Be able to draw the main
faults onto the blank
map of the Bay Area.
Bedrock: green
and purple
Unconsolidated
sediments
Soils of San
Francisco Bay Area
Bay mud: red
Ground motion
versus earth
material
Loma Prieta Earthquake
1989 Loma Prieta Earthquake
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October 17, 1989; 5:04 PM
Mw = 6.9
Oblique movement on the San Andreas Fault
Maximum offset of 3-4 feet
~ 25 miles of fault plane movement
15 seconds of shaking
6-10 billion dollars of damage
Oblique movement
Loma Prieta Earthquake
Failure of parapet and
upper floor, San Jose
Failure of parapet and arch,
unreinforced masonry, Los Gatos
Loma Prieta Earthquake
Structure shifted off the
foundation, Watsonville
Aptos
The Marina District
(soft story collapse)
Freeway failure
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Oakland, California
Cypress Structure
Redesigned in 1950
Insufficient number of
columns
• Columns not well connected
Photo Credit: H.G. Wilshire,
U.S. Geological Survey
Collapse of Soft Story
Soft Story Collapse
The San Andreas Fault
Location
Formation
History
The San Andreas Fault System
•Almost 800 miles long
•Includes the parallel and
subparallel faults
•North to Cape Mendocino
•South to the Salton Sea
Locked segments
produce a few
large events
About every
150 years
Creep segments
produce many
smaller events
Locked segments produce large events
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Fort Tejon earthquake
1857
225 miles along the SAF
Parkfield was the
epicenter
• About a magnitude 8
• Last “big one” in the Los
Angeles region
1906 Earthquake, epicenter
1906 San Francicso Earthquake
• 8 to 25 feet of offset
• 300 miles of fault rupture
1906 Earthquake
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April 18, 1906, 5:12 am
Mw =7.9
45-60 seconds of strong ground shaking
700-3000 fatalities
225,000 people homeless (pop.=400,000)
400 million dollars of damage
80 million from ground shaking
Modified
Mercalli
Scale
Parkfield, California: creeping segment
• On the straight portion
of the SAF
• Magnitude 6
earthquakes occur
every 25 years
1906 San Francisco Earthquake: locked
segment
Offset: amount of
displacement
Marin County, 16 feet of offset
Calaveras Fault: creep segment (not on the SAF but is
a great example how man-made structures respond)
From subduction to transform plate
boundary: 30 million years
• A subduction zone off the coast of California
produced the basement rocks
• The Sierra Nevada granites were formed
within the Earth’s crust
The Great Valley
Sierra Nevada
granites
The Coast
Ranges
How the current topography mimics
the 100 million year old tectonic
setting tectonic setting
Coast Ranges
Sierra Nevada
Great Valley
granites
Ridge and transform subduct
Subduction
Transform
fault
ridge
Migration of the SAF
Volcanic Rocks become progressively
younger form south to north
• Neenach volcanic rocks:
– Southern California
• 23 million years old
• Sonoma volcanic rocks:
– North of San Francisco
• 3-5 million years old
Pinnacles and the Neenach volcanic rock
offset
Geology of the
Pinnacles, CA
Initiation and migration of the San
Andreas Fault: offset and deformation
Neenach volcanic rocks formed
23 million years ago
Displacement or offset by
the San Andreas Fault
The Pinnacles National
Monument is the current
location
New Almaden Mercury Mine, South
San Jose
• Migration of the San
Andreas Fault
• Caused deformation
and metamorphism of
rocks
• Concentration of
mercury
• About 11 million years
ago
Cinnabar: HgS
Fault Movement
• Is calculated in centimeters/year
• The San Andreas Fault moves about 3.4 cm/yr
• The rate is calculated by dividing the amount of
displacement or offset by time
Calculate the Rate of Movement
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Offset is approximately 320 kilometers
Formation is approximately 20 million years
What is the rate in centimeters per year?
1000 meters = 1kilometer
100 centimeters = 1 meter
Two left-stepping areas
on the SAF: Santa Cruz
Mountains and the
Transverse Ranges
The Big Bend
Little bend: Santa Cruz Mountains
Extension creates basins
Other landforms associated with the
San Andreas Fault
Sag pond: depression between two
transform faults.
Fault plane intersects the water table.
Water moves against gravity to fill the
depression
Pressure ridge and sag pond
Linear Valley, Crystal Springs Reservoir
Scarp: where the fault plane is
exposed on the Earth’s surface
The San Andreas Fault
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Right lateral strike-slip fault
Capable of producing greater than M7.5 eqs
Left-stepping bends produce mountains
Right-stepping bends produce valleys
Many landforms are produced that define the
topography of the Coast Ranges
• Evolved from a mid-oceanic ridge and
transform fault subducting about 30 mya