Popular Science Monthly/Volume 69/July 1906/Our Greatest Earthquakes
|OUR GREATEST EARTHQUAKES
By MYRON LESLIE FULLER
UNITED STATES GEOLOGICAL SURVEY
PROBABLY few people are aware that the greatest earthquake our country has experienced since its settlement was not the destructive shock at Charleston in 1886, or even the recent terrifying manifestation at San Francisco, but was, on the contrary, the now almost forgotten earthquake of New Madrid, the first tremors of which took place on the sixteenth of December, 1811. Strange is that trait of human nature by which even the most appalling of nature's manifestations slip rapidly from the memory, so that only a hundred years later little but tradition remains of the earthquake which changed the configuration of extensive areas of the Mississippi Valley, raising some portions, depressing others, shifting the course of streams, draining old swamps at one point and forming new lakes at others. All this and more, however, took place during the successive vibrations which shook the New Madrid region almost continuously for a period of many months in 1811 and 1812.
The New Madrid Earthquake
The night of December 15, 1811, fell quiet and peaceful, and the settlers retired little dreaming of the impending catastrophe. At two o'clock in the morning, however, they suddenly awoke to find the houses over their heads groaning and cracking, chimneys falling, furniture thrown about, and the earth rocking and trembling. Groping their way to the open fields they huddled together until morning, the shock which succeeded shock at short intervals in the darkness keeping them from returning to their tottering houses. At New Madrid, on the Mississippi, the French population were dancing away the night when the shock came and instantly terminated the revelings, joy being replaced by terror as they rushed from the buildings to the open, where catholics and protestants alike knelt in supplication during the remaining hours of the night.
Daylight brought little relief. At seven a rumbling like distant thunder was heard and in an instant the earth was convulsed so that no one could stand. Looking at the ground the terrified people saw it rise and fall, as earth waves, like those upon the sea, rushed past, waving the trees until their branches interlocked and causing yawning cracks to open where the surface was bent and the swells burst. Giants of the forest were split for forty feet up the stump, half standing on one side of the fissure and the remainder on the other. In one instance a crack opened in a cellar, swallowing a large number of castings just received from Pittsburg and temporarily stored away there.
Some of the earthquake rents were of great size, having widths of thirty feet or more, while some are reported as many as five miles in length. Others were circular in form, making basin-like depressions up to several hundred feet in diameter. Into some of these cracks rushed the waters from swamps and bayous, while elsewhere small streams or even rivers left their old beds and made new channels through the cracks. In one instance, a settler living on a neck of land lying within a great bend or ox-bow started at daybreak the morning after the quake to go to his well which the night before had been in his yard. But no well was there! Instead the river was at his door. Glancing across the water, however, the well could be seen on the further side. During the night a crack had been formed between the house and the well and had been taken possession of by the waters, leaving both unharmed though on opposite sides of the stream.
Accompanying the cracking in many instances there seemed, according to one observer, "a blowing out of the earth, bringing up coal, wood, sand, etc., accompanied with a roaring and whistling produced by the impetuosity of the air escaping from the confinement... trees being blown up, cracked and split, and falling by thousands at a time. The surface settled and a black liquid rose to the belly of the horses." The atmosphere was saturated with 'sulphurous vapor,' due to the gases escaping from the decaying vegetation and coaly matter (lignite) deep below the surface in the deposits of the prehistoric Mississippi. These gases tainted the air for miles and so affected the streams and rivers that the waters, even to a distance of one hundred and fifty miles below, could not be used for several days. The intense darkness caused by these vapors in the night, and the murky purplish tinge imparted to the atmosphere by day, produced a vivid and never to be forgotten impression on every one who passed through the experience.
It was along the Mississippi that the destruction reached a maximum. A traveler on a flatboat, tied up to the bank about forty miles below New Madrid, speaking of the first shock, says that the men, wakened by the quake, sprang to the deck thinking the Indians had made an attack. After daylight, as they were preparing to depart, "a loud roaring was heard, sounding like steam escaping from a boiler. This was accompanied by a violent agitation of the shores and tremendous boiling up of the waters in huge swells which tossed the boats so violently that the men with difficulty could keep upon their feet. The sandbars and points of islands gave way, swallowed up in the tumultuous bosom of the river, carrying down with them the cottonwood trees cracking and crashing, tossing their arms to and fro, as if sensible of their danger, while they disappeared beneath the flood. From the check given to the current by the heaving bottom, the river rose in a few minutes five or six feet and again rushed forward with redoubled impetuosity, hurrying along the boats, now let loose by the horror-stricken boatmen, as in less danger on the water than on the land."
Whole islands disappeared. Captain Sarpy of St. Louis, with bis family and considerable money aboard, tied up at an island on the evening of the fifteenth of December, 1811. In looking around they found that a party of river pirates occupied part of the island and were expecting Sarpy with the intention of robbing him. As soon as the latter found that out he quietly dropped lower down the river. In the night the earthquake came, and next morning when the accompanying haziness disappeared, the island could no longer be seen; it had been utterly destroyed as well as its pirate inhabitants.
Few scientists were in the region during the period of shocks, but we are fortunate in having handed down to us a realistic picture from the pen of the great naturalist Audubon.
The vibrations did not cease for over a year from December sixteenth, the date memorable for the first shock. During the succeeding three months 1,874 shocks were recorded, of which eight were violently destructive, ten very severe and thirty-five generally alarming. In fact, this earthquake is famous all over the world as one of the few instances of almost incessant shaking for a period of many months in a region remote from the seat of any volcanic action.
Some, like the first, were accompanied by disruptions of the surface and by changes in elevation of the ground. The country which before the shocks was level, with occasional small prairies, was sadly changed. In places old bayou-lakes were drained so that corn could be planted in their bottoms, while elsewhere lakes of considerable size were created. The surface for hundreds of acres was covered by the sand thrown up with the water from the fissures. Even to this day this can be recognized in the forest, where it occurs as barren spots upon which little will grow.
A few more years and a century will have passed since the shocks so vividly described took place. From a wilderness with a few scattered settlements, the region has become, in the northern part at least. a populous farming region with numerous prosperous towns. The great city of Memphis has appeared near the limits of the earthquake region at the south, while St. Louis with its hundreds of thousands of people is but a little distance outside the area to the north.
Notwithstanding the development of the region and modification of the surface by nearly a hundred years of cultivation, the watchful eye can still detect evidences of the powerful forces which so strongly affected the area in 1811. Throughout all the country from New Madrid in Missouri southward to beyond the Arkansas line, and from the Mississippi river westward to the highlands of Crowleys Ridge, there is hardly an open field which does not show one or more low swells of light sand standing out in marked contrast with the dark soil constituting the ordinary surface. These are the well-known 'sand blows' produced by the actual eruptions of sand and water from considerable depths through cracks in the clayey surface deposits.
Some of the cracks were of considerable length, giving rise to the long narrow 'sand blows' while others were very short, almost all the water and sand coming from a single point. In such instances little cones or craterlets, as they are called, consisting of low mounds of sand with depressions in the center were often formed.Even more conspicuous, though less numerous, are the great cracks formed in the earth at the time of the quake. Few people have seen these in their full development, as they are hidden in tangles of vines in the as yet almost untouched hardwood forests on the bottoms of northeastern Arkansas. Turning northward from the lumber town of Parkin about thirty miles due west of Memphis and following the old 'De Soto trail' in a few hours one reaches the southern portion of the earthquake area and is in the midst of earthquake features of surprising magnitude. The region is low and is frequently submerged for weeks in the spring by backwater from the sluggish rivers, while in summer the cane brakes in the more open spots and the thickets of poison ivy and other vines in the forests present additional obstacles to the explorer. Wild turkeys, deer, wild cats and even wolves are still
found. There are no roads, and for many miles not even a solitary settler is to be seen, but with a good horse and a guide familiar with the cracks, blows and 'sand slews' the region can be penetrated and the earthquake features examined. It is in the depths of these forests along the St. Francis river that the cracks reach their greatest development. How wide they may have been when first formed and how deep, no one can tell. The originally steep banks have crumbled and the fissures partly filled until at the present time they resemble a deep ditch more than a crack. Yet some of these ditch-like depressions are still thirty feet or more across and so deep that a man on horseback can not see over the top, even when he has succeeded in scrambling or sliding down the steep sides. From cracks of this size there are all gradations down to little ones of only a half a foot in depth, but all are still distinctly recognizable. Most of them are within a quarter or half a mile from some river and have a general north-south direction, as if the surface of the land shifted bodily towards the waterways, leaving great rents in the ground where the materials parted. One of the smaller of the cracks is shown by one of the illustrations of the present article (Fig. 1).
Fissures of another, but equally conspicuous type are the land-slide cracks formed where steep slopes, such as those along the east side of Reelfoot Lake in western Tennessee, occurred within the earthquake area. Here the bluffs, which are several hundred feet in height, were literally shaken to pieces by the shocks, the trees uprooted, overturned, or prostrated, and great masses of earth precipitated down the steep hillsides. Figure 2 shows some of the scarps thus formed, while another shows trees overturned at the same time (Fig. 3). Sometimes the original trunks are decayed and gone, all perhaps but a projecting stump, but shoots from the original have often taken their place as giants of the forest.
The features for which the New Madrid earthquake is most renowned, however, are the swamps and lakes which resulted from the warping of the surface. The former may be seen at many places in southern Missouri and northern Arkansas. In the view of such a swamp here given, the water growth in the foreground and the dead trunks of the old trees are in contrast to the dense growth of new timber winch is gradually reclaiming the swamp (Fig. 4).
Reelfoot Lake, the most noted single feature resulting from the earthquake, is a shallow body of water between the Mississippi river and the Chickasaw Bluffs in western Tennessee. It has a length of about twenty-five miles, a width of about five miles, and a depth of twenty-five feet or more. Previous to the earthquake, it is said, no lake existed, the lands being of the ordinary type of fertile bottoms
characteristic of the Mississippi, and had early been granted by the Spanish to certain favored individuals. Through the land ran Reelfoot Creek, a little stream rising in the highlands on the east.
After the earthquake all was changed. A warping of the surface occurred across the course of the little Reelfoot Creek, the channel in the lower portion being lifted above its old level so water no longer flowed through it. while the upper part sank and was soon covered by the waters that collected behind the barrier. The old channel can still be traced by soundings across the lake and the landmarks bounding the early grants made out beneath the waters, while to the south the nearly dry bed of the unlifted creek may be traced to the Mississippi (Fig. 5).
Comparison with Later Earthquakes
In earthquake studies one of the most fascinating lines of research is the investigation of the relative intensities of the different shocks. Of the various evidences, that afforded by the resulting disturbances of the surface conditions is most reliable. Accounts of those who have felt the shocks are unreliable in determining intensities, since the feelings experienced at such a time are largely dependent upon nervous temperament, and upon previous experiences with earthquakes. One feeling a shock for the first time is often seriously disturbed by tremblings to which a resident of an earthquake country would not pay the slightest attention.
5. Old and Young Growth of Cypress in Reelfoot Lake, Tenn. The large stumps without enlarged butts are the old trees killed by the New Madrid earthquake. The living trees with enlarged butts are mainly subsequent growth.
A comparison of the effects produced upon artificial structures and upon the earth's surface by our three great earthquakes, New Madrid, Charleston and San Francisco, seems to show that of the various types of phenomena associated with earthquakes, nearly all were more strongly developed at New Madrid than at either of the other localities.
The length of the period of marked disturbance at San Francisco was only a few minutes during the eighteenth of April of this year. The Charleston earthquake occurred, after a preliminary tremor a day or two before, but not felt in the city on August 31, 1886, the severe shocks being confined to a few hours, although not entirely ceasing for three months. In New Madrid, on the other hand, the vibrations, which began on December 16, 1811, continued almost unceasingly for several months, while for more than a year they recurred at frequent intervals. Even at the present time there is probably not a year goes by without a distinctly recognizable shock.
In San Francisco few of the better class of buildings were destroyed, and in Charleston, although the damage was great, few buildings collapsed completely, and the cabins were seldom more than shaken from their foundations. In the New Madrid region there were no high buildings, one story log or frame houses being the rule, but notwithstanding this many are said to have been shaken to pieces by the relatively intense shock.
Again, no progressive wavelike undulation of the surface of any magnitude was recognized at the time of the shock at San Francisco, but at both Charleston and New Madrid the surface rose and fell in waves several feet in height. In Charleston the forests were but little affected, but at New Madrid the trees were often thrown together upon the ground in confused heaps or snapped sharply off near the ground as by an axe.
The streams in the vicinity were little affected by the San Francisco shock, and even at Charleston few if any permanent changes resulted from the earthquake, but in the New Madrid region the effect of the disturbance was very marked. The courses of some of the streams were changed—the water following new cracks instead of the old channels. Others were deflected by warpings of the surface, and still others by sharp uplifts or faults, giving rise to swamps or bodies of open water.
Cracks were formed by the earthquake in each locality. In San Francisco except along the immediate line of faulting they were few in number, small in size, and limited to small tracts of especially soft ground or to the steeper hillsides. In Charleston they occurred over an area several miles in diameter, but were usually under an inch across, except near the rivers. At New Madrid, on the other hand, they extended over an area many times as great, extending from southern Missouri nearly to Memphis, a distance of over one hundred miles, and from one side of the Mississippi Valley to the other, and were often many feet in width. No sand is reported to have been thrown from the cracks at San Francisco except in rare cases, but at Charleston numerous craterlets such as shown in one of the accompanying illustrations were formed, from which large amounts of sand and water flowed out quietly upon the surface (Fig. 6). At New Madrid the sand and water not only came out more frequently and covered a larger area, but were ejected with violence, sometimes reaching, according to observers, to the very tree tops.
Little change of level occurred at either San Francisco or Charleston, but in the New Madrid region great areas sank and were covered by water, one of them now covered by Eeelfoot Lake being over twentyfive miles long and more than five miles wide.
Cause of the Shocks
The shocks in each case have had their origin in the breaking and slipping of hard rocks underneath. All rocks of the earth's crust are subjected to stresses of different kinds, such as may be produced by the weight of overlying material, by the shrinking of the earth's interior, or by other causes, and the time comes when their strength is no longer sufficient to resist them, and a break occurs, usually accompanied by a crushing of the rock along the fracture or by a slipping of one part of the rock over the other. It is this slipping or crushing which gives rise to the vibrations known as earthquakes.
In the San Francisco region this slipping is constantly going on and minor shocks have been of frequent occurrence. It was only a slightly larger slip than usual which produced the recent disastrous shake. In Charleston the slipping was mainly at one time, no preliminary shocks of importance were felt and few occurred afterwards, except during a short period immediately following the earthquake, but in the New Madrid country the quaking has continued for several hundred years at least. Both the Charleston and New Madrid earthquakes occurred in regions where the earth's crust is being overloaded—in the one instance by the sediments brought down by streams from the Appalachian Mountains and in the other by the floods of the Mississippi—and the fracturing is believed to have resulted from the readjustment of the harder rocks to the increasing load.
A Look into the Future
As to what the future holds in store for the three regions we can only judge by the past. The Charleston earthquake, as far as shocks of any intensity in recent times are concerned, is unique in the Atlantic Coastal region. The equilibrium has probably been regained since the slip of 1886, and it may be ages before another occurs. San Francisco, on the other hand, is in an earthquake region, shocks are of common occurrence, and another of an intensity equal to or greater than the recent disturbance may occur at any time, although, on the other hand, the temporary adjustment brought about by the recent slip tends to decrease the danger of an immediate severe shock.
In the New Madrid area, however, the earthquake of 1811-12 was only one of a series. Cracks may be found with trees fully 200 years old growing in their bottoms, indicating early shakes of equal if not greater intensity than the last. Nor has the movement yet ceased. Every year there are one or more shocks, sufficient to shake objects from shelves, and to seriously affect wells and springs. Only last summer the newspapers were full of accounts of such a shock in southeastern Missouri and adjoining regions—the very area of the New Madrid earthquake. If there have been two or more strong shocks with an intensity far greater than the Charleston quake, and if the readjustment is not completed as is positively indicated by the recent shocks, then there is every reason to believe that disturbances of equal severity may occur in the future. Such quakes, it goes without saying, would be disastrous to such towns as Hickman in Kentucky, Caruthersville, New Madrid, Campbell and others in Missouri, all of which are in the area of disturbance. The larger cities of Cairo and Memphis, although outside the main area, would also probably suffer severely, as they are built on soft deposits overlooking the Mississippi in situations favoring easy slipping towards the streams. Such spots were often severely fissured by the early quake, large masses slipping into the river, and what has occurred once may occur again. St. Louis would also probably be severely shaken, but its buildings are less liable to destruction from a shock originating in the New Madrid area because of the remoteness from the point of disturbance.
- Published by permission of the director of the United States Geological Survey.