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Ausbrooks talks about earthquakes |
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Monday, 05 January 2009 |
The Malvern Rotary Club met Wednesday at the Western Sizzlin’ for their monthly meeting. Their guest speaker was Scott Ausbrooks, with the Arkansas Geological Survey and works in the Geohazards Division.
Rotary member George McClure introduced Ausbrooks. Ausbrook spoke about Earthquakes in Arkansas.
Ausbrook spoke about the recent earthquakes that were felt in the Magnet Cove area. He gave a short lecture on Earthquakes 101.
Ausbrook explained that earthquakes are the shifting of the earth’s plates, and that 95 percent of the world’s earthquakes are along the edge of plate boundaries. He explained that when a fault moves, energy is released and transfers through the earth in the form of seismic waves, and although earthquakes are associated with faults, not all faults produce earthquakes.
He stated that the point within the crust where the energy is released is called the focus. This is what the portable seismic stations that have been recently placed in the area are helping to locate. Ausbrooks stated that by finding the focus of these quakes and the epicenter, the geological survey will be better able to determine whether the earthquakes are caused by a fault line or from hydrothermal or water movement below the earth’s surface. He stated that the focus and the epicenter are two different points that occur during an earthquake. The epicenter is the location on the earth’s surface that is exactly above the focus. The shaking that occurs during an earthquake is caused by the transmission of seismic waves.
Ausbrooks stated that a study of the geology of Arkansas reveals thousands of faults. The faults are all differnet sizes and these faults have one thing in common- they are inactive. It was explained that in Arkansas, the faults in which earthquakes occur are not visible at the surface. They are deep in the earth, covered by thousands of feet of unconsolidated sediment.
When asked how the Geological Survey detected the earthquakes in Magnet Cove, Ausbrooks stated that the seismic sensors in North Eastern Arkansas detected the quakes, but were very unreliable in the pinpointing of the actual epicenter of the quakes. Ausbrooks stated that when all the seismic activity occurred in November, there were two events that were recorded in Prairie County and on in Northern Saline County, but he believes they may have actually been centered here in the Magnet Cove area where the others were located. He further mentioned that the earthquakes in Prairie and Saline Counties were not felt by any residents in those counties, and that the Geological Survey was relying on measurements from the seismographs in the northeast corner of the state.
Ausbrooks stated that the seismic stations that have been placed in the Magnet Cove area, are being checked every two weeks. He stated that they were checked before Christmas, and should be checked sometime in the next week. If they show any signs of significant activity, they will remain in place. This will help further pinpoint the exact locations of the recent earthquakes. If there is little or no activity within this time period, the stations will be removed from the area. MDR asked Ausbrooks that if there were further activity, would permanent seismographs be placed in the area? Ausbrooks stated that the Governor has allocated $300,000 for six permanent seismographs for the state, of which one would be placed permanently here in Hot Spring County.
Ausbrooks gave a brief history of previous earthquakes in Arkansas.
It was on Dec. 16, 1811, when residents living near New Madrid, Mo., were awakened early in the morning by a major earthquake. Buildings were destroyed, and people were frightened, and the land was changed drastically. He stated the earthquake was felt as far away as New Orleans, La., Boston, Mass., Charleston, S.C., Detroit, Mich., Washington, D.C., and Quebec, Canada. It was estimated that this event was over 7.0 in magnitude. This was the first of many earthquakes that occurred through February of the following year.
Since then, at least 20 damaging earthquakes have occurred in northeastern Arkansas and southeastern Missouri. An estimated 6.0 earthquake was reported near Marked Tree, Arkansas in January 1843. The earthquake caused damage to brick structures and the land subsided and formed new lakes. An earthquake with the magnitude of 5.0 was recorded in March 1976 in Poinsett County. It was felt as far away as Illinois, Kentucky, Tennessee, Alabama, Missouri and in Little Rock, Ark., for an area measuring over 174, 000 sq. miles.
Hundreds of aftershocks were and have been reported since the earthquake.
Ausbrooks reported that during the 1811-1812 earthquakes
the series of strong earthquakes that shook northeastern Arkansas had severe effects on the land. The effects included formation of sunken land, which created new lakes such as Big Lake in Mississippi County. Huge amounts of sand were brought up from depth and placed on the land surface from sand blows or sand boils. Large fissures opened up on the land surface.
It was even reported that the mississippi river had waterfalls, and flowed backwards when these earthquakes occurred.
The earthquakes of 1811 and 1812 had a magnitude of 7.0 or more and was felt in an area of over 200,00 sq. miles, and that the 7.0 magnitude earthquakes in California only were felt in an area covering 14,000 sq. miles.
Ausbrooks explained that the area of the New Madrid fault in northeastern Arkansas and southeaster Missouri is covered with unconsolidated sediment. This sediment acts as a muffler to the vibrations, holding the tremors in the rocks and stone formations in the earth. The rock and stone formations are cold and hard in this area, and carries the vibrations further than the areas involved in the California earthquakes.
Another concern is the liquefaction of the ground sediment, the caving of riverbanks and landslides are believed to be major problems along with flooding from the mississippi river, if another major earthquake were to occur.
If an earthquake today with the same magnitudes as the ones in 1811-1812 were to occur, the events would cause very widespread destruction. States surrounding northeast Arkansas would be affected, but the most severe effects would be centered in northeast Arkansas and southeastern Missouri. It is believed that a major earthquake of 6.0 or more were to occur, the effects might reach as far as Little Rock and Pine Bluff.
We cannot predict when an earthquake will occur, so it is best to be prepared. Planning at the State, City and County levels is underway in the event that the New Madrid seismic fault zone produces another major earthquake. If you live in northeastern Arkansas, chances are you have already felt an earthquake. If a major earthquake occurs, and it will. It will affect all those living in or near the New Madrid fault zone. Always be prepared. Talk to your family and friends about the hazards of earthquakes, and what to do in the event of an earthquake.
If an earthquake occurs, remain calm. If inside a building, stand in a sturdy doorway, or get beneath a sturdy table or desk.
If driving, stop in a safe area. Do not park in an underpass or on an overpasses. If you are outside stay away from trees, telephone poles and electrical lines, and stand away from buildings.
The Natural State has felt its fair share of earthquakes.
The New Madrid earthquake zone has been active for hundreds of years. The Arkansas Geological Survey asks, What if another series of major earthquakes were to strike this region again? Would your business or family be prepared?
The Arkansas Geological Survey generates seismicity maps based on data provided by the Center for Earthquake Research and Information (CERI) at the University of Memphis, the Department of Earth and Atmospheric Sciences at Saint Louis University, the United States Geological Survey, and the Arkansas Center for Earthquake Education and Technology Transfer at the University of Arkansas in Little Rock.
For more information on Arkansas Earthquakes and the New Madrid fault and seismic zone, contact the Arkansas Geological Survey at www.geology.arkansas.gov.
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Last Updated ( Wednesday, 14 January 2009 )
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