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Earthquakes
Pt Reyes Station 1906
Earthquakes
Ground shaking caused by the sudden release of accumulated strain
by an abrupt shift of rock along a fracture in the earth.
You Live in Earthquake Country—
Be Prepared
Don’t expect someone to
be there to help you!
• Have an emergency kit and a plan.
• Emergency kit: water and food (+3 days)
blankets, coat 1st aid kit, you are set if
you have camping gear (shelter, water
filter, light, stove, etc.)
• Plan: Know what the dangers are (gas
lines, tsunami risk, fire risk, etc..), and how
to react. Know where to go and where you
will meet.
+10,000 Fatalities
2010
Since 1970
2004
2005
2008
Japan 2011
~25,000 fatalities
(almost all due to
the associated
tsunami).
2003
2001
Turnagain Arm subsidence, Alaska 1964 Earthquake.
Sumatra, 2004
+250,000 dead (mostly from associated tsunami).
Turkey, 1999,
+17,000 dead
Sichuan, China 2008
+80,000 deaths
San Francisco 1906 and 1989 (inset)
Earthquakes most frequently occur
near plate boundaries---Why?
2
1
Discuss each location.
6
8
10
5
4
9
3
Intra-plate Earthquakes
New Madrid Seismic Zone
Ancient failed
rift (aulacogen)
1. Stress (pressure) is continually applied to rocks or preexisting faults. This stress most likely comes from plate
tectonics and is at a plate boundary.
2. Stress builds where strong rocks or locked faults withstand
it. (Friction is the internal force that locks a fault, making its
two sides stick together.)
3. Rocks and faults deform elastically (strain) in response to
the building stress.
4. Stress finally builds higher than rock strength or fault’s
friction and rock breaks or fault slips.
5. Stress is released in the form of energy waves that move
outward in all directions.
6. The rocks or faults elastically rebound and strain energy is
released.
7. After the rocks or faults have settled back into place, the
stress begins building again.
Types of Fault Movement Behavior
• Fault Creep: Slow, gradual and gentle
displacement along the fault. Segments of the
San Andreas Fault show this behavior.
• Locked Faults: Faulted blocks are locked, thus
stress builds and rocks strain (elastically
deform). When the rocks break or friction
between the blocks is exceeded, the blocks
suddenly slip releasing the strain through elastic
rebound (like a rubber band). This is an
earthquake!
Simon Street, Hayward
Body Waves (P and S
waves): Body waves travel
through the Earth. Body
waves are faster than surface
waves and are recorded
globally.
Surface Waves (Love and
Rayleigh waves): Surface
waves are analogous to water
waves and travel along the
Earth's surface. Surface
waves are low frequency, long
duration, and can have large
amplitudes, thus they can be
the most destructive type of
seismic wave.
Surface Waves
P-Waves
Primary or Pressure Wave
P-waves are compressional waves that travel through all materials including fluids
such as gases and liquids.
P-waves are the fastest waves (5 to 8 km/s) in the Earth’s crust, so they
are felt first.
The speed increases with increased density and stiffness thus P-waves travel through
granite in the crust at about 5 km/s and the deep mantle at about 13 km/s.
S-Waves
Secondary or Shear Wave
S-waves are shear waves so they do not travel through fluids.
S-waves are slower (2-5 km/second in the Earth’s Crust) than P-waves, so they
arrive after the P-waves.
The velocity of S-waves ranges from 2–3 km/s in light sediments and 4–5 km/s
in the Earth's crust up to 7 km/s in the deep mantle.
Seismic Wave Motion
Turn to Neighbor:
Draw cross-section through Earth’s
surface, where a fault comes through.
Explain fault, epicenter, and focus.
Seismographs
How are earthquake locations
pinpointed around the world?
Arrival of P waves
The time lag
between the
arrival of the
P waves and
the S waves…
?increases
with distance
from the
epicenter.
10 km
30 km
70 km
110 km
140 km
280 km
410 km
S waves
A single
seismograph
can determine
the distance to
an earthquake
epicenter.
Three or more
seismographs
are needed to
locate an
earthquake
epicenter.
1
2
3
Magnitude
There are different ways for measuring the magnitude of an earthquake.
The Richter Scale: A quantitative way of measuring an earthquake.
Other quantitative ways have been developed (the Moment Magnitude Scale
being in favor in the U.S.) and they are calibrated to the popular Richter Scale.
The Modified Mercalli Scale: A qualitative way of measuring an earthquake.
Richter Scale
AMPLITUDE
(of the waves)
increases 10X
for each increase of 1 in magnitude
ENERGY
increases 30X
for each increase of 1 in magnitude
MAGNITUDE 6 = 10X AMPLITUDE OF 5
=30X ENERGY RELEASE OF 5
MAGNITUDE 6 = 100X AMPLITUDE OF 4
=900X ENERGY RELEASE OF 4
MAGNITUDE 8 = 30X ENERGY RELEASE OF 7
MAGNITUDE 8 = 900X ENERGY RELEASE OF 6
MAGNITUDE 8 = 27,000X ENERGY RELEASE OF 5
MAGNITUDE 8 = 810,000X ENERGY RELEASE OF 4
MAGNITUDE 8 = 24,300,000X ENERGY RELEASE OF 3
Magnitude
The Richter Scale and Moment Magnitude
Richter magnitude scale is based on amplitude of shaking as measured on a
seismometer. It is limited in it’s effectiveness to magnitudes of 6.8.
The Richter scale becomes unreliable on large earthquakes.
The Richter scale is older and now mostly replaced by the moment magnitude scale.
Moment magnitude scale is based on crust character, rupture length and amount
of offset. The moment magnitude scale was developed in the 1970s to succeed
the Richter magnitude scale. The moment magnitude scale is better suited for
studying large earthquakes.
Even though the formulae are different, the new scale (moment magnitude scale)
retains the familiar continuum of magnitude values defined by the older one.
The moment magnitude scale is calibrated to give similar values for medium sized
Earthquakes.
MOMENT MAGNITUDE SCALE IS
BASED ON SEISMIC MOMENT
SEISMIC MOMENT
(LENGTH OF FAULT RUPTURE)
X (DEPTH TO FOCUS)
X (TOTAL AMOUNT OF SLIP ALONG RUPTURE)
X (STRENGTH OF ROCK)
Modified
Mercalli Scale
Qualitative Description of the event.
NE Japan Tomography
Honshu
Sea of Japan
Pacific
Ocean
0
Moho
100
Zone of Melt Generation
-4%
0%
5%
Relative P-wave velocity
Seismic Gaps:
fault segments
with likely
accumulated
strain.