Earthquakes, Part Two

 

The composition of the earths crust and the physical features of an area will affect the intensity of an earthquake, and this has been a real concern to seismologist. During the earthquakes of late December 1811 and early 1812 the seismic waves were felt in Washington DC as if they were having a magnitude 5 earthquake, also in New York and New Orleans they felt them as if they were having a magnitude 4 earthquake. Generally speaking, unconsolidated soil amplifies shaking and can make an earthquake feel much stronger than it is, and it will cause more damage in these areas.

 

Now, let’s look at some interesting figures that Charles Richter developed in 1935 and are still used to this day to measure earthquakes strength. The Richter scale is open ended and logarithmic which means there are no upper and lower limits to the scale, and that every time the magnitude goes up by one unit, the amount of energy this represents increase thirty  times. The following are measurements of the Richter scale and the equivalent amount of TNT each represents:

 

    RICHTER                            TNT ENERGY

1.    1.0                                         6 ounces

2.    1.5                                         2 pounds

3.    2.0                                       13 pounds

4.    2.5                                       63 pounds

5.    3.0                                     397 pounds

6.    3.5                                   1000 pounds

7.    4.0                                         6 Tons             equal to a small atomic bomb.

8.    4.5                                       32 Tons

9.    5.0                                     199 Tons

10.  5.5                                     500 Tons

11.  6.0                                   6270 Tons

12.  6.5                                31,550 Tons

13.  7.0                              199,000 Tons

14.  7.5                           1,000,000 Tons

15.  8.0                           6,270,000 Tons

 

As you can see from these figures, earthquakes can produce and unleash a tremendous amount of energy that will produce waves of destruction in there paths. Speaking of waves, there are four types of seismic waves and they are grouped into two categories according to the way they travel from the epicenter. They are called “P” waves and “S” waves that are also called “Body Waves” and these are the fastest moving waves--- also, there are the “Love Waves” and “Rayleigh waves” which are responsible for the strongest ground shaking and the destruction of buildings. These combined wave vibrations produce several different effects and the one effect that is of most interest to us is liquefaction... Strong ground motion during an earthquake can cause water saturated, unconsolidated soil to act more like a dense fluid than a solid. Water saturated soils, granular sediments such as silts, sands, and gravel that are free of clay particles which is what covers a large area east of the Rockies are susceptible to liquefaction. Buildings that are not shook apart, will simply putting it, sink into the earth because the earth will loose its load bearing capacity. Also, as David Norris mentioned back in November when we had Part one on earthquakes, there are populated areas in the New Madrid Fault Zone where studies show that the ground will fall away changing elevations drastically. During the earthquakes of 1811 and 1812, Lake St. Francis, which is in the wildlife management area 600, located 15 miles southeast of Jonesboro in Craighead County, was formed. The lake is shown to be 40 miles long and one half mile wide. Two other lakes were also formed during this time, Lake Reelfoot in Tennessee and Big Lake near Miniala  Arkansas. The ground sank away and the elevations dropped at least twenty feet in these areas. The Mississippi embayment fall line area, in which the New Madrid seismic zone is located extends from Cairo, Ill south through northeastern Arkansas, western Kentucky, western Tennessee, then westward to include the lowland area of southern Arkansas, eastern Oklahoma and northeast Texas.

 

So far, we have learned about the effects of earthquakes and these are generally referred to as primary effects. Now, let’s examine a few of the secondary effects of earthquakes. There will be fires, gas pipe lines will rupture, water supply lines will rupture which will render fire hydrants useless, power lines will be downed, and communications towers for your fire and police departments will be down or damaged and useless, trains may derail and the tanker cars may rupture and release dangerous or deadly chemicals, medical hazards from hospitals, clinics and laboratories may be released, raw sewage, and even carcinogens may be released. These are just a few of the secondary effects of an earthquake, but hopefully these are enough to alert you to what we could be facing some day.  We need to have family plan’s, we need to have survival supplies that will last for up to at least a week. We should check with our local and county agencies to see what their emergency plan is for earthquakes, and with that said , I’ll venture to say that there are some local and county governments in  or near the New Madrid Fault Zone that does not have a plan in effect.

 

Will we be prepared for the next damaging earthquake? The little towns that were shaken up in the early 1800s are now major cities, Nashville, Tennessee—St. Louis Mo. –Little Rock, Arkansas and Memphis, Tennessee. Approximately 80 million people now live in areas where the earthquake risk is deemed high, and another 130 million people live in areas deemed moderate risk. The city of Memphis, with over ½ million people is situated only 25 miles from the New Madrid Fault. Memphis is built on a Simi consolidated rock covered with crumbly soil and flood plain deposits making Memphis incredible vulnerable to liquefaction.

 

And now a NOTE of interest, THE PRISM OF UNCONSOLIATED MATERIAL WHICH IS A PART OF THE MAKE UP OF A FAULT LINE IS APPROXIMATELY 3200 FEET THICK AND COVERS THE TERRAIN FROM LITTLE ROCK TO THE TENNESSEE RIVER WHICH IS LOCATED ABOUT 80 MILES EAST OF MEMPHIS.

 

 

After the first session we had on earthquakes, Mike Nettles our State Races Officer sent me an e-mail. One suggestion that he had for us was that our repeaters need to be secured properly so that they will not fall off of shelves, fall over and so forth.

 

 

                                                                End

 

 Written by Tom Harris, k5wth