Global seismicity
Earthquake epicenters (locations) are shown by the colored dots. Note locations
and concentrations of activity. Compare with volcano locations.
Plate collisions and volcanoes
• Ocean-ocean
• Ocean-continent
• Continentcontinent
India-Asia
continent-continent collision
Rock Behavior
• How rocks respond to applied forces (stress)
• Stress – force per unit area (lbs per sq in)
• Response is termed “strain”
– Elastic deformation (reversible)
– Ductile deformation (flow)
– Brittle deformation (fracture)
Rock breakage
• Fracture
– Separation only; no vertical movement
• Fault
– Vertical and/or horizontal motion
Note offset in rocks
Footwall
block
Hanging wall
block
Note different fault motions
Normal or dip-slip
fault
Strike-slip fault
Reverse fault
Strike-slip motion on the plates
Note relative position of features
Subduction process
Shallow-focus: 0 to 70 km depth
Intermediate-focus: 70 to 300 km depth
Deep-focus: 300 to about 700 km depth
P- and S-wave motion
Note changes in amplitude of the
three wave arrivals
Seismogram of EQ near recording
station
Selected seismic stations in the US
Earthquake magnitudes
• Measures the “size” of an EQ
• Four types of measurements
– Local (ML) – original Richter scale
– Body-wave (Mb) – P-wave amplitude
– Surface-wave (Ms) – Rayleigh-wave amplitude
– Moment (Mw) – considers amount of strain
energy release along entire fault rupture.
Comparison of magnitude scales
The Big Ones
Japan Mar 2011
9.0
Intensity scale
• Measures damage caused by seismic energy
• Established by Mercalli (Italian) in 1902;
modified in 1931 to reflect enhanced building
standards in US
• Uses Roman numerals (I – XII)
• Values depend on EQ magnitude, distance
from source, bedrock type, building material
and style, duration of shaking
Isoseismal map of Dec 1811 EQ near
New Madrid, MO
Earthquakes don’t kill peoplebuildings do
• Many deaths in older regions on Earth due to
poor quality construction, especially through
trans-Mediterranean/Asiatic belt
• Secondary events (aftershocks) destroy
already weakened structures
• Surface waves produce the greatest damage
Benefits of EQs
• Changing natural resource paths
– Groundwater
– Oil and natural gas
– Exposures of minerals
• Natural mitigation
– Small events lessen likelihood of large events
Short-term predictions
• Precursors
– Events that imply an EQ; usually small magnitude
events, often in swarms
– Foreshock (sometimes) – main shock – aftershock
• Ground deformation
• Water level changes in wells
• Seismic gaps
– Greatest potential for large events in the gap
Sample seismogram showing P, S, and surface waves
Effects of EQs
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•
•
•
•
•
Shaking and ground rupture
Liquefaction
Regional elevation changes
Landslides
Fires
Disease
Humans cause earthquakes by
• Crustal loading by dams and reservoirs
• Injection of liquid waste
• Underground nuclear explosions
Human caused Eqs near Denver, CO
Earthquakes in the United States
Where do most
EQs occur in US
and why?
What’s happening
in eastern &
central US
Subduction zones in western North
America
• Alaska
– Subduction of Pacific
Plate
• Pacific Northwest
(BC-WA-OR-CA)
– Subduction of Juan de
Fuca Plate and smaller
Gorda Plate
– Cascadia Subduction
Zone
– Volcanoes on land
Subduction Zone Earthquakes
• Largest EQs worldwide
– 9 of the 10 largest earthquakes (1904-2008)
were related to plate subduction
• One in Tibet was due to India hitting Asia
– These 9 occurred along Circum-Pacific “Ring of
Fire”
– Five EQs were located in northern Pacific
[Japan-Kamchatka-Aleutians]
Examples of Subduction Zone Earthquakes
• Chile 1960 (Mw = 9.5): Nazca Plate diving under
South American Plate; tsunami producer
• Alaska 1964 (Mw = 9.2): Pacific Plate dives
beneath North American Plate; tsunami producer
• Mexico 1985 (Ms = 8.1): Cocos Plate dives
beneath North American Plate
• Indonesia 2004 (Mw = 9.1): India Plate dives under
Burma Plate; major tsunami producer
• Japan 2011 (Mw = 9.0): Pacific Plate dives under
North American Plate; major tsunami producer
Seismic Gap, Mexico, Sept 19,1985 Ms = 8.1
• Mexico earthquake
filled Michoacan
seismic gap
• Guerrero gap
remains
• Major aftershocks
of Ms =7.5 and 7.3
within a month
Fig 2.20a
Average annual worldwide frequency of EQs
magnitude 6.0 or greater
• M 6.0 – 6.9
Strong
• M 7.0 – 7.9
Major
15-20
• M 8.0+
Great
1 every 2-3 years
100
San Andreas Fault System
• Movement
occurs on many
faults
• Displacement is
distributed over
a wide zone
• Right-lateral
strike-slip motion
San Andreas: Earthquake Probabilities
• Probability of
major earthquake
(1988-2018)
• Use historical and
sag-pond data to
calculate
recurrence
intervals
Loma Prieta, Oct 17,1989
[World Series EQ]
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•
•
•
•
•
Magnitude 6.9; 67 killed
Epicenter at Loma Prieta, highest
peak in Santa Cruz Mountains
100 km SSE of San Francisco
Section of San Andreas that moved
in 1906 EQ ruptured again
Marina district in SFO was built on
rubble from 1906 EQ; mud was
pumped in to fill holes;
very unstable “land”
Game 3 halted by Commissioner;
after 10-day recess, series
continued in Oakland – Oakland
swept series 4 games to 0.
Fig 2.18a
Seismic Wave Amplification near Oakland
Plate boundary is not
a discrete line, but
rather a zone the
width of the Bay
Area
Notice the many
major faults that
are parallel to
the San Andreas fault
Location of Hayward fault
Univ of California stadium
in Berkeley
The trace of the
Hayward fault
runs through
goal posts
Left side is
moving N [top of
image]
30 yrs of
activity;
crosses are
epicenters
New Madrid, Missouri, 1811-2
• Series of three earthquakes in
Mississippi River valley (Dec
1811-Feb 1812)
• 4 main events, 3 w/
magnitudes > 8.0 !
• Knocked down chimneys 400
miles away!
• Shook windows 800 miles
away!
• Neotectonic analysis indicates
earlier EQs in 500, 900, 1300,
1600 AD  recurrence
interval of 200-400 years.
• Why did it affect such a large
area?
EQs associated with volcanoes
• Those connected with active subduction such
as western U.S.
– Much larger magnitudes due to brittle rock
• Isolated volcanism such as Hawaii
– Usually lower magnitude due to molten rock
Foci of Hawaiian EQs