APPENDIX D
and expansion of the rock. The S waves travel more slowly
than P waves, arrive after the P waves, and exhibit particle
GROUND MOTION BACKGROUND DATA
motion transverse to the direction in which they travel.
Both P and S waves move through the body of the earth and
D-1. Earthquake Source and Earthquake Size
are thus called body waves. Body waves are followed by
Definition
surface waves that travel along the earth' surface and have
s
motion that is restricted to near the earth' surface.
s
a. A Simple Earthquake Source Model. The actual
release of earthquake energy along a fault plane in the crust
e. Earthquake Size.
of the earth is a very complex phenomenon. All the
physical processes that occur just before, during and after a
(1) Magnitude. Among the various quantitative
seismic event are still not completely understood, and
measures of earthquake size, magnitude is undoubtedly the
considerable research is going on to better describe this
most successful and widely used. The basic concept of
phenomenon. However, for engineering purposes, the
magnitude is to compare sizes of earthquakes in a relative
above complex phenomenon is idealized, and Figure D-1(a)
manner. In his definition of magnitude, Richter (Richter,
gives the resulting simplified model representation of the
1958) rates an earthquake relative to a standard size
earthquake source. In this model, an earthquake is caused
earthquake by comparing their maximum amplitudes
by the sudden release of energy accumulated during
recorded by the same type of seismometer at the same
tectonic processes. The energy is released via faulting
distance to the epicenter,
(rupture) of rock along a plane (the fault plane). Part of the
energy is converted into elastic energy carried by seismic
A(∆)
waves and thus the shaking that is felt during earthquakes.
M = log
(D-1)
Ao (∆)
b. Earthquake Location. Even though a substantial
where ) is the distance from observation location to the
volume of the earth' crust is involved in the energy
s
epicenter (epicentral distance), A and A@denote the
release, it is generally assumed that the faulting initiates at
recorded maximum amplitudes of an earthquake and the
a discrete point (the hypocenter or focus) and then spreads
standard size earthquake, respectively. The standard size
over a larger area (Figure D-1(a)). The term epicenter is
earthquake is defined as to have A@= 1:m (10-6 meter or
used to denote the point on the earth' surface directly
s
3.3x10-6 feet) recorded by a Wood-Anderson seismometer
above the hypocenter. In recent times (since the
at ) = 100 km (62 miles). Tables were constructed
installation of seismographs), the locations of the
empirically to reduce from 100 km (62 miles) to any
hypocenter are determined by means of instruments.
distance. A graphical representation of the table is given in
Before the advent of instrumentation, the epicenter was
Figure D-3. Since the scale is logarithmic, an increase of
located by means of finding the region of most intense
one step on the magnitude scale increases the amplitude
shaking. Quite often, the field epicenter (region of intense
scale by a factor of 10 (see Figure D-3). Richter magnitude
shaking) and the instrumentally located epicenter do not
scale was originally defined for local earthquakes in
coincide.
southern California; the definition has been adopted and
expanded to become applicable to other regions using
c. Types of Faulting. Figure D-2 shows the three basic
different type of instruments. Richter magnitude is only
types of faulting. They are defined by the sense of relative
used for shallow local () < 600 km or 375 miles)
displacement between the two adjoining blocks along the
earthquakes, hence it is also called the local magnitude
fault plane. In a normal fault, the upper block slides
downward relative to the lower block. In a reverse fault,
the upper block rides up. In a strike-slip fault, one block
distant earthquakes () > 600 km or 375 miles). Surface-
moves horizontally past the other. Any faulting may be
wave magnitude Ms is usually based on the amplitude of 20
described as a combination of these three basic types of
seconds period surface waves recorded at distances of
faulting.
thousands of kilometers, where seismograms are dominated
by surface waves. Body wave magnitude is based on the
maximum amplitude of 1 second period P-waves.
d. Types of Seismic Waves. Seismic waves generated
by an earthquake source are of three main types: P, S, and
(2) Seismic moment. As more is known about the
surface waves (Figure D-1(b)). The P wave has the fastest
earthquake source mechanism and about the size of
travel speed and its particle motion involves compression
earthquake events, it is becoming increasingly clear that the
D-1
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