Distribution and Tsunamigenic Potential of
Submarine Landslides in the Gulf of Mexico
J.D. Chaytor, D.C. Twichell, P. Lynett, and E.L. Geist
Abstract The Gulf of Mexico (GOM) is a geologically diverse ocean basin that
includes three distinct geologic provinces: a carbonate province, a salt province,
and canyon to deep-sea fan province, all of which contain evidence of submarine
mass movements. The threat of submarine landslides in the GOM as a generator of
near-field damaging tsunamis has not been widely addressed. Submarine landslides
in the GOM are considered a potential tsunami hazard because: (1) some dated
landslides in the GOM have post-glacial ages and (2) recent seismicity recorded
within the GOM. We present a brief review of the distribution and style of submarine landslides that have occurred in the GOM during the Quaternary, followed by
preliminary hydrodynamic modeling results of tsunami generation from the East
Breaks landslide off Corpus Christie, TX.
Keywords Hydrodynamic modeling • bathymetry • canyons • fans • carbonate
• salt
1
Introduction
Submarine landslides have been studied in the Gulf of Mexico (GOM) for two reasons: first, as a hazard to offshore hydrocarbon extraction infrastructure and transportation. Second, when more deeply buried, they can serve either as hydrocarbon
J.D. Chaytor ()
Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA
e-mail:jchaytor@whoi.edu
D.C. Twichell
U.S. Geological Survey, Woods Hole, MA 02543, USA
P. Lynett
Texas A&M University, College Station, TX 77843, USA
E.L. Geist
U.S. Geological Survey, Menlo Park, CA 94025, USA
D.C. Mosher et al. (eds.), Submarine Mass Movements and Their Consequences,
Advances in Natural and Technological Hazards Research, Vol 28,
© Springer Science + Business Media B.V. 2010
745
746
J.D. Chaytor et al.
reservoirs or barriers in reservoirs. The threat of submarine landslides to generate
tsunamis has not been widely addressed for the GOM region. We present a brief
review of the literature on the distribution and style of submarine landslides that have
occurred in the GOM during the Quaternary and the results of some preliminary
landslide-generated tsunami modeling.
2
Setting
The GOM is dominated by three distinct geologic provinces: a carbonate province,
a salt province, and canyon to deep-sea fan province (Fig. 1). Three particular
aspects of the basin’s evolution that should be considered in an assessment of landslide activity include: (1) Jurassic-aged Louann salt that was deposited during the
early stages of basin formation (Salvador 1991), (2) the development and growth of
extensive carbonate reef tracts during the late Jurassic and Cretaceous (Bryant et al.
1991), and (3) the siliciclastic sediment input during the latest Mesozoic and
Cenozoic (Buffler 1991).
Fig. 1 Shaded bathymetry of the Gulf of Mexico. Landslide deposits are marked in red. The three
primary geologic provinces of the region are highlighted by the dashed lines. EB-East Breaks
Landslide, MC-Mississippi Canyon, BC-Bryant Canyon, EMF-East Mississippi Fan, BF-Bryant
Fan. Bathymetry derived from Armante and Eakins (2008)
Distribution and Tsunamigenic Potential of Submarine Landslides
747
The Louann salt underlies large parts of the northern GOM continental shelf and
slope. Under the upper and middle slope the salt is shaped into a network of ridges
and thin salt sheets that are interrupted by sub-circular basins (mini-basins) which
have thin salt or no salt underlying them. Campeche Knolls in the southwest corner
of the GOM has an irregular morphology that is similar to that of the northern
GOM slope, due to sediment loading of an underlying salt deposit (Fig. 1; Bryant
et al. 1991).
During the Mesozoic, an extensive reef system developed around much of the
margin of the GOM basin (Sohl et al. 1991). This reef system is exposed along the
Florida Escarpment (FE) and the Campeche Escarpment (Fig. 1). These escarpments stand as much as 1,500 m above the abyssal plain floor, and have average
gradients that commonly exceed 20° and locally are vertical. Reef growth ended
during the Middle Cretaceous (Freeman-Lynde 1983; Paull et al. 1990), and subsequently the platform edges have been sculpted and steepened by a variety of erosional processes (Freeman-Lynde 1983; Paull et al. 1990; Twichell et al. 1996).
Three fan systems formed during the Pliocene and Pleistocene, the Bryant Fan
(Lee et al. 1996), Mississippi Fan (Weimer 1989), and Eastern Mississippi Fan
(Weimer and Dixon 1994). The Mississippi Fan is the largest of these three fans,
and covers most of the eastern half of the deep GOM basin (Fig. 1). Mississippi
Canyon (MC) has retained its morphologic expression on the slope, but the canyons
that supplied sediment to Bryant and Eastern Mississippi Fans have been largely
erased by subsequent salt tectonism and depositional processes (Weimer and Dixon
1994; Lee et al. 1996).
3
Distribution of Submarine Landslides
Submarine landslides have occurred in each of the three provinces of the GOM
basin although they vary in style and size. This report will focus on landslides that
are thought to have occurred during the Quaternary Period as they have the greatest
implications for modern tsunami hazards.
3.1
Carbonate Province
Landslides on the West Florida Slope (WFS) above the FE are sourced in Tertiary
and Quaternary carbonate deposits (Fig. 2a). Mullins et al. (1986) mapped large
collapse scars along the WFS, one of which is 120 km long and 30 km wide, with
300–350 m relief. While the total volume of material removed from this feature is
around 1,000 km3, there were at least 3 generations of failures, with most of the
sediment removal occurring prior to the middle Miocene. Along the southern part
of the WFS, Doyle and Holmes (1985) and Twichell et al. (1993) mapped another
extensive area of the slope that has undergone collapse (Fig. 2a). Here the scarps
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J.D. Chaytor et al.
Fig. 2 (a) Landslide scar in carbonate platform above the central Florida Escarpment, (b) Upper
part of the East Breaks Landslide, and (c) Mississippi Canyon, the source area of the landslide
deposits comprising the Mississippi Fan. Red dashed polygons outline the landslide source zones
(excavations) used to calculate landslide volume and area given in Table 1. Bathymetry data available at the NOAA National Geophysical Data Center
are still exposed on the seafloor and have 50–150 m relief and are 10–70 km in
length. Some of the mass-movement deposits are on the slope above the FE while
the remainder were likely transported farther and deposited at the base of the FE.
The cross-cutting of the headwall scarps indicates that these landslides are composed of several smaller failure events (Twichell et al. 1993).
3.2
Salt Province
Landslide deposits, most of which are relatively small, have been mapped throughout the salt province using GLORIA imagery (Rothwell et al. 1991) as well as with
high-resolution sidescan sonar, bathymetry, seismic profiles, and cores (Lee and
Distribution and Tsunamigenic Potential of Submarine Landslides
749
George 2004; Orange et al. 2004; Sager et al. 2004; Silva et al. 2004; Tripsanas
et al. 2004). The largest of these failures, the East Breaks landslide (Fig. 2b),
occurs in the northwestern GOM, is 114 km long, 53 km wide, covers about
2,250 km2, and has been interpreted to consist of at least two debris flows
(Rothwell et al. 1991). This landslide, which incises a shelf-edge delta, lies offshore of the Colorado River system and formed during the last lowstand of sea
level (Piper and Behrens 2003).
The remaining landslides within the salt province are considerably smaller and
cover areas ranging from 4–273 km2 (Fig. 1) and are sourced from the walls of the
mini-basins or the Sigsbee Escarpment. While information is limited on the age of
landslides in the salt province and failure of mini-basin walls throughout the province may still be active, Tripsanas et al. (2004) indicates that most of the landslides
sampled in the salt province occurred during oxygen isotope stages 2 through 4
(18,170–71,000 yr BP) during the last lowstand of sea level, when salt movement
due to sediment loading may have been more active in certain locations.
3.3
Canyon/Fan Province
Recent studies of the MC and Mississippi Fan (Twichell et al. 1996;
Twichell et al. in press) reveal evidence of landslides at several scales.
Turbidity current deposits and thin debris flow deposits associated with
channel-levee development have been mapped and sampled on the distal fan
(Twichell et al. 1992; Schwab et al. 1996). Some of these deposits are relatively small: covering areas less than 331 km 2, and have volumes less than
1 km 3 (Twichell et al. in press). At the other extreme is a large landslide
complex that covers approximately 23,000 km 2 of the middle and upper fan
(Fig. 1) and reaches 100 m in thickness (Walker and Massingill 1970;
Twichell et al. 1992). The total volume of this deposit cannot be accurately
estimated because of inadequate seismic coverage, but, assuming an average
thickness of 75 m, the volume would be 1,725 km 3. Seismic profiles and
GLORIA imagery suggest that this feature consists of at least two separate
events (Twichell et al. in press). MC (Fig. 2c) appears to have been the
source area for these landslide deposits (Walker and Massingill 1970;
Coleman et al. 1983; Goodwin and Prior 1989). Borings and seismic data
from the head of MC (Goodwin and Prior 1989) indicate that there were
alternating episodes of canyon filling and excavation between 19,000 and
7,500 ybp, and Coleman et al. (1983) estimate total volume of sediment
removed was approximately 8,600 km 3.
The Eastern Mississippi Fan system also has a relatively large landslide that
partially buries the channel that supplied this fan. This landslide deposit is approximately 154 km long, as much as 22 km wide, and covers an area of 2,410 km2. The
volume of the deposit and its age are unknown.
750
4
J.D. Chaytor et al.
Preliminary Analysis of Potential Landslide-Generated
Tsunamigenic Sources
In the GOM, potential tsunami sources are primarily earthquakes along the
Caribbean plate boundary and submarine landslides, although the importance of
earthquakes within the region is being evaluated (e.g., Knight 2006). Submarine
landslides in the GOM are considered a potential tsunami hazard for two reasons:
(1) some dated landslides in the GOM have post-glacial ages (Coleman et al. 1983)
and (2) recent seismicity recorded within the GOM (Dewey and Dellinger 2008).
The MC and East Breaks landslides, among the largest landslides in the GOM,
occurred after the end of the last glacial maximum during post-glacial transgression
(Piper and Behrens 2003). In addition, the Mississippi River still deposits large
quantities of water-saturated sediments on the shelf and slope, making them vulnerable to over-pressurization and slope failure (Dugan and Flemings 2000).
On February 10, 2006, seismograms recorded an earthquake offshore southern
Louisiana (Dewey and Dellinger 2008). While source analyses of this event cannot
distinguish between a fault source and landslide, the occurrence of a magnitude 4.2
earthquake reveals the potential for ground shaking and slope failure.
4.1
Submarine Landslide Characteristics
We define three submarine landslides within the geologic provinces of the northern
GOM that may have had tsunamigenic potential: (1) East Breaks; (2) Mississippi
Canyon; and (3) Florida Escarpment. The U.S. gulf coast would be affected primarily by the shoreward-directed phase of the tsunami emanating from the East
Breaks and MC landslides, and would be affected primarily by the outgoing tsunami from a landslide sourced from above the FE. A tsunami from a landslide on
the FE would have a significant directivity effect that scales with the speed of
downslope motion of the landslide (up to the phase speed of the tsunami). The
characteristics and the parameters of landslides considered in preliminary tsunami
source analysis are given in Table 1.
Landslide volume calculations were based on measuring the volume of material
removed from the landslide source area using a technique similar to that applied by
Chaytor et al. (2009). In the case of the East Breaks landslide, the source area may
be somewhat larger, but bathymetry is not available for the entire landslide.
4.2
East Breaks Landslide Tsunami Modeling
Preliminary simplified hydrodynamic modeling of the East Breaks landslide has
been performed to investigate the upper limits of tsunami wave height generated by
this source. For modeling purposes, a 200 m resolution elevation grid was used,
2
Piper and Behrens (2003)
Trabant et al. (2001)
3
Rothwell et al. (1991)
4
Coleman et al. (1983)
5
Doyle and Holmes (1985)
1
River delta/salt
province
Mississippi canyon River delta/fan
system
Florida
Carbonate platform
Escarpment
East breaks
Landslide
Age (years)
1
Max
2
Partial can- 10,000–25,000 22 (60)
yon fill
Partial can- 7,500–11,0004 426
yon fill
None visible Early Holocene 16
or older5
Post failure
Geologic setting sediment
520
3687
648
–
–
Max
21
Min
Single event volume
(km3)
–
–
421
Min
Single event area
(km2)
91 (130)3
∼160
∼300 (upper 297 (442)
canyon)
∼150
?
Run-Out
Distance
(km)
Excavation
Depth (m)
Table 1 Characteristics of the landslides source regions considered in this study. Numbers in parentheses are values from other investigators
Distribution and Tsunamigenic Potential of Submarine Landslides
751
752
a
J.D. Chaytor et al.
b
(m)
x 105
6
Time after landslide (min) = 0
25
(m)
x 105
6
Time after landslide (min) = 13
20
5
20
5
15
10
4
15
10
4
0
5
y (m)
y (m)
5
3
0
3
–5
2
–10
–15
1
–5
2
–10
–15
1
–20
0.5
1 1.5 2 2.5 3 3.5 4 4.5 5 5.5
x (m)
x 105
–25
25
–20
0.5
1 1.5 2 2.5 3 3.5 4 4.5 5 5.5
x (m)
x 105
–25
Fig. 3 Results of two-dimensional simulation of the East Break landslide (headwall depth is
160 m) at (a) 0 min (initiation) and (b) 13 min. Note the radial spreading pattern
made up of a combination of data from the U.S. Army Corp of Engineers ADCIRC
(shallow water) and the NOAA 2-min ETOPO2 (deep water) databases. Two conservative assumptions were made: (1) the time scale of seafloor motion is very
small when compared to that of the generated tsunami, and (2) bottom roughness
and associated energy dissipation in deep water are negligible.
In initial runs, model conditions of a landslide source width of ∼12 km and length
of 50 km are used. As the excavation depth of the landslide is ∼160 m, a trough elevation of 160 m is used as the hot-start initial free water surface condition (i.e., the free
water surface response matches the change in the seafloor profile exactly), based on
the knowledge of the characteristics of landslide generated waves (e.g., Lynett and
Liu 2002). From this, model results (Fig. 3) indicate that wave components (backgoing and outgoing) with amplitudes greater than 20 m is initially generated.
Attenuation from radial spreading is important, with wave energy predicted to propagate in all directions away from the source. Further modeling and analysis is required
to determine if the entire coastline of the GOM would be impacted by a tsunami
generated by this source. The magnitude of the tsunami at the coast would be determined by shallow water amplification and energy dissipation of the waves.
Furthermore, using landslide durations predicted from the mobility analysis rather
than hot-start conditions we expect to see a change in tsunami generation efficiency.
5
Concluding Statements and Future Work
Timing of landslides in the GOM needs to be refined to determine the likelihood of
modern landslides and potential for future landslide-generated tsunamis. For example, it is not known if the Mississippi Fan landslides are associated with glacial
Distribution and Tsunamigenic Potential of Submarine Landslides
753
meltwater floods that discharged into the GOM (e.g., Aharon 2003), or whether
they occurred more recently. Available age dates indicate that this large landslide
complex is younger than 11,100 ybp, but the minimum age is still unknown
(Twichell et al. in press). Additional modeling of potential sources is required to
determine the magnitude of the initial waves, the amount of shallow water amplification, and the level of run-up expected at points around the GOM.
Acknowledgments This work was funded by U.S. Nuclear Regulatory Commission grant
N6480, Physical study of tsunami sources. These findings express the views of the authors and are
not necessarily those of the U.S. NRC. This manuscript benefited from reviews by Debbie
Hutchinson, Kathy Scanlon, and Matt Hornbach.
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