Chapter 5: EARTHQUAKES
&EARTH’S INTERIOR
Earthquakes & earthquake hazards
• Earthquake
– Sudden release of energy
• Seismology
– Scientific study of earthquakes
& seismic waves
• Seismic waves
– Shock waves traveling out in all
directions from earthquake’s
source
Earthquake
Hazards &
Predictions
• Primary hazard – ground
motion
– Collapsing buildings,
bridges & other structures
– Aftershocks
Earthquake Hazards
&Predictions
• Secondary hazards
–
–
–
–
Landslides
Fires
Ground liquefaction
Tsunamis
Earthquake Hazards
&Predictions
Earthquake Hazards
&Predictions
• Recent Tsunami
– 2004 Sumatra
– 2011 Japan
– Magnitude 9.0
– Magnitude 9.1
– ~25 ft.
– ~ 30 ft
– 225,000 casualties – 15,000 casualties
Earthquake
prediction
• Short-term prediction &
early warning
– Precursor phenomena
– Foreshocks
• Long-term
forecasting
• Paleoseismology
– Study of prehistoric
earthquakes
Earthquake
Readiness
• Mercalli Intensity Scale
– Based on observations & damage
to structures
– Subjective
The Science of Seismology
• Seismograph
– Instrument to detect & measure vibrations of Earth’s surface
• Seismogram
– Record made by a seismograph
Seismic waves
• Body wave
– Travels through Earth’s interior
• Surface wave
– Travels along Earth’s surface
• Focus
– Where rupture
occurs & earthquake’s
energy is first released
• Epicenter
– Point on Earth’s surface
directly above
earthquake’s focus
Locating earthquakes
• P (Primary) or Compressional waves:
– Alternating pulses of compression & expansion
– Pass through all states of
matter
• S (Secondary) or Shear
waves:
– Side to side or up &down
movement
– Pass only through solids
– Travel slower than P waves
Locating
earthquakes
• Surface or L (Love) wave:
– Pass around Earth, not
through it
– Cause most ground shaking
& damage
– Slower than P & S waves
Locating earthquakes
• Finding the epicenter
– S – P lag time
– Triangulation (3 points needed)
Measuring
Earthquakes
• Richter Magnitude Scale
– Magnitude vs. Intensity
– Based on amplitudes of seismic
waves shown on seismograph
– Logarithmic scale
• 10 fold increase in amplitude =
1 unit increase
• 32-fold increase in energy =
1 unit increase
Measuring Earthquakes
• Moment Magnitude Scale
– Based on rupture size, rock properties, & amount of displacement
along fault surface
– Used for large earthquakes
Studying Earth’s Interior
• Seismic
discontinuity
– Boundary inside
Earth where seismic
wave velocities
change abruptly
Studying Earth’s Interior
• 3 things can happen to seismic waves if they meet a boundary:
– Refraction: waves bend as they pass from one material to another
– Reflection: some or all of wave energy bounces back
– Absorption: some or all of wave energy is blocked
How geologists look
into Earth’s interior
• Direct observation
– Drilling
– Xenoliths
How geologists look
into Earth’s interior
• Indirect observation
– Magnetism
– Astronomical observations
• Density
– Meteorites
A Multilayered Planet
• Crust
– Outermost compositional layer of solid Earth, part of
lithosphere
• Lithosphere = brittle
& solid
– Thickness varies
• 8 km (oceanic)
• 45 km (continental)
– Moho boundary
A Multilayered Planet
• Mantle
– Middle compositional layer of Earth, between core & crust
– Asthenosphere: mantle where rock is near melting
• Plastic & weak
– Mesosphere
– Mantle-core
boundary
A Multilayered Planet
• Core
– Innermost layer
• Where magnetic field is generated &
most geothermal energy resides
– Outer core (liquid)
– Inner core
(solid)