Transcript What are Earthquakes? - Lancaster City Schools

videos
Three Basic Types of Plate
Boundaries
Divergent
Transform
Convergent
USGS Graphics
Divergent boundaries
USGS sea-floor maps
New crust is generated as the plates
pull apart.
Occurs at spreading ocean ridges
and at continental rifts.
Earthquakes are shallow and small.
Example:
 East Pacific Rise (moving
apart at about 15 cm/year)
Examples:
 Atlantic mid-ocean ridge
 Basin and Range, USA
 African Rift Valley
 Northern Red Sea
Convergent Plate Boundaries
Ocean /Ocean convergence (Marianas)
Ocean /Continent convergence (Cascades)
Plates push together.
A) The denser plate
subducts (is pushed
under the other), or
B) two continental plates
crunch together to
form high mountains.
Continent/Continent Collision (Himalayas)
Earthquakes along Convergent Zones with
Subducting Oceanic Lithosphere
Shallow earthquakes:
The most destructive of these
occur between the plates
on the plate boundary.
Shallow earthquakes also occur
within the subducting plate
and within the overriding
plate near the plate boundary.
Intermediate and Deep earthquakes:
The depth range defined as “intermediate” is 100 – 300 km deep
while “deep” earthquakes are in the 300 – 700 km depth
range. Intermediate and deep earthquakes occur only within
the subducting oceanic lithosphere.
Transform Boundaries
Lithosphere is neither produced nor destroyed as
the plates slide horizontally past each other. Stress
can build up and when released, cause earthquakes.
Strike-Slip Fault
Example:
San Andreas Fault, California
Strike-slip fault between
two spreading ridges
allows the two plates to
move apart.
Deforming Earth’s Crust
Types of stress: Tensional, Compression, Shear
Undeformed beds: no stress applied.
Extension makes faults and regional
thinning. (Ex., Basin & Range.)
Compression makes faults and folds.
(Ex., Rocky Mountains.)
Shearing displaces layers horizontally
and can result in strike-slip faulting.
(Ex., San Andreas Fault, California.)
What is the Elastic Rebound Theory?
•
Explains how energy is
stored in rocks
– Rocks bend until the
strength of the rock
is exceeded
– Rupture occurs and
the rocks quickly
rebound to an
undeformed shape
– Energy is released in
waves that radiate
outward from the
fault
What are
Earthquakes?
• The shaking or trembling caused by the sudden
release of energy along fault lines.
• Usually associated with faulting or breaking of
rocks.
• Faults are found at all three kinds of
plate boundaries as well as within the
plates!!
Where does the energy come
from?
The edges of the fault lines tend to stick together
due to friction while the rest of the block is
moving.
The energy that would normally cause the blocks
to slide past one another is being stored up.
When the force of the moving blocks finally is
greater than the force of friction of the jagged
edges of the fault, it “unsticks” and all that stored
energy is released!
Where Do Earthquakes Occur and How Often?
~80% of all earthquakes occur in the circum-Pacific belt
– most of these result from convergent boundary activity
– ~15% occur in the Mediterranean-Asiatic belt
– remaining 5% occur in the interiors of plates and on
divergent boundaries
– more than 150,000 quakes strong enough to be felt are
recorded each year
Map, Seismic Risk
What are Seismic Waves?
• Energy that travels in all
directions in the form of
waves. This energy is
released when the rock
fractures or slips due to a
build up of stress.
• Two types:
– Body waves
• Primary (P) and Secondary (S)
– Surface waves
• Rayleigh and Love
The Focus and Epicenter of an Earthquake
•
•
•
The point within Earth
where faulting begins is
the focus
The point directly above
the focus on the surface
is the epicenter.
The epicenter has the
POTENTIAL to have
the greatest damage
since it receives the
most energy.
Two Types of Body Waves
(1) P or primary waves
• fastest waves
• travel through solids,
liquids, or gases
• compressional wave,
material movement is in
the same direction as
wave movement
Copyright © Houghton Mifflin Company. All rights reserved.
Geology, 10a–15
S Waves
(2) S or secondary waves
• slower than P waves
• travel through solids
only
• shear waves - move
material perpendicular
to wave movement
Copyright © Houghton Mifflin Company. All rights reserved.
Geology, 10a–16
Surface Waves: R and L waves
•
Surface Waves
– Travel just below or along the ground’s surface
– Slower than body waves; rolling and side-to-side
movement
– Especially damaging to buildings
Two Common Types
of Surface Waves
Copyright © Houghton Mifflin Company. All rights reserved.
Geology, 10a–18
How do scientists detect earthquakes?
When an earthquake occurs the seismic waves travel through the Earth to the
seismic station where the information is transmitted to distant computers.
A seismograph
detects and records
EQs.
A seismogram is the
EQ record.
How is an Earthquake’s Epicenter Located?
Seismic wave behavior
– P waves arrive first, then S waves, then L and R
– Average speeds for all these waves is known
– After an earthquake, the difference in arrival times at a
seismograph station can be used to calculate the distance
from the seismograph to the epicenter.
S-P tutorial
How is an Earthquake’s Epicenter Located?
Time-distance graph
showing the average
travel times for P- and Swaves. The farther away a
seismograph is from the
focus of an earthquake,
the longer the interval
between the arrivals of
the P- and S- waves
How is an Earthquake’s
Epicenter Located?
•
•
•
Three seismograph stations
are needed to locate the
epicenter of an earthquake
A circle where the radius
equals the distance to the
epicenter is drawn
The intersection of the circles
locates the epicenter
How are the Size and Strength of an Earthquake Measured?
• Magnitude
– Richter scale
measures total
amount of
energy released
by an
earthquake.
– Richter Scale
rates
earthquakes
from 0 to 10
(
Strongest earthquake was in 1960, centered in
Chile and was rated a 9.5!
How are the Size and Strength of an Earthquake
Measured?
Intensity
• Modified Mercalli Scale is a
subjective measure of the kind of damage
done and people’s reactions to it
• Depends on magnitude of quake as well as
soil types, time of day of the quake, building
styles and whether the area is densely
populated.
• Ranges from 1 to 12.
Modified Mercalli Intensity Map
1994 Northridge, CA earthquake, magnitude 6.7
The Economics and Societal Impacts of EQs
•
•
•
•
Damage in Oakland, CA, 1989
Building collapse
Fire
Tsunami
Ground failure
Earthquake Depth and Plate Tectonic
Setting, Selfos, Iceland (June 200)
Source: AP Photo/Morgunbladid, Sverrir Vilhelmsson
Copyright © Houghton Mifflin Company. All rights reserved.
Geology, 10a–29
Effects of
Mexico City
Earthquake
Source: Albert Copley/Visuals Unlimited
Copyright © Houghton Mifflin Company. All rights reserved.
Geology, 10a–30
(from Kobe Univ.) Buildings founded on liquefied ground will lean or topple.
Can Earthquakes be Predicted?
Earthquake Prediction Programs
– include laboratory and field studies of rocks before, during,
and after earthquakes
– monitor activity along major faults
– produce risk assessments