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Epicenter
A

Focus

Fault plane
FIGURE 19.14 Simplified diagram of a fault illustrating
component parts and associated earthquake terminology.
B
Earthquake
longitudinal (compressional) wave called a P-wave, (2) the trans-
Fault verse (shear) wave called an S-wave, and (3) up-and-down (crests
and troughs) waves that travel across the surface called surface
waves, which are similar to a water wave that moves across the
solid surface of Earth. S- and P-waves provide information about
the location and magnitude of an earthquake, and they also pro-
vide information about Earth’s interior.
Seismic S- and P-waves leave the focus of an earthquake at
C
essentially the same time. As they travel away from the focus,
they gradually separate because the P-waves travel faster than the
S-waves. To locate an epicenter, at least three recording stations
measure the time lag between the arrival of the first P-waves and
the first slower S-waves. The difference in the speed between the
two waves is a constant. Therefore, the farther they travel, the
greater the time lag between the arrival of the faster P-waves and
the slower S-waves (Figure 19.16A). By measuring the time lag
and knowing the speed of the two waves, it is possible to calcu-
late the distance to their source. However, the calculated distance
provides no information about the direction or location of the
source of the waves. The location is found by first using the cal-
culated distance as the radius of a circle drawn on a map. The
place where the circles from the three recording stations inter-
sect is the location of the source of the waves (Figure 19.16B).
D
Seismographic data can also be used to calculate the depth
FIGURE 19.13 The elastic rebound theory of the cause of of the earthquake focus. Earthquakes are classified into three
earthquakes. (A) Rock with stress acting on it. (B) Stress has caused groups according to their depth of focus:
strain in the rock. Strain builds up over a long period of time.
(C) Rock breaks suddenly, releasing energy, with rock movement 1. Shallow-focus earthquakes occur within the depth of
along a fault. Horizontal motion is shown; rocks can also move the continental crust, which is from the surface down to
vertically. (D) Horizontal offset of rows in a lettuce field, 1979, 70 km (about 45 mi) deep.
El Centro, California. (D) Photo by University of Colorado; courtesy National 2. Intermediate-focus earthquakes occur in the upper part
Geophysical Data Center, Boulder, Colorado.

of the mantle. This is defined as between the bottom

of the crust down to where the seismic wave velocities
The point on Earth’s surface directly above the focus is called increase because of a change in the character of the mantle
the earthquake epicenter (Figure 19.14). materials. Based on this definition, the upper mantle is

Seismic waves from an earthquake are detected and mea- 70 to 350 km (45 and 220 mi) deep.
sured by an instrument called a seismometer (Figure 19.15). 3. Deep-focus earthquakes occur in the lower part of the upper
Seismic waves were introduced in chapter 18. These waves radiate mantle. This is defined as the boundary where a deeper

outward from the earthquake focus, spreading in all directions change in the seismic wave velocities indicates still another
through the solid Earth’s interior as do the sound waves from an change in the character of mantle materials. Th is boundary
explosion. The seismometer detects three kinds of waves: (1) the occurs 350 to 700 km (about 220 to 430 mi) deep.
19-9 CHAPTER 19 Building Earth’s Surface 485