New Madrid Study Sheds New Light

Seismic Zone Good at Hiding Its Faults

A recent geologic study may shed some light on a mid-western U.S. seismic zone that is much discussed but little understood.

The New Madrid zone, which runs northeast-southwest through eastern Arkansas, western Tennessee, southwest Missouri and western Kentucky, has been the site of several significant earthquakes.

Unfortunately, the region is far away from known active tectonic forces, and as such is a mystery when compared to the data available for seismically active regions like California and around the Pacific Rim.

That's a big reason why Margaret Guccione, professor of geosciences at Fulbright College at the University of Arkansas, and AAPG member Ron Marple with WesternGeco decided to take a closer look at the Bootheel lineament surface rupture in the New Madrid seismic zone.

Their recently completed work combined resistivity and shallow core samples in an effort to prove that the Bootheel lineament is in fact a major fault — knowledge that could help scientists better understand the distribution of earthquakes in the seismic zone, which encompasses both Memphis and St. Louis.

"Researchers have speculated that one or more faults might occur within the New Madrid seismic zone, the site of earthquakes in 1811 and 1812 that rang bells in Boston and temporarily blocked the flow of the Mississippi River," Guccione said.

"However, faults that have mostly horizontal movement and any that extend to the surface have been obscured by liquefaction of sand in the new Madrid seismic zone."

Finding Fault

Liquefaction is caused by shaking, saturated sand during an earthquake. The liquefied sand may build up pressure and explode toward the surface, causing the soil and clay to collapse into the resulting hole. Collapse of the soil can look similar to a fault.

The New Madrid seismic zone has only two features that have been suggested as possible ruptures — the Reelfoot scarp and the Bootheel lineament.

The Bootheel lineament is a 125-kilometer-long linear feature that can be seen on aerial photography and satellite, she said. The lineament runs sub parallel to the New Madrid seismic zone, but classifying the feature as a fault has been complicated by extensive sand liquefaction along the lineament, making any surface tectonic offset difficult to pinpoint.

The Bootheel lineament is recognized by:

  • A contrast in sand blows on opposite sides, generally denser to the southeast.
  • Shallow linear depressions.
  • Continuous or discontinuous linear bodies of sand.
  • Truncated paleochannels.

This study's purpose was to examine a filled paleochannel visible on aerial photographs east of the lineament that is truncated by the lineament, but not obvious west of the lineament.

Guccione and Marple felt this could provide a piercing point to test their hypothesis.

Image Caption

The Bootheel lineament in Missouri and Arkansas, in relation to the New Madrid seismic zone.

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A recent geologic study may shed some light on a mid-western U.S. seismic zone that is much discussed but little understood.

The New Madrid zone, which runs northeast-southwest through eastern Arkansas, western Tennessee, southwest Missouri and western Kentucky, has been the site of several significant earthquakes.

Unfortunately, the region is far away from known active tectonic forces, and as such is a mystery when compared to the data available for seismically active regions like California and around the Pacific Rim.

That's a big reason why Margaret Guccione, professor of geosciences at Fulbright College at the University of Arkansas, and AAPG member Ron Marple with WesternGeco decided to take a closer look at the Bootheel lineament surface rupture in the New Madrid seismic zone.

Their recently completed work combined resistivity and shallow core samples in an effort to prove that the Bootheel lineament is in fact a major fault — knowledge that could help scientists better understand the distribution of earthquakes in the seismic zone, which encompasses both Memphis and St. Louis.

"Researchers have speculated that one or more faults might occur within the New Madrid seismic zone, the site of earthquakes in 1811 and 1812 that rang bells in Boston and temporarily blocked the flow of the Mississippi River," Guccione said.

"However, faults that have mostly horizontal movement and any that extend to the surface have been obscured by liquefaction of sand in the new Madrid seismic zone."

Finding Fault

Liquefaction is caused by shaking, saturated sand during an earthquake. The liquefied sand may build up pressure and explode toward the surface, causing the soil and clay to collapse into the resulting hole. Collapse of the soil can look similar to a fault.

The New Madrid seismic zone has only two features that have been suggested as possible ruptures — the Reelfoot scarp and the Bootheel lineament.

The Bootheel lineament is a 125-kilometer-long linear feature that can be seen on aerial photography and satellite, she said. The lineament runs sub parallel to the New Madrid seismic zone, but classifying the feature as a fault has been complicated by extensive sand liquefaction along the lineament, making any surface tectonic offset difficult to pinpoint.

The Bootheel lineament is recognized by:

  • A contrast in sand blows on opposite sides, generally denser to the southeast.
  • Shallow linear depressions.
  • Continuous or discontinuous linear bodies of sand.
  • Truncated paleochannels.

This study's purpose was to examine a filled paleochannel visible on aerial photographs east of the lineament that is truncated by the lineament, but not obvious west of the lineament.

Guccione and Marple felt this could provide a piercing point to test their hypothesis.

"In the early 1990s Ron (Marple) was part of a research group looking at the New Madrid seismic zone, and they noticed on aerial photography and satellite imagery this long linear feature," Guccione said. "It wasn't continuous, but they could trace it from about New Madrid, Missouri, down into Arkansas near Jonesboro."

The feature was identified by differences in the kinds and amounts of sand blows on either side, she added. In addition, in some locations there are small depressions along the lineament.

"These aren't big," she said, "just enough to hold water when it rains. They also saw some small inactive filled-in channels that come right up to the lineament and are then truncated. It was these features that delineated the feature as a lineament."

Marple and his fellow scientists suspected this feature was a fault, but they couldn't be sure. They did some seismic reflection work across the area, which did identify a fault in the deep subsurface, but the fault could not be imaged in the shallower sediments.

The scientists then trenched across the lineament in one location, but there was so much liquefaction and displacement of layers that it was impossible to tell if it was simply liquefaction or tectonic forces at work.

"When the sand explodes to the surface in these sand blows, whatever is at the surface has to drop down to fill the void and sand covers it," Guccione said. "So, the resulting feature can look like a fault due to displacement or just ground failure due to the sand blows."

Channel Resistivity

Marple decided to locate a channel that terminates at the lineament and then attempt to locate the same channel on the other side at an offset position. He knew that was likely since the New Madrid seismic zone is made up of strike slip or lateral movement.

He enlisted Guccione to participate in the project to acquire and study core in the area.

The scientists chose a location near Carruthersville, Mo., near the Mississippi River in the middle of the Bootheel lineament where a channel was identified. Initially Marple acquired resistivity measurements across the channel where it can be seen on the east side of the Bootheel lineament.

The resistivity showed the channels very nicely, but when he moved to the west side of the lineament the channel was no longer visible and the resistivity was no help.

"On the west side all Ron got were consistent, uniform resistivity readings," Guccione said. "There is so much clay present on that side of the lineaments that it obscures any channel."

Despite this setback Guccione went ahead with her portion of the study, and with the assistance of University of Arkansas geocience students conducted coring operations over the span of three years. The group drilled over 100 cores up to eight meters deep in a 300 by 300 meter area to reconstruct the geologic history of the lineament for the past 12,000 years.

She examined the composition of the cores and used radiocarbon dating to determine the age of the material in the cores.

"We started drilling on the east side, where we could see the channel," she said. "With this approach we knew what the channel fill looked like, what was on either side of it, how deep it was and about how wide it was before moving to the west side of the lineament where the channel is obscured."

Buried Treasure

The channel is fairly well defined on the east side, since the channel's edge fill is clay and it contrasts nicely with the surrounding sands. The original surface material was sand from the Mississippi River, and as the channel cut through it filled in on the edges with sandy clays.

The west side, however, is a completely different story.

"We think we did finally find the channel, but it wasn't easy — the channel is mostly buried and it cut through clay, so there is not the contrast between the sandy clay channel fill and the surrounding material," she said.

"This was the most difficult project I have ever worked on."

Guccione found that on the east side of the lineament the sediment is sand almost to the surface, with just a thin layer of clay from flood deposits of the Mississippi River. This area has remained sandy because it is topographically higher.

"But when you get to the Bootheel lineament there is an abrupt change in stratigraphy and elevation in the sediments," she said, "all over a distance of just six to 10 meters."

On the west side of the lineament the sediment that is nearly on the surface to the east is about four to five meters deeper, overlain by clay from Mississippi flood deposits, another layer of sand — which was interpreted as a very large blow that erupted at the surface — and then another layer of clay.

"This combination of middle sand and upper clay basically bury the channel," she said, "so nothing can be seen on the surface."

The cores show that sand deposited by the Mississippi River has subsided a total of 10 feet on the west side of the fault during at least two earthquakes, creating a depression in the ground.

  • During the first earthquake, which occurred between 12,000 and 10,500 years ago, there is evidence for faulting but no liquefaction at the study site. Flooding of the Mississippi deposited a thick clay at the bottom of the depression but on the other side of the lineament only a thin layer of clay was deposited on the uplifted sand.
  • During the second earthquake, one that is widely recognized throughout the New Madrid seismic zone and dated to about 1470 AD, both faulting and liquefaction caused additional subsidence west of the fault.

This time liquefied sand partially filled the depression along the fault. Clay subsequently buried the sand to fill the remaining depression.

The channel of a small stream that eroded across the lineament between the first and second earthquakes was displaced by lateral movement along the fault during the 1470 AD earthquake.

Conclusions

According to the authors, the coring project supports the suggestion that the lineament is a surface rupture and that it has both vertical and horizontal offset, with the east side moving relatively up and south and the west side moving down and north.

The boundary, or lineament, between the uplifted area and the depressed area is very narrow at less than six meters wide.

There is a record of at least three seismic events in the past 10,500 years and a total vertical offset of more than three meters in that time. There also is evidence of a total horizontal offset greater than 13 meters in the last 2,500 years. This movement is consistent with the seismic zone regionally.

The research, however, did not determine if there was any offset during the major earthquake of 1811.

The scientists drew a series of cross sections through time to uncover these findings:

  • During the Late Pleistocene, glacier melting brought sediments into the area via the braided stream of the Mississippi.

    Sometime after the sand was deposited there was some type of rupture and vertical motion 10,300 to 12,300 years ago.

    Following the rupture, clays likely filled in to make the ground surface flat, according to Guccione.

  • The next event occurred between 1450 and 1470 AD, and this was likely the biggest rupture in the area.

    During this event a huge pile of sand erupted to the surface, and the west side of the lineament dropped down.

  • Once again clay filled in to make the surface flat.

    Based on these data, the authors concluded there was about three meters of vertical movement in the area.

  • The next seismic event was in 1811 and there are some liquefaction features at the surface consistent with this time frame.

    "The earthquake was definitely felt in this location along the Bootheel lineament, but we don't see any vertical motion so there is no concrete evidence that the lineament ruptured," Guccione said. "There likely was lateral motion that we can't see.

    "To me, this movement is a very strong argument that there is a surface fault," Guccione added. "We can demonstrate three meters of vertical offset in the last 10,000 years and more than 10 meters of lateral offset in the last 2,500 years. This vertical movement will now have to be taken into account for the structure and tectonics of the area.

    "I feel very confident that the Bootheel lineament is actually a fault."

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