It is very thin and rigid. Rigid means that it is a solid. If you can imagine an apple. It would have the thickness of the skin of an apple. There is two types of crust – continental crust and oceanic crust.
Important notes:
Thin crust + uppermost mantle is rigid. This forms the “lithospheric plates”.
partially molten mantle acts as a lubricant. This enables the plates to move freely along the surface of the Earth.
Crust
Outermost layer
Very thin and rigid
Continental – granite
Density = 2.8 g/cm3
Oceanic – basalt
Density = 3.0 g/cm3

5. Evidence of Internal Structure
Density
calculate density of Earth
Speculate on probable compositions
Basalt
Granite
Meteorites
Use composition and age to determine composition and age of Earth
By calculation, the Earth’s density is 5.8 gm/cm3. In the Sierras, composed mostly of granite, represents the thin part of Earth’s interior, i.e. skin of an apple. If you calculate the Earth’s surface using granite, it comes to 2.8 gm/cm3. What is this telling us? There are heavier rocks in the Earth. We know the age of the earth is 4.6 Ga. People who study meteorites determined their age to be about 4.5 Ga – same as Earth. Meteorites are composed of Iron and Nickel. It is hypothesized the Earth’s interior is composed of the same elements. P-waves travel 4x faster than S-waves and also travel through different material. These also give us some clue on the properties of the Earth’s interior.
Seismic waves
Travel times and direction give indication of internal structure of Earth

6. Types of Seismic Waves P waves S waves Primary waves
Push and pull movement
Travel fastest (~ 6 km/sec)
Travel thru solids and liquids
S waves
Secondary waves
Move side-to-side
Slower (~ 4 km/sec)
Travel thru solids only

7. Seismic Waves Through Earth
Scientists use waves generated by earthquakes to determine the Earth’s interior. that the outer core of the earth is liquid.
Earthquakes generate P-waves and S-waves within the earth. Shadows occur on the opposite side of the earth from the earthquake epicenter because the outer core reflects S-waves, and bends P-waves. S-waves are reflected because they cannot travel through liquids, and they cast a larger shadow than the bent P-waves. Geologists and seismologists determined the size of the outer core by using the 154-degree arc of the S-wave shadow and measurements taken on the surface of the earth.

8. Parts of an Earthquake Focus Epicenter
place within earth where EQ originate
Epicenter
Location on surface directly above focus
Energy radiates in all directions from the focus
Energy is in the form of waves – “seismic waves”

9. Seismic Waves Locating the Epicenter
Seismograph
Instruments around the world record EQ
Records Earth movement by stationary mass on rotating drum
Seismogram
Locate an epicenter
Determine magnitude

10. Seismogram Help determine the epicenter
Measure the distance between P- and S-waves
This is the time difference in arrival times

11. Locating the Epicenter
Plot the time difference on y-axis (time interval)
Trace plot to blue line to determine distance to epicenter

12. Locating an Epicenter Triangulation
Measure that distance around the seismic station
The epicenter may be located anywhere on that line
A minimum of 3 seismic stations are needed to determine the epicenter

13. Earthquake Strength Richter Scale
A measure of the strength of an earthquake or the amount of energy released

14. Determining Magnitude
Measure amplitude of the strongest wave
Amplitude is the height on paper
Plot distance between p- and s-wave
Plot amplitude
Connect plots to determine magnitude

15. Earthquake Intensity Modified Mercalli Scale
Effects of an EQ vary from place to place
There is only 1 magnitude, but intensity can vary
Subjective scale

16. Intensity Map – Loma Prieta 1989

17. Oakland, 1989
The Yanks and Mets are currently playing in the fisrt subway World Series since However, 11 years ago, the bay-side cities of Oakland and San Francisco played in a World Series shocker (sorry). The 7.1 magnitude Loma Prieta earthquake occurred on October 17, This powerful quake, the strongest in San Francisco since 1906, struck just before the third game of the series. Across the bay area, the quake was responsible for 62 deaths, and damage estimates neared the 5 billion dollar mark. A number of lives were lost when elevated highways, such as the Cypress viaduct of Interstate HIghway 880 in Oakland (pictured above), collapsed or "pancaked." H. G. Wilshire of the U.S.G.S took this remarkable photograph. In addition, the shaker triggered a 4 foot tsunami in Monterey Bay. Parts of San Francisco were dark for three days, and the World Series didn't resume until October 27. For the record, the Oakland As swept the San Francisco Giants in the 89' Series, 4 games to 0.
The part of the Cypress freeway structure in Oakland, California, that stood on soft mud (dashed red line) collapsed in the 1989 Loma Prieta earthquake, killing 42 people. Adjacent parts of the structure (solid red) that were built on firmer ground remained standing. Seismograms (upper right) show that the shaking was especially severe in the soft mud. (Photograph by Lloyd S. Cluff)
Oakland is farther from epicenter, but scored a 9 on Intensity Scale

18. You are now a Seismologist!!
Virtual Earthquake Introduction:
Virtual Earthquake: