This document discusses the nature and mechanics of earthquakes, defining key vocabulary such as fault, seismic wave, focus, and epicenter. It describes the causes and locations of earthquakes in relation to tectonic plate boundaries, lists the different types of faults, and explains seismic waves and their impact on Earth's structure. Additionally, it covers methods for locating epicenters and measuring earthquake magnitude using various scales.

1. Earthquakes
Chapter 15 Lesson 1
P530-540

2. Vocabulary
• Earthquake(531) – the vibrations in the ground that
result from movement along breaks in Earth’s
lithosphere
• Fault (533) – a break in Earth’s lithosphere where one
block of rock moves toward, away from, or past
another
• Seismic wave (534) – energy that travels as vibrations
on and in Earth
• Focus (534) – These waves originate where rocks first
move along the fault, at a location inside Earth
• Epicenter (534) – the location on Earth’s surface
directly above the earthquake’s focus

3. Vocabulary
• Primary wave (535) – Also called p-waves, it causes particles in
the ground to move in a push-pull motion similar to a coiled
spring
• Secondary wave (535) – Also called S-waves, it causes particles to
move up and down at right angles relative to the direction the
wave travels
• Surface wave (535) – causes particles in the ground to move up
and down in a rolling motion
• Seismologist (536) – scientists that study earthquakes
• Seismometer (537) – an instrument that measures and records
ground motion and can be used to determine the distance seismic
waves travel
• Seismogram (537) – a graphical illustration of seismic waves

4. What are Earthquakes?
• Earthquakes are the vibrations in the ground that
result from movement along breaks in Earth’s
lithosphere
• These breaks are called faults
• The forces that move tectonic plates also push
and pull on rocks along the fault and if these
become big enough the blocks of rock on either
side of the fault can move horizontally or vertically
• The greater the force the larger and more
disastrous the earthquake

5. Where do Earthquakes Occur?

6. Earthquakes and Plate Boundaries
• Earthquakes result from the build up and release
of stress along active plate boundaries
• Some earthquakes occur more than 100km below
Earth’s surface
• The deepest earthquakes occur at convergent
plate boundaries
– Here the denser oceanic plate subducts into the
mantle
– These earthquakes release tremendous amounts of
energy

7. Earthquakes and Plate Boundaries
• Shallow earthquakes occur along divergent
plate boundaries, like mid-ocean ridges
• Shallow earthquakes also occur along
transform boundaries
• Along continental convergent boundaries,
earthquakes of varying depths occur

8. Rock Deformation
• When a force such as pressure is applied to
rock along plate boundaries, the rock can
change shape. This is called rock deformation.
• Eventually rocks can be deformed so much
that they break and move.
• This is a lot like bending a stick until it breaks

9. Faults
• When stress builds in places like a plate
boundary, rock can form faults.
• A fault is a break in Earth’s lithosphere where
one block of rock moves toward, away from,
or past another
• When rocks move in any direction along a
fault, an earthquake occurs
• The direction depends on the force applied

10. Types of Faults

11. Reverse Fault
• Forces push two blocks of rock
together.
• The rock above the fault moves
up relative to the block of rock
below the fault
• Occurs at convergent
boundaries

12. Normal Fault
• Forces pull two blocks of
rock apart.
• The rock above the fault
moves down relative to the
rock below the fault
• Occurs at divergent plate
boundaries

13. Strike-Slip
• Two blocks of rock slide
horizontally past each other
in opposite directions
• Occurs at transform plate
boundaries

14. Earthquake Focus and Epicenter
• When rock moves along a fault, they release energy that
travels as vibrations on and in Earth called seismic waves
• These waves originate where rocks first move along the
fault, at a location inside Earth called the focus
– An earthquakes focus can occur anywhere between Earth’s
surface and depths of greater than 600km
• Earthquakes are often referred to by their epicenter
– The epicenter is the location on Earth’s surface directly above the
earthquake’s focus

15. Seismic Waves
• During an earthquake, a rapid release
of energy along a fault produces
seismic waves
• Seismic waves travel outward in all
directions through rock
– Similar to a stone being dropped in
water, seismic waves move outward in
circles
– Seismic waves transfer energy through
the ground and produce the motion
that you feel during an earthquake
– The energy released is stronger near the
epicenter and decrease in energy and
intensity as you move outward

16. Types of Seismic Waves

17. Primary Waves (P-Waves)
• Causes rock particles to vibrate in the same
direction that waves travel
• Fastest seismic wave
• First to be detected and recorded
• Travels through solids and liquids

18. Secondary Waves (S-Waves)
• Causes rock particles to vibrate perpendicular to the
direction that waves travel
• Slower than P-waves, but faster than surface waves
• Detected and recorded after P-waves
• Only travels through solids

19. Surface Waves
• Cause rock particles to move in a rolling or
elliptical motion in the same direction that waves
travel
• Slowest seismic wave
• Generally causes the most damage

20. Mapping Earth’s Interior
• Scientists that study earthquakes are called
seismologists
• They use the properties of seismic waves to
map Earth’s interior
• P-waves and S-waves change speed and
direction depending on the material they
travel through

22. Inner and Outer Core
• Through extensive earthquake studies,
seismologists have discovered that S-waves
cannot travel through the outer core
• This discovery proved that Earth’s outer core
is liquid unlike the solid inner core
• By analyzing speed of P-waves traveling
through the core, seismologists also
discovered that the inner and outer cores are
composed of mostly iron and nickel

23. The Mantle
• Seismologists also have used seismic waves to
model convection currents in the mantle
• The speeds of seismic waves depend on the
temperature, pressure, and chemistry of the rocks
that the seismic waves travel through.
• Seismic waves tend to slow down as they travel
through hot material
– For example, seismic waves are slower in areas of the
mantle beneath mid-ocean ridges or near hotspots
– Seismic waves are faster in cool areas of the mantle
near subduction zones

24. Locating an Earthquakes Epicenter
• An instrument called a seismometer measures and records
ground motion and can be used to determine the distance seismic
waves travel
• Ground motion is recorded as a seismogram, a graphical
illustration of seismic waves
• Seismologists use a methods involving the speed and travel times
of the waves to determine the distance to the earthquake
epicenter from at least three different seismometers, this is called
triangulation

25. How to Find an Epicenter?
Step 1:
Determine the number of seconds between the arrival of the first P-
wave and the first S-wave on the seismogram.
This is called lag time.
Lag time = (arrival time of 1st S-wave) – (arrival time of 1st P-wave)

26. How to Find an Epicenter?
Step 2:
Use a graph showing lag time versus distance.
Use the lag time to find the distance.

27. How to Find an Epicenter?
Step 3:
Using a ruler and a map scale, measure the distance between the
seismometer and the earthquakes epicenter.
Draw a circle with a radius equal to the distance.
When three circles are plotted, the epicenter will be where the
three circles intersect.

28. Lab
• http://www.glencoe.com/sites/common_asse
ts/science/virtual_labs/ES09/ES09.html

29. Determining Earthquake Magnitude
• Scientists use 3 different scales to measure
and describe earthquakes:
– The Richter Scale
– The Modified Mercalli Scale
– Moment magnitude scale

30. The Richter Scale
• The Richter magnitude scale uses the amount of
ground motion at a given distance from an earthquake
to determine magnitude
• It begins at zero, but there is no upper limit to the
scale
• Each 1 unit increase represents ten times the amount
of ground motion recorded on a seismogram.
– For example, a magnitude 8 earthquake produces 10 times
greater shaking than a magnitude 7
• The largest earthquake ever recorded was a magnitude
9.5 in Chile in 1960. The earthquake and following
tsunamis left nearly 2000 people dead and 2 million
people homeless.

32. Moment Magnitude Scale
• Seismologists use the moment magnitude scale to measure
the total amount of energy released by the earthquake.
• The energy released depends on the size of the fault that
breaks, the motion that occurs along the fault, and the
strength of the rocks that break during an earthquake.
• The units in this scale are exponential
• For each increase of one unit on the scale, the earthquake
releases 31.5 times more energy
– That means a magnitude 8 earthquake releases more than 992
times the amount of energy than that of a magnitude 6
earthquake.

34. The Modified Mercalli scale
• Another way to measure and describe an earthquake is
to evaluate the damage that results from shaking
– Shaking is directly related to earthquake intensity
• The Modified Mercalli scale measures earthquake
intensity based on descriptions of the earth’s effects
on people and structures
– The Mercalli scale ranges from I, when shaking is not
notice able, to XII, when everything is destroyed.
• Local geology also contributes to earthquake damage.
– In an area covered by loose sediment, ground motion is
exaggerated.
• http://elearning.niu.edu/simulations/images/S_portfolio/Mercal
li/Mercalli_Scale.swf

37. *Side Notes: Roman Numerals
• V=Values: X = 10 ; V = 5 ; I = 1
• Add similar values that are next to one another such as III (1+1+1
= 3)
• Add a smaller value that comes after a larger value, such as XV (10
+ 5 = 15)
• Subtract a smaller value that precedes a larger value, such as IX
(10 – 1 = 9)
• Use the fewest possible numerals to express the value (X rather
than VV)
• Counting goes like
– I, II, III, IV, V, VI, VII, VIII, IX, X, XI, XII, XIII
*Practice: What is the value of Roman numeral XVI? XIV?

38. Earthquake Risk
• Not all earthquakes occur near plate boundaries
• Seismologist study the probability of an
earthquake at a given location
• Areas that experienced earthquakes in the past
will likely experience earthquakes again
• The New Madrid Fault in the central United
States has a history of severe earthquakes in
1811-1812 registering magnitudes of 7.8 and
8.1
• However, on average only about 10 earthquakes
with magnitudes greater than 7.0 occur
worldwide each year.