Earthquakes
Earthquake Terms
• An earthquake is a trembling of the
Earth caused by a sudden release of
energy stored in subsurface rock units
(on the Moon these are called
moonquakes).
• Earthquake activity is also referred to
as seismic activity.
• The energy is generally released along
fault lines as sections of ground move.
Earthquake Terms
• The point of energy release is called the
focus. The point on the Earth directly
above the focus is called the epicenter.
• Surface waves move along the surface
of the Earth while body waves
penetrate the Earth’s interior. Surface
waves cause most of the structural
damage.
Waves
• The types and sizes of waves generated
by an energy release can provide
information about the location of the
quake and the magnitude.
• Earthquakes are recorded by an
instrument known as a seismograph.
Types of Waves
• Primary or P-waves – cause compression
and dilation along axis of propagation
Types of Waves
• Shear or S-waves – cause vertical
motion along axis of propagation
Seismographs
• P-waves travel faster than S-waves.
Seismographs
• The time between the arrival of the P wave
and S-wave is the S-P time interval. It is
related to the distance of the detecting
device from the epicenter.
S-P
Interval
If you know
the S-P
interval you
can determine
the distance
from the
recording
station to the
epicenter.
When you find the distance from the epicenter for 3
stations, you can pinpoint the epicenter. You draw a circle
from each station with a radius that is the distance to the
epicenter for each station. Where the 3 circles intersect
is the epicenter.
Magnitude
• To determine the magnitude of an
earthquake you need the distance to the
epicenter and the strength (amplitude)
of the S-wave from the seismogram.
Magnitude
For any seismic station, the
distance to the epicenter and
the amplitude of the S-wave
will estimate the magnitude.
Using the nomogram shown
here, draw a line connecting
the distance and amplitude
for a the station. Where the
line crosses the magnitude
scale is the approximate
magnitude.
For a distance of 220 miles
and an amplitude of 50 mm,
the magnitude is 5.3.
Magnitude and the Richter Scale
Now…
• Back to the “Shake, Rattle,
and Roll” activity and locate
an earthquake yourself!