Because intensity is a measure of the severity of shaking at a given location, it varies geographically for any given earthquake. In other words, different observers at different locations may experience different effects. Assigning values to levels of earthquake intensity allowed early researchers to make some rudimentary discoveries about earthquakes and the distribution of shaking associated with them.
They did this by surveying people immediately after an earthquake had struck the area, and collecting reports of what those people experienced in the earthquake. Were they frightened by the shaking? If they were sleeping at the time, were they awakened? Was anything knocked over and damaged? Did they notice trees shaking, doors swinging open, or walls creaking? The answers to these questions allowed investigators to assign an intensity to the location of each survey response. With enough reports, they could create contour maps of the variations in intensity across an area. Points of similar intensity were grouped into zones bounded by lines called isoseisms, analogous to the isobars seen on weather maps of barometric pressure.
The first crude isoseismal maps showed that intensity zones tend to form a "bull's-eye" pattern of concentric circles, grading from the highest intensity in the innermost circle to the lowest intensity (shaking not felt at all) at the outer edges. The center of this pattern, and consequently the point of greatest intensity, was designated as the epicenter, and was assumed to be the point on the surface directly above the underground source of the earthquake.
The initial, idealized view of isoseisms forming neat concentric
circles (as shown on the map of Los Angeles in 1920, at right) did not
last. The more detailed isoseismal maps became, the clearer it became that
the distribution of intensity was not so regular and predictable.
In spite of this, isoseismal maps still allowed researchers their first
means of locating earthquakes and rating them by "maximum intensity",
a measure of their relative power.
Learn about plotting isoseisms, and deduce the location
of an earthquake's epicenter by using written
reports to assemble an intensity map online!