Download Earthquake intensity, elastic rebound theory and plate boundaries.

Earthquake
Science
(Seismology)
• Quick review of last lecture
(fault, epicentre, seismic waves,
magnitude)
• Earthquake Intensity
• Elastic-rebound theory
• Fault classification
• Earthquakes and plate boundaries
• Benioff-Wadati zone
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.
Seismic Waves 101
Fast
P waves
Primary (or compressional)
S waves
Secondary (or shear)
Love
waves
Slow
Rayleigh
waves
Body waves: travel
through Earth.
Surface waves: travel
around the surface of
Earth.
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 ~ 1000*6.6 - 1500 = 5100 km
... D ~ 5100/111 ~ 46 degrees
MS = log10(A) + 1.656log10(D) + 1.818
= 7.4
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)
Earthquake Intensity Map
A map of
earthquake
intensity is also
called an isoseismal
map.
New Madrid Earthquake sequence of 1811-1812
http://www.abag.ca.gov/bayarea/eqmaps/doc/mmi.html
http://www.abag.ca.gov/bayarea/eqmaps/doc/mmi.html
http://www.abag.ca.gov/bayarea/eqmaps/doc/mmi.html
http://www.abag.ca.gov/bayarea/eqmaps/doc/mmi.html
http://www.abag.ca.gov/bayarea/eqmaps/doc/mmi.html
Recently,
intensity
measurements
have been
carried out using
the web.
Internet
Intensity
Map
Produced using
web response
forms.
A map of
earthquake
intensity is also
called an isoseismal
map.
Parkfield, CA, 28 Sept., 2004 M = 6
Elastic Rebound Theory
Fault
Fence
Original position
Stress Accumulates
Earthquake: stress
released
Rebound to original
shape
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
Elastic Rebound Theory:
The Earthquake Cycle
For an active fault,
earthquakes will
repeat periodically.
1460
1855
Turakirae Head, New Zealand
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 ...
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
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
Normal fault
• Occurs where
plates are pulling
apart under
tension
• Divergent plate
boundary
• Example: Iceland
Strike-slip fault
• Occurs where
plates are sliding
past each other
• Transform plate
boundary
• Example: San
Andreas Fault,
California
Thrust (reverse)
faults
• Occurs where
plates are pushing
together under
compression
• Convergent plate
boundary
• Example: Japan
Benioff-Wadati Zone
• Dipping zone of
earthquakes
• Occurs at a
convergent plate
boundary