Introduction
An earthquake measuring 4.1 on the Richter Scale shook west-central Minnesota on June 4, 1993. Although this earthquake was not powerful enough to have been felt beyond this area, it was important to the scientific community because earthquakes in the Upper Midwest are rare. Most earthquakes occur in a zone, the Ring of Fire, that surrounds the Pacific Ocean, but there are smaller fault zones all over the world. West-central Minnesota happens to lie on one of these.
The focus of the earthquake was probably somewhere beneath west-central Minnesota and within the Great Lakes Tectonic Zone, a zone faulting which extends southwest from Lake Superior to perhaps New Mexico or Arizona. The epicenter, the point on the earth's surface immediately above the focus, was near Collis, Minnesota (personal communication, Dr. Val Chandler, Minnesota Geological Survey, June 1993; and newspaper reports.
We developed this research project to answer such questions as: Where was the earthquake most intense? What caused the earthquake? What were the effects? How did thos involved react? Was there a difference in the way men and women reacted to the earthquake? In the days following the earthquake, geologists from the Minnesota Geological Survey interviewed people in west-central Minnesota to try to answer some of these questions. During the past six weeks, we have extended that survey by interviewing an additional 179 people.
Objectives
Originally we had two objectives. Our first objective was to extend the work of the Minnesota Geological Survey. Our second objective was to determine if there were significant differences in the way the men and women responded to the earthquake. We felt this would be an interesting addition to our research because of the stereotypes present in society regarding the differential sensitivities related to gender.
As we conducted our research, we added some supplemental objectives: to determine how people reacted to the earthquake, and how long the earthquake lasted.
Methodology
The format of the survey we used was based on one developed by the Minnesota Geological Survey (MGS). We modified the MGS survey form to reflect the objectives of our research. Specifically, we added questions regarding any common reactions or perceptions to the earthquake, and differences among reactions based on gender.
We included the towns of Morris, Donnelly, Herman, Wheaton, Dumont, Graceville, and Chokio in our survey (see map). On the basis of what the Minnesota Geological Survey had learned through their research, we knew that the epicenter of the earthquake was somewhere in this area. We randomly selected homes and interviewed at least 25 people per town. Our results were biased, in part, by the fact that some people were not at home, some people were in an area where the earthquake was not felt, and some people, although they had felt the earthquake, were not in their home towns when it occurred.
After we completed our survy, we used a computer spreadsheet to compile and analyze our data. We searched for similarities in the reactions and personal observations of people within the same towns. We used the Modified Mercalli Intensity Scale, a verbal and qualitative description of an earthquake's magnitude based on personal observations, to assign intensities to each area.
Data
The age of the people surveyed ranged from 9 to nearly 90; approximately 3/5ths of those surveyed were female (See gender chart).
Most people that we surveyed were indoors on the first floor of their homes when the earthquake occurred; most were sitting or standing. A small portion of the people were indoors when the quake struck were at some place other than their homes. These people were in places such as the local high school (attending graduation), at a neighbor's house, or in a cafe. Of those who were not indoors, 17% were outside and 5% were in vehicles during the quake (see location chart).
Approximately 90% of those surveyed heard a definite noise. Sixty-five percent of those surveyed described the noise as a rumble, as opposed to the 35% who described it as a loud sharp noise. Some popular responses were, "I thought the furnace exploded", "I thought it was a sonic boom", and "I thought a plane crashed".
About 23% of those surveyed said they felt vibrations. Most described these vibrations because the shock was absorbed by some object, such as a cushioned seat or vehicle. The second most common observation, cited by 14% of those surveyed, was the rattling of windows, dishes, and doors.
Although the actual length of the earthquake is still undetermined, about one half of the people surveyed estimated that it lasted between 2 and 6 seconds. The other half of the people estimated it was over 9 seconds long; 29% of those surveyed believed it lasted for more than 15 seconds.
Of the people surveyed, 10% reported damage to their houses. Some of the damage reported was: cracks in the basement walls, cracks in the front steps, a slanted door frame, and cracks in the interior walls. Most of the people said that they weren't sure if there was any damage because there were too many cracks present before the earthquake to notice a difference.
Our data on the differences in the way men and women reacted to the earthquake did show a significant difference. Although approximately 64% of the couples said they noticed it simultaneously, 28% said that the women noticed it first, as opposed to the 8% who said that the man noticed it first. From these data, we calculate that at the 95% confidence level there is a statistically significant relation between gender and sensitivity.
On the basis of our survey results, we estimate that the Modified Mercalli Intensity, was approximately VII in the Wheaton and Herman areas, because respondents reported cracks in concrete and cases of people losing their balance. We estimate that in Graceville, Dumont, and Chokio the intensity was in the upper ranges of VI on the Mofified Mercalli Scale because pictures fell off walls, plaster cracked, and furniture moved. Finally, we estimate the earthquake intensity to be in the lower ranges of VI on the Modified Mercalli Scale in Morris and Donnelly because respondents reported that windows, dishes, and glassware rattled. (See the earthquake intensity map).
See the complete numerical summary of our data.
Discussion
There has always been a suspicion that men and women react differently when faced with the same situation. Although the number of male-female pairs in our survey was small, our data supports, in part, the stereotypical perception that women are more sensitive than men to sudden changes. However, it is important to note that nearly 2/3rds of the time, males and females noticed the earthquake simultaneously.
The most common reactions to the earthquake were indifference and confusion, two very dissimilar reactions. The majority of people who responded with indifference explained that they had been through an earthquake before, either the 1975 Morris earthquake or, as in the case of several people, earthquakes in California. As for the people who responded that they were confused, their reasons were varied. There were people who had never experienced an earthquake before, people who said that they were not expecting because earthquakes only happen in places like California, and a great number of people who thought the noise was an explosion or a sonic boom.
Geologists estimate that, in general, earthquakes last anywhere from less than 1 second to almost 1 minute. The people we surveyed were no different, with their estimations ranging from 0.1 seconds to 10 minutes. One possibility for such an enormous variance in responses is simply the fallibility of human judgment, especially in times of confusion, shock or fear (to which 55% of the respondents admitted).
Whether true or just superstition, the concept that animals have the ability to predict unusual events in nature has been around for decades. On May 6, 1976, for example, the peasants of Friuli, a small town in the Peruvian Andaes, reported that their cattle became restless and their dogs barked right before an earthquake, registering at 6.5 on the Richter Scale, struck at 9 P.M. (Tributsch, 1976). Similarly, on June 4, 1993, one person in Chokio noted that his dog started running around and barking before the earthquake struck and an identical case was reported in Dumont. Even though our cases might seem insignificant to some people, we believe it is necessary to draw attention to them because of this theory.
Conclusion
On the basis of the data we collected interviewing 179 people, the earthquake of June 4, 1993, was felt over a wide area of west-central Minnesota. Most people in this area felt the vibrations from it and heard a rumble when it struck. Although little damage was noted, we estimated the earthquake to have been a VI or VII on the Modified Mercalli Intensity Scale on the basis that foundations were cracked, furniture was moved, and hanging objects fell off walls. We also concluded that the earthquake was strongest around Wheaton.
With a 95% confidence, we know that there is a direct correlation between gender and perception of the earthquake, with women more commonly reacting to the earthquake before men. From the wide range in responses, we learned that even eyewitness accounts are not as reliable as society believes them to be. Because people are human, there is always room for error; either through confusion or, just plain memory loss, there is going to be some degree of inaccuracy.
Lastly, we noticed that animals reacted to the earthquake before it occurred, thus supporting the theory that animals are more sensitive to natural phenomena that humans.
This report is included in this site with the permission of Dr. Peter Whelan, owner of the data and reports resulting from this project. Using their data, I recreated the pie charts to enhance their appearance on the Web. I also located sources on the Web to link to some of the figures referenced in the report, such as the Ring of Fire and the Modified Mercalli Intensity Scale. --Pam Gades, April 2001.