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Earthquakes
What Are Faults
& What Causes Them??
• When subjected to stress (forces acting on rock) rocks will strain
(deform).
• Three different stresses(forces):
• Compression: Force that pushes masses together (squeezes)
• Tension: Force that pulls rocks apart
• Shear: Force that have rocks slide past each other
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When enough stress (force) accumulates rocks can deform no longer.
The rocks will then fracture along surfaces called faults.
Faults: cracks in rock where movement occurs
The vibrations produced by faulting are called earthquakes.
They have reached their elastic limit.
Stress vs. Strain
Elastic Rebound
• Elastic energy stores up (i.e. stretched rubber band) due to
stresses & friction btwn plates
• When force exceeds friction, snap!!! It returns to
original shape
Faults
• Normal: caused by tension (divergence), one block moves
down relative to the other
• Reverse: caused by compression (convergence), one block
moves up relative to the other
• Strike-slip: caused by shear stress (transform), no vertical
movement. Blocks slide past each other
Seismic Waves
• Energy released by an earthquake is
released in seismic waves.
• 3 main types
• P-waves (primary waves)
• S-waves (secondary waves)
• L-waves (surface waves)
Seismic Waves
• P-waves
• Particles that move back and forth in the same direction as the wave.
• Compressional waves (similar to sound waves).
• Fastest moving
• S-waves
• Particles move at right angles to the direction of the wave. (transverse
waves)
• Moderate speed
• L-waves
• Surface waves roll around along the surface, like ocean waves
• Do most of the damage during an earthquake.
• Slowest moving waves
Types of Waves
Examples of Earthquakes
Alaska – 1964
1. Subduction boundary Pacific under North
American plate
2. Severe due to triggering of
other breaks
Result –
10,000 after shocks,
tsunamis (600 km/hr and
21 m high)
New Madrid (1811 & 1812)
• Location –
• Missouri
Size –
• 8.6, 8.4, 8.7
• Changed course of
Mississippi River
• 2 new waterfalls
Significance! –
• Not at a plate boundary
• Cause: 3 faults deeply buried
by Mississippi sediments
• Class: mid-plate earthquakes
(not many)
Earthquakes and Earth’s Interior
Seismology
• Seismology: The study of earthquakes
• Seismograph: instrument to measure earthquakes
• Seismologists: scientist that study earthquakes
Seismometer/Seismograph
Locating an Earthquake
Locating the epicenter
- Triangulate between 3 or more seismographs
Terminology
• 1. Focus –
• Where earthquakes
actually occur below the
surface
• 2. Epicenter –
• Point on earth’s surface
directly above focus
Focus & Epicenter
• Focus: The point under the surface where the earthquake
actually occurs.
• Epicenter: The point on the surface directly above the
focus
Earth’s Structure
Seismic waves allow us to “look”
at Earth’s interior
• P-Waves: can travel through liquids & solids
• S-waves: can travel through solids only
• Waves changes speed & direction when going through
different materials
• These changes indicate a different layer of Earth
Earth’s Structure
• P-waves are received on the other side of Earth
• Due to p-waves ability to travel through all materials
• S-waves don’t travel through the core
• They can not pass through liquids
• Thus there is an absence of them on the opposite side of
the Earth (s-wave shadow zone)
• This tells us the outer core is liquid
Earth’s Structure
Earth’s Structure