BASIC EARTHQUAKE PRINCIPLES - Geotechnical Earthquake Engineering - page 17

FIGURE 2.13 Illustration of a reverse fault. For a reverse fault, the hanging wall block has moved upward
with respect to the foot wall block. ( Adapted from Namson and Davis 1988. )
● Fault Scarp: This generally only refers to a portion of the fault that has been exposed at
ground surface due to ground surface fault rupture. The exposed portion of the fault often
consists of a thin layer of “fault gouge,” which is a clayey seam that has formed during the
slipping or shearing of the fault and often contains numerous slickensides.
2.2 SEISMOGRAPH
Most earthquakes are caused by the release of energy due to sudden displacements on
faults. This is not to imply that all ground movement of a fault will produce an earthquake.
For example, there can be fault creep, where the ground movement is unaccompanied by
an earthquake. The major earthquake is characterized by the buildup of stress and then the
sudden release of this stress as the fault ruptures.
A seismograph is an instrument that records, as a function of time, the motion of the
earth's surface due to the seismic waves generated by the earthquake. The actual record of
ground shaking from the seismograph, known as a seismogram, can provide information
about the nature of the earthquake.
The simplest seismographs can consist of a pendulum or a mass attached to a spring,
and they are used to record the horizontal movement of the ground surface. For the pendu-
lum-type seismograph, a pen is attached to the bottom of the pendulum, and the pen is in
contact with a chart that is firmly anchored to the ground. When the ground shakes during
an earthquake, the chart moves, but the pendulum and its attached pen tend to remain more
or less stationary because of the effects of inertia. The pen then traces the horizontal move-
ment between the relatively stationary pendulum and the moving chart. After the ground