Transcript 19.1 Forces Within Earth

Honors Earthquakes Chp. 8
Earthquake: vibration of Earth produced by rapid
release of energy, usually along a break in Earth’s
crust
Focus: where
earthquake originates
(deep in Earth)
Epicenter: point directly
above focus on Earths
surface
Fault: fracture along which
movement occurs, in the
lithosphere
3 types of faults:
1.
2.
3.
Normal Fault: horizontal
tension, hanging wall drops
down foot wall (foot wall
become higher point)
Reverse fault: (Thrust):
horizontal compression,
hanging wall pushed up
over foot wall (foot wall
becomes lower point)
Strike-slip fault: horizontal
shear
Foot
wall
Foot
wall
Hanging wall
Hanging wall
Cause of Earthquakes
Stress & Strain
• Stress: force per unit
area causes fractures
• Strain: deformation of
material due to stress
Lead to:
• Elastic deformation:
material will return to
normal if stress is
decreased
• Elastic limit: point
between elastic &
ductile deformation
• Ductile Deformation:
permanent deformation
due to stress & strain,
occurs after elastic limit
is exceeded
• Failure: breakage,
Earthquake
Elastic Rebound Hypothesis:
Aftershock: smaller
earthquakes (could
be hundreds) that
follow a major
earthquake, hinder
rescue efforts
Foreshock: small
earthquakes that
happen before a
major earthquake
Measuring Earthquakes
• Seismographs: sensitive
instrument measures
vibration
• Seismogram: (seismos =
shake, gramma = what is written)
written record produced by
seismometer of all 3 waves
Seismic Waves
1. Surface waves (Lwaves, complex
waves): Rock moves
up & down and side
to side; side to side
motion most
destructive, most
destructive wave
overall, slowest
moving wave (90%
slower than S waves)
2. Primary waves (Pwaves, longitudinal):
squeezes & pulls rock
in direction of traveling
wave, fastest wave
(1.7 X faster than S
waves)
3. Secondary waves
(S-waves, transverse):
rocks move at right
angle in relation to
wave direction
Time-Travel Graph
• S-wave: travel slower
than P-waves
• P-waves: Travel
faster than S-waves
What is it used for?
Locating an Earthquake
1. Determine P and S wave arrival time to seismographs
2. Calculate the difference between P & S wave arrival times
3. Determine epicentral distance
4. Minimum of three stations needed to determine epicenter
Seismic Belts
• Areas of greatest seismic activity, narrow & follow
plate boundaries
– 80% in Circum-Pacific Belt
– 15% in Mediterranean-Asian Belt
Measuring Earthquakes
Intensity: amount of
earthquake shaking at a
given location (not
quantitative)
Magnitude: amount of
energy released from
source of earthquake
(quantitative)
Richter scale
• 1935
– Charles Richter
– Cal Tech (California
Institute of Technology)
– Only useful for small
shallow earthquakes
within 500 km of epicenter
– Used mostly by news
reporters
Richter scale: based on
amplitude of largest
seismic wave generated,
use logarithmic scale
– Wave size increases by a
factor of 10
• Magnitude 6 wave size is
10X lager than magnitude 5,
and 100X larger than a
magnitude 4
– Energy released increased
by a factor of 32X
• Magnitude 6 energy is 32X
greater than magnitude 5,
and 1024X greater than a
magnitude 4
Moment magnitude scale: used by most scientists,
derived from amount of displacement
• Calculated using:
–
–
–
–
seismograph data size
average movement along fault
Area of surface breakage
Strength of rock
See table 1 page 227 in book
Modified Mercalli scale:
measures intensity on I
to XII scale, XII
greatest damage
earthquakes can
cause
• The same earthquake
can have different
Mercalli scale ratings
(depends on distance
from epicenter) Why?
Depth & Focus related to earthquake magnitude
• Deeper the focus smaller the vibrations
Crust
darker
area
Lithosphere
Hole thing
Destruction from Earthquakes
Depends on:
• Intensity (why intensity & not magnitude?)
• Duration of vibration (material, magnitude)
• Nature of material under structure or area
– Liquefaction: loosely compacted earth & saturated
sediments
• Liquefies during an earthquake causing settling
• Design of structure
– Unreinforced masonry (brick) buildings most at
threat
• Wood and steel have more flexibility
• Generally why is wood better than steel?
Tsunamis: waves created by rapid displacement of water,
with long wavelength off shore and short wavelength
near shore (travel very fast off shore, slow near shore)
Amplitude
increases
in shallow
Water only
Tsunami warning systems are common in the Pacific Ocean & were not
put in the Indian Ocean until after the 2002 Sumatra Tsunami
Tsunamis are created by:
• Earthquakes activity
• volcanic activity
• Landslides
• Asteroids or meteor impacts
Tsunami damage is caused by huge surge that pushes
inland long distances due to waves long wavelength
High ground is the safety zone during a tsunami
Before
After
Other dangers resulting from earthquakes:
• Landslides
• Ground subsidence
• Fires
Predicting Earthquakes:
Shore-range predictions: monitor & measure; uplift,
subsidence, strain in rock, water level, pressure in
wells, radon gas emissions & electromagnetic
properties in rock. Have not been successful
Long-range predictions: forecasts occurrence within
30 to 100+ years by studying historic records &
seismic gaps, based on the idea earthquakes are
cyclical, help in determination of building codes; Are
predictions accurate? What is a seismic gap?
Earth’s Layers Defined by Composition
Crust: thin rock outermost layer
– Oceanic: 7 km, basalt & gabbro (density 3 g/cm3)
– Continental: 8-75km (average 40km), granitic
(grandiorite), less dense than oceanic (density
2.7 g/cm3)
Mantle: 82% of Earth’s volume, dominant rock type
peridotite, extends to depth of 2890 km, density 3.4
g/cm3
Core: center sphere composed of iron-nickel greatest
concentrations of metal), extreme pressure density
13 g/cm3
Earth’s Layers Defined by Physical Properties
Lithosphere: outer most layer crust and upper mantle,
cool ridged shell, 100 km in thickness
Asthenosphere: small amount of melting due to temp
& pressure, weak layer because near melting point
Upper mantle: lower lithosphere & asthenosphere
Lower mantle: more ridged layer, rocks very hot
capable of gradual flow
Outer Core: liquid layer, flow generates Earth’s
magnetic field, 2260 km thick
Inner Core: solid due to pressure, 1220 km thick
Discovering Earth's layers
Mohorovicic’s evidence of
1909: velocity of seismic
waves increased
abruptly below 50km (the
Moho)
Moho: boundary between
crust and mantle
More evidence:
P waves can travel through
a liquid (but they lose
velocity)
S waves can not travel
through liquid
Shadow zones:
P wave: due to refraction in core, 105 degrees to
140 degrees
S wave: due to outer core (cannot travel through
liquid), waves stop at 105 degree mark
Asthenosphere
Higher viscosity
103 degrees
Outer
core
low viscosity
Outer core
Lower viscosity
Inner core
solid
Discovering Earth’s Composition
• Drilling (direct observation)
• Seismic data
– P & S wave shadow zones
• Nuclear test
– 1960 allowed for measurement of inner core
Kola Peninsula Deepest Well Cool Facts:
Began to drill in 1970
– 5 years to drill 7000 m (1975)
– 9 years to drill the next 5000 m! (1984)
– Stuck at 12,000 m in 1989
– Reached 12,262 m in 1994
– At the bottom of the well…
• Rocks 2.9 billion years old
• Temperature 190°C ~ 2X the temp of boiling
water
Local Faults: