The southeastern
United States has been hit by a series of strange unexplained quakes
- most recently, the 2011 magnitude-5.8 earthquake near Mineral,
Virginia that shook the nation's capital.
Researchers have
been baffled, believing the areas should be relatively quiet in terms
of seismic activity, as it is located in the interior of the North
American Plate, far away from plate boundaries where earthquakes
usually occur.
Now, they believe
the quakes could be caused by pieces of the Earth's mantle breaking
off and sinking into the planet.
A new study found
pieces of the mantle under this region have been periodically
breaking off and sinking down into the Earth.
This thins and
weakens the remaining plate, making it more prone to slipping that
causes earthquakes.
The study authors
conclude this process is ongoing and likely to produce more
earthquakes in the future.
'Our idea supports
the view that this seismicity will continue due to unbalanced
stresses in the plate,' said Berk Biryol, a seismologist at the
University of North Carolina at Chapel Hill and lead author of the
new study.
'The [seismic] zones
that are active will continue to be active for some time.'
The study was
published in the Journal of Geophysical Research – Solid Earth, a
journal of the American Geophysical Union.
'Based on distinct
variations in the geometry and thickness of the lithospheric mantle
and foundered lithosphere, we propose that piecemeal delamination has
occurred beneath the region throughout the Cenozoic, removing a
significant amount of reworked/deformed mantle lithosphere,' the team
wrote.
Compared to
earthquakes near plate boundaries, earthquakes in the middle of
plates are not well understood and the hazards they pose are
difficult to quantify.
The new findings
could help scientists better understand the dangers these earthquakes
present.
Today, the
southeastern U.S. is more than 1,056 miles from the nearest edge of
the North American Plate, which covers all of North America,
Greenland and parts of the Atlantic and Arctic oceans.
But the region was
built over the past billion years by periods of accretion, when new
material is added to a plate, and rifting, when plates split apart.
Researchers
initially suspected ancient fault lines or pieces of old plates
extending deep in the mantle following episodes of accretion and
rifting could be responsible for earthquakes in the area.
'This region has not
been active for a long time,' Biryol said.
'We were intrigued
by what was going on and how we can link these activities to
structures in deeper parts of the Earth.'
'This was an
interesting finding because everybody thought that this is a stable
region, and we would expect regular plate thickness,' Biryol said.
At first, they
thought the thick, old rocks could be remnants of ancient tectonic
plates.
Shaking from the
magnitude 5.8 earthquake near Mineral, Virginia on August 23, 2011
was felt by more people than any other earthquake in U.S. history,
according to the U.S. Geological Survey. Researchers now believe the
quakes could be caused by pieces of the Earth's mantle breaking off
and sinking into the planet.
Shaking from the
magnitude 5.8 earthquake near Mineral, Virginia on August 23, 2011
was felt by more people than any other earthquake in U.S. history,
according to the U.S. Geological Survey. Researchers now believe the
quakes could be caused by pieces of the Earth's mantle breaking off
and sinking into the planet.
But the shapes and
locations of the thick and thin regions suggested a different
explanation: through past rifting and accretion, areas of the North
American Plate have become more dense and were pulled downward into
the mantle through gravity.
At certain times,
the densest parts broke off from the plate and sank into the warm
asthenosphere below.
The asthenosphere,
being lighter and more buoyant, surged in to fill the void created by
the missing pieces of mantle, eventually cooling to become the thin,
young rock in the images.
Volcanoes were once
active in the southeastern. U.S. Mole Hill, pictured here, is a mound
of volcanic rock in the Shenandoah Valley in Virginia that formed
from an active volcano 48 million years ago (a relatively recent
event, in geological time scales).
Volcanoes were once
active in the southeastern. U.S. Mole Hill, pictured here, is a mound
of volcanic rock in the Shenandoah Valley in Virginia that formed
from an active volcano 48 million years ago (a relatively recent
event, in geological time scales).
WHAT THE MAP SHOWS
This map shows the
study area in detail, showing various tectonic attributes of the
Southeastern United States.
BMA =
BrunswickMagnetic Anomaly, BRE = Blue Ridge Escarpment,
CFA = Cape Fear
Arch, OS = Orangeburg Scarp
ETSZ =
EasternTennessee Seismic Zone
NMSZ = New Madrid
Seismic Zone
GCSZ = Giles County
Seismic Zone,
SCSZ = South
Carolina Seismic Zone
CVSZ = Central
Virginia Seismic Zone
V1 = Eocene Virginia
Volcanics,
V2 = Jackson,
Mississippi Volcanism,
V3 = Offshore
Louisiana Volcanism.
Seismicity is from
the Virginia Tech Seismological Observatory catalog (1900–2014).
A new study found
pieces of the mantle under this region have been periodically
breaking off and sinking down into the Earth.
This thins and
weakens the remaining plate, making it more prone to slipping that
causes earthquakes.
The study authors
conclude this process is ongoing and likely to produce more
earthquakes in the future.
'Our idea supports
the view that this seismicity will continue due to unbalanced
stresses in the plate,' said Berk Biryol, a seismologist at the
University of North Carolina at Chapel Hill and lead author of the
new study.
'The [seismic] zones
that are active will continue to be active for some time.'
The study was
published in the Journal of Geophysical Research – Solid Earth, a
journal of the American Geophysical Union.
'Based on distinct
variations in the geometry and thickness of the lithospheric mantle
and foundered lithosphere, we propose that piecemeal delamination has
occurred beneath the region throughout the Cenozoic, removing a
significant amount of reworked/deformed mantle lithosphere,' the team
wrote.
Compared to
earthquakes near plate boundaries, earthquakes in the middle of
plates are not well understood and the hazards they pose are
difficult to quantify.
The new findings
could help scientists better understand the dangers these earthquakes
present.
Today, the
southeastern U.S. is more than 1,056 miles from the nearest edge of
the North American Plate, which covers all of North America,
Greenland and parts of the Atlantic and Arctic oceans.
But the region was
built over the past billion years by periods of accretion, when new
material is added to a plate, and rifting, when plates split apart.
Researchers
initially suspected ancient fault lines or pieces of old plates
extending deep in the mantle following episodes of accretion and
rifting could be responsible for earthquakes in the area.
'This region has not
been active for a long time,' Biryol said.-
WHAT ARE TECTONIC
PLATES?
Tectonic plates are
composed of Earth's crust and the uppermost portion of the mantle.
Below is the
asthenosphere: the warm, viscous conveyor belt of rock on which
tectonic plates ride.
Earthquakes
typically occur at the boundaries of tectonic plates, where one plate
dips below another, thrusts another upward, or where plate edges
scrape alongside each other.
The world's tectonic
plates: Earthquakes typically occur at the boundaries of tectonic
plates, where one plate dips below another, thrusts another upward,
or where plate edges scrape alongside each other.
The world's tectonic
plates: Earthquakes typically occur at the boundaries of tectonic
plates, where one plate dips below another, thrusts another upward,
or where plate edges scrape alongside each other.
Earthquakes rarely
occur in the middle of plates, but they can happen when ancient
faults or rifts far below the surface reactivate.
These areas are
relatively weak compared to the surrounding plate, and can easily
slip and cause an earthquake.
'We were intrigued
by what was going on and how we can link these activities to
structures in deeper parts of the Earth.'
'This was an
interesting finding because everybody thought that this is a stable
region, and we would expect regular plate thickness,' Biryol said.
At first, they
thought the thick, old rocks could be remnants of ancient tectonic
plates.
Shaking from the
magnitude 5.8 earthquake near Mineral, Virginia on August 23, 2011
was felt by more people than any other earthquake in U.S. history,
according to the U.S. Geological Survey. Researchers now believe the
quakes could be caused by pieces of the Earth's mantle breaking off
and sinking into the planet.
Shaking from the
magnitude 5.8 earthquake near Mineral, Virginia on August 23, 2011
was felt by more people than any other earthquake in U.S. history,
according to the U.S. Geological Survey. Researchers now believe the
quakes could be caused by pieces of the Earth's mantle breaking off
and sinking into the planet.
But the shapes and
locations of the thick and thin regions suggested a different
explanation: through past rifting and accretion, areas of the North
American Plate have become more dense and were pulled downward into
the mantle through gravity.
At certain times,
the densest parts broke off from the plate and sank into the warm
asthenosphere below.
The asthenosphere,
being lighter and more buoyant, surged in to fill the void created by
the missing pieces of mantle, eventually cooling to become the thin,
young rock in the images.
Volcanoes were once
active in the southeastern. U.S. Mole Hill, pictured here, is a mound
of volcanic rock in the Shenandoah Valley in Virginia that formed
from an active volcano 48 million years ago (a relatively recent
event, in geological time scales).
Volcanoes were once
active in the southeastern. U.S. Mole Hill, pictured here, is a mound
of volcanic rock in the Shenandoah Valley in Virginia that formed
from an active volcano 48 million years ago (a relatively recent
event, in geological time scales).
The researchers
concluded this process is likely what causes earthquakes in this
otherwise stable region: when the pieces of the mantle break off, the
plate above them becomes thinner and more prone to slip along ancient
fault lines.
Typically, the
thicker the plate, the stronger it is, and the less likely to produce
earthquakes.
According to Biryol,
pieces of the mantle have most likely been breaking off from
underneath the plate since at least 65 million years ago.
Because the
researchers found fragments of hard rocks at shallow depths, this
process is still ongoing and likely to continue into the future,
potentially leading to more earthquakes in the region, he said.
Read more:
http://www.dailymail.co.uk/sciencetech/article-3573759/Giant-chunks-Earth-s-mantle-FALLING-causing-quakes-southeastern-coming-warn-researchers.html#ixzz495NcaEeg
