Features of Earthquakes
Chapter 11, Section 2
Focus
 The point on a fault where rocks “break” and
energy is released.
Epicenter
 Point on the surface of the Earth directly
above the focus.
Seismograph
 Instruments that register seismic waves and
record horizontal and vertical ground
movement. http://scign.jpl.nasa.gov/learn/eq7.htm
 Produce a paper record of the seismic event
that is called a seismogram.
Magnitude
 The height (or amplitude) of the lines on a
seismogram indicate the magnitude or
strength of an earthquake.
 Most commonly measured with the Richter
magnitude scale.

For each increase of 1.0 on the scale:



The height of the line on the seismogram is 10x
greater.
The amount of released energy is 32x greater.
A magnitude 9.5 earthquake is the largest ever
recorded (southern Chile – 1960).
Locating an Epicenter
 Because s and p waves travel at different
speeds, they each reach a seismograph
station at different times.
 The difference in time can be converted into a
distance.
 Using three seismograph stations, the
location of the epicenter can be determined
through triangulation.
Tips for Your Assignment:
 Finding amplitude of
S-wave:

The distance
between the center
line and the highest
peak on the
seismogram.
Tips for Your Assigment:
 Finding S-P lag time:

Time between the
start of the p-wave
and the start of the
s-wave.
Tips for Your Assignment:
 Finding distance:

Use the chart to convert
s-p lag time into
distance.
Tips for Your Assignment:
 Finding magnitude:

Line up distance and swave amplitude to
determine magnitude.
Tips for Your Assignment:
 Finding epicenter location:


Draw a circle from each station with a radius
equal to the distance of the station from the
epicenter.
Epicenter is located where the three circle
cross.