Earthquakes
Chapter 6
Elastic Rebound Theory
• Rocks on each side of the fault are moving slowly
• If the fault is “locked”, stress increases
• Rocks fracture and spring back to their original
shape
• Rocks along the fault release energy as seismic
waves
• Focus: area on the fault where the slippage
first occurs.
• Epicenter: point on the earth’s surface
directly above the focus
Focus Depths
Shallow-Focus Earthquakes
– Within 70 km of the surface
– Causes the most damage
Intermediate Focus Earthquakes
– Within 70-300 km of the surface
Deep-Focus Earthquakes
– From 300-650 km of the surface
– Occur in subduction zones
– Occur farther inland
3 Major
Earthquake
Zones
Pacific Ring of Fire
Mid-Ocean Ridges
Eurasian-Melanesian
Mountain Belt
• Fault Zones:
– Groups of
interconnected faults
– Form at plate
boundaries because of
the intense stress that
results
• San Andreas Fault
•
3 Types of
Seismic Waves
P waves
• Primary waves
– Fastest
– First recorded by a seismograph
– Travel through solids and liquids
– Faster through more rigid material
– Compression waves
• Cause rock particles to move
together and apart along the
direction of the waves
P-Wave movement
S waves
• Secondary Waves
– Second waves recorded by a
seismograph
– Only travel through solids
– Can’t be detected on the side of the
earth opposite the earthquake’s
epicenter.
– Shear waves
• Cause rock particles to move at
right angles to the direction in
which the waves are traveling
•
S-Wave movement
Surface Waves
• Slowest moving wave
• Last to be recorded by
seismograph
• Travel similar to ocean waves
• Cause the surface to rise, fall and
turn
• Very destructive: especially in
loose earth
Surface Waves movements
Finding the
Epicenter
of an
Earthquake
• Analyze
difference in
arrival times
between P waves
and S waves.
• Need info from 3
seismograph
stations at
different
locations
Measuring Earthquakes also called a
Seismic Event
Three standard measures of the "size" of
the earthquake are commonly used:

Richter Magnitude Scale (based on energy
released as measured by seismometers) less than 3.5
Moment Magnitude Scale (based on total energy
released by an EQ or seismic event.)
Modified Mercalli Intensity Scale (based on
damage caused as assessed by people).

Magnitude
•Measure of the energy released
by an earthquake
•The amount of ground motion
•Higher the magnitude the larger
the seismic event.
Moment Magnitude Scale
• Introduced 1979 to replace Richter Scale.
• Each step is an increase of 10 x 10…
– A 4 is 10000 times a 3
– A 3 is 1000 times a 2
– A 2 is 100X a 1.
Moment Magnitude Scale
• One advantage of this scale is that it does
not saturate at the upper end.
– For example: with Richter Scale a major EQ
lasting 1 minute could be a 9, while the same
EQ lasting 5 min is still a 9, in fact still a 9 if it
lasted 10 minutes.
• So, moment magnitude is most often used
to estimate large earthquake magnitudes.
• The USGS uses Richter for EQs smaller
than 3.5.
Modified Mercalli Scale
• Modified Mercalli (MM) Scale based on
1909 model of volcano damage.
• Lower steps on the MM scale deals how
the EQ is felt by people.
• Higher steps are based on observed
structural damage.
Modified Mercalli Scale
• I. People do not feel any Earth movement.
• II. A few people might notice movement if they are at rest and/or on the
upper floors of tall buildings.
• III. Many people indoors feel movement. Hanging objects swing back
and forth. People outdoors might not realize that an earthquake is
occurring.
• IV. Most people indoors feel movement. Hanging objects swing. Dishes,
windows, and doors rattle. The earthquake feels like a heavy truck hitting
the walls. A few people outdoors may feel movement. Parked cars rock.
• V. Almost everyone feels movement. Sleeping people are awakened.
Doors swing open or close. Dishes are broken. Pictures on the wall
move. Small objects move or are turned over. Trees might shake. Liquids
might spill out of open containers.
• VI. Everyone feels movement. People have trouble walking. Objects fall
from shelves. Pictures fall off walls. Furniture moves. Plaster in walls
might crack. Trees and bushes shake. Damage is slight in poorly built
buildings. No structural damage.
Modified Mercalli Scale
• VII. People have difficulty standing. Drivers feel their cars shaking. Some
furniture breaks. Loose bricks fall from buildings. Damage is slight to moderate
in well-built buildings; considerable in poorly built buildings.
• VIII. Drivers have trouble steering. Houses that are not bolted down might shift
on their foundations. Tall structures such as towers and chimneys might twist and
fall. Well-built buildings suffer slight damage. Poorly built structures suffer
severe damage. Tree branches break. Hillsides might crack if the ground is wet.
Water levels in wells might change.
• IX. Well-built buildings suffer considerable damage. Houses that are not bolted
down move off their foundations. Some underground pipes are broken. The
ground cracks. Reservoirs suffer serious damage.
• X. Most buildings and their foundations are destroyed. Some bridges are
destroyed. Dams are seriously damaged. Large landslides occur. Water is thrown
on the banks of canals, rivers, lakes. The ground cracks in large areas. Railroad
tracks are bent slightly.
• XI. Most buildings collapse. Some bridges are destroyed. Large cracks appear in
the ground. Underground pipelines are destroyed. Railroad tracks are badly bent.
• XII. Almost everything is destroyed. Objects are thrown into the air. The ground
moves in waves or ripples. Large amounts of rock may move.
Magnitude Scales
• Richter Scale
Richter scale no.
No. of earthquakes
per year
Typical effects of this magnitude
< 3.4
800 000
3.5 - 4.2
30 000
4.3 - 4.8
4 800
Most people notice them, windows rattle.
4.9 - 5.4
1400
Everyone notices them, dishes may break, open
doors swing.
5.5 - 6.1
500
Slight damage to buildings, plaster cracks, bricks
fall.
6.2 6.9
100
Much damage to buildings: chimneys fall, houses
move on foundations.
7.0 - 7.3
15
7.4 - 7.9
4
> 8.0
One every 5 to 10 years
Detected only by seismometers
Just about noticeable indoors
Serious damage: bridges twist, walls fracture,
buildings may collapse.
Great damage, most buildings collapse.
Total damage, surface waves seen, objects thrown
in the air.
Destruction to Buildings and Property
• 1. Most buildings are not designed to
withstand shaking from an EQ.
• 2. Type of ground beneath affects the way
buildings respond to EQ.
• 3. Buildings on loose soil and rock, much
more likely to be damaged.
• 4. Buildings on firm soil or bedrock are
much more likely to survive.
Tsunamis
Form in one of two ways
1. Faulting, causes a sudden drop or rise in
ocean floor, displaces a large volume of sea
water. This volume of water churns up and
down as it adjusts to the change. This sets into
motion a series of low, long waves that
develop into tsunamis.
Tsunamis
• 2. Underwater landslide causes tons of
debris to drop down to sea floor, which
causes water to be displaced, then this
sets into motion a series of low, long
waves that develop into tsunamis.
Earthquake Warnings and Predictions
Smiley face denotes success of research.
1. Animal research continues to determine if they can tell us
something. 
2. Using past EQ dates scientists approximate future dates. 
3. Sensors placed at faults help to measure strain on rocks. 
4. SEISMIC GAPS_ a place where fault is locked & does not move
lets strain build up surrounding it. These are believed to be future
EQ 
5. Slight tilting of ground before EQ. 
6. Magnetic & electrical properties of rocks change 
7. Increase in natural gas seepage from rocks
8. Decrease in P wave speed, longer the decrease stronger the EQ 
