EARTHQUAKES (PART SIX)

The New Madrid Fault System extends 120 Miles southward from the area of Charleston, Missouri, and Cairo, Illinois, through New Madrid and Caruthersville, following Interstate 55 to Blytheville and on down to Marked Tree, Arkansas. It crosses five state lines and cuts across the Mississippi River in three places and the Ohio River in two places. The Fault is Active, averaging more than 200 measured events per year. Tremors large enough to be felt (2.5-3.0 on the Richter scale) are noted annually. Every 18 months the fault releases a shock of 4.0 or more, capable of local minor damage. Magnitudes of 5.0 or greater occur about once per decade, can do significant damage and be felt in several states.

What can be done to protect ourselves? Education, planning, proper building construction, and preparedness are proven means to minimize earthquake losses, deaths, and injuries. While we still have time, we can get ready and cut our losses, or we can do little or nothing and be caught unprepared. We cannot prevent the coming of an earthquake, but we can prevent it from being a major disaster.

What everyone should know about earthquakes.

Words like Sand boils, Liquefaction, Epicenter, Hypocenter, Aftershock, Foreshock and Fault, we hear them, after earthquakes, but what do these terms mean? What do they mean for what we felt and what we will feel the next time? Do we really understand what seismologists are saying?

Sand boils, liquefaction, disappearing highways:

Liquefaction is difficult for newcomers to grasp. It can cause a section of heavy interstate highway, or your house to disappear into the sandy soil when the subsurface sand becomes water-saturated loses its friction, and pressure is applied. The band of unconsolidated soil that runs from 80 miles over into Tennessee all the way back to Little Rock , Arkansas is going to be the greatest threat to liquefaction. During the past thousands of years this alluvial deposit has reached the depth of approximately 3500 feet . 

Human casualties could be in the thousands, per county in both Arkansas and Missouri .

Planners expect sections of I-55 which connects St. Louis and Memphis and U.S. 60 in Missouri to be unusable in the Bootheel after a major quake because of liquefaction on roads and bridges.

Epicenter:

The epicenter is the point on the earth's surface vertically above the hypocenter (or focus), point in the crust where a seismic rupture begins.

Hypocenter:

The hypocenter is the point within the earth where an earthquake rupture starts. The epicenter is the point directly above it at the surface of the Earth.

Aftershocks:

Aftershocks are earthquakes that follow the largest shock of an earthquake sequence. They are smaller than the main shock. Aftershocks can continue over a period of weeks, months, or years. In general, the larger the main shock, the larger and more numerous the aftershocks, and the longer they will continue.

Foreshocks:

Foreshocks are relatively smaller earthquakes that precede the largest earthquake in a series, which is termed the main shock. Not all main shocks have foreshocks.

Fault:

A fault is a fracture along which the blocks of crust on either side have moved relative to one another parallel to the fracture.

Not all faults will cause earthquakes. But, if there is a sudden rupture and movement of rock along a fault line, the vibrations we call an earthquake will result. We think of the earth’s rocky surface as hard and impregnable. However, rock under extreme pressure can have elastic properties. Try simulating an earthquake with a shallow plastic mould filled with gelatin. If you gently stretch the surface by pulling on the sides of the mould, you begin to stress the gelatin, just like rock can be stressed by geological forces. What do you think will happen when the gelatin is stretched as tight as possible and a small slit is made in the surface? The rupture rips through the gelatin, and the gelatin quivers as it snaps back into a relaxed state. The same thing happens during an earthquake when a sudden rupture occurs along a fault. As the rupture travels through the rock, energy is released that creates the motions associated with an earthquake. If a fault rupture is shallow enough, the fault line may appear on the earth’s surface.

Arkansas and Missouri are at great risk of experiencing a large damaging earthquake in the not too distant future. Recent studies suggest that this threat is as great today as at any time in the last hundred years. Each day that goes by brings us one day closer to a catastrophe the like of which we have not seen in modern times. In today's modern society, Arkansas and Missouri are even less resistant to the potential for damage than they were in the early 1800s. Cities like Little Rock , Springfield , Kansas City, and Memphis most likely will experience very significant damage. Traffic on the Mississippi River would be disrupted. Bridges, highways, railroads, pipelines, water supplies, electricity and communications networks which did not exist in 1811 and 1812, would be badly damaged or destroyed.

Since there is nothing we can do to stop or prevent earthquakes our only hope is to understand them and the forces that they produce, and to be mentally and materially ready when one does occur.

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U.S. Census Bureau 2005 Population Estimates for Arkansas

The orange print denotes that that county is in the northeast quarter of the State.

The counties listed above are the three most eastern from the north to the south parts of the State. The population of this area is equal to 25% of the total population of Arkansas .

Estimates of Damage from an Earthquake on the Southern Portion of the New Madrid Fault Zone (NMFZ)