Transcript Earthquakes

Aim: What is an earthquake?
Do now: In your notes, explain where most
earthquakes occur? Why?
I. Earthquakes
- are vibration of the Earth
produced by the release
of energy.
- Caused by moving
plates at the boundaries.
A. Focus - the origin or starting point of the
earthquake underground. Energy is released
in all directions.
B. Epicenter – the location on the surface
directly above the focus.
II. Causes of Earthquakes
• Earthquakes are caused by the release
of potential energy stored in rock.
• Energy is
released in
all directions
A. Elastic Rebound Hypothesis
- Deformation of rocks (stressing/bending)
cause them to build up pressure (energy).
- When they break, the stored energy is
released which results in the vibrations.
B. Aftershocks and Foreshocks
•
movements of the crust that follow a
major earthquake called aftershocks.
•
small vibrations called foreshocks
come before a major earthquake.
Anchorage, Alaska
Closure:
Why do most earthquakes cause little
damage and loss of life?
- Because most occur in
areas that are not
populated.
L.A. Earthquake
Earthquake Destruction
AIM: How are Earthquakes
Measured?
Do now:
If you were
asked to measure a
recent Earthquake,
what information,
observations or
data would you
use?
I. Measuring Earthquakes
• Seismology – the study of earthquake
waves.
• Seismographs –
instruments that
record earthquake
waves. Data is
recorded on a
seismogram.
Seismogram/Seismograph
• Seismogram – a graph produced by
recording the ground motion.
*Seismos = shake
* Gramma = what is
written
1. Richter Scale: Measures the magnitude
(strength) of an earthquake.
2. Moment Magnitude
- estimates the energy released by
earthquakes.
II. Earthquake Waves
A. Surface Waves – are seismic waves
that travel along the Earth’s outer
layers.
- These are the most destructive
earthquake waves.
B. Body Waves
•
Waves that travel
through the Earth’s
interior. This
includes P and S
waves.
• These waves help us
understand the Earth’s
interior
epicenter
1. P-waves
(Primary / Compression Waves)
- Are push-pull waves that compress and
expand in the direction that the waves
travel.
- Travel through solids, liquids, and gases.
- Have the greatest velocity of all earthquake
waves.
2. S- Waves
(Secondary / Transverse Waves)
- Shake particles at right angles to the
direction that they travel. (up and
down)
- Travel only through solids. Cannot go
through liquids.
- Slower velocity than P waves.
•A seismogram
records all three
types of seismic
waves – surface
waves, P waves
and S waves.
•P & S Wave Animation
III. Earthquake Zones
• About 95% of the major earthquakes
occur in a few narrow zones.
– Circum-Pacific Belt – the outer edge of the
Pacific Ocean (Ring of Fire)
– Mediterranean-Asian Belt - Mediterranean
Sea
Aim: How do we locate Epicenters?
Do Now:
1) From the side table obtain the two
tan sheets.
2)Describe the causes of earthquakes.
Earthquakes are caused by the rapid release of
energy stored in rock that has been subjected to
great forces (at plate boundaries where plates
move).
This releases of energy causes the vibrations of
an earthquake.
I. Locating an Earthquake
epicenter
• The difference in arrival times of P and S
waves provides a way to locate the epicenter.
• The greater the time interval between the
arrival of the P wave and S wave, the greater
the distance to the earthquakes epicenter.
Example:
A) S-wave - P-wave = 18 sec
B) S-wave - P-wave = 35 sec
II. Distance to Epicenter
A) Find the difference in arrival time
between the first P wave and the first S
wave on the seismogram.
S-wave Arrival
-
P-wave Arrival
B) Using ESRT page 11 and a piece of scrap
paper, find the equivalent distance that
matches the difference in time.
**ESRT Page 11 and a piece of scrap paper **
S-wave – P-wave =
00:04:45
1) Use the time axis to
mark off this difference
in time. Always use 0
to start.
2) Slide the scrap
paper up the chart to
match the P and S
wave lines with the
marks you made.
3) Once they are
matched, as close as
you can, read down
from this intersection
to find the equivalent
distance.
“Spread Technique”
III. P- Wave
Travel Time
(or s-wave travel
time)
-Use the distance
from the previous
step to find the travel
time.
- Move up to the
wave you are
looking for. (either P
or S)
IV. Origin Time
-The origin is when the earthquake
began. (start)
- Subtract the arrival time (P- wave
or S- wave) from the travel time (Pwave or S- wave)
Origin (start)
Travel Time
Arrival (end)
V. Epicenter Location
• Data from three or more seismographs
must be used to find the exact location
of an earthquake epicenter.
Epicenter
- Use the distance
to epicenter value
and a protractor to
draw these circles.