Seismology: The Instrumental Study of
Earthquakes
Marek Chichanski, Ph.D.
Christopher DiLeonardo, Ph.D.
Earth & Space Sciences
De Anza College
E
arthquakes are dynamic events that can alter the landscape, bring down
civilizations, and change the course of history. They are constant reminders of
the awesome forces involved in the ongoing movement of the lithosphere
beneath our feet. Scientists study these events by looking at the seismic waves
radiating out from the focus (center point of rupture generating the earthquake). The
study of seismology is an attempt to understand these events by studying and
interpreting the instrumental recordings of seismic waves.
Lone Pine Fault Scarp
The Lone Pine earthquake of
1872 was one of largest in
California history. Because of
the damage caused and the
geographical area over which
it was felt it is estimated to
have been between a
Magnitude 7.6 and 8.0 event,
similar in size to the great San
Francisco earthquake of 1906.
Movement along this fault
scarp outside of Lone Pine,
California in the area of the
Alabama Hills records this
dynamic event in the
landscape. Photo of student
on fault scarp from Foothill-De
Anza field excursion.
Objectives
By completing this exercise you will be able to:
•
•
•
•
Read and interpret a seismogram.
Using travel time curves and seismograms determine the focal distance for an earthquake.
From focal distances determine the epicenter of an earthquake.
Determine the Richter Magnitude (ML) of an earthquake.
© 2010 C. G. DiLeonardo & M. Chichanski
Materials: Pencils; eraser; millimeter-scale ruler/straight edge, drawing compass.
Determining the Epicenter of an Earthquake
The three seismograms on the following page are from an earthquake that occurred somewhere
in the Bay Area. Using these seismograms, you will determine the location of the earthquake's
epicenter and plot the epicenter on the map. (See the map titled `Three seismographic stations
and major faults of the Bay Area'.) To find the location of the epicenter, do the following FOR
EACH OF THE THREE SEISMOGRAMS:
Step
1
Measure the S-P lag time to the nearest 0.1 seconds. (Note that you have a time scale
on each seismogram that will allow you to do this. Think about how to identify the P
and S wave arrivals on each seismogram, and how to measure the time interval
between them.)
Step
2
Take the S-P lag time to the graph (`TRAVEL TIME CURVES'), and use it to figure
out how far away that seismograph was from the epicenter. (Note: Think about
which of the graph's axes shows travel time, and which one shows distance. How can
you use a number on one axis, plus a line on the graph, to get the corresponding
number on the other axis? Which of the three lines is the correct one to use?)
Step
3
When you have drawn all three circles, they should intersect at one point, and this is
the earthquake epicenter.
1.
Did the earthquake occur on one of the Bay Area's major faults? If so, which one?
2.
If you only had the data from Mt. Hamilton, would you have been able to determine
which fault was the source of the earthquake?
2.
If you If you add the Berkeley data to the Mt. Hamilton data, does it become easier to
guess on which fault the earthquake occurred?
Estimation of Magnitude
The well-known Richer magnitude scale was developed in 1935 by Charles Richter of the
California Institute of Technology as a means of comparing the magnitude of earthquakes on a
quantitative scale that is independent of the damage caused. Richter defined the magnitude of an
earthquake (ML) in such a way that as the amplitude of the recorded seismic wave increases by a
Introductory Geology Laboratory
Seismology 2
factor of ten, the magnitude goes up by 1 unit. The energy released by the earthquake, however,
guess up by a factor of 31.5 with each whole number increase in the Richter magnitude.
The size of a wave is called its amplitude. It decreases as the wave moves away from its point of
origin. As an example, think of decreasing loudness of a sound wave as it travels from its
source. Because seismographic stations are located at various distances from any given
earthquake epicente, Richter devised a way of compensating for the attenuation (reduction in
amplitude). The figure on the next page is a nomogram for obtaining the magnitude of a local
earthquake once the epicentral distance and amplitude have been determined.
4.
Let’s use the Mount Hamilton (MHC) seismogram from the previous problem. It was
recorded on a standard seismograph, and the actual ground movement was 1/2800 of the
amplitude traced on the paper. Measuring the peak-to-peak distance on the seismogram
and dividing by 2 most accurately determines the amplitude.
What is the maximum amplitude of the seismic waves on the MHC record? _______mm
5.
What is the distance from MHC to the epicenter? ____________________km
6.
What is the local Richter magnitude of the earthquake? ML = ____________________
7.
Suppose a different seismogram was recorded at Mt. Hamilton with the same maximum
S-wave amplitude as the value you just measured, but the epicentral distance fort he
earthquake was 450 km. What ML value would you assign for that earthquake?
ML=_________________
Acknowledgements
Seismic traces adapted from data provided by the Oklahoma Geological Survey.
About the Earth Discovery Project
The Earth Discovery Project is a collaborative effort to integrate hands-on discovery-based
learning with modern research tools in undergraduate geoscience education. The approach is to
develop and disseminate a comprehensive set of learning resources and experiences supporting
systemic educational reform. The logo of the Earth Discovery Project portrays the earth as a threedimensional puzzle. The globe used in the logo is from NASA’s Blue Marble Project. The Blue
Marble is a unique view of the earth, which integrates numerous data sets to construct a “truecolor” three-dimensional globe.
Introductory Geology Laboratory
Seismology 3
Three Seismographic Stations & Faults of the Bay Area
Nomogram for Determining Richter Magnitude
Example
For an earthquake 20 km away the S-wave
amplitude is measured as 20 mm. By connecting
the distance bar to the amplitude bar a line
crosses the magnitude bar at 3.