Following established procedure after significant United States earthquakes, the Coast and Geodetic Survey carried out a postal canvass to delineate the felt area and determine the extent of damage associated with the earthquake. The initial request for intensity data consisted of approximately 2,000 questionnaires mailed to postmasters in a broad area of the central United States immediately after the earthquake. The survey was divided into several categories based on epicentral distance. Questionnaires were mailed to: (1) every post office within 50 miles of the preliminary epicenter, (2) selected localities in the 50- to 100-mile range, and (3) every county seat out to 500 miles. Several hundred additional queries were mailed later to compensate for the adjusted position of the instrumental epicenter.
These data are the basis of the generalized isoseismals shown in Figure 3. In addition to the felt area indicated in the figure, there were isolated felt reports from people in tall buildings at more distant localities, such as Boston, Massachusetts; Mobile, Alabama; and southern Ontario. The isoseismals drawn on the figure depict intensity in the engineering sense, i.e., they represent the actual intensity experienced as accurately as possible, subject to the inherent uncertainties of intensity data (Richter, 1958, p. 142).
The isoseismals shown on the map form a series of irregular concentric rings with conspicuous finger-like projections where major river valleys are encountered. The instrumental epicenter lies near the northwest margin of the strongest shaking, which corresponds to portions of the Ohio and Wabash Valleys and glacial lake plains along their tributaries in Illinois, Indiana, and Kentucky. Two separate areas of intensity VII are shown. The largest includes floodplains and terrace landforms in the Ohio and Wabash Valleys and their tributaries, the Little Wabash River, Skillet Fork, and the Saline River. A second, smaller area of intensity VII coincides with swampy ground along the Big Muddy River which empties into the Mississippi. Fingers of intensity VI which project down into the VII area represent uplands. The map also shows anomalies, such as the outlier of intensity VI in the Mississippi Valley below Cairo, which correlate with deep alluvium. Intensity patterns due to azimuthal variations in energy radiated at the source are not evident. If they exist, they are apparently masked by ground factors.
The configuration of the isoseismals in Figure 3 shows remarkable similarity to distribution of intensities associated with an intensity VI earthquake which shook the area surrounding Mt. Carmel, Illinois on November 7, 1958 (United States Earthquakes, 1958). The limits of the felt area of the 1958 shock closely resemble the outer boundary of the damage zone (intensity V or greater) of the more recent shock, and the combined area of V and VI associated with the Mt. Carmel earthquake covers nearly the same region as that assigned VI and VII of the 1968 event.