Earthquakes In Arkansas
Numerous earthquakes occur every year throughout the State of Arkansas, but most go unnoticed. Earthquakes that are felt can be startling, and serve as good reminders that Arkansas is located near one of the most hazardous earthquake zones in the country. Earthquakes have been historically documented in Arkansas, as far back as 1699, by missionaries traveling down the Mississippi River near Helena (Phillips County), Arkansas. Although, it is uncommon for major earthquakes to occur far away from active tectonic boundaries, earthquakes associated with the New Madrid seismic zone (NMSZ), an active earthquake zone extending from Cairo, Illinois, into Marked Tree (Poinsett County), Arkansas, have been some of the largest earthquakes to ever strike North America.
What Causes An Earthquake?
Earthquakes are caused by movement along geologic faults, or fractures in the Earth's crust. When a fault moves, energy is released and transfers through the earth causing the shaking that is experienced during an earthquake. Arkansas has hundreds, if not thousands of faults. Most of these faults are considered inactive. However, faults associated with the New Madrid seismic zone are active, and deeply buried beneath many layers of unconsolidated sediment and sedimentary rock, making them almost impossible to identify on the Earth's surface. These faults exist within a failed rift zone, known as the Reelfoot Rift, which developed in the Earth's crust over 600 million years ago.
Types Of Faults
Strike Slip Fault
If the movement of a fault is predominately horizontal, the fault is considered a strike-slip fault. The San Andreas Fault zone is one of the most famous examples of a strike-slip fault in the United States.
Dip Slip Faults
If the movement along a fault is predominately vertical, the fault is considered to be a dip-slip fault, or in other words the displacement occurs along the dip plane of the fault. There are two main types of dip-slip faults; Normal faults and Reverse. In order to determine the differences between dip-slip faults it's important to understand the terms hanging wall and footwall.
Normal faults occur when the foot wall is displaced upward with respect to the hanging wall.
Reverse faults occur when the hanging wall is displaced upward with respect to the footwall.
Where Do Earthquakes Occur?
Most earthquakes occur at plate boundaries. Earthquakes associated with faults on plate boundaries are called interplate earthquakes. About 5% of earthquakes are intraplate earthquakes and occur in the center of a tectonic plate.
Magnitude Scale
There are two scales in common use that give some measure of the size of an earthquake. These scales are quite commonly confused but measure very different parameters of the seismic event. The first scale, and the one most commonly heard on the news, is a magnitude scale. The magnitude scale was first developed in the 1930’s and is an objective instrumental measurement. It is based on the amplitude and amount of displacement of an instrument record trace calibrated and corrected with known distance, magnification and instrument factors. The magnitude scale is a logarithmic scale, meaning that each unit of increase (1, 2, 3…) of the scale value indicates a ten-fold increase in the value being measured.
Therefore, if a magnitude 3 earthquake shows a record trace displacement of 1mm, then a magnitude 4 earthquake will show a trace displacement of 10mm. Each increase in magnitude corresponds with about 33 times more released energy. For example a magnitude 1 earthquake is equal to 56 kilograms of explosive energy. A magnitude 2 earthquake is then equal to 1,800 kilograms of explosive energy (approximately 56 times 33). A magnitude 7 earthquake releases 56,000,000,000 kilograms of explosive energy compared to a magnitude 8 earthquake which releases 33 times more energy. That’s approximately 1,800,000,000,000 kilograms of energy! As one can see, the amount of energy released in an earthquake drastically increases the higher the magnitude of the earthquake.
Intensity Scale
The other scale often used to indicate earthquake size is the Modified Mercalli Intensity scale. Intensity is a qualitative measure of the strength of ground shaking at a particular site. It is a subjective scale relying on the observations of people trained to relate the earthquake effects to a numeric scale from I-X+ (Roman numerals are used to distinguish this as an empirical scale). Because this is an observational scale, the intensity will decrease as a function of the distance from the epicenter. Most people (but not all) in an area experiencing Intensity III effects will just feel the earthquake shock. Intensity VII areas are indicated when damage is slight to moderate. Some people may have found it difficult to stand during the quake; chimneys, windows and plaster walls will be cracked; furniture may have overturned. Although damage may be moderate in an area of Intensity VII, it will be mostly architectural damage—i.e. it will look bad but almost all damage will be superficial. Beyond Intensity VII we start seeing structural damage and some collapse buildings. At first only poorly designed structures are destroyed, but as the Intensity level reaches X damage increases significantly.
The Modified Mercalli Intensity Scale
| MMI Value | Perceived Shaking | Potential Damage | Full Description |
|---|---|---|---|
| I. | Not Felt | None | Not Felt |
| II. | Weak | None | Felt by persons at rest, on upper floors, or favorably placed. |
| III. | Weak | None | Felt indoors. Hanging objects swing. Vibration like passing of light trucks. May not be recognized as an earthquake. |
| IV. | Light | None | Hanging objects swing. Vibration like passing of heavy trucks; or sensations of a jolt like a heavy ball striking the walls. Standing motor cars rock. Windows, dishes, doors rattle. Glasses clink. Crockery clashes. In the upper range of IV, wooden walls and frame creak. |
| V. | Moderate | Very Light | Felt outdoors; direction estimated. Sleepers wakened. Liquids disturbed, some spilled. Small unstable objects displaced or upset. Doors swing, close, open. Shutters, pictures move. Pendulum clocks stop, start, change rate. |
| VI. | Srong | Light | Felt by all. Many frightened and run outdoors. Persons walk unsteadily. Windows, dishes, glassware broken. Knickknacks, books, etc., off shelves. Pictures off walls. Furniture moved or overturned. Weak plaster and masonry cracked. Trees, bushes shaken (visibly, or heard to rustle). |
| VII. | Very Strong | Moderate | Difficult to stand. Noticed by drivers of motors cars. Furniture broken. Damage to masonry, including cracks. Weak chimneys broken at roof line. Fall of plaster, loose bricks, stones, tiles, cornices (also unbraced parapets and architectural ornaments). Waves on pond water, turned with mud. Small slides and caving in along sand or gravel banks. Large bells ring. Concrete irrigation ditches damaged. |
| VIII. | Severe | Moderate/Heavy | Steering of motor cars affected. Damage to masonry. Twisting, fall of chimneys, factory stacks, monuments, towers, elevated tanks. Frame houses moved on foundations if not bolted down. Branches broken from trees. Changes in flow or temperature of springs and wells. Cracks in wet ground and on steep slopes. |
| IX. | Violent | Violent/Heavy | Some Masonry destroyed, heavily damaged or completely collapsed. General damage to foundations. Frame structures, if not bolted, shifted off foundations. Frames racked. Serious damage to reservoirs. Underground pipes broken. Conspicuous cracks in ground. In alluvial areas sand and mud ejected, earthquake fountains, sand craters. |
| X. | Extreme | Very Heavy | Most masonry and frame structures destroyed with their foundations. Some well-built wooden structures and bridges destroyed. Serious damage to dams, dikes, embankments. Large landslides. Water thrown on banks of canals, rivers, lakes, etc. Sand and mud shifted horizontally on beaches and flat land. Rails bent slightly. |
In general earthquakes smaller than magnitude 2.5 will not be felt in most situations, they are simply too small and will certainly do no damage. Earthquakes between magnitude 2.5 and 5 will be increasingly felt but generally do little damage. When earthquakes scale to over magnitude 5 their effects become highly significant. Earthquakes of magnitude 5 to 6 will almost always cause some damage, but most of it will be architectural rather than structural. An earthquake of magnitude 6 can cause significant architectural damage and some structural damage and collapse of poorly-built structures. The area of damage will tend to be roughly bulls-eye shaped, with the greatest damage in the immediate epicentral area and lessening damage radially away from that area. This general pattern will be distorted more or less by variations in the nature of the bedrock and topography of the region in relation to the source of the earthquake and the exact manner in which it releases its energy. An earthquake of magnitude 7 will cause near total devastation in the epicentral area and cause structural damage and collapse of poorly-built structures over a much larger area (remember, this earthquake is 30 times more powerful than a magnitude 6 event). The great earthquakes, those over magnitude 8, will destroy most of the infrastructure in a very large area, several 10s of miles in diameter, and can cause structural damage and collapse of poorly-built structures as much as 100 miles away. Great earthquakes can still be felt many hundreds of miles away.
Seismic Waves
Two main types of seismic waves are generated in an earthquake.
Body waves are seismic waves that travel through the interior, or the body, of the earth. There are two main types of body waves; primary waves and secondary waves. Primary waves, or P waves, are compressional waves and travel faster than Secondary, or S waves. As a result, P waves are first to be detected by a seismic station after an earthquake has occurred, thus “primary”. Secondary waves, or S waves, travel in a side-to-side motion and move through the rock perpendicular to the direction the wave is traveling. S waves travel slower than P waves are recorded by seismic stations after the P wave arrival.
Illustration courtesy of the USGS
Surface waves travel through the very outer layers of the earth’s crust and arrive at a seismic station after the P and S waves. Two main types of surfaces waves are Raleigh and Love waves. Raleigh waves travel in a circular or rolling motion through the earth’s crust, similar to that of an ocean wave on the water. Love Waves are a type of surface wave that travels in a horizontal, side-to-side motion through the outer layer of the earth’s crust.
Determining An Earthquake Epicenter
The amount of time that passes between the arrival of the Primary wave and the Secondary wave allows seismologists to determine the distance from the seismic station to the earthquake epicenter. When a seismic station records a P wave arrival, the longer it takes for the S wave to arrive, the farther away the earthquake epicenter.
