Earthquake Science

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Earthquake Science (Seismology)
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• Quick review of last lecture (fault, epicentre,
  seismic waves, magnitude)
• Earthquake Intensity
• Elastic-rebound theory
• Fault classification
• Earthquakes and plate boundaries
• Benioff-Wadati zone

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Earthquake Terminology
Fault A surface across which two blocks can move
relative to each other.
Focus The location on a fault where earthquake
rupture initiates.
Hypocentre The calculated position of an
earthquake focus.
Epicentre That point on the Earth's surface
directly above the hypocentre.
Magnitude A measure of the strength of an
earthquake, as determined by seismographic
observations.
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Seismic Waves 101
P waves
Primary (or compressional)
S waves
Secondary (or shear)
Body waves travel through Earth.
Love waves
Rayleigh waves
Surface waves travel around the surface of Earth.
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Surface-wave magnitude formula (most common)
MS log10(A) 1.656log10(D) 1.818
A amplitude in microns (mm) D epicentral
distance in degrees (divide by 111 to go from km
to degrees)
672 mm
A 672 mm (measured) D 10006.6 - 1500 5100
km ... D 5100/111 46 degrees MS log10(A)
1.656log10(D) 1.818 7.4
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Intensity

• Intensity is a measure of the effects of an
  earthquake on the natural and built environment.
• While the magnitude of an earthquake is
  constant, the intensity changes with epicentral
  distance.
• In N.A., the Modified Mercalli intensity scale
  is used (I-XII)

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Earthquake Intensity Map
A map of earthquake intensity is also called an
isoseismal map.
New Madrid Earthquake sequence of 1811-1812
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http//www.abag.ca.gov/bayarea/eqmaps/doc/mmi.html
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http//www.abag.ca.gov/bayarea/eqmaps/doc/mmi.html
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http//www.abag.ca.gov/bayarea/eqmaps/doc/mmi.html
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http//www.abag.ca.gov/bayarea/eqmaps/doc/mmi.html
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http//www.abag.ca.gov/bayarea/eqmaps/doc/mmi.html
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Recently, intensity measurements have been
carried out using the web.
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Internet Intensity Map
A map of earthquake intensity is also called an
isoseismal map.
Produced using web response forms.
Parkfield, CA, 28 Sept., 2004 M 6
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Elastic Rebound Theory
Fault
Fence
Original position
Stress Accumulates
Earthquake stress released
Rebound to original shape
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Elastic Rebound Theory
1. Stress on a fault slowly accumulates 2. During
an earthquake, stress on the fault is released 3.
After an earthquake, stress begins to
re-accumulate
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Elastic Rebound Theory The Earthquake Cycle
For an active fault, earthquakes will repeat
periodically.
1460
1855
Turakirae Head, New Zealand
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Earthquakes and Plate Boundaries
More than 90 of earthquakes occur near plate
boundaries in fact the earthquake distribution
can be used to outline tectonic plates ...
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Earthquakes and Plate Boundaries
Although there are exceptions rare earthquakes
do occur in stable continental regions
New Madrid Earthquake sequence of 1811-1812
Seismic Intensity Map
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Three basic categories of faults
Normal fault upper block drops down
Thrust (reverse) fault upper block pushes up
Strike-slip fault blocks slide past each other
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Normal fault

• Occurs where plates are pulling apart under
  tension
• Divergent plate boundary
• Example Iceland

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Strike-slip fault

• Occurs where plates are sliding past each other
• Transform plate boundary
• Example San Andreas Fault, California

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Thrust (reverse) faults

• Occurs where plates are pushing together under
  compression
• Convergent plate boundary
• Example Japan

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Benioff-Wadati Zone

• Dipping zone of earthquakes
• Occurs at a convergent plate boundary