Download Chapter 8 Earthquake Notes

Do Now: Earthquake Questions
• How does an earthquake happen?
• What are 3 basic types of faults?
• What equipment is used to record/measure
earthquakes?
• What is the center of the earthquake called?
How do we locate it?
• What are the different types of quake waves?
What are their characteristics?
• Describe the scale used to measure quakes.
Fault Types:
Normal Fault –
- Pieces of crust are pulled apart
- Hanging Block slides down
Thrust Fault –
- Pieces are pushed together
- One block pushed up fault line
Strike-Slip Fault –
- Blocks grind next to each other
on same plane
Earthquakes
• Earthquake: shaking or trembling of the
ground caused by the sudden release of
energy, usually as a result of faulting,
which involves the displacement of rocks
along fractures
• Aftershocks: earthquakes from continued
adjustments along a fault; usually smaller
than the initial quake
Elastic Rebound Theory
• Rocks undergoing deformation bend and
store energy
• When strength of rock is exceeded, they
rupture and release energy – the
earthquake
• Rocks rebound to original, undeformed
shape
Fault
Fence
Original position
Rupture and release of energy
Deformation
Rocks rebound to original
undeformed shape Stepped Art
Fig. 8-1a, p. 151
Seismology
• Seismology: the study of earthquakes
• Seismic waves: energy from earthquakes
• Seismographs: detect, record, and
measure earthquakes
• Seismogram: record from a seismograph
• Earthquakes occur along faults, where
movement is stored as energy in rocks
• Most faults related to plate movements
Focus and Epicenter
•
•
•
•
Focus: point where energy is first released
Epicenter: point on surface above focus
Shallow-focus: 0-70 km below surface
Intermediate focus: 70-300 km below
surface
• Deep-focus: >300 km below surface
• 90% less than 100 km below surface
Earthquakes and
Plate Boundaries
• Divergent and transform boundaries:
always shallow-focus
• Convergent boundaries:
– shallow-, intermediate-, and deep-focus
– Beniorr-Wadati zones: foci along subducted
plate
Major Earthquake Regions
• Plate boundaries: convergent, divergent,
and transform
• 80% in Pacific Ring of Fire
• 15% in Mediterranean-Asian belt
• 5% in plate interiors and ocean spreading
ridges
• Intraplate: from compression of plate along
margins
Seismic Waves
• All waves generated by an earthquake
• Body waves
– P-waves
– S-waves
– Travel faster through less dense, more elastic
rocks
• Surface waves
– R-waves
– L-waves
P-Waves
•
•
•
•
Primary waves
Fastest seismic waves
Travel through solids, liquids, and gases
Compressional/push-pull: expand and
compress material, like sound waves
S-Waves
•
•
•
•
Secondary waves
Slower than P-waves
Travel only through solids
Shear waves: move material perpendicular
to direction of wave movement
• Create shear stresses
Undisturbed material
Surface
Undisturbed
material
Primary wave (P-wave)
Direction of wave movement
Wavelength
Secondary wave (S-wave)
Focus
Stepped Art
Fig. 8-7, p. 156
Surface Waves
• Travel at or just below the surface
• Slower than body waves
• R-waves (Rayleigh waves)
– Particles move in elliptical path, like water
waves
• L-waves (Love waves)
– Faster than R-waves
– Particles move back forth in horizontal plane
perpendicular to direction of travel
Earthquake Magnitude
• Quantitative measure: amount of energy
released
• Richter Magnitude Scale: total amount of
energy released at earthquake source
• Measure amplitude of largest seismic
wave
• Logarithmic: each whole-number increase
is a 10-fold increase in amplitude, but a
30-fold increase in energy
Earthquake Magnitude, cont.
• Richter Magnitude Scale underestimates energy
of very large quakes
– Only measures peak energy, not duration
• Seismic-moment magnitude scale:
– Strength of rocks
– Area of fault rupture
– Amount of movement of rocks adjacent to fault
• 1964 Alaska earthquake:
– 8.6 Richter
– 9.2 seismic-moment