Download Chapter 11- Earthquakes

Continuation of plate tectonicsconvection in the asthenosphere
is still the driving force of moving
lithospheric plates.
 At
plate boundaries where pressure builds up
and eventually breaks rock
• Usually not in the middle of plates (1%)
• We call these Intraplate earthquakes
 Faults
occur where boundaries meet and rocks
“pass their elastic limit”
• Earthquakes often occur near these fault lines
• Three types of faults
 See
Exploring Earth Website
 1.
Normal faults
• Caused by TENSIONAL forces
• Forces PULL OR STRETCH rocks
2.
Reverse- caused by COMPRESSIONAL forces
- rocks are PUSHED toward each other
3.
Strike-slip fault
- Caused by SHEAR forces
- Rocks move PAST EACH OTHER
with little up or down motion.
 For
each type of fault:
• NAME
• DEMONSTRATE
• DESCRIBE how the rocks move at
the fault line – away, toward, past
• FORCES - compressional,
tensional or shear
• Type of BOUNDARY
• FOOTWALL/HANGING WALL
Point
where plate movement occurs
and energy is originated is called
the focus (can be miles below the
surface).
Point on Earth’s surface above the
focus is called the epicenter
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Let’s see if we can figure out from our data first!!!
Now let’s consult a textbook’s website and see what
you think.
Normal (divergent): shallow
Strike-slip (transform): shallow
Intraplate: shallow
Reverse (convergent)
• C/C - shallow
• C/O - all depths (up to 700 km – 435 mi)
 Focus Depths increase farther from the coast
 Due to subduction
 All
types of seismic waves (s,p and
surface) are detected by
seismographs and recorded on
seismograms.
 An
earthquake creates seismic waves
that travel away from the epicenter of
an earthquake.
• Remember, epicenter is the place on the Earth’s
surface directly above where the earthquake
occurred.
 Seismic
waves are detected by an
instrument called a seismograph,
which our book calls a seismometer.
 Energy shown on a seismogram.
 Studied by seismologist!
 Three
types of seismic waves:
 Primary
waves (P waves) travel fastest
• ~6 km/sec in the upper crust
• Move particles back and forth in the same
direction
• Cause little destruction
 Secondary
waves (S-waves) travel slower,
cause more damage
• ~3.5 km/sec in the upper crust
• Move particles back and forth at a ninety
degree angle to wave motion
 Surface
waves travel slowest, cause the most
damage
• Move particles side-to-side and in a swaying
motion
 Where
does each wave type originate?
 The
difference in arrival time
between p-waves and s-waves can
be timed to determine how far away
from the seismograph station the
earthquake occurred.
 With
at least three stations
reporting, we can pinpoint the
earthquake’s location using
TRIANGULATION (see next slide or
page 312 for example).
 How
far away is the epicenter of an
earthquake if there is 5 minutes between
wave arrival time?
 If
an earthquake occurs 4700 miles away
from a seismograph station:
• How long after the earthquake will the P-wave
arrive?
• How long after the earthquake will the S-wave
arrive?
• What will be the difference in arrival time?
 In
three steps, seismologists can find the
epicenter of an earthquake.
• 1. Find the time difference between P&S wave
arrival
• 2. Use the time difference to find the distance
from the seismograph station to the epicenter
 Gives a radius around the city
• 3. Compare with minimum three locations to
find epicenter
 Richter
Scale – 1935
 Measures earthquake magnitude
 Determined by height of waves from
seismogram
 Scale is infinite (depends on
sensitivity of equipment)
 Largest recorded - 9.5 1960 Chile
Earthquake
 Base-10 logarithm - √1000 ≈ 31.6 x
 Quantitative
 Mercalli
Scale – late 1800’s - modified
• Measures earthquake intensity
• Determined by human observation and
structural damage
• I – XII
• Qualitative
 Mercalli
 What
Scale
happens to Mercalli Intensity as
distance from epicenter increases?
 Magnitude
 Shallow/deep
 Liquefaction
 Tsunami
 Time
of day
 Shadow zone
 Landslides/mudslides/avalanches - terrain
 Structures (codes, poverty, expected?)
 Gas Pipes/Fires
 Warning – Pacific Tsunami Warning Center
 Duration
(how long did shaking occur?)
 Government Aid
 By Permission Only
 Location, date and time
 Pictures
 Damage (cost), deaths, injuries
 Magnitude
 Difference in P/S waves in Punxsy
 Search USGS Top Ten for list of earthquakes.
Also a list on p. 318.
 Other research
 Plates involved
 Occur
far from plate
boundaries
 Faults can still exist,
even where two
tectonic plates do not
meet
 Example: New Madrid
Fault
 Explain
how Primary and Secondary
waves are used to pinpoint the location of
an earthquake’s epicenter
 Calculate
the difference in earthquake
magnitude using the Richter Scale