Chapter 8
Chapter 8: A Violent Pulse: Earthquakes
Animation
This animation shows the differences between the three types of faults and illustrates how they are formed. View 1 shows a normal fault, View 2 shows a reverse fault, and View 3 shows a strike-slip fault. For more information, see “Faults in the Crust” starting on page 202 and Figure 8.3 in your textbook.
The photo shows a wooden fence built across the San Andreas Fault. During the 1906 San Francisco earthquake, a slip on the fault broke and offset the fence; the displacement of the fence indicates that the fault is strike-slip, as we see no evidence of up or down motion. The amount the fence was offset indicates the displacement on the fault. For more information, see page 217 and Figure 8.16 in your textbook.
Faults are fractures along which one block of crust slides past another block. Sometimes movement takes place slowly and smoothly, without earthquakes, but other times the movement is sudden, and rocks break as a consequence. For more information, see Geology at a Glance: Faulting in the Crust on page 204 in your textbook.
Seismologists distinguish between different types of seismic waves based on how they move, and whether they travel along the Earth’s surface (surface waves) or pass through its interior (body waves). This animation shows two types of body wave motion: View 1 shows shear body waves (also called S-waves) and View 2 shows compressional body waves (P-waves). For more information, see page 207 and Figure 8.6 in your textbook.
Seismologists use two basic configurations of seismographs, one for measuring horizontal ground motion, like the one shown in this animation, and the other for measuring vertical ground motion. Both work on the principle of inertia as described by Newton’s law, which states that an object at rest tends to remain at rest unless acted on by an outside force. Thus, during an earthquake, vibrations cause the frame of the seismograph to move. The pendulum apparatus remains fixed as the paper cylinder moves back and forth beneath it. For more information, see page 208 and Figure 8.7 in your textbook.