Download ppt. - Science with Ms. Braget

How Do Stress Forces Affect
Rock?
 The movement of earth’s
plates creates powerful forces
that squeeze or pull the rock in the crust – these forces are
examples of stress

Stress – a force that acts on rock to change its shape or
volume
How Does Stress Effect the
Earth’s Crust?
Deformation – any change
in the volume or shape or earth’s
crust
 Three kinds of stress in the crust:
– Shearing – stress that pushes a
mass of rock in two opposite directions
– Tension – pulls on the crust,
stretching rock so that it becomes
thinner in the middle like warm bubble
gum
– Compression – squeezes rock
until it folds or breaks like a giant trash
compactor
What Is a Fault?
Fault – a
break in earth’s
crust where slabs
of crust slip past each
other; These usually
occur at plate
boundaries
A miner walks on the foot wall and looks up at the hanging wall!
B
A
Hanging wall
moves down
Hanging wall
moves up
What type of fault?
What type of fault?
Normal Fault
Reverse Fault
What Kind of Faults Are
There?
 Three Kinds:
–Strike-slip faults
–Normal Faults
–Reverse Faults
What Are Strike-slip Faults?
 Strike-slip faults
–Shearing forces cause
rocks to slip past each other
sideways with little up
and down Motion;
– Ex. San Andreas fault in California
What Are Normal Faults?
 Normal faults
–Tension forces cause the rocks
to form the fault at an angle
– One block is above the fault
–Hanging wall – the half of
the fault that lies above
–Footwall – the half of the
fault that lies below
– Ex. Rio Grande rift valley
What Are Reverse Faults?
 Reverse faults
–compression forces
cause the rocks to move
towards each other
– Same structure as normal fault but the
blocks move in opposite direction;
hanging wall move up
– Ex. Appalachian Mountains and
Mount Gould in Glacier National Park
How Are Mountains Effected
by These Forces? (Continued)
Anticlines – a fold
upward into an arch
Syncline – a fold
downward into an arch
Plateaus – a large area of
flat land elevated high above
sea level
How Are Mountains Effected
by These Forces?
Fault-block mountains – normal
faults uplift a block of rock
Folding – bends in the rock that form
when compression shortens and thickens part
of the earth’s crust. Ex. Himalayas
How Does the Energy of an
Earthquake Travel Through Earth?
Earthquakes
–the shaking and
trembling that results
from the movement of
rock beneath earth’s
surface. most begin in
the lithosphere.
How Does the Energy of an
Earthquake Travel Through Earth?
Focus – the point
beneath the earth’s
surface where rock that
is under stress breaks,
triggering an
earthquake
Epicenter – the
point on the earth’s
surface directly above
the focus
What Are Seismic Waves?
Seismic Waves –
vibrations that travel through Earth
carrying the energy released during
an earthquake
– They move like ripples on a pond
– They carry the energy of an earthquake
away from the focus, through Earth’s
interior, and across the surface
– The energy is greatest the the
Epicenter
What Are the Different Kinds
of Seismic Waves?
 Three categories:
–P waves
–S waves
–Surface waves
 P waves and S waves are sent out
focus
from the
; Surface waves
develop when the waves reach the
surface
What Are P Waves?
primary waves
– The first waves to arrive
 P waves are
– Earthquake waves that
compress and expand the
ground like an accordion
– Cause buildings to contract and expand
What Are S Waves?
 S waves are
secondary
waves
– Earthquake waves that vibrate from side
to side as well as up and down
– These waves shake the ground
back and forth
– Shake structures violently
–Cannot move through liquids
What Are Surface Waves?
 When P waves and S waves reach the
surface some are transformed into
surface waves
– Surface waves move more
slowly than P waves and S
waves
severe
– Produce the most
ground movements
– Can make the ground roll like ocean
waves or shake buildings from side to
side
How Do Scientists Detect
Seismic Waves?
Seismograph – records
movements
the ground
caused by seismic waves as they
move through the Earth
How Do Scientists Measure
Earthquakes?
 There are at least 20 different measures for rating
earthquakes, three are:
–Mercalli
–Richter
–Moment Magnitude
Magnitude – a measurement of
earthquake strength based on seismic waves
What Is the Mercalli Scale?
 Rated earthquakes according to
intensity
–Intensity: strength of
their
ground motion in a given place
 Not a precise measurement
 Describes how earthquakes
affect people, buildings, and the
land surface
What Is the Richter Scale?
 A rating of the size of
seismic waves as measured
by a particular type of
seismograph
 The rating 1-10 is a single
number that represents the
earthquakes magnitude.
 Accurate measurements for
small, nearby earthquakes not
large, distant earthquakes
Richter Scale
 Less than 2.0 not felt.About 8,000 per day
2.0-2.9
3.0-3.9
4.0-4.9
5.0-5.9
6.0-6.9
7.0-7.9
8.0-8.9
9.0-9.9
10.0
Generally not felt, but recorded.
Often felt, but rarely causes damage.
Noticeable shaking of indoor items, rattling
noises. Significant damage unlikely.
Can cause major damage to poorly constructed
buildings over small regions. At most slight damage to
well-designed buildings.
Can be destructive in areas up to about 160 kilometres
(100 mi) across in populated areas.
Can cause serious damage over larger areas.
Can cause serious damage in areas several hundred
miles across.
Devastating in areas several thousand miles across.1
per 20 years
Never recorded; Extremely rare (Unknown)
The largest recorded earthquake in the world was a magnitude 9.5 (Mw) in Chile
on May 22, 1960.
What Is the Moment
Magnitude?
rating system that
estimates the total energy
A
released by an earthquake
 Can be used to rate earthquakes
of all sizes, near or far
 Below
5.0 – little damage
 Above 5.0 – great destruction
How Do Scientists Locate the
Epicenter?
 Geologists use
seismic waves
– P waves arrive first
– S waves arrive close behind
– Scientist measure the
difference in arrival times
• The farther away an earthquake is the
greater the time between their arrival
– Scientists draw three circles using data
from seismographs set at different
stations to see where they intersect – the
epicenter
How do seismographs work
Seismic waves cause a
simple seismographs drum
to vibrate which in turn
causes the pen to record the
drum’s vibrations.
Seismogram: is the record of
the earthquakes seismic
waves produced by a
seismograph.
What patterns do seismographic
data reveal?
 Geologists have created maps of where
earthquakes occur around the world.
– They mark the earthquakes epicenter.
 The maps show that earthquakes often occur
along plate boundaries.
 Geologists still can’t predict when an earthquake
will occur, but they do know areas that are at high
risk for earthquakes and can try too prevent heavy
damage and loss of life by retrofitting and
reinforcing buildings and roads in those areas.
The latest seismic activity
Earthquake “belts”
 95% of energy from earthquakes from thin zones
(plate edges)
 Some are quite deep (subduction zones)
The Pacific Rim of Fire
 This notorious zone is
characterized by
subduction zones
 Earthquakes and
volcanoes here are
particularly violent
 friction from
subduction produces
large destructive quakes