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."