On November 9, 1968, an earthquake shook a 580,000 square-mile area of the central United States, including all or portions of 23 states. At its center in south-central Illinois, the quake reached maximum intensity VII on the Modified Mercalli Scale (Wood and Neumann, 1931). The earthquake caused minor damage in the nearest metropolitan centers, Evansville, 50 miles to the east and St. Louis, 110 miles northwest, and also caused alarm in Chicago, over 270 miles to the north. The following source coordinates were determined by ESSA's National Earthquake Information Center:
|Origin Time:||17:01:40.9 G.M.T.|
|Latitude:||37.96 degrees North|
|Longitude:||88.46 degrees West|
|Focal Depth:||19 kilometers|
The epicenter lies within a predominantly agricultural region containing significant oil, coal, and gas resources. Statistics published by the United States Census Bureau show a gradually declining population in the area since about 1910. This is reflected in the style of architecture and overall condition of buildings within the region. A few of the larger towns have held their own or grown larger. However, many of the smaller towns and villages have experienced steady loss of population since the turn of the century. A typical example is Hamilton County, which covers an area of 435 square miles and contains the instrumental epicenter. Between the census of 1900 and 1960, the population of the county decreased from 20,197 to 10,010.
The south-central Illinois earthquake renewed interest in an unanswered seismological problem: the explanation of the extremely large felt areas associated with comparatively moderate shocks in the central United States. The quake also suggested that the seismicity of the middle Mississippi Valley, the scene of catastrophic earthquakes at New Madrid, Missouri, in 1811-1812, needs review. In addition, the recent shock provides a unique opportunity to study the amplification effects of a wide variety of soils. From the engineering and planning point of view the earthquake serves as a reminder of the latent seismic risk in the mid-continent.
Following the earthquake, the U.S. Coast and Geodetic Survey and St. Louis University initiated a cooperative investigation of damage and intensity in the felt area. This study consisted of an extensive questionnaire program conducted by the United States Coast and Geodetic Survey and a field reconnaissance of the epicentral area carried out by the authors. This paper contains the results of the study, together with a description of the physical environment of the epicentral area and correlation of intensity and ground conditions.