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The movement of earthquake waves through the Earth’s interior has
given us a better picture of the Earth’s interior.
P-waves are faster than S-waves. The density and elasticity of
rock affects the speed of the waves.
When P-waves hit the core, they are refracted and slow down.
This creates a P-wave shadow 103º -143 º from the focus
S-waves can’t travel through liquids.
When S-waves hit the outer liquid core, they stop. This creates a
S-wave shadow at locations greater than 103º from the focus of
the earthquake.
Speed of Seismic Waves:
*Boundary between crust and
upper mantle (Moho)
*P-waves travel at 6.75 km/sec in
the crust and 8 km/sec below this
boundary.
*The depth varies depending if it is
under continents (20 - 90 km) or
under ocean floors (5 to 10 km).
*Speed also increases with depth
in the mantle except for a low
velocity zone at 100-250 km. This
corresponds to the asthenosphere.
• Focus: where
“slippage” first occurs
• Epicenter: directly
above the focus on the
Earth’s surface
• The arrival times of P
and S waves
determines the
distance to an
earthquake
• The difference in arrival
times of P and S waves
determines the distance of
the earthquake to a
seismograph station.
• A circle representing the
distance is drawn around
the station.
• This is done with three
stations, and where the
circles cross is the exact
location of the epicenter.
But they are clustered along the Pacific Coast. Why?
Movement along the San Andreas Fault
•Tectonic Creep is the slow continuous movement along a
fault zone that is not accompanied by felt earthquakes.
•Locked faults are sections that are not moving (locked
due to friction). Pressure builds up in these sections until it
overcomes the friction and the energy is released in an
earthquake (elastic rebound).
Offset fence
Point Reyes
• 150 miles long
• Illinois, Missouri,
Arkansas, Kentucky,
Tennessee
• Faults formed when N.A.
began to split (failed)
• Flooded by ancient sea
(covered by sediment)
• Thousands of E-Q since
1970’s
• 1811-1812: 4 of largest
NA major E-Qs felt
 Two lithospheric plates
come together
• Yellow squares indicate
earthquake depth
• Oceanic plate subducts
beneath continental (more
dense)
• Older oceanic plate
subducts beneath younger
oceanic plate
• Shallow to deep
earthquakes
• No subduction occurs
when a continent collides
with another continent
(same density); folding
and buckling occurs along
the continental margins
• Shallow earthquakes
• Transform boundaries
(plates slide past one
another) also create
relatively shallow
earthquakes
• Plates separate
• Mid-Atlantic Ridge
(MAR)
• New crust formed
• Shallow earthquakes
• Generally less
frequent
The strength of an earthquake can be measured in two different
ways:
Intensity:
qualitative assessment of the extent of
damage done by an earthquake; subjective;
Modified Mercalli Intensity scale ( I to XII)
Magnitude: quantitative measurement of the amount
of energy released by an earthquake;
Richter Magnitude scale (1 to 10);
each step in the Richter Scale is an increase
of 10-fold in movement and a 30-fold
increase in energy
Fault Scarp Central California
scarp
Linear
valley
Linear
ridge
Offset stream
Earthquake Damage depends on many factors:
The size of the Earthquake (magnitude)
The distance from the focus of the earthquake
The types and properties of the materials at the site
The nature of the building
Soil and Earthquake Damage:
•Soil thickness: shallow soils may not perform as well as deep
•Soil saturation: saturated soils perform less well than dry
•Soil grain size and sorting: well-sorted, fine grained sands and
silts are the most likely to liquefy
•Types of bedrock: unweathered igneous rocks are better than
weak fractured rock
•Areas where ground may settle or slide
Nature of building and Earthquake Damage:
•Type of construction (size and use of building)
•Seismic design considerations
•Architectural simplicity of building
• Rock, earth or debris
flows on slopes due to
gravity
• Major geologic hazard
because widespread
• $1-2 billions/year in
damage
• Fisheries, tourism, timber
harvesting, mining and
energy production
impacted
• As development ,
damage 
• Appliances, furniture
and gas, chemical and
electrical hazards
present
• Water pools can  fire
hazard
• Leaking gas lines,
damaged or leaking
propane containers,
and leaking gas tanks
San Francisco, 1906
• Earthquake struck at
5:13 AM
• “Liquified” ground
beneath the city
exacerbated potential
for fires
• Fires burned for four
days
• Occur when faulting or
landslide occur on the
ocean floor
• Seafloor permanently
uplifted or down-dropped,
creating “sloshing” of
water
• Displaced water gains
height as approaches shore
• Floating debris and strong
currents create damage
Indonesia, 12/26/2004
• Picture taken by
newlyweds vacationing
when tsunami stuck; both
were killed when tsunami
came inland
• The destruction of coral
reefs along shorelines (for
shrimp “farms”) increased
destruction from tsunami
as waves took longer to
“break”
It is difficult to accurately predict earthquakes.
Analysis of past earthquake patterns, measurement of movement,
and the distribution of faults have allowed scientists to create
seismic risk maps.
Along the San Andreas fault, geologists are looking for areas along
faults that are currently inactive. This probably means that these
regions are locked and that energy is building up.
Tilting of rocks on either side of fault-lines is also a sign of
pressure build-up.
Earthquake risk in the Bay Area
Green:
Stable bedrock
Orange:
Unstable bedrock
Yellow:
Unconsolidated
Soil
Red:
Mud and Fill