Transcript Earthquakes PowerPoint

EARTHQUAKES
Earthquakes are the shaking, rolling or
sudden shock of the earth’s surface.
Earthquakes happen along "fault lines" in the
earth’s crust. Earthquakes can be felt over large
areas although they usually last less than one
minute.
Earthquakes cannot be
predicted -- although
scientists are working on it!
Most of the time, you will notice an
earthquake by the gentle shaking of
the ground. You may notice hanging
plants swaying or objects wobbling on
shelves. Sometimes you may hear a
low rumbling noise or feel a sharp
jolt. A survivor of the 1906 earthquake
in San Francisco said the sensation
was like riding a bicycle down a long
flight of stairs.
RICHTER SCALE
Small: 5.0 to 5.9
Moderate: 6.0 to 6.9
Major: 7.0 to 7.9
Great: 8.0
or greater
SAN FRANCISCO
EARTHQUAKE
April 18, 1906
3000 deaths
28,000 buildings
destroyed
(most by fire)
$10B damage
“The whole street was
undulating as if the waves
of the ocean were coming
toward me.”
“I saw the whole city
enveloped in a pile of dust
caused by falling buildings.”
“Inside of twelve hours half
the heart of the city was
gone”
Convergent Boundaries
What happens when plates collide? It
depends how the plates are moving when
they meet:
When two plates collide head-on, they push
each other up and form mountains. That's
how the Himalayas and other great
mountain ranges (including the Rockies,
long ago) were created.
Divergent Boundary
Another boundary is the Divergent
boundary: this is where the
plates separate and move apart. They
often form a rift zone. Most are
located on the oceanic floor where
new seafloor is created at the
separating edges. One example is the
Mid-Atlantic Ridge.
When two
plates slide
past each
other, they
create a
transform
fault, like
the San
Andreas
fault.
ELASTIC REBOUND
ELASTIC REBOUND
Over many years, rocks on opposite sides of the fault move,
but friction on fault "locks" it and prevents slip
Eventually strain accumulated overcomes friction,
and fault slips in earthquake
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
STRESS AND STRAIN: ROCK BEHAVIOR
Strain - the result of stress or deformation
elastic deformation - when stresses are removed, rock returns to original shape
plastic deformation - permanent deformation. when stresses are removed, rock
stays bent
rupture - breakage and fracturing of the rock, causing an earthquake.
Brittle materials break during elastic deformation.
The Focus and Epicenter of an
Earthquake
• The point within
Earth where
faulting begins is
the focus, or
hypocenter
• The point directly
above the focus
on the surface is
the epicenter
WHERE DO EARTHQUAKES OCCUR AND HOW OFTEN?
~80% of all earthquakes
occur in the circum-Pacific
belt
most of these result from
convergent margin activity
~15% occur in the
Mediterranean-Asiatic belt
remaining 5% occur in the
interiors of plates and on
spreading ridge centers
more than 150,000 quakes
strong enough to be felt are
recorded each year
THE ECONOMICS AND SOCIETAL IMPACTS OF EQS
•
•
•
•
Damage in Oakland, CA, 1989
Building collapse
Fire
Tsunami
Ground failure
BODY WAVES: P AND S WAVES
Body waves
Po r primary waves
fastest waves
travel thro ugh so lid s, liquid s,o r
gases
co mpressio nal wave, material
o
m vement is in the same
directio n as wave m
o vement
So r sec
o n
d ary waves
slo wer than P waves
travel thro ugh so lid so nly
shear waves - m
o ve material
perpen
d icular to wave
o vement
m
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
This is an image of a seismograph, an
instrument used to record the energy
released by an earthquake. When the needle
is moved by the motion of the earth, it
leaves a wavy line.
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