travel through various earth materials. Based on the difference in the arrival times of the
first P wave and the first S wave, seismologists determine the distance between the epicenter
and a seismometer. Once the distance to the epicenter is known, scientists can identify each
point that is that distance away. Let’s say that they know that an earthquake’s epicenter is
50 kilometers from Kansas City. When each point that is that distance away from Kansas
City is marked, the marks create a circle. This circle can be drawn with a compass.
To locate the earthquake epicenter, seismologists must have data from at least three seis-
mometers. A circle drawn at the correct distance to the epicenter from a second seismometer
will intercept the first circle in two places. A third circle showing the distance to the epi-
center from a third seismometer will intercept the other two circles at a single point. This
point is the earthquake epicenter (Figure7.41). While this method was extremely useful
for locating epicenters for decades, the technique has been replaced by digital calculations.
Figure 7.41: Circles are drawn with radii representing the distance from each seismic station
to the earthquake’s epicenter. The intersection of these three circles is the earthquake’s
epicenter. ( 24 )
Earthquake Intensity
People have always tried to quantify the size of and damage done by earthquakes. Early
in the 20thcentury, earthquakes could only be described in terms of what nearby residents
felt and the damage that was done to nearby structures. This was called theMercalli
Intensity Scaleand was developed in 1902 by the Italian seismologist Giuseppe Mercalli.
The Mercalli Scale is sometimes used today in conjunction with the more modern intensity
scales described below.
Table7.1shows an abbreviated description of the twelve Mercalli intensity levels: