In an earthquake, the earth moves in a nearly random fashion in all directions, both
horizontally and vertically. Measurements have been made of earthquake motions in a
number of instances. In general, those measurements which are of greatest interest are the
records of ‘strong motion’ earthquake accelerations, measured by the U.S. Coast and Geodetic
Survey for a number of earthquakes in California in the past three decades. These accelerations,
as a function of time, are available for motion in two horizontal directions as well as
in the vertical direction, at a number of locations for several earthquakes. From the timerecord
of the acceleration,‘the velocities and displacements can be computed by integration.
One of the most intense strong motion records available is that for the El Centro, California
earthquake of 18 May, 1940. The record for the north-south component of acceleration
of this earthquake is shown in Fig. I, which also shows the values computed for velocity and
displacement in the same direction. From the figure it can be observed that the maximum
ground acceleration in the direction of this measurement is about 0.32 g, the maximum ground
velocity 13.7 in/set, and the maximum ground displacement 8.3 in.
The general nature of earthquake motions is indicated by this figure. It can be noted
that the highest intensity peaks of acceleration have a relatively short period or a relatively
high frequency; the most important peaks in the velocity, however, have a longer period
which corresponds to a lower frequency; and the important peaks in the ground displacement
have a much longer period still. For the ground conditions at El Centro the length of single
loops of the highest intensities, in the various records, have durations of the order of the following
: for acceleration, about 0.1 to 0.5 set; for velocity, about O-3 to 2 set ; and for displacement about 1 to 4 sec. In other types of soil, the relative durations may differ, with softer soils in
general showing lower magnitudes of acceleration, but longer durations and much larger
displacements, than in Fig. 1.
It must be remembered that the El Centro earthquake is not the largest earthquake which
has been experienced, even in California. It happened only to yield the most intense record
at a point where a strong motion accelerograph was located.