Geology Reference
In-Depth Information
EXERCISE
6
Earthquake Epicenters,
Intensities, Risks, Faults,
Nonstructural Hazards
and Preparation
INTRODUCTION
Zachary Grey, writing in 1750, said "An earthquake is
a vehement shake or agitation of some considerable
place or part of the Earth, from natural causes,
attended with a huge noise, like thunder; and fre-
quently with an eruption of water, fire, smoke or wind.
They are looked upon to be the greatest and most
formidable phenomena of nature." Although our pre-
sent understanding of earthquakes is much more
refined, they are still considered to be formidable
phenomena. An earthquake is the ground shaking
caused by elastic waves propagating in the Earth
generated by a sudden release of stored strain
energy. The sudden release of stored strain energy is
the result of an abrupt slip of rock masses along a
break in the Earth called a
fault.
Most fault slip occurs
below the Earth's surface without leaving any surface
evidence. The place where this slippage occurs is
known as the
hypocenter
or
focus
of the earthquake,
and the point on the surface vertically above the
focus is the
epicenter.
In this exercise we review earthquake wave
types, locate an earthquake epicenter, determine
earthquake intensities, assess seismic risk, examine
fault types, and study fault zone characteristics.
waves travel through the Earth to the surface and are
known as body waves. Others travel along the
Earth's surface and are known as surface waves
(Figure 6.1).
One type of body wave is a compressional wave
in which the particles of rock vibrate back and forth in
the direction of wave travel; the motion is similar to
that of sound waves that alternately compress and
dilate the medium—solid, liquid, or gas—through
which they travel. Compressional waves are also
called longitudinal or
primary
waves (P waves); the
latter name is given because these waves appear first
on seismograms (Figure 6.2) that record earthquake
waves. Another type of wave is the shear or trans-
verse wave, in which the particles vibrate at right
angles to the direction of wave progress, in the same
manner as a wave moving along a stretched string
that is plucked. Because these waves are the second
waves to appear on the seismogram, they are called
secondary
waves (S waves).
After the body waves, another class of seismic
waves, the surface waves, arrive. They have frequen-
cies of less than 1 cycle per second and often
approximate the natural frequency of vibration in tall
buildings. Surface waves in general decrease in
amplitude more slowly than body waves. The surface
waves consist of
Love
waves (horizontal lateral
vibrations perpendicular to direction of transmission;
they travel forwards but shake sideways) and
Rayleigh
waves (rotational displacement of particles
to produce a wavy or undulating surface; they travel
up and down in small circles).
Earthquake Waves
The energy released at the focus of an earthquake
sets up several types of vibrations or waves that are
transmitted through the Earth in all directions. Some
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