10.2 Locating and Measuring
Earthquakes
Key Idea:
A seismograph is used to determine
the magnitude of an earthquake and
the location of its epicenter.
What is Seismograph?
 A seismograph is an instrument that detects
and records waves produced by
earthquakes.
 Scientists use data from seismographs to
locate the earthquakes’ epicenters , and
measure their magnitudes.
 There are more than 10,000 seismic
stations which monitor the seismic activity
world wide.
The Seismograph
 Fundamentally, a seismograph is a simple
pendulum. When the ground shakes, the
base and frame of the instrument move with
it, but inertia keeps the pendulum bob in
place. It will then appear to move relative to
the shaking ground. As it moves it records
the pendulum displacements as they
change with time, tracing out a record called
a seismogram.
The basic seismograph, as described
above
Notice the parts:
Base
Pendulum
Moving paper
Types of Seismographs
Because an earthquake produces different
types of waves, there are seismographs
which record each type of wave.
 Some record the side-to-side motion of the
bedrock (created by P waves)
 Others record the up-and-down motion
(created by the S waves).
Horizontal Seismograph (records P
waves)
Vertical Seismograph
 This types records the up-and-down
movements (S waves)
How is an Earthquake’s Epicenter Located?
Time-distance graph
showing the average
travel times for P- and Swaves. The farther away a
seismograph is from the
focus of an earthquake,
the longer the interval
between the arrivals of
the P- and S- waves
Locating the Epicenter
 Let’s imagine that a seismic station located in city
A records an earthquake that took place “a” miles
away. This earthquake could be anywhere around
City A, on a circle with a radius of “a” miles.
 Two other stations, in cities B and C record the
same earthquake, at distances “b” and “c”
respectively.
 To locate the epicenter, the scientists draw circles
centered at each seismic station, with radii of “a”,
“b”, and “c”. The point where they intersect, is the
epicenter of the earthquake.
How is an Earthquake’s
Epicenter Located?



Three seismograph stations
are needed to locate the
epicenter of an earthquake
A circle where the radius
equals the distance to the
epicenter is drawn
The intersection of the circles
locates the epicenter
The Magnitude of an Earthquake
 The magnitude is the amount of energy
released during an earthquake.
 The RICHTER scale of magnitude is one
widely used to measure the magnitude of
earthquakes.
 It was developed by Charles Richter of the
California Institute of Technology.
The Richter Scale
 The Richter scale is used to measure the
magnitude of earthquakes. The scale, more
formally known as the Richter magnitude test
scale, ranges from 0 to over 8.0. It is a logarithmic
scale, meaning each unit increase on the scale
corresponds to an absolute increase by an order
of magnitude, or factor of 10. Earthquakes less
than about 2 on the Richter scale are meaningless
as they can barely even be measured, much less
felt. An earthquake is not really newsworthy until it
hits about 5. At magnitudes of 7.0 or 8.0 the
damage can be catastrophic.