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Timothy Larson, earthquake expert
Early Tuesday (Feb. 21), residents of Missouri, southern Illinois and western Kentucky felt the shaking of a magnitude 4.0 earthquake. Geophysicist and earthquake expert Timothy Larson, of the Illinois State Geological Survey, a unit of the Prairie Research Institute at the University of Illinois, talked to News Bureau physical sciences editor Liz Ahlberg about the earthquake and the implications of seismic activity in the Midwestern United States.
The Midwest is usually associated with tornadoes, not earthquakes, in terms of natural disasters.
True, but we do have earthquakes around here. In Illinois, we have a magnitude 5.0 earthquake every 10 years or so and maybe one or two earthquakes strong enough to be felt every year. I have lived in Urbana for 30 years and have felt two earthquakes in that time. It is a very strange sensation, hard to forget.
Here in the central U.S. we have a hard time seeing where the faults are because the bedrock is covered with sediment. In and near Illinois there are three major clusters or zones of earthquakes: northern Illinois, southeast Illinois/southwest Indiana (also known as the Wabash Valley area), and south of Illinois is the New Madrid Zone. We just had a minor earthquake in the northern Illinois zone last month. It shook up quite a few people in the north suburbs of Chicago. By far the most active earthquake zone in the Midwest is the 150-mile long New Madrid Zone.
Where is the New Madrid Zone? How active is it?
It is defined by many hundreds of mostly very small earthquakes that cluster on three lines, two running southwest-northeast almost parallel to the Mississippi River and connected by a third line of quakes in the middle that runs northwest-southeast. The middle line runs through the town of New Madrid, Mo. The north end extends almost to Illinois. The earthquake of Tuesday, Feb. 21, was located at the north tip of the New Madrid Zone. Although most of the earthquakes in the New Madrid Zone are very small, this is also the earthquake zone that spawned the great earthquakes of 1811 and 1812. Three (or four) earthquakes occurred between December and February that winter, one major quake (and hundreds of aftershocks) on each of the three segments.
What does magnitude 4.0 really mean?
Earthquakes in the U.S. are rated by the energy that is released by the movement on the fault as recorded by seismometers. Charles Richter devised the scale in California based on the largest recorded motions on seismograms from a standard type of seismometer and scaled for distance from the epicenter. Because there is such a large range in the amount of energy in different earthquakes, the scale is logarithmic; that is, each unit increase represents a tenfold increase in the amount of energy. So when we say that the earthquake in southeast Missouri was magnitude 4.0 and the great earthquake last year in Japan was magnitude 9.0, we mean that the Japanese earthquake was roughly 100,000 times more powerful than the Missouri earthquake. Earthquake waves travel farther in the central part of the continent than they do on the margins like in Japan or California. For instance, the relatively small earthquake in Missouri was felt by people over 200 miles away from the epicenter. We have reports from 1811 and 1812 describing motions so strong along the east coast that people felt seasick.
What kinds of damage can this cause?
Initial reports from the magnitude 4.0 earthquake in southeast Missouri state that it caused very little damage. This is typical of magnitude 4.0 earthquakes. Within a few tens of miles from the epicenter most people feel the shaking, cups are tossed about and maybe some plaster or old brickwork is cracked, but that is usually the extent of it. My rule of thumb is that a magnitude 5 can be expected to cause minor damage, but rarely is it severe. If a magnitude 6 earthquake strikes near populated areas, then severe damage and some casualties can be expected. The damage depends on the location of the earthquake relative to population centers and the types of construction in the affected area. As soon as I heard that a magnitude 7 earthquake had occurred near Port au Prince, Haiti, I knew it would be catastrophic without seeing any news footage. That was a very big earthquake near a major population center with poor building codes.
How often do earthquakes of this magnitude occur in the New Madrid Zone?
Although we didn’t have seismographs back in 1812 to measure the big earthquakes, we do have a lot of information about them from newspaper accounts and personal journals. We also know that similar large earthquakes occurred in the same area several times over the past 1,000 years or so. Even a magnitude 6.0 or 6.5 earthquake would likely cause considerable damage. Whether they will recur is a point of ongoing research.
Could this be a portent of bigger quakes to come? Should people be worried?
Based on the record of past earthquakes, some scientists suggest the probability of a magnitude 6.0 or greater earthquake in the New Madrid Zone to be 25 to 40 percent in the next 50 years. If I heard on the morning news that there was a 40 percent chance of rain in the afternoon, I would probably take an umbrella with me. It’s similar with earthquakes. We have a credible prognosis of 40 percent chance of damaging earthquakes within the next several decades. That means if we take preventive measures now, we can reduce the damage when the earthquake strikes. Fortunately, agencies such as the U.S. Department of Transportation are working to strengthen bridges. Building codes are being upgraded so that critical structures in earthquake-prone areas are built to resist earthquake damage. Emergency responders are routinely training to deal with the widespread, multi-state chaos that will ensue in the days following a major earthquake. Finally, public events such as the Great Shakeout held earlier this month bring public awareness to the threat.
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