Spatial and Temporal Characteristics of Paleoseismic Features in

Spatial and Temporal Characteristics of
Paleoseismic Features in the Southern
Terminus of the New Madrid Seismic Zone in
Eastern Arkansas
Haydar J. AI-Shukri
University of Arkansas at Little Rock
Robert E. Lemmer
Leighton and Associates, Inc.
Hanan H. Mahdi and Jeffrey B. Connelly
University of Arkansas at Little Rock
ABSTRACT
INTRODUCTION
The focus of this study is to identify and characterize specific
features related to historic or prehistoric earthquakes south of
the southern terminus of the New Madrid seismic zone in
eastern Arkansas. Aerial photography, field surveys, and
trenching reveal the existence of several liquefaction features
(sand blows) and linear structures as far as south of Marianna,
Arkansas. This is more than 100 km from the currently active
segments of the New Madrid seismic zone. Radiocarbon dating indicts that the event(s) that generated some of these features took place about 5500 years B.P. The discovered
liquefaction features are significant because they are at a considerable distance from present-day earthquake activity. The
implication of this is that they either represent a new earthquake source not previously recognized or that they are
related to an earthquake(s) of very large magnitude in the
source region of the New Madrid seismic zone. These liquefaction features have very large dimensions (-110 by 60 m),
resembling features in the immediate vicinity of the New
Madrid seismic zone, implying that regardless of where the
source region was, the ground shaking had to be severe in
order to generate them. Detailed investigation of these features may have important implications for earthquake risk
mapping in the central United States, as they may provide
important constraints on the southern terminus of the New
Madrid seismic zone and the magnitude of the characteristic
earthquake in the region.
A basic assertion of paleoliquefaction studies is that largemagnitude earthquakes, of 6 or greater, may leave a record in
the form of liquefaction-related features. Of primary significance among these features are sand blows, which form as a
result of venting of sand-bearing water to the surface due to
earthquake-induced liquefaction. The presence of sand blows
in the geologic record provides opportunity to define the ages
of prehistoric events. Employing modern dating techniques
allows for evaluation of the recurrence times of large earthquakes and helps define the long-term behavior of seismogenic fault zones (Pavlides et al., 1999). The past 25 years
of research and data collection concerning the seismicity of
the central United States have resulted in a dramatic increase
in our knowledge of this region. Our understanding of earth~
quake hazards in the New Madrid seismic zone (NMSZ), the
Wabash Valley seismic zone, and other areas in the central
and eastern United States has been profoundly changed by
paleoseismic investigations. Hundreds of liquefaction features (Figure 1), believed to be the result of local earthquake
ground motion, and evidence of recent faulting were systematically surveyed and studied by several investigators (e.g.,
Schweig et al., 1992; Tuttle et al., 1996; Li et al., 1998; Tuttle,
1999; Guccione et al., 2000; Broughton et al., 2001; Tuttle et
al., 2002; Tuttle et al., 2005). Most efforts have concentrated
on locating and dating these features within the current area
of enhanced seismicity and the immediately adjacent areas.
Limited research has been conducted to locate and study such
features south of the currently known source region.
502 SeismologicalResearchLetters Volume76, Number4 July/August2005
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A Figure 1. Paleoseismologyin the New Madridseismiczone(afterTuttleet al., 2002). Historicand recentseismicityare alsoshown.For detailssee Tuttle
et al. (2002).
Seismological ResearchLetters July/August2005 Volume76,Number4 503
From the paleoseismic research conducted in the central
United States, it is clear that this area has experienced
repeated earthquakes in the last few thousand years. There is
also mounting evidence supporting a moment magnitude of
larger than 7 for some of these earthquakes. Due to the short
duration of historic archiving of natural events in the central
United States, paleoseismology is the primary source of information for seismic hazard mapping. Given the lack of knowledge about the spatial characteristics of prehistoric
earthquakes in areas beyond the current microseismicity
zones, and the uncertainty in recurrence periods and magnitudes, earthquake hazard will continue to be a debated issue
and a fair subject of criticism.
Until recently, the southernmost paleoseismic features
identified were located about 20 km southwest of Marked
Tree, Arkansas (Tuttle et al., 2002). More recently, however,
Tuttle (personal communication) has identified features as far
south as Madison, Arkansas (approximately 60 km southwest
of Marked Tree). Van Arsdale et al. (2003) found evidence of
widespread liquefaction features in the Memphis area east of
the Mississippi River. We conducted reconnaissance, including aerial photography interpretation and land surveys, and
trenching studies in eastern Arkansas. The surveys covered an
area from immediately west of Helena to Marked Tree in eastern Arkansas (Figure 2). These surveys indicate that the spatial distribution of sand blows extends at least to Helena.
These newly discovered features are more than 100 km
southwest of the southern terminus of the current area of
microseismicity, which ends at Marked Tree.
Several features located west and south of Marianna,
Arkansas (Figure 2) were found to be sand blows (AI-Shukri
et al., 2000). Two possible hypotheses might explain their
existence: (1) The liquefaction features may be the result of
earthquakes generated by a source in the Marianna area. This
source region currently shows little seismic activity, however,
and the potential for reactivation is unknown. Gravity and
magnetic analysis in this area (Hildenbrand, personal communication) indicate the existence of a major fault, the
"Arkansas transform fault", bordering the study area to the
south (Figure 2). (2) The liquefaction features might be
related directly (major NMSZ events) or indirectly (aftershocks of NMSZ events) to the currently active source
responsible for the NMSZ. This would place the southernmost features that we have identified approximately 100 km
from the source region. If this is the case, then sand blows of
this size and at this distance from the source region require a
"major" earthquake.
Given these observations, the questions that need to be
addressed are: Where is the southern terminus of the NMSZ?
What is the relationship between the recently discovered features, the historic earthquake series that took place in
1811-1812, and current microseismicity? If the issue of the
relationship between these features and the current New
Madrid earthquake activity can be resolved, how will these
new findings change our understanding of the magnitude of
the characteristic earthquake, the recurrence rate, and possi-
ble migration of seismicity throughout the region? We feel
that it is important to address these gaps in our knowledge
about this enigmatic region through comprehensive geological, geophysical, and geochronological research in east-central
and southeastern Arkansas.
GEOLOGICALAND TECTONIC SETTING OF THE
STUDY AREA
The study area (Figure 2) is located in the Western Lowlands
of the Mississippi embayment, immediately beyond the
southern terminus of the NMSZ. The Mississippi embayment is a broad south-southwest plunging trough extending
from westernmost Kentucky to the Gulf of Mexico. This
trough is filled with Cretaceous to recent sediments (sand,
silt, clay, and gravel) of shallow marine and fluvial origin.
These embayment sediments unconformably overlie Cambrian clastic and Ordovician carbonate rocks deposited in the
Reelfoot rift basin. The most prominent buried structure in
the upper Mississippi embayment is the Reelfoot rift, which
is a failed crustal rift of late Proterozoic to Early Cambrian
age. The aulacogen was probably subjected to a second episode of rifting and subsequent failure during the late Mesozoic (Burke and Dewey, 1973; Ervin and McGinnis, 1975).
The Reelfoot rift, which contains most of the earthquakes in
the NMSZ, is about 70 kilometers wide and has 2-3 km of
subsurface relief. It trends northeast-southwest for about
320 km from the Rough Creek graben in Kentucky south to
the Arkansas transform fault (Hildenbrand and Hendricks,
1995; Langenheim and Hildenbrand, 1997). Several buried
plutons border the rift to the northwest and southeast (e.g.,
Jonesboro pluton, Paragould pluton, and Covington pluton).
The Blytheville arch, a zone of arched and faulted strata, is
located in the center of the rift system (Hamilton and McKeown, 1988). The arch is approximately 15 km wide and
110 km long and has been mapped to within 20 km of the
study area.
In the last several thousands of years, the NMSZ has
been the source of several seismic events or sequences of
events that were large enough and had sufficient ground
motion to cause widespread and severe liquefaction similar to
that in 1811-1812 (Tuttle, 1999; Tuttle et al., 2002; Tuttle et
al., 2005). Some of these liquefaction features have been associated with the New Madrid seismic events of 1811-1812. In
the last few years, however, numerous other features have
been correlated with older events. Dating, measuring, and
interpretation of many large liquefaction features over a
broad region have led to the development of a New Madrid
event chronology (Tuttle, 1999; Tuttle et al., 2002). Kelson et
al. (1992, 1996), Tuttle and Schweig (1995), Li et al. (1998),
and Tuttle et al. (2002) have documented liquefaction features from at least two seismic events occurring prior to the
1811-1812 earthquakes. These two events occurred A.D.
900 _+ 100 years and A.D. 1450 _+ 150 years and were likely
similar in strength to those of 1811 and 1812 (Tuttle et al.,
2002). Two prior events in A.D. 300 _+200 years and 2350
504 SeismologicalResearchLetters Volume76, Number4 July/August2005
37.0 ~
32.5 ~
.94.0 ~
-89.5 ~
A Figure 2. Location of study sites (Nancy 1, Nancy 2, and Parkin 1). The map also shows the locations of Cox et al. (2004) study sites. Red crosses represent earthquake locations. Thick solid line (ATF) represents the approximate location of the Arkansas transform fault (Hildenbrand, personal communication).
B.C. _+200 years have also been interpreted from liquefaction
data (Tuttle et al., 2005)
The boundary of the study area extends from approximately 45 km southwest of Marked Tree, Arkansas to south
of Helena, Arkansas (Figure 2). The area is underlain by Cretaceous to Tertiary sediments that range in thickness from
775 m in the north to 930 m in the south (Cushing et al.,
1964; Boswell et al., 1965; Hosman et al., 1968). These sediments are mantled by fluvial and eolian deposits, including
multiple Pleistocene braided stream surfaces of the Mississippi River that are crosscut and/or reoccupied by Holocene
meander belts of smaller streams (Boswell et al., 1968; Blum
et al., 2000).
ANALYSISAND RESULTS
An aerial survey in eastern Arkansas reveals numerous features throughout the research area that appear to have seis-
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A:
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,A, Figure 3. Aerial photograph of a linear feature near Parkin, Arkansas
(indicated by the arrows). Dark solid line indicates the location of the trench
(see Figure 2 for location).
mogenic origins (Figures 3 and 4). These features are light
sandy patches surrounded by dark silty soil and are easily distinguishable from the air (and on the ground) (Figure 4).
Most of the features south of Interstate 40 are semicircular to
elliptical and are relatively large. The region north of Interstate 40 yielded fewer and more widely spaced features than
the Marianna area. This region, however, has experienced
more flooding than the areas to the south, and this may have
obscured additional features. Ground reconnaissance surveys
were also performed. The intent was to evaluate features identified from the air to determine which features warranted further investigation and evaluation. Evaluations included
measuring feature size and orientation and determining nearsurface stratigraphy with a hand auger and digging test pits.
Trenching Results
Four trenches at three sites (Figure 2) were excavated. The
dimensions of these trenches range between 35-95 m long;
they were 1.2 m wide and 3 m average depth. Two of the sites
were near Marianna, Arkansas (Nancy 1 and Nancy 2), and
one was near Parkin, Arkansas (Parkin 1). One trench was
excavated at Nancy 1 and two trenches were excavated at
Nancy 2. Parkin 1 was trenched to investigate a 1.5-km-long
linear feature (Figure 3). The lineament trends N56~ and
has a ground surface that is 2.75 km higher on the southeast
side. No fault was observed in the trench, but sand and clay
layers tilted to the northwest were observed on the down side
of the lineament. The work at this site was not completed due
to heavy rain and flooding that forced the research team to
close the trench. Both the Nancy 1 and Nancy 2 sites were
confirmed to be sand blows of seismogenic origin (Figures 5,
6, and 7). Below is a description of the three trenches excavated through these sand blows:
506
Seismological Research Letters Volume 76, Number4
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,A Figure 4. Aerial photograph of a sand blow (Nancy 1) just east of Marianna, Arkansas. Dark solid line indicates the location of the trench shown
in Figure 5.
Nancy 1, T-1
A trench oriented approximately N38~ was excavated across
the long axis of an elliptically shaped sand blow (Figure 4).
The excavation was 59 m long and ranged from 1.85-2.25 m
deep. The east wall was cleaned, gridded at a 50-cm spacing,
logged in detail, and photographed. Important portions of
the west wall were also cleaned and photographed. Logging
revealed four distinct lithologic layers (Figure 5). The lowermost layer is a gray to brownish clay, which extended the
entire length of the trench. The excavation exposed the upper
1.5 m of this layer, denoted as Unit A. The clay is plastic, is
slightly iron-stained, and exhibits a moderately developed soil
structure (blocky) with rootlet pores; small charcoal fragments were present. At least 45 vertical to near vertical sand
dikes cutting across Unit A are connected to the overlying
sand layers. The sand dikes range in width from < 1 cm to
20 cm. Radiocarbon dating of charcoal (Beta B-149264) collected 5 cm from the top of Unit A (Figure 5) yielded a twosigma calibrated date of 4850--4800 B.E Overlying the clay
(Unit A) are two sand layers denoted as Units B and C, which
are distinguished from one another by their degree of cementation. Unit B is moderately cemented, while Unit C is loose.
Both units are a light gray, fine-medium-grained sand with
yellowish brown iron staining or mottling throughout. Overlying both sand units is a plow zone of highly disturbed silty
sand which ranges in thickness from 13-35 cm.
Nancy 2, T-1
A 38-m-long trench oriented N66~ was excavated across a
sand blow located at the Nancy 2 site (Figure 6). This excavation averaged 2.1 m in depth. The northwest wall was
cleaned, gridded at a 50-cm spacing, logged, and photographed. Portions of the southeast wall were also cleaned and
photographed to assist in interpretation. Four distinct layers
were exposed in the wall, all of which extended the complete
July/August2005
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A Figure 5. Log and photographs of the east wall of the trench excavated in the sand blow at site Nancy 1, T-1. See Figure 2 for location. The photographs
show a number of nearly vertical dikes cutting through a thick clay layer (Unit A).
length of the excavation. The oldest layer, denoted as Unit A,
was exposed at the bottom of the trench wall and floor. Only
the upper 15-75 cm of the lowermost layer were observed.
Unit A is a light brown and light bluish mottled clay. This
unit is plastic and heavily iron stained. Many charcoal fragments are present and the layer has a moderate blocky soil
structure. Overlying this unit is Unit B. The contact between
Units A and B is gradational. Unit B is also a clay, which
ranges in thickness from 20-100 cm. The unit is plastic with
a minor amount of charcoal and rootlet pores present. It has
a moderate to well developed blocky soil structure. Radiocarbon dating of organic sediments (Beta 149266) collected
10 cm from the top of Unit B (Figure 6) yielded a two-sigma
calibrated date of 5660-5580 B.P. Cutting across both clay
units are five sand dikes feeding the overlying sand blow.
These discordant features include one thin near-vertical dike,
three shallowly dipping thin dikes, and one wide (15 cm)
shallowly dipping (10-15~
dike striking N25~ All
dikes dip to the northwest except one, which dips to the
northeast at 30 ~ The overlying sand (Unit C) is from
22-118 cm thick. The sand was loose, light colored, and fine
to medium grained with a minor amount of silt present.
Charcoal fragments were scattered throughout the unit.
Above the shallowly dipping dike numerous clay clasts of
Unit A and several rounded pebbles (4 cm in diameter) occur
in the lower portion of the sand blow unit. Clay clasts of Unit
A were also observed in the near-horizontal dike. Overlying
Unit C is a plow zone of highly disturbed silty sand that
ranged in thickness from 7-18 cm.
Nancy 2, T-2
A 34-m-long trench oriented N32~ was excavated across a
second sand blow at the Nancy 2 site (Figure 7). This trench
averaged 1.9 m deep. The east wall was cleaned, gridded at a
50-cm spacing, logged in detail, and photographed. Portions
of the west wall were also cleaned and photographed to assist
in interpretation. Exposed in the trench wall were four distinct layers, all of which extend the complete length of the
excavation. Only the upper 20-50 cm of the lowermost layer,
denoted as Unit A, was exposed. Unit A is a light brown and
light bluish mottled clay. This unit is plastic and heavily iron
stained. Overlying this unit is Unit B. The contact between
Seismological Research Letters July/August2005
Volume 76, Number4
507
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,i Figure 6. Log and photographs of the trench at Nancy 2 T-1. The top panel represents the log of the entire length of the trench. The lower panel represents
a close-up view of the first 10 m of the trench. $1 indicates the location of the sample collected for dating. The two photographs are of the two largest dikes
observed in the trench.
Units A and B is gradational. Unit B is also a clay that ranged
in thickness from 30-75 cm. This unit is light bluish gray
with white zones in the upper 4 cm for most of its exposure.
In the southern 12 m of the trench, the white zone extends
throughout the thickness of Unit B. Cutting across both clay
units are three sand dikes feeding the overlying sand blow.
These disconcordant features include two thin vertical dikes
dike strikand one wide (20 cm) shallowly dipping (44~
ing N35~ The sand blow (Unit C) is from 65-130 cm
thick. The sand is pale yellow, loose fine to medium grained
with a minor amount of silt present. Radiocarbon dating of
charcoal (Beta 149267) collected 15 cm from the top of Unit
B (Figure 7) yielded a two-sigma calibrated date of
5590-5450 and 5410-5330 B.P. At the northern end of the
trench a tree-root cavity in Unit B is filled with sand. Above
the shallowly dipping dike numerous clay clasts of Unit A
occur in the sand blow. Overlying Unit C is a plow zone of
highly disturbed silty sand that ranged in thickness from
7-15 cm.
508
Seismological Research Letters Volume 76, Number4
Geophysical Investigation (GPR)
Geophysical techniques have already proven to be a powerful
tool to study earthquake-related features such as sand blows
(Wolf el:. al., 1998; Tuttle et al., 1999) and faults (Sexton et
al., 1992). Ground-penetrating radar (GPR) technology is
known to work well in dry, sandy conditions. It is widely utilized in environmental and engineering sciences.
We conducted GPR surveys at several sites near Marianna, including Nancy 1 and Nancy 2. Because sand thickness in most of the features is no more than 2 m, we used a
400-MHz antenna in all of the GPR surveys. This antenna is
designed to give the best resolution in the upper 5 m of soil,
similar to that in the study area. Data acquisition was along
parallel profiles 5 m apart. These profiles were run normal to
the long axes of the sand deposits in all three sites. The data
reduction procedure included removal of the direct and
ground-surface effects; band-pass, high-pass, and low-pass filtration to remove noise; gaining control to enhance the signal
of desired reflectors; profile migration to remove high
July/August2005
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20
10
30
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Plow Zone
Sand (C)
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,A, Figure 7. Trench log and GPR profile at Nancy 2 T-2. Top panel represents a ground-penetrating radar profile run parallel but at about 10 m west of the
trench. A 400-MHz antenna was used to collect the GPR data. Second panel represents the log of the east wall of the trench. $1 indicates the location of the
sample collected for dating. The photographs at the right and center show low-angle sand dikes. The photograph to the left shows clay clasts within the sand
blow that were ripped from the underlying clay layer during venting of sand-bearing water. The dashed line represents the contact between Unit A and Unit B.
parabolas and improve depth determination; and threedimensional visualization for signal enhancement and identification.
The upper panel of Figure 7 shows a GPR profile from
site Nancy 2. This profile is parallel and about 4 m to the west
of trench 2 of this site. In this profile, the contact between the
yellow and the purple represents the contact between the
sand and the clay. Note the similarities in the morphology of
this contact between the GPR profile and the trench log.
Note also the disruption to this contact in the area of the sand
dikes.
trench at Parkin 1 was excavated for the purpose of studying
a 1.5-km-long linear feature identified through interpretation
of aerial photography and land survey. The lineament trends
N56~ and the ground surface on the southeast side of the
lineament is 2.75 m higher in elevation. No fault was discovered in the trench, but sand and clay layers tilt to the northwest, suggesting possible subsurface faulting. Geophysical
work (GPR) was conducted at three sites near Marianna.
Results indicate that GPR is a very promising, cost-effective
tool for sand-blow studies.
SUMMARY AND CONCLUSIONS
Due to the paucity of radiocarbon dates of the liquefaction
features discovered in this study, it is possible to suggest three
viable earthquake sources: (1) The liquefaction features are
the result of an earthquake generated by a local source that
might not be directly relatedto NMSZ seismicity. (2) The
liquefaction features are due to major New Madrid events. (3)
These features formed as a result of aftershocks near the study
area possibly triggered by mainshocks within the NMSZ.
Due to the lack of a complete record of earthquake activity in
Source Region Scenarios
Four trenches at three sites were excavated during the summer and fall of 2000. Two of these sites were near Marianna,
Arkansas (Nancy 1 and Nancy 2), and one was near Parkin,
Arkansas (Parkin 1). Each of the trenches exposed a fine- to
medium-grained sand overlying a thick clay unit. The surficial sand deposit was found to be connected to numerous vertical- to shallow-dipping sand dikes up to 20 cm wide. The
Seismological Research Letters July/August2005
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509
the central United States, and the difficulty of precisely dating these features, it might be difficult to isolate a single scenario as the most likely one. The sizes of these features (some
more than 100 m in diameter) and the aerial extent of the
liquefaction field (which has a minimum radius of approximately 5 km) may indicate that no matter what the source
region or the epicentral distance is, severe ground shaking has
taken place in the area.
In reference to the first case scenario, Cox et al. (2004)
studied earthquake-related liquefaction features approximately 100 km to the south and southwest of our study area.
That study described multiple episodes of ground shaking in
the last 6,000 years and suggested that some episodes of
liquefaction correlate with the chronology of the NMSZ
events, while other episodes may not. Although it is focused
on an area at a considerable distance from Marianna, the
study raises the possibility that significant earthquake sources
may occur outside the NMSZ. Although inconclusive, the
dating of some of the sand blows discovered in this research
(-5500 B.P.) does not match with any event within the
NMSZ. This increases the possibility that the source region
might be closer to the study area.
There is strong evidence that major earthquakes may
cause liquefaction features at considerable distances from
their epicenters. The 1811-1812 New Madrid earthquakes
induced liquefaction about 240 krn from their inferred epicenters, including areas near the mouth of the Arkansas River
(Johnston and Schweig, 1996). More recently, the M w 7.6
2001 Bhuj, India earthquake induced liquefaction up to
250 km from its epicenter (Rajendran and Rajendran, 2002;
Tuttle et al., 2002). The features discovered near Marianna,
Arkansas are only about 100 km from the present-day seismicity of the NMSZ. This makes it impossible to rule out the
second scenario, that a major NMSZ earthquake may by the
energy source that caused these features. One observation
that is not in favor of this scenario is the lack of large liquefaction features between Marianna and the NMSZ. Aerial and
repeated ground-reconnaissance surveys failed to find sand
blows as large and as concentrated as the ones discovered in
the study area. This, however, might be attributed to burial
by repeated flooding or to soil conditions, site amplifications,
and other factors that can influence soil liquefaction.
There is increasing evidence that supports the hypothesis
that triggered earthquakes (afiershocks) may be generated at
considerable epicentral distances from a major earthquake.
Recent research in the central United States (Hough, 2001;
Hough and Martin, 2002; Hough et al., 2004) suggested or
concluded that damaging earthquakes occurred at considerable distances (200-500 km) from the NMSZ. Hough and
Martin (2002) stated that one aftershock (17 December
1811) of the 16 December 1811 earthquake had a M w of
approximately 6.1. They presented accounts suggesting that
the epicenter for this aftershock might be beyond the southern end of the NMSZ, possibly close to the modern city of
Memphis, Tennessee. Given the margin of error in constrain-
ing the epicentral region, especially to the south, this aftershock might have taken place much closer to the liquefaction
field near Marianna, Arkansas. An earthquake of this magnitude has, without a doubt, the energy to cause local liquefaction and sand blows. El
ACKNOWLEDGMENTS
We thank M. Egan, C. Clegg-Scala, and S. Eyuboglu for field
assistance; and Mrs. Nancy Apple and Mr. Mike Ragsdale for
providing their farmland for trenching. The research was partially supported by the U.S. Geological Survey /NEHRP
(Award # 00-HQ-GR-0066) and the Arkansas Science &
Technology Authority (Award # 00-B-38). Eugene Schweig
and Martitia P. Tuttle contributed greatly to improve this
article.
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Department of Applied Science
University of Arkansas
2801 S. University Avenue
Little Rock, AR 72204
[email protected]
(H.J.A.-S., H.H.M., J.B. C.)
Leighton andAssociates, Inc. Irvine, CA
(R.E.L.)
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