Finding an Earthquakes Epicenter
Background
Earthquakes occur because of a sudden
release of stored energy. This energy
has built up over long periods of time
as a result of tectonic forces within the
earth. Most earthquakes take place
along faults in the upper 25 miles of
the earth's surface when one side
rapidly moves relative to the other
side of the fault. This sudden motion
causes shock waves (seismic waves)
to radiate from their point of origin
called the focus and travel through the earth. The epicenter is the point on the surface
directly above the focus. It is these seismic waves that can produce ground motion which
people call an earthquake.
The rocks that seismic waves travel
through quickly organize the waves
into two types; longitudinal and
transverse. The longitudinal
waves, also known as primary or P
waves, travel fastest, at speeds
between 1.5 and 8 kilometers per
second in the Earth's crust. The
transverse waves, also known as
secondary or S waves, travel more
slowly, usually at 60% to 70% of
the speed of P waves.
Since waves travel outward from
the source in all directions, it is
possible to track an earthquake back to its source. The greater the earthquake damage an
area suffers, the closer the area is to the earthquakes epicenter. One way to measure and
track this damage is the modified Mercalli scale.
Purpose
To find the epicenter of an earthquake using the Mercalli scale.
Materials
A pencil and colored pencils
Procedure
1. Use the modified Mercalli scale to rate the severity of the earthquake in each
location listed in the data table. Write the Roman numeral rating for each in
column 3 of the data table.
2. Draw lines on the map connecting areas of matching earthquake severity.
Label each zone
3. Color each zone with a different color
Name___________________________________________ D____________ P_____
Finding an Earthquakes Epicenter
Background
Earthquakes occur because of a sudden release of
_____________. This energy has built up over long
periods of time as a result of tectonic forces within
the earth. Most earthquakes take place along faults in
the upper 25 miles of the earth's surface when one
side ________________________ relative to the
other side of the fault. This sudden motion causes
shock waves (_____________________) to radiate
from their point of origin called the
_______________ and travel through the earth. The ______________is the point on the
surface directly above the focus. It is these seismic waves that can produce ground
motion which people call an earthquake.
The rocks that seismic waves travel through
quickly organize the waves into two types;
longitudinal and transverse.
The______________ waves, also known as
primary or ___ waves, travel fastest, at
speeds between 1.5 and 8 kilometers per
second in the Earth's crust. The
___________________ waves, also known
as secondary or _____ waves, travel more
slowly, usually at 60% to 70% of the speed
of P waves.
Since waves travel outward from the source
in all directions, it is possible to track an earthquake back to its source. The greater the
earthquake damage an area suffers, the closer the area is to the earthquakes epicenter.
One way to measure and track this damage is the modified Mercalli scale.
Purpose
To find the epicenter of an earthquake using the Mercalli scale.
Materials
A pencil and colored pencils
Procedure
1. Use the modified Mercalli scale to rate the severity of the earthquake in each
location listed in the data table. Write the Roman numeral rating for each in
column 3 of the data table.
2. Draw lines on the map connecting areas of matching earthquake severity.
Label each zone
3. Color each zone with a different color
Map and Answer Sheet
Analysis and Conclusions
1. What cities were nearest the epicenter of the earthquake?
__________________________________________________________________
__________________________________________________________________
2. Where was the focus of the Earthquake?
__________________________________________________________________
__________________________________________________________________
3. Explain the process that causes seismic waves.
__________________________________________________________________
__________________________________________________________________
__________________________________________________________________
4. Earthquake waves travel out from the focus in all _________________________.
5. Earthquakes happen because of a release of _____________________________.
6. Transverse earthquake waves are called ________________________________.
7. Longitudinal earthquake waves are called _______________________________.
8. Explain 3 ways the picture below is similar to what happens during an earthquake.
A.______________________________________________________________________
________________________________________________________________________
________________________________________________________________________
B.______________________________________________________________________
________________________________________________________________________
________________________________________________________________________
C.______________________________________________________________________
________________________________________________________________________
________________________________________________________________________
Mercalli Scale of Earthquake Intensity
I.
Not felt except by a very few under especially favorable conditions.
Usually only felt very close to the epicenter
II.
Felt only by a few persons at rest, especially on upper floors of
buildings. Delicately suspended objects may swing.
III.
Felt quite noticeably by persons indoors, especially on upper floors
of buildings. Many people do not recognize it as an earthquake. Standing
motor cars may rock slightly. Vibration similar to the passing of a truck.
Duration estimated.
IV.
Felt indoors by many, outdoors by few during the day. At night,
some awakened. Dishes, windows, doors disturbed; walls make cracking
sound. Sensation like heavy truck or train passing. Standing motor cars
rocked noticeably.
V.
Felt by nearly everyone; many awakened. some dishes, windows
broken. Unstable objects overturned. Pendulum clocks may stop.
VI.
Felt by all, many frightened. Some heavy furniture moved; a few
instances of fallen plaster. Damage slight.
VII. Little damage in buildings of good design and construction; slight
to moderate in well-built ordinary structures; considerable damage in
poorly built or badly designed structures; some chimneys broken.
VIII. Damage slight in specially designed structures; considerable
damage in ordinary substantial buildings with partial collapse. Damage
great in poorly built structures. Fall of chimneys, factory stacks,
columns, monuments, walls. Heavy furniture overturned.
IX.
Damage considerable in specially designed structures; welldesigned frame structures thrown out of plumb. Damage great in
substantial buildings, with partial collapse. Buildings shifted off
foundations.
X.
Some well-built wooden structures destroyed; most masonry and
frame structures destroyed with foundations. Rails bent.
XI.
Few, if any structures remain standing. Bridges destroyed. Rails
bent greatly.
XII. Total Destruction. Lines of sight and level are distorted. Objects
thrown into the air.