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Australia: The Land Where Time Began |
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Deglaciation and seismic activity
According to the author¹ there is strong evidence linking deglaciation and seismic activity, with the rebound of the lithosphere and increased earthquake activity, though it is still not possible to pin down the exact role deglaciation played in either the 1356 destruction of Basle of the early 19th century or the seismic sequence that occurred in New Madrid in 1811/1812. During the last ice age ice sheets had spread across Europe and North America, stabilising the active faults beneath them and suppressing earthquake activity. This effect can be seen in Greenland and Antarctica where there are presently low levels of seismic activity, though it has yet to be shown if this lack of activity is the result of earthquake faults in the underlying crust, or they are present but locked by the massive weight of the ice.
According to the author¹ there is a large consensus that a combination
of spectacular uplift and increased earthquake activity, during which
accrued tectonic strain on faults beneath and adjacent to ice sheets is
released, results from the melting of large ice sheets. The author¹
suggests Lapland, a transnational wilderness region stretching across
Arctic Scandinavia and Finland is probably the best evidence for this
relationship. The thickest part of the Fennoscandian Ice Sheet was
located here, and it is this place where the relationship between
faulting and surface features, such as meltwater channels that are
indicative of ice wastage, indicate that major faults moved within a few
years to a few decades of being uncovered by the melting of the
overlying ice, a response that is virtually geologically instantaneous.
It is suggested by the size of the faults, up to 150 km long, and in
some cases with movement of 15 km in a single event, that hey would have
been the source of some really enormous earthquakes higher than
magnitude 8. Similar faults, though on a lesser scale, have been
reported from Canada. This suggests that the rebound stresses following
the melting of the Fennoscandian Ice Sheet appear to be higher than
those associated with the melting of the ice covering much of North
America.
Difficulties arise when trying to fix relationships between the
earthquake faults and the surface features associated with the
melting of the ice sheets when searching for evidence beyond the limits
of ice sheets for an indisputable increase of earthquake activity
associated with deglaciation. Faults are searched for that cut across
sediment of the Late Pleistocene or Holocene, or landforms
characteristic of the so-called periglacial environment of the land
areas around the ice sheets, though accurate dating of the faults and
determining whether the cause was tectonic or glacial rebound is
extremely difficult. Increased earthquake activity resulting from
deglaciation can also be looked for in the sediments, marine and
freshwater, the sequence being looked at to determine if it has been
subjected to disturbance by severe shaking. Even moderate earthquakes
can stir up unconsolidated sediment, down to about magnitude 5, thereby
providing one of the best indicators of post-glacial volcanic activity.
It has been found that lake sediments in Canada, Sweden and Scotland
deposited in the latest stages of the last ice age, or shortly after,
all contain evidence of a strong seismic response to wholesale ice sheet
melting.
When evaluating the current risk of earthquake, especially in the stable
interior of Europe and North America, it is essential to know the
extent, if any, the current pattern of earthquake activity and fault
movement is a reflection of post-glacial-bounce-back. Are the interiors
of Europe and North America more subject to fault movement and
earthquake prone as a result of post-glacial-bounce-back?
It has been suggested by Robert Muir-Wood that the patterns of current
earthquake activity around the margins of the former ice sheets, those
of the UK being included, can be explained by the combination of
stresses related to tectonics and crustal rebound. According to Muir’s
suggestion the glacial rebound forces reinforce the background stresses
that result from the movement of tectonic plates in some places around
the former ice sheets. The 2 forces work against each in other places,
‘shadow zones’, resulting in the suppression of earthquakes. Resulting
from this interplay characteristic earthquake patterns have been found
in the forebulge surrounding the edge of the former ice sheets that is
comprised of 2 zones, called quadrants that are seismically active,
alternating with 2 quiescent quadrants in which there is little seismic
activity as well as in the lithosphere beneath the ice sheets. In the
lithosphere surrounding the British ice sheets the more seismically
actively zones cover parts of central and western Scotland, though there
is a more notable swaths across almost the entire area of England and
into the northern part of France. There is significantly more earthquake
activity in the area of this swath than to the west in Ireland or to the
east, the North Sea, magnitude 5 earthquake occurring about every
decade, the latest of which was in Market Rasen in Lincolnshire in 2008.
In the 14th and 16th centuries earthquakes
occurred in the Dover Straits, in offshoots of the north-western Rhine
Graben that extends into the quadrant that is seismically active, with
the result that this provides the opportunity for the stresses arising
from glacial rebound to interact with those associated with stretching
across the Graben, that possibly explains these events and providing the
conditions required for future earthquakes. According to the author the
low level of earthquake activity in northern Germany is explained by use
of the Muir-Wood model by its location within one of the shadow zones
that are seismically quite, associated with the Fennoscandian Ice Sheet.
During the Early Holocene there was a bout of earthquake activity
further to the south, as well as the large historical earthquakes that
are historical, such as the Basel quake, the explanation of which has
been suggested to be their location in one of the seismically active
zones associated with the reduction of the ice mass covering the Alpine
Region, one of the smaller ice sheets. The earthquakes that occurred in
New Madrid and Charleston, South Carolina earthquake of 1886 are all
located in a Muir-Wood seismically active quadrant southeast of the
Laurentide Ice Sheet.
Weight of water
In 1962 construction began on a dam across the Koyna River in the Indian
state of Maharashtra at a point where it flows through a deep valley
200 km to the south of Mumbai, with the intension of providing a secure
water supply and generating electricity. The water impounded by the dam
formed Lake Shivajisagar that occupied a 50 km stretch of the valley. The
area around the dam was considered to be geologically stable, the area
not being known as one that was prone to earthquakes. Seismic activity
began to be noticed once the dam was filled, the size and frequency of
the tremors increased from 1963, the magnitude of the tremors increasing
until they were exceeding magnitude 3 in November and December 1967,
comparatively small compared to earthquakes in seismically active areas,
they were occurring in an area that was not previously seismically
active. On 11 December a magnitude 6.3 earthquake struck near the site
of the dam, though the dam held it had undergone a severe shaking.
Though the water wasn’t released catastrophically from the dam there
were more than 200 deaths and up to 2,000 injured as a result of the
earthquake. The relatively low number of deaths was more the result of
the low population density of the area. There was a large amount of
damage in nearby villages such as Koyna Nagar. Fissures, rubble and
landslides cut bridges and roads, though the power station within the
dam survived relatively well, but power supplies to Poona and Mumbai
were interrupted.
Problems with storing water
The Hoover Dam on the Colorado River was constructed in 1931to form Lake
Mead, the largest reservoir in The USA with more than 35 km³ of water
and a surface area of almost 650 km². Before the dam was built the area
had no record of earthquake activity, as was the case with the Koyna
Dam, but as the dam was filling there were more than 600 earthquakes in
the area of the dam by the time the water level reached 150 m in 1939,
the biggest of which being magnitude 5.
According to the author¹ it is now apparent that such a response in the
underlying crust is not restricted to a few localities with
more than 100 dams being built in previously seismically quite
areas where seismic activity increased following the construction of
the dam. In the 1950s damaging earthquakes occurred as the Kariba Dam
was being built on the Zambezi River between Zimbabwe and Zambia. At the
time of its construction the Kariba Dam reservoir behind it was the largest volume of water in a dam in the world, with a volume of 180
billion metric tonnes of water. Strong seismic activity accompanied the
filling of the dam, with more than 20 of them being higher than
magnitude 5. The Hsingfengkiang Dam in China triggered a magnitude
6 earthquake that caused serious cracks to appear in the dam in 1962. In 1966 when the Kremasta reservoir in Greece was accompanied by
a sequence of hundreds of earthquakes that culminated in an earthquake
of almost magnitude 6.
Pressure addition
The author1 suggests it is now apparent that the accumulation
of large volumes of water behind dam walls is very effective at
triggering earthquakes, apparently in the opposite sense to that of glacial activity.
The increase of seismic activity along rejuvenated faults as
deglaciation occurred at the end of the last ice age. In the case of
large dams it is the addition of water that is responsible for
triggering the seismic activity. At first sight it appears that the
weight of the addition of
large volumes of water is causing the earthquake activity associated
with the construction of dams, but research has shown that the
commencement of seismic activity is not the result of the weight of the
water, no seismic response being detected at the deepest part of the
dam, even though it is the part of the dam that has the greatest weight
of water. Most earthquakes have been found to be occurring around the
margins of the reservoir. The author1 suggests this indicates
that the faults directly beneath the water are being stabilised by the
weight of the water as occurred during the glacial phases when ice sheets
stabilised them, though faults around the margins are encouraged to
become active. Support has been found for this proposal in the Pakistani
section of the Himalayas where the Tarbela reservoir has dammed the
Indus River. In this reservoir it has been found that earthquake
activity on a small scale on faults beneath the reservoir occur more
often during the dry season when the water level is low, suggesting that
the weight of the water stabilises the underlying faults when the
reservoir is full. The reason for the siting of the earthquakes around
the margins has been found to result from water being forced into
fractures of all sizes by the pressure of the water when the reservoir
begins to fill. The rock is weakened by the new water being forced into
it increasing the pressure applied to the pore water that is already
present. The increased fluid pressure on any faults that are present and
at a suitable critical state can trigger movement, causing an earthquake
by the reduction of the stresses that have until that point is reached
kept the fault stable. According to the author1 when water
infiltrates the rock it raises the pore pressure and this lubricates the
faults allowing them to move more easily.
According to the author1 in some cases it is possible to
observe a pattern of the results of changing pore pressure and changing
levels of seismic activity as the level of seismic activity changes in
line with the pore pressure, the seismic activity increasing as the
reservoir fills and decreasing as the water level falls, as in a dry
season. More than 40 yeas after the construction of the Koyna Dam the
levels of seismic activity are still raised above those prior to the
construction of the dam. The dam at Koyna has been studied by Pradeep
Talwani, of the University of South Carolina, a founder of the study of
‘reservoir induce seismicity’ RIS, finding that the levels of seismicity
display a recognisable pattern, seismic activity increasing in the wet
season, June-August, and decreasing in the dry season covering the
remainder of the year. In the rainy season when the lake water levels
are high the largest earthquakes occur 6-8 weeks after the water level
begins to rise. The proposed link between the water level in a large
water body and the level of seismicity in the local area is strongly
supported by the observed timing of water level rise and the rise of
seismicity; the lag following the rising of the water and the onset on
increased seismicity is explained, according to the author1,
by the time required by the increasing pore pressure in the rocks on the
margins of the reservoir to achieve sufficiently high values to promote the movement
of faults. He says causal relationships are commonly recognised between
changing water levels, 2 examples of which have been observed at Lake
Meade and the Lake Monticello reservoir in South Carolina.
In 2008 a 7.9 earthquake struck in Wenchuan County in China's Sichuan Province causing 70,000 deaths and making 4 million homeless.
In China large, destructive earthquakes are common, including some of
the most lethal and destructive to be recorded. Seismic strain
accumulates slowly in the area of this large quake, with no major
earthquakes being known of in the Holocene, though Wenchuan had
experienced more than 6 earthquakes prior to the ‘big one‘of 2008. The Zipingpu
Dam had been opened 4 years before the 7.9 quake, the epicentre
of which was 21 km from the dam. The reservoir was 120 m deep and was
suggested by many as the cause of the magnitude 7.9 earthquake that was
one of the most devastating in the first decade of the 21st
century. According to the author1 research was carried out by
Christian Klose of Columbia University, the findings of the study
suggesting the earthquake could have been triggered by the 300 million
tonnes of water in the reservoir. According to Klose’s proposal the
altered crustal stress may have brought closer on a fault that was already
in a critical state. This proposal is strongly disputed, with other
researchers arguing that the dam may have caused small, more local quakes
but could not have been even partly responsible for initiating the 7.9
earthquake that occurred at depth of 20 km.
According to the author there has been much speculation that the Three
Gorges Dam in Hubei Province, China, with its 40 km3 of water
and a length of 600 km, could possibly promote a seismic response that
was serious enough to cause the dam wall to give way. As the water level
in the dam rose, reaching 175 m in 2009, the predicted seismic level of
activity has increased, Though it is not yet known if the seismic
activity will reach a level at which the dam wall will be damaged, or in
the worst case, give way.
It is now well established that when pore pressures in rocks are forced
up, but in spite of this water is being pumped into the crust at high
pressure to tap geothermal energy. At Basle in Switzerland a borehole
was drilled to access geothermal energy on the 650th
anniversary of the earthquake that flattened Basle in 1356. Following a
magnitude 3.4 earthquake, that caused $US 9 million in damage, which was
followed by other earthquakes, the project was eventually shut down to
prevent further earthquakes. The aim of the project had been to use high
pressure water to open fractures to make the rock more permeable so that
as more water was pumped into the ground it would be heated and
extracted at the surface where the steam would be used to drive
turbines.
A wetter world
As the global temperature rose rapidly at the close of the last ice age
the vast ice sheets melted and the meltwater caused sea levels to rise
and the climate became more humid and warmer. As the continental ice
sheets melted and drained into the oceans the crust readjusted to the
unloading with an increase in earthquake activity. Places where large
bodies of meltwater accumulated were also characterised by a seismic
response, the place which the author1 suggests is probably
the best known location for this association is the Lochaber area in the
western Scottish Highlands, are the valleys of Glen Roy and Glen Spean
near the town of Fort William and close to Ben Nevis. Characteristic
of these valleys are the parallel horizontal lines following the
contours of the topography that are preserved shorelines, that were
incised into the sides of the valley by glacial meltwater lakes at
various levels, that were fed by glaciers covering much of Scotland in
the last ice age, and that drained catastrophically 12,000-11,000 years
ago during the Younger Dryas when the Scottish glaciers that had been
retreating once again advanced. These ‘parallel roads of Glen Roy’ have
been the source of a number of legends in the vacuity. According to one
of these the Celtic giant Fingal carved them to facilitate his hunting
trips. Another legend says the ancient kings of Scotland constructed
them for unknown reasons.
Over the last 150 years there have been many studies of these
shorelines, as well as the deposits associated with them, that have
resulted in a detailed picture of the lakes and their evolution being constructed. Ice was the
source of these lakes and also formed the barriers that dammed them.
During the Younger Dryas about 11,500-12,000 years ago during which
glaciers that had been retreating in Scotland began advancing again. The
warming of the Holocene caused the ice to retreat again after a bit more
than 500 years, the warming eventually melting the ice component of the
barriers impounding the water allowing the lakes to empty. Analysis of
ancient lake sediments and shorelines by Adrian Palmer et
al. of Royal Holloway
College, University of London, and the results indicate how the water
levels in the lakes rose progressively as the glaciers retreated. The
water levels fell as lakes were drained in 3 catastrophic drainage
events similar to the jökulhlaups of Iceland.
These lake sediments show evidence that as the water levels were
dropping they were strongly churned, a feature of the sediments first
noted during a study in the 1980s by Philip Ringrose, that he attributed
to liquefaction of the sediment, that was soft and waterlogged, as the
result of an earthquake. The author1 suggests it is possible
that as a result of unloading as the ice dams melted faults released by
the lessening pressure could have moved to cause earthquakes of up to
magnitude 6. An alternative explanation that was also thought possible
by Ringrose and others is that the seismic activity indicted by the
sediments was actually triggered by the sudden reduction of pressure
above the faults as the water broke through the ice dams. As has been
demonstrated by the Tarbela reservoir in Pakistan, high lake levels
would have stabilised the underlying faults, the loss of the lake water
reducing the pressure on the faults that could then move more easily.
In North America the effects on earthquake activity of sudden lake
discharge has also been recognised, though on a much larger scale. The
Basin and Range Province, a vast region of narrow
mountain ranges that are separated by broad valleys aligned in
a north-south direction, are situated just to the east of the Rocky
Mountains. Some of the faults that bound the mountain chains and valleys
display evidence that is believed to indicate they were particularly
active at the end of the last glacial phase. This area was not buried
beneath a large ice sheet during the ice age so the large-scale melting
of ice cannot be an explanation for the increased seismic activity. Such
an ice cap covered the Yellowstone Plateau that is nearby, and much of
the region was covered by 2 very large lakes, the largest being Lake
Bonneville, ancestral to the Great Salt Lake of the present. The lake
that was about the same size as Lake Michigan, the second largest of the
Great Lakes covered an area of more than 52,000 km2 at its
greatest extent, at which time it covered most of Utah and some of
Nevada and Idaho, with a depth of 350 m, being present for almost 20,000
years it was long-lived, though it eventually succumbed to higher
temperatures and reduced precipitation. The water level reached a peak
about 17,000 years ago at which point it breached the dam, draining
through Red Rock Pass in Idaho as an immense flood. It has been
estimated to have formed a wall of water more the 120 m high moving at
more than 110 km/hour across the Snake River Plain basalt gouging the
Snake River Canyon to a depth of 180m on its way to the Columbia River,
and eventually into the Pacific Ocean. A difference between Lake
Bonneville, and the smaller Lake Lahontan to its west, and the much
smaller lakes in Glen Roy and adjacent valleys was that whereas the
Scottish lakes were glacier fed, the American lakes were not.
The water of the American lakes
came from a change in the climate to conditions where there was less
evaporation and increased rainfall leading to the accumulation of water
in areas that would previously have been dry. According to the author1
evidence has been found that there have been dozens of predecessors of
Lake Bonneville over the previous 3 million years apparently indicating
that there were previously many times when there were conditions
conducive to the formation of lakes.
There is a clear coincidence between the shrinkage of Lakes Bonneville
and Lahontan and an outbreak of earthquake activity on local faults, as
in the case of meltwater lakes in the Scottish Highlands. The
relationship between the large lakes of the Basin and Range Province
from the Late Pleistocene, and the activity of the local faults, such as
the Wasatch Fault along the eastern margin of Lake Bonneville, has been
studied by Andrea Hampel, Ruhr University, Bochum, Germany, with others.
The Wasatch Fault is a very active major tectonic structure almost 400
km long on which there have been powerful earthquakes about every 350
years, and according to the author1 many have concerns that
the next earthquake could severely damage Salt Lake City, as well as
other population centres along its length, especially as the buildings
of the area are constructed on the soft sediments of the floor of the former
Lake Bonneville. According to the author1 a strong seismic
shaking is likely to cause liquefaction of these sediments and at least
1 estimate puts the possible death toll to as much as 6,000 and damages
up $40 billion. When Lake Bonneville was full during the Late
Pleistocene the Wasatch Fault was quiescent, then 17,000 years ago the
lake volume was reduced catastrophically and a burst of earthquake
activity followed soon after. According
to Hampel et al. this
resulted from slippage of parts of this fault at double the normal rate
that occurred at this time; a direct consequence of the load reduction
as the weight of the water dropped. Once unloading occurred the crust
rebounded by up to 70 m, and it was this that freed the fault and
initiated a burst of earthquake activity. At Lake Lahontan, that held a
lower volume of water, the rebound that resulted from catastrophic
lowering of the volume led to a rebound of not more than 20 m and a
seismic response of faults in the vicinity of this lake that were
similar though less marked than on the Wasatch Fault.
Alaska
The author1 suggests there is plenty of evidence of the
association between earthquakes and water, either liquid or solid,
coming from a period of rapid climate change marking the latest part of
the Pleistocene and the Holocene. Evidence is also seen on the building
of dams and artificial impounding of water in reservoirs. According to
the author1 increases of earthquake activity was a natural
response to natural changes that resulted from the wasting of ice sheets
that were kilometres thick and the spontaneous draining of vast lakes.
He suggests that to discover if the climate changes
of the present are capable of triggering a seismic response evidence
should be looked for in the parts of the world that are experiencing ice
loss at the most rapid rate or the accumulation of glacial meltwater,
such as in Alaska.
In Alaska there are 43 volcanoes that have been active historically,
such as Pavlof, and there are also snow-capped peaks, permafrost and
glaciers, as is the case in Iceland. A wholesale melting of permafrost
is occurring in Alaska as a result of a 3º C rise of temperature over
the last 50 years that has led to an increasing problem in cities such
as Fairbanks of structures including buildings, bridges and roads
becoming misaligned or deformed as the ground beneath them become
increasingly soggy. So far the Bagley Ice Field (aka the Bagley Ice
Valley) at 200 km long, 10 km wide and up to 1 km thick in places is
still the largest non-polar ice field in North America, though it is
rapidly shrinking. Many valley glaciers are fed by this icefield,
branching out from the parent mass, such as the Bering Glacier, among
the biggest in the USA. The Bering Glacier and all the other valley
glaciers are retreating, and even the parent ice field is beginning to
be affected by rising temperatures and precipitation changes, thinning
by several hundred metres since 1900. According to the author1
this retreat is reversed every couple of decades, but the retreat
is soon resumed. It has been estimated that it is now adding 30 km3
to the Pacific Ocean annually.
At the surface dynamic change can be seen in the glacier retreat and
permafrost thawing, both of which will become more apparent as the Earth
warms. What can’t be seen are the natural tectonic forces at play
beneath Alaska, as well as around the margins of the state. The powerful
geological forces beneath Alaska are hinted at by the many active
volcanoes and the high level earthquake activity, the largest of which was Good Friday
Earthquake of 1964, the second largest seismic event ever recorded. The
largest of all the tectonic plates is the Pacific Plate and it is being
subducted to the northwest beneath the North American Plate at 5 cm/year
beneath southern Alaska. This rate of subduction is high for plate
movements and the massive collision between the 2 plates has pushed up
peaks to more than 5,000 m a bit less than 15 km of from the Gulf of
Alaska. There are many active faults and many earthquakes. Since the end
of the 19th century there have been 7 earthquakes larger than
magnitude 8 as well as the 1964 event, and serious damage was caused by
4 of them. There are annually 4,000 large, shallow earthquakes, making
up 6 % of all the earthquakes of this type around the Pacific Rim.
Jeanne Sauber, Space Flight Center, NASA, Maryland and Bruce Molnia,
USGS, have been studying how the continued wastage of glaciers and ice
fields in southern Alaska are influencing the stability of faults
situated at the subduction plate boundary immediately underlying the
region, particularly if the seismic record reflects the glacier
shrinkage. The ice across much of the region has been thinned by
hundreds of metres, possibly 1 km in areas of greatest ice loss has
resulted from progressive melting that is driven by a warming climate
since the beginning of the 20th century. As the ice melted
over this period, deep beneath the surface the segment of plate boundary
fault lying deep beneath the surface was accumulating strain and in 1979
a magnitude 7.2 earthquakes struck. Sauber and Molnia were trying to
determine if the wholesale loss of overlying ice mass that occurred over
about the previous century directly caused the earthquake, or possibly
have a role in its triggering. They concluded that the amount of ice
mass loss over this period, a pressure fall that is comparable to 20
times the pressure of the Earth’s atmosphere, would have certainly been
sufficient to influence the type of thrust faulting that was
characteristic of the 1979 quake. According to the author1
this would indicates that the ice loss would have facilitated the
slippage of the fault, possibly bringing forward the timing of the event
though not actually causing it. It would have been the deep tectonic
forces that actually drove the slippage.
The author1 also suggests that in southern Alaska it also
becoming apparent that short term fluctuations of ice mass is
influencing earthquake activity. In the 1990s the Bering Glacier surged
and 14 km3 from the Bagley Ice Field source was moved rapidly
to the snout of the glacier. It was suggested by Sauber and Molnia et
al. that a noticeable rise in
the number of earthquakes of less than magnitude 4 in the underlying
crust resulted from the unloading. Since then, according to the author1,
in the early years of the 21st century a significant rise in
the rate ice loss coincided with an increase in the number of seismic
shocks beneath Ice Bay on the coast immediately south of the Bagley Ice
Field.
The author1 says the findings in Alaska are of great
significance for seismic hazard evaluation throughout the world in
places where glaciers are melting as it indicates that even 10,000 years
after the close of the last ice age the loss of ice mass is still
facilitating, if not directly causing, earthquakes. At locations, such
as Alaska, at high latitudes where active faults capable of producing
large, destructive earthquakes, are covered by ice, as well as in
comparable situations away from the poles where the terrain is covered
by glaciers, such as the Andes in South America and the Alps in New
Zealand, the author1 suggests it is advisable that the
seismic hazard should be re-evaluated in light of the findings in
Alaska. When evaluating the seismic hazard of region it is usual to
base it on the seismic record of the place over time, and not take into
account changes of environmental factors such as ice cover at the time
of the evaluation, and past behaviour of both the ice and the crust. As
the continued survival of the glaciers of the world increasingly seems
unlikely the evaluation of the likely consequences of their loss on the
active faults beneath them has taken on a greater importance.
Crustal rebound
According to the author¹ the city of Basle in Switzerland is situated
in a placein which the geology is particularly complicated where there are stresses
that stretch the crust along a zone of weakness, the Rhine Graben, that
comes into contact with collisional forces caused by the uplift of the
Alps, the continued rise of which is in turn caused by the collision of
Italy with southern Europe and continues to move north. It was a result
of Basle being situated on this highly complicated geology that in 1356
there were a series of massive earthquakes and many aftershocks, some of
which were large. The author¹ says most discussion has focused on
whether the
earthquakes of 1356 were a result of the rupture of a compressional (or
thrust) fault south of the city, that was associated with Alpine
deformation, or a tensional (or extensional) fault in the Rhine Graben to
the north, an ancient rift that is slowly pulling Europe apart,
that was a source of the earthquakes. There is now some degree of
consensus that the earthquakes were associated with rupturing of a fault
within the graben. It now appears the earthquake of 1356 was not
unprecedented in comparatively modern history, as there is evidence of
at least 2 other comparable events that occurred in the past 10,000
years that were identified during excavations across the fault. There
are also other places in western Europe that are now known to be at risk
of earthquakes that are large and potentially destructive. There have
been many other earthquakes in the northern end of the Rhine Graben such
as a 5.8 event that occurred close to the town of Roermond, the
Netherlands, in which there was a lot of damage. It has been revealed by
fault excavation studies that in about the past 20,000 years there have
been 3 larger quakes in the region, and it is suggested by research at
other places in and around the northern segment of the graben that
faults capable of generating substantial earthquakes crisscross the
area. The author¹ suggests the threat of seismic activity may even reach
as far as the UK, some faults extending to the west beneath the Dover
Straits that have been associated with damaging quakes that occurred in
1382 and 1580. In the 1580 event 2 people were killed in London by a 5.5
magnitude earthquake that caused damage across southeast England. Over
about the past 500 years there has been little seismic activity in the
area affected by the 1580 earthquake but there remains the potential for
earthquakes of comparable size to occur in the future. The population of
London is now 50 times greater than it was at the time of the 1580
earthquake, as pointed out by Roger Musson of the British Geological
Survey, an earthquake of comparable size would now cause a much greater
loss of life and the damage would be in the billions of pounds.
It is now known that the crust of Europe is much less stable than is
generally believed, and there is evidence that Europe is now more
seismically active that in was in the past. A burst of seismic activity
is believed to have occurred in central Switzerland at the time of the
Late Pleistocene-Holocene transition when the ice sheets of northern
Europe were retreating and the Alpine glaciers were also retreating. It
has been suggested that the destructive earthquake of 1356 may be the
result of the close of the ice age with the unloading of Europe
following the loss of the ice sheets, the accumulated stresses on faults
that were previously imprisoned were released leading to European
earthquakes in a similar manner to the unloading led to volcanic
activity in France and Germany when the ice retreated. Isostatic rebound
is the key mechanism involved in the rebound of the lithosphere
following unloading as the ice sheets melted, and has been used to
explain the earthquakes in Canada, Europe and the UK that occurred in
post-glacial times. Once released from the ice there is a vigorous
response as the faults can again move resulting in earthquakes.
According to the author¹ isostasy is the principle of buoyancy that has
been applied to the outer layers of the Earth. The brittle lithospheric
plates are of relatively low density and float on the asthenosphere that
is denser, and being plastic deforms more easily than the lithosphere.
At any particular point the lithosphere floats at a particular level in
the asthenosphere depending on density and thickness of the lithosphere. The
lithosphere under high mountains pushes deeper into the asthenosphere
than the surrounding lowlands. A state of gravitational or 'isostatic'
equilibrium exists between the lithosphere that floats on the
asthenosphere and the asthenosphere beneath it under ideal
circumstances, though circumstances are not often ideal on the dynamic
Earth. As a result of erosion unloading may cause the mountains to
rise higher in the asthenosphere. Where the eroded material is
accumulated in deposits it increases the weight of the lithosphere which
then sinks deeper into the asthenosphere.
The lithosphere is said to be in a state of disequilibrium where is is
loaded by ice sheets that are kilometres thick, being forced downwards
by the weight of the ice to a degree where its level in the
asthenosphere is out of balance with its mass. Isostatic rebound occurs
when the ice melts and the lithosphere is unloaded, though at a slow
rate. Post-glacial-rebound is the term used for the isostatic rebound
that resulted from the unloading as the ice melted, to distinguish it
from the broader lithospheric uplift that can occur for a number of
other reasons, glacial isostatic readjustment is a more accurate term,
as in the case of subsidence of continental margins that are loaded as a
result of sea level rise following the end of a glacial phase.
The thickness of the ice sheets across North America and Europe reached
up to 3 km at the height of the last glaciation. The uppermost part of the
mantle, that is hot and
plastic, was pushed aside as the enormous mass of the overlying ice
sheets pushed the lithosphere deeper into the asthenosphere. As the
lithosphere was unloaded at the end of the glaciation as the ice sheets
were melting the lithosphere flexed back up and mantle material flowing
back to its previous position, caused continued long-term uplift, a
process that was very slow as a result of the low viscosity of the
mantle, the uplift following the ice loss continuing at the present in
the land area in much of the Northern Hemisphere, such as Canada and
Scandinavia. The initial rate of uplift was up to several centimetres
per year, though at the present it is not more than 1 cm per year. The
author1 suggests it could take another 10,000 years before for the
lithosphere beneath the deepest parts of the former ice sheets finishes
rising.
Examples of physical evidence for rebounding of the lithosphere
following the close of the last glacial phase, of which there is a great
deal, include marine shore lines and elevated lakes in areas that had
previously been glaciated, as well as otherwise unexplained uplift
patterns. In about the 12th century an arm of the Baltic Sea was
separated from the marine environment by progressive uplift in Sweden
close to the centre of the vast Fennoscandian Ice Sheet that was centred
on Scandinavia, in the process transforming the separated arm of the
Baltic into a freshwater lake, Lake Mälaren, on the shores of which is
Stockholm, the capital of Sweden. The northern arm, the Gulf of Bothnia,
will eventually be transformed into a freshwater lake. At Kvarken, where
the water has a maximum depth of 25 m, about 80 km separates Sweden from
Finland. It has been estimated that it should take more than 25,000
years at the current uplift rate of a bit under 1 cm/year for the
northern part to be cut off to form a freshwater lake. According to the
author¹ the time should be even longer because of a slowing of the
uplift rate over time and rising sea level resulting from anthropogenic
climate change.
The author¹ suggests Lake Ontario in North America appears to have been
formed in a similar manner. The sea flowed into the St Lawrence Valley
after the Laurentide Ice Sheet melted, and Lake Ontario formed a bay of
this flooded river valley. Soon after the sea flooded into the St
Lawrence Valley uplift separated the bay and it was transformed into the
freshwater lake of the present. Uplift has been continuing rapidly in
the region, rapidly enough to tilt the bed of the lake to the south, river
valleys being converted into bays and causing erosion rates to increase,
and this is causing concern among property owners along the shoreline. In the UK
post-glacial-rebound continues to cause the UK to pivot across its
middle, the south subsiding as Scotland and Northern England are
uplifted following the melting of the ice sheets that covered them
during the last glaciation. The author¹ suggests that as a consequence
of this movement, by 2100 parts of Scotland could have risen by about a
further 10 cm while the southeast of England could have subsided
by about an extra 5 cm, which would in effect add to the sea level rise
that will have occurred by then. It has been established that
post-glacial-rebound on a widespread scale has actually occurred, but is
there a connection between earthquake occurrence and this lithospheric
uplift? The author¹ suggests that a good place to look for evidence of
this link is the biggest ice sheet, the Laurentide Ice Sheet in North
America.
New Madrid
In the putatively stable central parts of North America, a long way from
the known earthquake areas such as California and Alaska, and similar to
the case of Western Europe, earthquakes took many by surprise. Shortly
before Christmas in 1811 there were severe jolts that shook the upper
Mississippi Valley in an area of land, where there were previously
unknown faults, between St Louise, Missouri to the north and Memphis,
Tennessee to the south. This was followed on the 16th of
December by a major earthquake of magnitude 7-8 centred in north-eastern
Arkansas. The population of the area was sparse at that time so property
damage was small. At the settlement of New Madrid by the Mississippi
River, a bit north of the epicentre, the quake caused the collapse of
the river banks and toppled apple trees. The shaking has been estimated
to have reached as much as magnitude 9 where the present city of Memphis
is located, that is comparable to the magnitude of the earthquake that levelled Basle 450 years earlier, and would have caused massive damage
if it occurred at the present. Eliza Bryan, an eyewitness, later wrote of
the terror of the population of the settlement.
This earthquake proved to be the beginning of one of the most extraordinary
seismic events in the history of the USA. This was followed by major
shocks every few minutes and eventually, at around breakfast time, a
second major quake about the same size as the first, followed by more
aftershocks, and other major quakes on 23rd
of January and February 7th, the last possibly being
the largest of all. A great deal of property damage was caused in St
Louis, with ruptures opening in the ground instantaneously forming
waterfalls on the Mississippi, though they were only temporary. Church
bells in Richmond, Virginia, more than 1,000 km away, were caused to
ring by the earthquakes as the cold, brittle crust of central USA
transmitted the seismic waves very efficiently and Windows were rattled
in Washington, DC, 1,400 km away, and they were felt in New York and
Boston. Aftershocks that were strong enough to be felt continued for
more than 5 years.
The source of the New Madrid earthquakes is not known as the area is a
very great distance from any plate margins, leading seismologists to
look for other possible underlying causes. Research has been focused on
the Laurentide ice sheet that covered much of the northern part of North
America during the last glacial maximum, in particular on the
possibility of post-glacial-rebound being responsible for the earthquake
activity many years after the retreat of the glaciers and subsequent
unloading as the ice melted.
According to the author¹ a problem with a possible role played by
deglaciation in the triggering of the New Madrid earthquakes is that the
New Madrid Seismic Zone (NMSZ) was never covered by ice during the
glaciation. At the height of the glaciation the glaciers didn’t come
closer to the NMSZ than a few hundred kilometres to the North. The
application of stress to rocks surrounding the edges of the ice sheets
stretches a long way from the glacier margins. When the ice sheets push
the lithosphere down into asthenosphere the mantle is pushed to the side to make room for the
continental crust. The lithosphere flexes upwards to compensate around
the edge of the depressed area, and this uplift is helped by the outward
flow of the mantle resulting in the hollow formed beneath the ice being
surrounded by a contrasting ‘forebulge’ extending well beyond the ice
sheet front. As deglaciation occurs the lithosphere and the forebulge
subsides gradually in a wave moving outwards from the area of the former
ice sheet. It has been suggested by Robert Muir-Wood that the perfect
conditions
for the release of tectonic strain that had accumulated during
glaciation might be provided by just such as collapse of a forebulge
with the result that a corresponding cascade wave of seismic activity
migrated progressively away from the ice sheet’s former margins.
Muir-Wood also suggested that as applied to North America this model
could also explain an earthquake that struck Charleston, South Carolina,
in 1886, that was large and caused a lot of damage, as well as others
off the Canadian coast. Similar ideas have also been developed by
Patrick Wu of University of Calgary, Canada and Paul Johnston, ANU,
Canberra, involving rolling waves of seismic activity travelling away
from the margin of the Laurentide Ice Sheet over time. They have
developed a model that tries to predict the arrival in specific
locations in North America of this earthquake activity pulse. According
to the author¹ their model predicts accurately the nature and timing of
earthquakes that occurred in Quebec and Indiana, though it has problems
with those at New Madrid, predicting the timing of the New Madrid
sequence but failing to predict the rate at which the rebound stresses
decay over distance from the margins of the former ice sheet, suggesting
they would be not be large enough to trigger magnitude 8 earthquakes in
this area. The magnitudes of the New Madrid earthquakes were later
scaled down to about 7 by Susan Hough of the USGS in 2010, a level that
can be predicted for the New Madrid quakes by post-glacial-rebound.
According to the author¹ there is apparently good evidence that seismic
activity was triggered by the melting of the ice sheets that covered
North America; this doesn’t necessarily apply to the earthquakes that
struck New Madrid in 1811 and 1812, though it remains to be proved that
they
actually resulted from post-glacial-rebound. He
also suggests that the most scientifically reasonable explanation for
the New Madrid earthquakes is that deglaciation and tectonic forces were
involved in the triggering of the New Madrid earthquakes by the
lithospheric bounce-back being just sufficient to nudge faults that had
been weakened by the gradual accumulation of stresses that resulted from
the continuous movement of tectonic plates. The result of GPS studies to
estimate the rapidity of the accumulation of strain on the NMSZ faults
suggests that such strains are accumulating extremely slowly, with
fault movements being of a few tenths of a mm per year. The is more than
100 times slower than strain accumulates on the San Andreas Fault in
California. According to the author¹ this has been used to play-down that
likelihood of another quake on the scale of those of 1811 and 1812
occurring. Some have suggested that earthquakes that have occurred in
the NMSZ more recently are very late aftershocks resulting from the
resettling of the crust after the 2 big quakes of the early 19th
century. Alternatively it is has been suggested that the NMSZ may be
closing down.
A cautious approach has been taken by the USGS, with the possibility of
another large earthquake in the area continuing to be a possibility.
They consider that about a decade of strain measurements should not
override a record of persistent seismic activity in the NMSZ that covers
4,500 years. There have been more than 4,000 earthquakes detected on the
instruments of USGS since the mid-1970s, at least 1/year that was big
enough to be felt. The USGS have not found any sign that the activity is
diminishing over time, which indicates that the continued tremors are
not simply adjustments of the crust. The USGS have not downgraded the
perceived risk of another major earthquake in the NMSZ because the
possible consequences of any large earthquake that did occur, as the
FEMA has warned that a large quake in the area could possibly cost the
US more than Hurricane Katrina in New Orleans in 2005. Estimates of the
probability of another earthquake comparable to the magnitude of those
in the early 19th century have been put at 10 % by the USGS.
It has been suggested by Muir-Wood that the wave of post-glacial strain
relief might continue moving south eventually increasing the risk of
large earthquakes in Georgia, Arkansas and the northern part of
Louisiana.
Overview1
As in the case of volcanic activity, it is not possible to isolate
seismic activity from what occurs on the surface. Water, either solid or
liquid, has been shown to be capable of having an influence on potential
effects of hazardous geological activity and eliciting responses from
the Earth’s interior. Studies of the post-glacial period, as well as of
the seismic effects of dam construction, have shown that the addition or
removal of sufficiently large amounts of water can significantly affect
the activity of the underlying faults and adjacent faults. When an ice
sheet is formed or a glacier is thickened the additional pressure will
stabilise the underlying faults, their activity being suppressed and the
earthquake incidence reduced along that fault. The opposite effect
results from unloading, the removal of the ice, the faults being freed
up and the facilitating of fault movement resulting in a seismic
response. The loading and unloading effects of water are comparable, though
with the added complication of pore pressure is increased in the rocks
adjacent to new or growing dams or lakes leading to destabilisation of
nearby faults.
Assuming present trends in climate change continue until temperature and
sea level rises are comparable to those of the post-glacial period it is
likely, according to the author1 that at least some of the
many faults around the Earth will respond to the changed redistribution
of global water that will no doubt occur as the world continues to get
warmer. Among the phenomena that have the potential to trigger a seismic
reaction in the underlying crust are melting high latitude ice sheets,
the wasting of glaciers in mountainous regions, sea levels that are
rising rapidly, dams built to store water to prepare for predicted water
shortages. The author suggests it is not certain that a rise of global
earthquake activity would be detectable, and any increase in the numbers
or frequency of earthquakes on a planet-wide scale will be hidden in the
statistical ‘noise’ associated with the 2 million earthquakes that
occur, on average, every year. The author1 suggests that on a
regional or local scale it may be more obvious when there is a response
from underlying faults, especially if and when the Greenland and West
Antarctica ice sheets begin to disintegrate. It is possible that the
crust underlying these vast ice sheets may be relatively stable with
few active faults capable of triggering an earthquake, though it is also
possibly that the ice may be the only stabilising influence on the
faults, in which case they may respond with a burst of seismic activity
if the ice sheets disintegrate. Even large earthquakes in Greenland or
West Antarctica may not be seen as much of a problem for populations on
other continents but their effects could be felt much further away. If
large earthquakes occur in a watery environment they can trigger
tsunamis if the crust is unstable enough to fail and generate a
landslide, and such tsunamis can travel large distances across the ocean
and cause great damage even thousands of kilometres from the site of the
landslide. As Scandinavia continued shedding its ice sheets in the early
part of the Holocene isostatic rebound promoted earthquakes that are
believed to have triggered massive submarine glacial sediment slides off
the Norwegian coast that generated tsunamis that travelled across the
northeast Atlantic to the Shetland Islands and mainland Scotland. If
such a burst of earthquake activity occurs in Greenland in the future it
could prove to be disastrous for coastal communities in Iceland and
Newfoundland.
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Climate Change |
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |