Paleoliquefaction
Encyclopedia
Paleoliquefaction is the term used to describe liquefaction
features attributed to seismic events occurring before measurements or detailed records were kept of earthquakes. The study of these features can tell us a great deal about the seismicity of regions where large earthquakes happen infrequently. This is a subset of the broader field of paleoseismology
.
Paleoliquefaction studies in areas like the New Madrid Seismic Zone
, and the Wabash Valley
have helped scientists and engineers determine the severity of ground shaking to expect when planning for future earthquakes.
article, this phenomenon has the effect of a soil losing its shear strength and behaving like a fluid. This liquefied soil (most often sand) flows like water, leaving several kinds of evidence behind in the geologic record.
, where the non-liquefied layer is fractured by the water pressure in the liquefied layer. Lateral spreading, where portions of the non-liquefied layer move either towards relief in the topography or down-slope if the ground isn't level, can cause cracks to open in the non-liquefied layer and subsequently fill with liquefied soil. Additionally, the motion of the earthquake waves at the surface can cause cracks to form in a non-liquefied layer. These cracks fill with soil from the liquefied layer below. Dikes can range in size from an inch in width to several feet, depending on the severity of the liquefaction and the strength of the non-liquefied layer.
Dikes that don't result in sand boils (see below) can only be found in profile view because they don't leave evidence at the ground surface. They can be discovered by digging exploratory trenches in areas where moderate levels of liquefaction are thought to have occurred, or by studying the banks of streams that have cut down through existing strata.
, a significant portion of the ground can be covered by sand carried to the surface by sand boils http://pubs.usgs.gov/of/of98-488/
Soil liquefaction
Soil liquefaction describes a phenomenon whereby a saturated soil substantially loses strength and stiffness in response to an applied stress, usually earthquake shaking or other sudden change in stress condition, causing it to behave like a liquid....
features attributed to seismic events occurring before measurements or detailed records were kept of earthquakes. The study of these features can tell us a great deal about the seismicity of regions where large earthquakes happen infrequently. This is a subset of the broader field of paleoseismology
Paleoseismology
Paleoseismology looks at geologic sediments and rocks, for signs of ancient earthquakes. It is used to supplement seismic monitoring, for the calculation of seismic hazard...
.
Paleoliquefaction studies in areas like the New Madrid Seismic Zone
New Madrid Seismic Zone
The New Madrid Seismic Zone , sometimes called the New Madrid Fault Line, is a major seismic zone and a prolific source of intraplate earthquakes in the southern and midwestern United States, stretching to the southwest from New Madrid, Missouri.The New Madrid fault system was responsible for the...
, and the Wabash Valley
Wabash Valley
The Wabash Valley is a region with parts in both Illinois and Indiana. It is named for the Wabash River and spans the middle to the middle-lower portion of the river and is centered at Terre Haute, Indiana...
have helped scientists and engineers determine the severity of ground shaking to expect when planning for future earthquakes.
Paleoliquefaction features
As described in the liquefactionSoil liquefaction
Soil liquefaction describes a phenomenon whereby a saturated soil substantially loses strength and stiffness in response to an applied stress, usually earthquake shaking or other sudden change in stress condition, causing it to behave like a liquid....
article, this phenomenon has the effect of a soil losing its shear strength and behaving like a fluid. This liquefied soil (most often sand) flows like water, leaving several kinds of evidence behind in the geologic record.
Dikes
Dikes are intrusions of a lower liquefied sand that penetrate strata of an upper, non-liquefied soil. This penetration can occur due to hydraulic fracturingHydraulic fracturing
Considerable controversy surrounds the current implementation of hydraulic fracturing technology in the United States. Hydraulic fracturing, or fracking, is the process of utilizing pressurized water, or some other liquid, to fracture rock layers and release petroleum, natural gas, or other...
, where the non-liquefied layer is fractured by the water pressure in the liquefied layer. Lateral spreading, where portions of the non-liquefied layer move either towards relief in the topography or down-slope if the ground isn't level, can cause cracks to open in the non-liquefied layer and subsequently fill with liquefied soil. Additionally, the motion of the earthquake waves at the surface can cause cracks to form in a non-liquefied layer. These cracks fill with soil from the liquefied layer below. Dikes can range in size from an inch in width to several feet, depending on the severity of the liquefaction and the strength of the non-liquefied layer.
Dikes that don't result in sand boils (see below) can only be found in profile view because they don't leave evidence at the ground surface. They can be discovered by digging exploratory trenches in areas where moderate levels of liquefaction are thought to have occurred, or by studying the banks of streams that have cut down through existing strata.
Sand boils
Sand boils occur when a dike completely penetrates the non-liquefied layer above it and reaches the ground surface. The water pressure in the liquefied layer causes an eruption of liquefied soil at the ground surface, often resembling a volcano. This can carry large amounts of sand to the surface, covering areas tens of feet or more in diameter. This makes sand boils an easy paleoliquefaction feature to locate using aerial photography. In many areas of the New Madrid Seismic ZoneNew Madrid Seismic Zone
The New Madrid Seismic Zone , sometimes called the New Madrid Fault Line, is a major seismic zone and a prolific source of intraplate earthquakes in the southern and midwestern United States, stretching to the southwest from New Madrid, Missouri.The New Madrid fault system was responsible for the...
, a significant portion of the ground can be covered by sand carried to the surface by sand boils http://pubs.usgs.gov/of/of98-488/