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earthquake_location.ppt
1. What are
Earthquakes?
• The shaking or trembling caused by the sudden
release of energy
• Usually associated with faulting or breaking of
rocks
• Continuing adjustment of position results in
aftershocks
2. 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
4. P and S waves
– P or primary waves
• fastest waves
• travel through solids,
liquids, or gases
• compressional wave,
material movement is
in the same direction
as wave movement
– S or secondary waves
• slower than P waves
• travel through solids
only
• shear waves - move
material
perpendicular to
wave movement
5. 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
6. How are the Size and Strength of an Earthquake Measured?
• Modified Mercalli Intensity Map
– 1994 Northridge, CA earthquake,
magnitude 6.7
• Intensity
– subjective measure
of the kind of
damage done and
people’s reactions
to it
7. How are the Size and Strength of an Earthquake Measured?
• Magnitude
– Richter scale
measures total amount
of energy released by
an earthquake;
independent of
intensity
– Amplitude of the
largest wave produced
by an event is
corrected for distance
and assigned a value
on an open-ended
logarithmic scale
8.
9. Where Do Earthquakes Occur and How Often?
~80% of all earthquakes occur in the circum-Pacific belt ( ring of
fire)
– 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
10. The Economics and Societal Impacts of EQs
Damage in Oakland, CA, 1989
• Building collapse
• Fire
• Tsunami
• Ground failure
12. What is an Epicenter?
• A point on the surface of the Earth directly
above the FOCUS of the earthquake.
•The point within the Earth from which
earthquake waves originate.
13.
14. Use the data from the recording
stations:
• Station A: San Francisco, California
P-Wave arrival 3:02:20 S-Wave arrival 3:06:30
What is the time
difference
between P and
S wave arrivals?
15. Use the data from the recording
stations:
• Station B: Denver, Colorado
P-Wave arrival 3:01:40 S-Wave arrival 3:05:00
What is the time
difference
between P and
S wave arrivals?
16. Use the data from the recording
stations:
• Station C: Missoula, Montana
P-Wave arrival 3:01:00 S-Wave arrival 3:03:00
What is the time
difference
between P and
S wave arrivals?
17. Difference in arrival times:
San Francisco: 4:10
Denver, Colorado: 3:20
Missoula, Montana:
2:00
18. TAKE A PIECE OF PAPER, AND MARK
OFF THE DIFFERENCE IN ARRIVAL TIME
2800Km
4:10
19. MOVE THE PAPER UNTIL THE TWO
TICK MARKS LINE UP WITH THE P
AND S CURVES
WHEN TICK MARKS LINE UP, GO
STRAIGHT DOWN AND READ THE
EPICENTER DISTANCE
EPICENTER DISTANCE
OF 2800 KM
22. Recording Board
Difference in arrival times:
San Francisco: 41:0
2,800km
1,000
2,000
3,000
4,000
5,000
Open your compass to
the EXACT distance on
the scale.
24. How is an Earthquake’s Epicenter Located?
Time-distance graph
showing the average
travel times for P- and S-
waves. 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
