Big Bend in the San Andreas Fault
* Many faults in Southern California
other than San Andreas (SAF)
* Most are right-lateral (like SAF) but
many in So-Cal are not
(thrust, normal, blind thrust)
* Right-lateral strike slip motion common
in northern California but things are
different in LA at the “Big Bend”
* Bend in SAF causes intense compression
and formation of the transverse ranges
(San Gabriels, Santa Susanas...)
* Pressure causes rocks here to deform
and break alleviating stress and
creating non-uniform slip events.
Big Bend in the San Andreas Fault
North America
Plate
Right-lateral strike-slip
motion along the SAF
becomes compressional
at a bend in the fault line.
Pacific Plate
Northridge Earthquake, 1994
- January 17, 1994, 4:30 am
- Magnitude = 6.9
- Blind thrust fault
- Difficult to find or predict
- Duration 10-20 seconds
CSUN Parking Structure
Earthquakes
Chapter 16
1989, Loma Prieta earthquake, Mw = 7.2
Historic Earthquakes
1906 San Francisco (7.8)
- 280 miles of displacement
- Shaking ~1 minute
- Damaged water mains, fires spread and caused many deaths (3000).
Historic
Earthquakes
1989 Loma Prieta, Mw = 7.2
- Shaking for 15 seconds
- Death toll 63
Historic
Earthquakes
1964 southern Alaska (Mw = 9.2)
- Shaking for 3 minutes
- Rupture 350,000 square miles
- Death toll from quake 15 (remote area)
- Tsunami, landslides 100 more
Historic Earthquakes
1994 Northridge
- 1.8 miles from CSUN
- Shaking 40 seconds
- Damaged all 53 CSUN buildings
- Damaged 300 other schools
- Lower story buildings collapsed
- 4 interstate hwy's closed for months
(Golden State, Santa Monica fwys)
- 15000 people live in tents for days
- 10000 without water, elect, gas
- Arid climate did not cause liquifaction of soils – shaki
(compared to 1964 Alaska and 1989 Loma Prieta).
- Landslides in Santa Susana, Santa Monica, San Gabrie
blocked roads and traffic, damaged water lines and ho
- Sylmar – Olive View Hospital – rebuilt from 1971 to c
- Aftershocks 5.9 1 min after main shock, 5.6 11 hrs aft
Historic
Earthquakes
2002 inland Alaska – Denali Fault
- Propagated east 7000 miles/hr
- Offset streams, glaciers, landslides
- Trans-Alaska Oil Pipeline no
serious damaged - pre-engineered
Historic Earthquakes
2004 Sumatra – Andaman E.Q.
- Mw = 9.3
- Second largest recorded on Earth
- Major damage in Sumatra
- Tsunami damage spread far to
Indonesia, Thailand, Sri Lanka,
India, East Africa
- Death toll 220,000 from force
of tsunami wave
Faults and Crater Offsets on the Moon
Moon Quakes
Moon quakes were recorded by a seismometer from the
Apollo 16 mission
INSIGHT Mission to Mars
* The INSIGHT missionwill deploy one seismometer to measure Mars quakes!
* Do you think this is a good use of our research dollars ? Why or Why not ?
Earthquakes
• An earthquake is a trembling or shaking of the
ground caused by the sudden release of energy
– Tectonic forces produce stresses on rocks that
eventually exceed their elastic limits, resulting in
brittle failure
• Energy is released during earthquakes in the
form of seismic waves
• Elastic rebound theory - earthquakes
are a sudden release of strain stored in
rocks that bend until they finally break
and move along a fault
Seismic Waves
• Focus (or hypocenter) - the point of initial
breakage and movement along a fault,
where seismic waves originate
• Epicenter - point on Earth’s surface
directly above the focus
• Two types of seismic waves are produced
during earthquakes
– Body waves - travel outward from the focus in
all directions through Earth’s interior
– Surface waves - travel along Earth’s surface
away from the epicenter
Body Waves
• P wave - compressional body wave in which
rock vibrates back and forth parallel to the
direction of wave propagation
– Fast (4 to 7 kms/sec) wave that is the first or
primary wave to arrive at recording station
following earthquake
– Pass through solids and fluids
• S wave – shearing body wave in which rock
vibrates back and forth perpendicular to the
direction of wave propagation
– Slower (2 to 5 km/sec) wave that is the
secondary wave to arrive at recording
station following earthquake
– Pass through solids only
Surface Waves
Surface waves are slower than
body waves but have the larges
amplitude
• Love waves - side-to-side motion
of the ground surface
– Can’t travel through fluids
• Rayleigh waves - ground to moves
in an elliptical path opposite the
direction of wave motion
– Extremely destructive to buildings
Measuring Earthquakes
• Seismometers - used to measure seismic waves
• Seismographs - recording devices used to produce
a permanent record of the motion detected by
seismometers
Measuring Earthquakes
• Seismograms - permanent paper (or digital) records of
the earthquake vibrations
– Used to measure the earthquake strengths
Locating
Earthquakes
• P- and S-waves leave E.Q.
focus at the same time
• P-wave gets farther and farther
ahead of the S-wave with distance
and time from the earthquake
• Travel-time curve - used to
determine distance to focus
– based on time between first P- and
wave arrivals
S-
Locating Earthquakes
• One station gives distance to E.Q.
but not direction
• Plotting distances from 3 stations on
a map, as circles with radii equaling
the distance from the quake. Point
where 3 circles overlap locates the
earthquake epicenter
• Depth of focus beneath Earth’s
surface can also be determined
– Shallow focus
0-70 km deep
– Intermediate focus 70-350 km deep
– Deep focus 350-670 km deep
Measuring the Sumatra – Andaman Earthquake
A Global Seismic Network (GSN)
is maintained around the Earth
to monitor earthquake activity
as well as nuclear testing.
Earthquake locations and depth
can be determined within minutes
of an earthquake using this
GSN network.
Measuring the
“Size”
of Earthquakes
• Earthquake “size”
measured two ways intensity and magnitude
• Intensity - a measure of
the effects an earthquake
produces (on both
structures and people)
– Modified Mercalli scale
Measuring the “Size” of Earthquakes
• Magnitude is a measure
of the amount of energy
released by an earthquake
– Richter scale
– Body waves
– Surface waves
• Moment magnitude - more objective measure of energy released by a
major earthquake
Mo = m* U * A
– Uses rock strength, surface area, fault rupture distance
– Smaller earthquakes are more common than large ones
Earthquake Magnitudes
“How Big is Big ? ”
Earthquake magnitudes are logarithmic
M = 10
R
(M = ground movement, R = Richter scale)
If R = 1,
If R = 2,
then M = 10
then M = 100
So this means that a magnitude 7 earthquake will have 10 times
as much ground movement as a magnitude 6 earthquake.
Location and Size of Earthquakes
in the U.S.
• Earthquakes occur throughout the
U.S., but are much more common in
the western states and Alaska
• Largest seismic risks or hazards exist
near the plate boundary along the U.S.
Pacific coast (e.g., San Andreas fault),
and around New Madrid, Missouri
• Seismic risk determined based on
the assumption that large future
earthquakes will occur where they
have occurred in the past
Earthquake locations since 1977
Effects of Earthquakes
• Earthquakes produce several types of
effects, all of which can cause loss of
property and human life
– Ground motion is the familiar trembling and
shaking of the land during an earthquake
• Can topple buildings and bridges
– Fire is a problem just after earthquakes
because of broken gas and water mains and
fallen electrical wires
– Landslides can be triggered by ground shaking,
particularly in larger quakes
– Liquefaction occurs when water-saturated soil
or sediment sloshes like a liquid during a quake
– Permanent displacement of the land surface
can also occur, leaving fractures and scarps
World Earthquake Distribution
• Most earthquakes occur in narrow geographic belts - plate
boundaries
• Most important concentrations in circum-Pacific and MediterraneanHimalayan belts
World Earthquake
Distribution
• Nearly all intermediate- and deep-focus earthquakes occur
in Benioff zones
- inclined seismic activity associated with descending oceanic plate at subduction
E.Q.'s are caused by plate interactions along tectonic plate boundaries
Plate boundaries are identified and defined by earthquakes
Earthquakes
Tectonics
Earthquakes occur at each of the three types of plate boundaries:
-At divergent boundaries, tensional forces - shallow-focus quakes on normal faults
-At transform boundaries, shear forces - shallow-focus quakes along strike-slip faults
-At convergent boundaries, compressional forces produce shallow- to deep-focus
quakes along reverse faults
First Motion of Ground Motion
Observed on a Seismogram
Compression (thrust faults)
Extension (normal faults)
Activity
Building Codes for Earthquakes
Best materials are:
-Strong
-Flexible
-Light
Good examples:
Steel
Wood
Reinforced concrete
(rebar steel core)
1999, Izmit Turkey
Poor examples:
Simple concrete
Brick – chimneys often fall
(in moderate E.Q.'s)
Heavy roofs (Tile)
Strong liquefaction can supersede
all these precautions...
1964 Niigata, Japan - liquifaction
Buildings and Bridges...
1995 Kobe, Japan
Elevated highway knocked over by
strong horizontal jolt.
Damage to bridge and 88,000 buildings
costs exceeded $400 billion
Tsunami Waves Created by Earthquakes
Tsunami waves are generated
by submarine earthakes
Long low waves are formed
above displaced seafloor
that travel for miles along the
base of the seafloor.
Displacement of seawater
surface can be as little as
a few centimeters in the middle
of the ocean.
Displacements increase
dramatically as they approach
a continental shelf
Tsunami Travel time in the Pacific Ocean
Earthquake in Alaska may form a tsunami in the Pacific
Monitoring Tsunami Waves
Tsunami in Sumatra, 2004