Minnesota earthquakes strong enough for anybody to notice them are somewhat unusual. They arouse interest and are reported by newspapers and television. The June 4, 1993 event in west-central Minnesota was no exception.

It was recorded on seismographs by the National Earthquake Information Center (NEIC) in Boulder, CO, which measured a magnitude (Richter scale) of 4.1 with a probable epicenter several miles northeast of Graceville.

Intensity (Mercalli scale) is a measure of the amount of shaking observed at different locations. Although local soil conditions can cause higher values at distant sites, intensity generally decreases with distance from an epicenter (location on the land surface above the focus of an earthquake). Thus, reports of what people felt are useful in locating an epicenter. They also provide information on the depth of the focus and on crustal structure.

The Minnesota Geological Survey (MGS) publicly requested information from anyone who had felt the earthquake and in response received 17 phone calls 115 letters, and 16 completed questionnaires. MGS also interviewed 71 people, and additional intensity information was obtained by Prof. Peter Whelan and his students at the University of Minnesota, Morris.

The results indicate that the earthquake was felt over about 26,800 square miles. The outer parat of this area reported intensities equivalent to II to III (Roman numerals are used to avoiding confusing intensity with magnitude). At this level, people report vibrations like passing trucks and gentle swaying of suspended objects. Intensity IV effects include rattling of dishes and sensations like something heavy striking the building. Intensity V includes some objects being upset. A few places inside the June 4, 1993 intensity V area may have experienced shaking at the upper limit of that level. The location of the largest of these subareas north of NEIC's epicenter suggests that the true epicenter may be slightly farther to the north or northwest. A loud "explosion" heard near there led some people to suspect that a propane gas tank had exploded or that a local crop duster had crashed his plane nearby. Such loud noises are fairly common for earthquakes that have a focus several miles below the surface, and the intensity data suggest a depth of about six miles. A reconnaissance investigation failed to find any evidence of a surface rupture. No aftershocks were reported by anyone.

Like most earthquakes in the upper Midwest, the recent event probably reflects small reactivation of an ancient fault in response to regional stress associated with the slow, westward movement of North America. Two faults cross deep under the area. One is the Morris fault, which is some 2,500 million years old and trends northeast; it is not exposed, but geophysical data indicate that it is in the bedrock surface several miles south of the recent epicenter, and that is slopes at a low angle beneath it. A second fault trends northwest, cutting across and offsetting the Morris fault.

The newest epicenter is about 20 miles northwest of the magnitude 4.8 Morris earthquake of July 9, 1975. The late Professor Harold Mooney on the University of Minnesota estimated that an earthquake as strong as the most recent event could occur about every 10 to 30 years in west-central Minnesota.