Why Is New Orleans Sinking

Why is New Orleans Sinking? Prof. Timothy M. Kusky Paul C. Reinert Endowed Chair of Natural Sciences Department of Earth and Atmospheric Sciences Saint Louis University St. Louis, MO 63103 Email: [email protected], web: http://www.eas.slu.edu/People/TMKusky/index.html © December 29, 2005 The tragic losses from Hurricane Katrina in New Orleans and the Gulf Coast have led many to inquire about why the Gulf coast is sinking, or subsiding, and what can be done to protect coastal residents. This note is intended to provide some background about the complexities involved in knowing what causes subsidence along some coasts, and many links and references are provided to guide readers who want to obtain further information. The first part of the text below is summarized from my 2003© book "Geological Hazards" A Sourcebook. 300 pp. An Oryx Book. Greenwood Press, Westport Conn., ISBN 1-57356-469-9. http://www.greenwood.com/books/BookDetail.asp?dept_id=1&sku=OXHAZGEO. The text provides some general background information on geologic and tectonic subsidence on deltas. The book includes many figures that can not be reproduced here for copyright reasons. The general background reading is followed by a new section specifically about the situation in New Orleans.

GEOLOGICAL HAZARDS: AN ORYX SOURCEBOOK Timothy M. Kusky, PhD. Chapter 11: Natural Geologic Subsidence Hazards "condensed extracts from original text"

Introduction Natural geologic subsidence is the sinking of land relative to sea level or some other uniform surface. Subsidence may be a gradual barely perceptible process, or it may occur as a catastrophic collapse of the surface. Subsidence occurs naturally along some coastlines, and in areas where groundwater has dissolved cave systems in rocks such as limestone. It may occur on a regional scale, affecting an entire coastline, or it may be local in scale, such as when a sinkhole suddenly opens and collapses in the middle of a neighborhood. Other subsidence events reflect the interaction of humans with the environment, and include ground surface subsidence as a result of mining excavations, groundwater and petroleum extraction, and several other processes. Compaction is a related phenomenon, where the pore spaces of a material are gradually reduced, condensing the material and causing the surface to subside. Subsidence and compaction do not typically result in death or even injury, but they do cost

Americans alone tens of millions of dollars per year. The main hazard of subsidence and compaction is damage to property. Subsidence and compaction directly affect millions of people. Residents of New Orleans live below sea level and are constantly struggling with the consequences of living on a slowly subsiding delta. Coastal residents in the Netherlands have constructed massive dike systems to try to keep the North Sea out of their slowly subsiding land. The city of Venice, Italy has dealt with subsidence in a uniquely charming way, drawing tourists from around the world. Millions of people live below the high-tide level in Tokyo. The coastline of Texas along the Gulf of Mexico is slowly subsiding, placing residents of Baytown and other Houston suburbs close to sea level and in danger of hurricane induced storm surges and other more frequent flooding events. In Florida, sinkholes have episodically opened up swallowing homes and businesses, particularly during times of drought. What are the causes of this sinking of the land, and where is most and least likely to occur? The driving force of subsidence is gravity, with the style and amount of subsidence controlled by the physical properties of the soil, regolith and bedrock underlying the area that is subsiding. Subsidence does not require a transporting medium, but it is aided by other processes such as ground water dissolution that can remove mineral material and carry it away in solution, creating underground caverns that are prone to collapse. Natural subsidence has many causes. Dissolution of limestone by underground streams and water systems is one of the most common, creating large open spaces that collapse under the influence of gravity. Groundwater dissolution results in the formation of sinkholes, large, generally circular depressions caused by collapse of the surface into underground open spaces. Earthquakes may raise or lower the land suddenly, as in the case of the 1964 Alaskan earthquake where tens of thousands of square miles suddenly sank or rose three to five feet, causing massive disruption to coastal communities and ecosystems. Earthquake – induced ground shaking can also cause liquefaction and compaction of unconsolidated surface sediments, also leading to subsidence. Regional lowering of the land surface by liquefaction and compaction is known from the massive 1811 and 1812 earthquakes in New Madrid, Missouri, and from many other examples. Volcanic activity can cause subsidence, as when underground magma chambers empty out during an eruption. In this case, subsidence is often the lesser of many hazards that local residents need to fear. Subsidence may also occur on lava flows, when lava empties out of tubes or underground chambers. Some natural subsidence on the regional scale is associated with continental scale tectonic processes. The weight of sediments deposited along continental shelves can cause the entire continental margin to sink causing coastal subsidence and a landward migration of the shoreline. Tectonic processes associated with extension, continental rifting, strike slip faulting, and even collision can cause local or regional subsidence, sometimes at rates of several inches per year.

Types of Surface Subsidence and Collapse Some subsidence occurs because of processes that happen at depths of thousands of feet beneath the surface, and is referred to as deep subsidence. Other subsidence is caused by shallow near-surface processes and is known as shallow subsidence. Tectonic subsidence is a result of the movement of the plates on a lithospheric scale, whereas human-induced subsidence refers to cases where the activities of people, such as extraction of fluids from depth, have resulted in lowering of the land surface. Compaction-related subsidence may be defined as the slow sinking of the ground surface because of reduced pore space, lowered pore pressure, and other processes that cause the regolith to

become more condensed and occupy a smaller volume. Most subsidence and compaction mechanisms are slow and result in gradual sinking of the lands surface, whereas sometimes the process may occur catastrophically, and is known as collapse. >>>snip

Human-Induced Subsidence Several types of human activity can result in the formation of sinkholes or cause other surface subsidence phenomena. Withdrawals of fluids from underground aquifers, depletion of the source of replenishment to these aquifers, and collapse of underground mines can all cause surface subsidence. In addition, vibrations from drilling, construction or blasting can trigger collapse events, and the extra load of buildings over unknown deep collapse structures can cause them to propagate to the surface, forming a sinkhole. These processes reflect geologic hazards caused by human’s interaction with the natural geologic environment. Mine Collapse Mining activities may mimic the formation of natural caves, since mining operations remove material from depth and leave roof materials partly unsupported. There are many examples of mines of different types that have collapsed, resulting in surface subsidence, sinkhole formation, and even the catastrophic draining of large lakes. The mining of salt has created sinkholes and surface subsidence problems in a number of cases. Salt is mined in several different ways, including the digging of tunnels from which the salt is excavated, and by injecting water into a salt deposit, removing the salt-saturated water, and drying it to remove the salt for use. This second technique is called solution mining, and is has less control over where the salt is removed from than the classical excavation style mining. Shallow sinkholes and surface subsidence are typical and expected around solution mining operations, for instance the saltmining operations near Hutchinson, Kansas, resulted in the formation of dozens of sinkholes, including one nearly 1,000 feet across that partly swallowed the salt processing plant. One of the most spectacular of all salt-mining subsidence incidents occurred on November 21, 1980, on Lake Peigneur Louisiana. The center of Lake Peigneur is occupied by a salt dome, forming Jefferson Island. This salt dome was mined extensively, with many underground tunnels excavated to remove the salt. Southern Louisiana is also an oil-rich region, and on the ill-fated day in 1980, an oildrilling rig accidentally drilled a hole into one of the mine shafts. Water began swirling into the hole, and the roof of the mine collapsed, setting the entire lake into a giant spinning whirlpool that quickly drained into the deep mine, as if it were bath water escaping down the drain. The oil drilling rig, ten barges, and a tugboat were sucked in to the mine shafts, and much of the surrounding land collapsed into the collapse structure, destroying a home and other properties. After the mine was filled with water, the lake gradually filled again, but the damage was done. Other types of mining have resulted in surface subsidence and collapse, including coal mining in the Appalachians and the Rocky Mountains. Most coal-mine related subsidence occurs where relatively shallow flat-lying coal seams have been mined, and the mine roofs collapse. The fractures and collapse structures eventually migrate to the surface leading to elongate trains of sinkholes and other collapse structures.

Ground Water Extraction The extraction of groundwater, oil, gas, or other fluids from underground reservoirs can cause significant subsidence of the land’s surface. In some cases the removal of underground water is natural. During times of severe drought, soil moisture may decrease dramatically and droughtresistant plants with deep root systems can draw water from great depths, reaching many tens of meters in some cases. In most cases, however, subsidence caused by deep fluid extraction is caused by human activity. This deep subsidence mechanism operates because the fluids that are extracted served to help support the weight of the overlying regolith. The weight of the overlying material places the fluids under significant pressure, known as hydrostatic pressure, that keeps the pressure between individual grains in the regolith at a minimum. This in turns helps prevent the grains from becoming closely packed or compacted. If the fluids are removed, the pressure between individual grains increases and the grains become more closely packed and compacted, occupying less space than before the fluid was extracted. This can cause the surface to subside. A small amount of this subsidence may be temporary, or recoverable, but generally once surface subsidence related to fluid extraction occurs, it is nonrecoverable. When this process occurs on the scale of a reservoir or entire basin, the effect can be subsidence of a relatively large area. Subsidence associated with underground fluid extraction is usually gradual, but still costs millions of dollars in damage every year in the United States. The amount of surface subsidence is related to the amount of fluid withdrawn from the ground, and also to the compressibility of the layer that the fluid has been removed from. If water is removed from cracks in a solid igneous, metamorphic or sedimentary rock, then the strength of the rock around the cracks will be great enough to support the overlying material and no surface subsidence is likely to occur. In contrast, if fluids are removed from a compressible layer such as sand, shale or clay, then significant surface subsidence may result from fluid extraction. Clay and shale have a greater porosity and compressibility than sand, so extraction of water from clay rich sediments results in greater subsidence than the same amount of fluid withdrawn from a sandy layer. One of the most common causes of fluid extraction related subsidence is the overpumping of groundwater from aquifers. If many wells are pumping water from the same aquifer the cones of depression (see Chapter 10) surrounding each well begin to merge, lowering the regional groundwater level. Lowering of the groundwater table can lead to gradual, irreversible subsidence. Surface subsidence associated with groundwater extraction is a serious problem in many parts of the southwestern United States. Many cities such as Tucson, Phoenix, Los Angeles, Salt Lake City, Las Vegas, and San Diego rely heavily on groundwater pumped from compressible layers in underground aquifers. The San Joaquin Valley of California offers a dramatic example of the effects of groundwater extraction. Extraction of ground water for irrigation over a period of fifty years has resulted in nearly thirty feet of surface subsidence. Parts of the Tucson Basin in Arizona are presently subsiding at an accelerating rate, and many investigators fear that the increasing rate of subsidence reflects a transition from temporary recoverable subsidence, to a permanent compaction of the water-bearing layers at depth. The world’s most-famous subsiding city is Venice, Italy. Venice is sinking at a rate of about one foot per century, and much of the city is below sea level or just above sea level, and prone to floods from storm surges and astronomical high tides in the Adriatic Sea. The city has subsided more than ten feet since it was founded near sea level. These aqua altas (meaning high-water in Italian) flood streets as far as the famous Piazza San Marco. Venice has been subsiding for a combination of reasons, including compaction of the coastal muds that the city was built on. One of the main causes of

the sinking of Venice has been groundwater extraction. Nearly 20,000 groundwater wells pumped water from compressible sediment beneath the city, with the result being the city sunk into the empty space created by the withdrawal of water. The Italian government has now built an aqueduct system to bring drinking water to residents, and has closed most of the 20,000 wells. This action has slowed the subsidence of the city, but it is still sinking, and this action may be too little too late to spare Venice from the future effects of storm surges and astronomical high tides. Mexico City is also plagued with subsidence problems caused by groundwater extraction. Mexico City is built on a several thousand foot thick sequence of sedimentary and volcanic rocks, including a large dried lake bed on the surface. Most of the groundwater is extracted from the upper 200 feet of these sediments. Parts of Mexico City have subsided dramatically, whereas others have not. The northeast part of the city has subsided about 20 feet. Many of the subsidence patterns in Mexico City can be related to the underlying geology. In places like the northeast part of the city that are underlain by loose compressible sediments, the subsidence has been large. In other places underlain by volcanic rocks, the subsidence has been minor. The extraction of oil, natural gas, and other fluids from the Earth also may result in surface subsidence. In the United States, subsidence related to petroleum extraction is a large problem in Texas, Louisiana, and parts of California. One of the worst-cases of oil field subsidence is that of Long Beach, California, where the ground surface has subsided 30 feet in response to extraction of underground oil. There are approximately 2,000 oil wells in Long Beach, pumping oil from beneath the city. Much of Long Beach’s coastal area subsided below sea level, forcing the City to construct a series of dikes to keep the water out. When the subsidence problem was recognized and understood, the city began a program of re-injecting water into the oil field to replace the extracted fluids and to prevent further subsidence. This re-injection program was initiated in 1958, and since then the subsidence has stopped, but the land surface can not be pumped up again to its former levels. Pumping of oil from an oil field west of Marina del Ray along the Newport-Ingelwood fault resulted in subsidence beneath the Baldwin Hills Dam and Reservoir, leading to the dam’s catastrophic failure on December 14, 1963. Oil extraction from the Inglewood Oil field resulted in subsidence related slip on a fault beneath the dam and reservoir, which was enough to initiate a crack in the dam foundation. The crack was quickly expanded by pressure from the water in the reservoir, which led to the dam’s catastrophic failure at 3:38 P.M. on December 14th, 1963. Sixty five million gallons of water were suddenly released, destroying dozens of homes, killing five people, and causing 12 million dollars in damage. >>>snip

Tectonic Subsidence Plate tectonics is associated with subsidence of many types and scales, particularly on or near plate boundaries. Plate tectonics is associated with the large-scale vertical motions that uplift entire mountain ranges, drop basins to lower elevations, and form elongate kilometer deep depressions in the Earth’s surface known as rifts. Plate tectonics also causes the broad flat coastal plains and passive margins (see Chapter 1) to slowly subside relative to sea level, causing the sea to move slowly into the continents. More local scale folding and faulting can cause areas of the land surface to rise or sink, although at rates that rarely exceed 1 centimeter per year. Extensional plate boundaries are naturally associated with subsidence, since these boundaries are places where the crust is being pulled apart, thinning, and sinking relative to sea level. Places where the continental crust has ruptured and is extending are known as continental rifts. In the United

States, the Rio Grande rift in New Mexico represents a place where the crust has begun to rupture, and it is subsiding relative to surrounding mountain ranges. In this area, the actual subsidence does not present much of a hazard, since the land is not near the sea, and a large region is subsiding. The net effect is that the valley floor is slightly lower in elevation every year than it was the year before. The rifting and subsidence is sometimes associated with faulting when the basin floor suddenly drops, and the earthquakes are associated with their own sets of hazards. Rifting in the Rio Grande is also associated with the rise of a large body of magma beneath Soccoro, and if this magma body has an eruption it is likely to be catastrophic. Large areas of the basin and range province of the southwestern United States are also subsiding. The region was topographically uplifted millions of years ago, and tectonic stresses are now pulling the entire region apart, causing locally rapid subsidence in the basins between ranges. Again, the main hazards from this type of subsidence are mainly associated with the earthquakes that sometimes accommodate the extension and subsidence. The world’s most extensive continental rift province is found in East Africa. An elongate subsiding rift depression extends from Ethiopia and Somalia in the north, south through Kenya, Uganda, Rwanda, Burundi, and Tanzania, then swings back toward the coast through Malawi and Mozambique (Figure 11.7). The East African rift system contains the oldest hominid fossils, and is also host to areas of rapid land surface subsidence. Earthquakes are common, as are volcanic eruptions such as the catastrophic eruption of Nyiragongo in Congo in January of 2002.Lava flows from Nyiragongo covered large parts of the town of Goma, forcing residents to flee to neighboring Rwanda. Subsidence in the East African rift system has formed a series of very deep elongate lakes, including Lakes Edward, Albert, Kivu, Malawi, and Tanganyika (Figure 11.7). These lakes sit on narrow basin floors, bounded on their east and west sides by steep rift escarpments. The shoulders of the rifts slope away from the center of the rift, so sediments carried by streams do not enter the rift, but are carried away from it. This allows the rift lakes to become very deep without being filled by sediments. It also means that additional subsidence can cause parts of the rift floor to subside well below sea level, such as Lake Abe in the Awash depression in the Afar rift. This lake and several other areas near Djibouti rest hundreds of feet below sea level. These lakes, by virtue of being so deep, become stratified with respect to dissolved oxygen, methane, and other gases. Methane is locally extracted from these lakes for fuel, although periodic overturning of the lakes water’s can lead to hazardous release of gases. Transform plate boundaries, where one plate slides past another, can also be sites of hazardous subsidence. The strike slip faults that comprise transform plate boundaries are rarely perfectly straight. Places where the faults bend may be sites of uplift of mountains, or rapid subsidence of narrow elongate basins. The orientation of the bend in the fault system determines whether the bend is associated with contraction and the formation of mountains, or extension, subsidence, and the formation of the elongate basins known as pull-apart basins. Pull-apart basins typically subside quickly, have steep escarpments marked by active faults on at least two sides, and may have volcanic activity. Some of the topographically lowest places on Earth are in pull-apart basins, including the Salton Sea in California, and the Dead Sea along the border between Israel and Jordan (Figure 11.8). The hazards in pull-apart basins are very much like those in continental rifts. Convergent plate boundaries are known for tectonic uplift, although they may also be associated with regional subsidence. When a mountain range is pushed along a fault on top of a plate boundary, the underlying plate may subside rapidly. In most situations erosion of the overriding mountain range sheds enormous amounts of loose sediment onto the underriding plate, so the land

surface does not actually subside, although any particular marker surface will be buried and subside rapidly.

Subsidence from Earthquake Ground Displacements Sometimes individual large earthquakes may displace the land surface vertically, resulting in subsidence or uplift. One of the largest and best-documented cases of earthquake induced subsidence resulted from the March 27, 1964 magnitude 9.2 earthquake in southern Alaska. This earthquake tilted a huge approximately 200,000 square kilometer area of the Earth’s crust. Significant changes in ground level were recorded along the coast for more than 1,000 kilometers, including uplifts of up to 11 meters, subsidence of up to 2 meters, and lateral shifts of several to tens of meters. Much of the area that subsided was along Cook Inlet, north to Anchorage, Valdez, and south to Kodiak Island (Figure 11.9). Towns that were built around docks prior to the earthquake were suddenly located below the high tide mark, and entire towns had to move to higher ground. Forests that subsided found their root systems suddenly inundated by salt water, leading to the death of the forests. Populated areas located at previously safe distances from the high-tide (and storm) line became prone to flooding and storm surges, and had to be relocated.

Compaction-Related Subsidence on Deltas and Passive Margin Coastal Areas Subsidence related to compaction and removal of water from sediments deposited on continental margin deltas, in lake beds, and in other wetlands poses a serious problem to residents trying to cope with the hazards of life at sea level. Deltas are especially prone to subsidence because the sediments that are deposited on deltas are very water-rich, and the weight of overlying new sediments compacts existing material, forcing the water out of pore spaces. Deltas are also constructed along continental shelves that are prone to regional-scale tectonic subsidence, and are subject to additional subsidence forced by the weight of the sedimentary burden deposited on the entire margin. Continental margin deltas are rarely more than a few feet above sea level, so are prone to the effects of tides, storm surges, river floods, and other coastal disasters. Any decrease in the sediment supply to keep the land at sea level has serious ramifications, subjecting the area to subsidence below sea level. Some of the world’s thickest sedimentary deposits are formed in deltas on the continental shelves, and these are of considerable economic importance because they also host the world’s largest petroleum reserves. The continental shelves are divided into many different sedimentary environments. Beaches contain the coarse fraction of material deposited at the ocean front by rivers and sea cliff erosion. Quartz is typically very abundant, because of its resistance to weathering and its abundance in the crust. Beach sands tend to be well-rounded, as does anything else such as beach glass, because of the continuous abrasion caused by the waves dragging the particles back and forth. Many of the sediments transported by rivers are deposited in estuaries, which are semi-enclosed bodies of water near the coast in which fresh water and seawater mix. Near shore sediments deposited in estuaries include thick layers of mud, sand, and silt. Many estuaries are slowly subsiding, and they get filled with thick sedimentary deposits. Deltas are formed where streams and rivers meet the ocean, and drop their loads because of the reduced flow velocity. Deltas are complex sedimentary systems, with coarse stream channels, fine-grained inter-channel sediments, and a gradation seaward to deep water deposits of silt and mud.

All of the sediments deposited in the coastal environments tend to be water rich when deposited, and thus subject to water loss and compaction. Subsidence poses the greatest hazard on deltas, since these sediments tend to be thickest of all deposited on continental shelves. They are typically finegrained muds and shales that suffer the greatest water loss and compaction. Unfortunately, deltas are also the sites of some of the world’s largest cities, since they offer great river ports. New Orleans, Shanghai, and many other major cities have been built on delta deposits, and have subsided several meters since they were first built (Table 11.1). Many other cities built on these very compactible shelf sediments are also experiencing dangerous amounts of subsidence. What are the consequences of this subsidence for people who live in these cities, and how will they be affected by increased rates of subsidence caused by damming of rivers that trap replenishing sediments upstream? How will these cities fare with current sea level rise, estimated to be occurring currently at a rate of an inch every ten years, with more than 6 inches of rise in the past century? Whatever the response, it will be costly. Some urban and government planners estimate that protecting the populace from sea level rise on subsiding coasts will be the costliest endeavor ever undertaken by humans. Table 11.1 Subsidence Statistics for the 10 Worst-Case Coastal Cities. City Los Angeles (Long Beach) Tokyo San Jose Osaka Houston Shanghai Niigata Nagoya New Orleans Taipei

Maximum Subsidence (m) 9.0 4.5 3.9 3.0 2.7 2.63 2.5 2.37 2.0 1.9

Area Affected (km2)

Tectonic Environment

50 3,000 800 500 12,100 121 8,300 1,300 175 130

Oilfield subsidence Delta Delta Delta Oilfield and coastal marsh Delta Delta Delta Delta

What is the fate of these and other coastal cities that are plagued with natural and human-induced subsidence in a time of global sea level rise? The natural subsidence in these cities is accelerated by human activities. First of all, construction of tall heavy buildings on loose, compactible water-rich sediments forces water out of the pore spaces of the sediment underlying each building, causing that building to subside. The weight of cities has a cumulative effect, and big cities built on deltas and other compactible sediment cause a regional flow of water out of underlying sediments, leading to subsidence of the city as a whole. New Orleans has one of the worst subsidence problems of coastal cities in the United States. Its rate and total amount of subsidence are not the highest (see table 11.1), but since nearly half of the city is at or below sea level, any additional subsidence will put the city dangerously far below sea level. Already, the Mississippi River is higher than downtown streets and ships float by at the second story level of buildings. Dikes keep the river at bay, and keep storm surges from inundating the city. Additional subsidence will make these measures unpractical. New Orleans, Houston, and other coastal cities have been accelerating their own sinking by withdrawing groundwater and oil from compactible sediments beneath the cities. They are literally pulling the ground out from under their own feet.

The combined effects of natural and human-induced subsidence, plus global sea level rise, has resulted in increased urban flooding of many cities, and greater destruction during storms. Storm barriers have been built in some cases, but this is only the beginning. Thousands of kilometers of barriers will need to be built to protect these cities unless billions of people are willing to relocate to inland areas, an unlikely prospect. What can be done to reduce the risks from coastal subsidence? First, a more intelligent regulation of groundwater extraction from coastal aquifers, and oil from coastal regions, must be enforced. If oil is pumped out of an oil reservoir then water should be pumped back in to prevent subsidence. Sea level is rising, partly from natural astronomical effects, and partly from humaninduced changes to the atmosphere. It is not too early to start planning for sea level rises of a few feet. Sea walls should be designed and tested before construction on massive scales. Consideration to moving many operations inland to higher ground should be considered. >>>snip

The New Orleans Situation Some people have wondered why structural and tectonic oriented geologists may be working on subsidence problems, when they have been told previously [by a marine biologist] that subsidence is caused [only] by near surface compaction of soils. The problem is more complex, as alluded to in T. Kusky’s op-ed piece in the Boston Globe (Time to Move to Higher Ground, Sept. 25, 2005), and follow-up segment on a 60-Minutes story on CBS (New Orleans is Sinking, Nov. 20, 2005), and in many scientific publications from scientists on all sides of the issues. The discussion above should make it clear that subsidence is a major, regional tectonic issue. Kusky’s main points were that 1) much of New Orleans is below sea level and subsiding rapidly; 2) the coast line is retreating toward the mainland as wetlands sink below sea level making urban New Orleans more vulnerable to storm surges; 3) sea level is rising and hurricanes are entering a more active and intense cycle; 4) the current system of coastal defenses and levees are inadequate to protect lives and property in New Orleans, and 5) if major and costly coastal defenses are not constructed we should begin a phased withdrawal or down-sizing of New Orleans, by not re-inhabiting the most devastated and deepest below sea level parts of the city. The cause of the subsidence can be best understood as follows. As the weight of deltaic and other sediments is added to the shelf/passive margin on the Gulf coast, it presses down and compacts underlying sediments. The weight also causes the entire thickness of the crust and lithosphere to sink (tectonic subsidence), much like dirt added to an iceberg would cause it to float lower in the water. As some sediments at great depth, particularly salts and muds, may be able to flow, they tend to move out toward the open Gulf at deep levels, where they can ooze out on the continental slope and form pressure ridges and fold belts. This process is much like sitting on a peanut butter and jelly sandwich, where the bread compacts and the jelly oozes out the sides. Another mechanism by which the weight of the sediments causes subsidence is that gravity and the weight triggers motion on long-lived curved or listric growth faults. These faults curve toward the open gulf, and sink slowly downward and outward as material is deposited on them, causing the surface above the faults to sink. Together, these processes all allow the delta and shelf to gradually grow outward and the surface to slowly sink. If sediments are not regularly added to the surface layer, then the surface will subside below sea level.

This sediment deprivation can be natural (as when a river switches course) or human-induced (as when a river has levees built on its bank not allowing sediment to overflow its banks).

Generalized cross-section across the Gulf of Mexico from north (USA) to south (Yucatan). Rock layers at depth were originally deposited near the surface and have subsided many kilometers below the surface over the past 200 million years of geological time. The weight of overlying rocks and sediments has pushed much of the salt out toward the gulf, and helped form salt diapirs and domes that puncture upward toward the surface (from Salvador, 1991). Much of the huge volume of salt rising beneath the Sigsbee Escarpment originated further north beneath the continental shelf. As the weight of overlying rocks pushed it out seaward, the entire shelf subsided to compensate for the loss of volume at depth. Subsidence of the surface is also accommodated by listric normal faults (not shown) and a general down-sagging of the entire thinned continental lithosphere (crust and upper mantle) in the region. In addition to these mechanisms of tectonic subsidence, near surface layers may lose water and become more compact as they get buried, as discussed below.

It is estimated that one-quarter to one-half of the subsidence of the Gulf coast is tectonic, related to the continuous deep compaction and seaward motion of salts and muds, and motion on deep listric normal faults. For details, see: http://www.coastalenv.com/Publications/ActiveGeologicalFaultsandLandChangeinSELA.pdf.

(http://qfaults.cr.usgs.gov/faults/FMPro?-db=us%20web%20fault%20database&-format=record2_detail.htm&lay=scientist%20input&-sortfield=Name&-op=eq&Number=1022&-max=2147483647&-recid=33845&-find) http://neic.usgs.gov/neis/bulletin/neic_gvbk_l.html)

Cross section from NW to SE across Louisiana, showing how sediments deposited 200 million years (Paleozoic) ago are now ~15 km below the surface of the shelf, sediments deposited 50 million years ago (Paleocene) are now 8-15 km below the surface, and sediments less than 5 million years old (Pliocene & Quaternary) are 0-5 km below the surface. This shows that subsidence is a long lived phenomenon, and hasn’t likely stopped or dramatically changed in the past 10 years (from Salvador, 1991).

The Gulf coast has been sinking for 200 million years. This cross-section through Louisiana shows many of the active growth faults in the southern Louisiana area, and the deep salt that flows toward the gulf. Motion on these faults is thought by some geologists to account for about half of the subsidence in the New Orleans and delta areas. From Gagliano, S. E.B. Kemp, K. Wicker, and K. Wiltenmouth, 2003, Active Geologic Faults and Land Change in Southeastern Louisiana, Coastal Env. Inc., Report to the US Army Corps of Engineers, New Orleans District, 2003.

Other possible causes of subsidence that together may constitute the other approximately 50% of the observed subsidence have received more attention, including compaction of surface sediments, compaction by oxidation of organic soils, loading by buildings, fluid extraction of oil and water, and lack of replenishment of the surface by Mississippi River floods. Some Louisiana scientists have suggested that the subsidence may have dramatically slowed since the last measurements were made, because some of the surface soils are now compacted. This idea, though it may be partly true, does not adequately consider that the main causes of subsidence are deeper. The component of subsidence caused by relatively recent compaction of near-surface sediments may have slowed recently, since little overbank muds or silts are being deposited since the river has been confined by levees. Thus, there is little material left to be compacted and the existing muds are already relatively compact. Such an observation does not, however, stop the underlying deep or tectonic subsidence and retreat of the shoreline. It is difficult to extrapolate current subsidence rates to the future however, as many other factors including sedimentation and changing conditions come into play. See for instance: http://geology.com/news/2005/10/rebuilding-new-orleans-subsidence-and.html and http://www.colorado.edu/hazards/o/maro02/maro02e.htm and http://www.geotimes.org/aug05/NN_sinkingGulf.html

Subsidence has many causes, including compaction of the surface layer, motion on seaward dipping curved listric faults (e.g., two recent earthquakes in Dec. 05, and the New Orleans East earthquake of 1987: http://neic.usgs.gov/neis/bulletin/neic_gvbk_l.html), salt motion towards the Gulf, deep tectonic compaction, and perhaps fluid extraction and loading by surface structures. The river flood deposits used to keep pace with the subsidence from these various mechanisms, but since the river was leveed more than 100 years ago the flood-muds are channeled out to the deep Gulf and no longer replenish the surface, which is now rapidly disappearing below sea level.

Diagram shows some of the near-surface phenomena that cause surface subsidence, including slumping, seaward movement of salts, listric growth faults, solution collapse, compaction of soils, dewatering, etc. (from Salvador, 1991).

This diagram shows many of the active growth faults in the southern Louisiana area. Motion on these faults is thought by some geologists to account for about half of the subsidence in the New Orleans and delta areas. From Gagliano, S. E.B. Kemp, K. Wicker, and K. Wiltenmouth, 2003, Active Geologic Faults and Land Change in Southeastern Louisiana, Coastal Env. Inc., Report to the US Army Corps of Engineers, New Orleans District, 2003.

Cross section from west of New Orleans shows the huge amount of subsidence accommodated by the listric normal growth faults and seaward movement of the Louann salt. From Gagliano et al., 2003.

Cover of report by Gagliano et al (2003) shows conceptual model where areas above active listric faults experience enhanced subsidence after faulting. These patterns are confirmed by aerial and satellite image analysis of the Louisiana marshes, compared with seismic and structural studies on the surface.

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