Seismology and the Earth’s Interior (18)

Presented By: Eric Zhou & Usman Hanif

What is “seismology”?● Seismology: Study of earthquakes and seismic waves; analyzing the relationship between both

natural phenomenons.

“Earthquake belts outline plate boundaries, the zones along which plates collide, diverge, or slide past one another.”

How is seismology studied?

Seismographs● Seismographs: an instrument that measures and records details of earthquakes, such

as force and duration; “a tool for peering into inaccessible regions”

Modern seismographs can detect ground displacements as small as 10-8 cm- This sensitivity however can not be utilized on earth b/c factors such as wind, ocean

waves and machinery continually shake the earth. - Could work on the Moon though :)

What is an earthquake?● Earthquakes are associated with large fractures, or faults in Earth’s crust and upper

mantle.● Elastic Rebound Theory (H. F. Reid)

- Blocks move in opp. direction, yet the friction from the overlying rock locks them together at the fault.

- As the rock continues to be moved/strained elastic energy is built up.

- Once the elastic energy from strain is > than the frictional bond, the blocks can no longer hold and rupture at the focus.

- The offset of the two blocks from the rupture is called the “slip” (can be as large as 50ft)

“Faults” In The Theory

Simply Speaking: The pressure holding together the blocks is so great that the frictional bond is actually stronger than the rock itself. ● Hence, it is more likely the rock break than slip along the fault.

-- To fix the theory we need a means of “lubricating” the fault in order to reduce the locking pressure. Geologists are currently working on this.

The Catch

Earthquakes - How Big And How Many?● The time required to build up enough elastic strain energy in the rocks along a fault is

colossal compared with the time it takes to release that energy during an earthquake.○ Time between great earthquakes: 50 -100 years○ Time elapsed during earthquake: few minutes

X 10,000● During earthquake, 109 ergs of elastic strain energy is

released per cubic meter. If fault 1000 km long, 100 km deep and distorts surveyed lines as far as 50 km -- energy released is equivalent to 10,000 nuclear explosions the strength of Hiroshima

MATH FACT

Richter Magnitude Scale

● Logarithmic○ Based on seismic wave magnitude

● Examples:

○ 9/28 Indonesia: 7.5 - 800+ dead

○ Largest recorded: 9.5

○ In space: Magnetar quake - 23https://www.nasa.gov/content/goddard/nasas-fermi-satellite-finds-hints-of-starquakes-in-magnetar-storm

Earthquakes - Where Do They Occur?

● Plate boundaries ○ Where plates separate○ Where plates meet○ Stress and tension in general

● Within plates○ Rarely

● Research done before○ More seismic observatories○ Use of computers - nukes vs quakes

Locating The Epicenter ● S and P wave time difference

○ P and then S

● Distance associated with time○ 3 or more used

Obtaining Stress Patterns

● Different plane junctions: different fault orientations and slip directions

○ Divergence - tension■ Normal fault - overlying block moves down the dip

of the fault plane○ Convergence - compression

■ Thrust fault - overlying block moves up the dip of the fault plane

○ Shear - friction■ Slip lateral fault - blocks move against each other

● Seismograms tell which quake mechanism is involved - find initial motion, radiates outwards

Earthquake Destructiveness - Can It Be Controlled?

● Prediction and Evacuation

● Better engineering practices

“YES WE CAN”

Insert Picture

Types of Seismic Waves

P-Waves cause a crest of compression to move through the block matching the wave velocity. ● Particles vibrate back and forth in

the direction of the wave propagation.

Action: Squeezing and Unsqueezing

S-Waves cause the vertical planes to shake up and down with the plane velocity.● Particles vibrate back and forth at

right angles to the direction of propagation.

Action: Right angle waves*shearing disturbance to the rock

Shadow RegionShadow Zones suggest that the core is molten because compressional waves decrease sharply in velocity and shear waves don’t even penetrate. ● Fluid elasticity resist and recover from

compressional waves allowing for the passing of P-waves but do not resist shearing (S-waves don’t ‘pass’)

**Information regarding wave paths and their travel times have been determined through analyzing the seismographic records of earthquakes all over the world**

Finding Earth Models From Travel Times And Vibration Frequencies

● P, S, Surface Wave travel times and frequencies○ Depends on elasticity and density of materials traveled through

● Use data to figure out what inside of Earth is like○ Math and stuff used

High Pressure and Shock Experiments

● Hydraulic Press

● Explosives

● Density, pressure○ Shock velocity

Composition, Structure, and State Of The Interior

-- The major divisions of earth’s interior — crust, mantle, and core — were discovered from reflected and refracted P and S waves.

Mohorovičić discontinuity: boundary between the crust and the mantle ● Chemical boundary

Principle of Isostasy: relatively light continent float on more dense mantle-- Nature has contrived that large topographic loads such mountains and continents are ‘compensated’ by buoyancy.

Thanks!