Laboratory Manual for Introductory Geology 4e

LEARNING
OBJECTIVES
■■
Understand how faulting
causes the ground to move
during earthquakes
■■
Recognize how earthquakes
cause damage to buildings and
other structures
■■
Learn how to locate an
earthquake’s epicenter and
determine its magnitude and
when it occurred
MATERIALS
NEEDED
■■
Sharp pencil
■■
Clear plastic ruler with
divisions in tenths of an inch
and millimeters (included in
the Geo Tools section at the
back of this manual)
■■
Architect’s compass
(or piece of string)
■■
Seismic overlay reading tool
(in your toolkit at the back of
this manual)
11.1 Introduction
Few things are as fearsome as a major earthquake. Unpredictable and enormously
powerful, a great earthquake destroys more than buildings and other structures.
It shakes our sense of safety and stability as it shakes the solid rock beneath
our feet. But it is not just the shaking that is dangerous. Devastating tsunamis
in 2004 and 2011 reminded us that oceanic earthquakes can ravage coastlines
thousands of miles from the earthquake origin. Landslides and mudslides triggered
by earthquakes can engulf towns and villages, and the loss of water when
rigid pipes break beneath city streets can cause health problems and make it
difficult to fight fires.
In our attempt to understand earthquakes, we have developed tools that reveal
Earth’s internal structure, define the boundaries between lithosphere plates, prove
that the asthenosphere exists, and track the movement of plates as they are subducted
into the mantle. In this chapter, we look at what an earthquake is and why
it causes so much damage, and we learn how seismologists locate earthquakes and
estimate the amount of energy they release. You will learn to read a seismogram
and use it to locate an earthquake, determine when it happened, and measure its
strength. First, let’s review some basic facts about the causes and nature of earthquakes
that are discussed in more detail in your textbook.
11.2 Causes of Earthquakes: Seismic Waves
Earthquakes occur when rocks in a fault zone break, releasing energy. The energy
is brought to the surface by seismic waves called body waves because they travel
through the body of the Earth. It is this energy that causes the ground to shake
initially.
The point beneath the surface where the energy is released is called the focus
(or hypocenter) of the earthquake. The point on the surface directly above the
focus is called the epicenter (FIG. 11.1). In most cases, the epicenter, being closest to
the focus, is the site of greatest ground motion and damage.
FIGURE 11.1 The focus and epicenter of an earthquake.
Epicenter B
Epicenter A
Focus B
Fault surface
Focus A
Seismic wave
The focus is the point on the fault where slip begins. Seismic energy
starts radiating from it. The epicenter is the point on the Earth’s
surface directly above the focus. Earthquake A just happened;
earthquake B happened a while ago.
276 CHAPTER 11 EARTHQUAKES AND SEISMOLOGY