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Earthquakes
What Are Faults?
• When subjected to stress (either tension or
compression) rocks will deform (bend).
• When enough force accumulates rocks can
deform no longer.
• The rocks will then fracture along surfaces
called faults.
• They have reached their elastic limit.
What Causes Them?
• Three different stresses (forces):
• Compression
• Tension
• Shear
• The vibrations produced by faulting are
called earthquakes.
Compression
Force that pushes masses together (squeezes)
Tension
Force that pulls rocks apart.
Shear
Force that has rocks slide past each other.
Elastic Rebound
Normal Fault
• Caused by tension are pulled apart.
• One slides passed the other
Reverse Fault
• Caused by compression
• One block is pushed over another block.
Strike-Slip Fault
• Caused by shear forces
• Slide passed one another
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).
• S-waves
• Particles move at right angles to the direction of the
wave. (transverse waves)
Surface Waves
• Surface waves roll around along the surface
• Do most of the damage during an
earthquake.
Types of Waves
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
• Due to the study of seismic waves,
scientists know:
• Earth has a solid inner core
• Liquid outer core
• Mantle
• Crust
Moho Discontinuity
• The Moho discontinuity is the boundary
between the crust and the mantle.
• Seismic waves speed up here (upper mantle
lower lithosphere) because the material is
more dense.
Seismology
• The study of earthquakes is the field of
seismology.
• The instrument to measure earthquakes is a
seismometer and the instrument that records
these earthquakes is the seismograph.
• The scientists that study these are
seismologists.
Magnitude
• The magnitude of an earthquake is
measured by the height of the line traced on
a seismograph.
• The Richter Scale is a scale that measures
the energy released by an earthquake.
• An increase of 1.0 is an increase of intensity
of 32x.