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The
Earth’s Structure |
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The Earth’s interior is composed of three major layers, similar
to an egg. The Earth’s shell is called the crust, which is made up
of more than a dozen major tectonic plates. The whites are called the
mantle, which is composed of solid rock. The yolk is called the core,
and is composed of liquid iron and other materials. |
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Inter-plate earthquakes |
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Although the mantle is solid rock, it slowly flows in a convection current
(1) because of heat dissipating from the core. This causes the plates
on the mantle to move several centimeters a year.
When plates move, they collide with each other or grind past each other
at plate boundaries, creating stress (strain energy). When sufficient
strain energy is stored, the plates try to return to their original position,
causing an earthquake. This is called an interplate earthquake. Interplate
earthquakes can reach magnitudes of over 8, and cause tsunamis and other
extensive damage over wide areas.
The Pacific Plate is subducting under the Okhotsk Plate at the bottom
of the sea east of Sendai. The Miyagi-oki Earthquake occurs when the Okhotsk
Plate, which is dragged down by the Pacific Plate, rebounds to its original
position. |
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Convection is a phenomenon that occurs when part
of a fluid is warmed. The warmed fluid expands, becomes less dense,
and rises upwards. Cold and denser parts of the fluid sink and fill
the space previously occupied by the warmed fluid. |
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Figure 1. Inter-plate Earthquakes |
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Plate A subducts under Plate B, dragging
Plate B with itself. |
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Plate B tries to return to its original
position, causing an earthquake. |
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Intra-plate Earthquakes |
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Strain energy caused by plate movement is also stored within the plates.
When weak parts of the bedrock cannot resist the energy, they slip, causing
intra-plate earthquakes.
The discontinuities caused by intra-plate earthquakes are called faults.
Faults that have caused earthquakes within the last 1.8 million years
are called active faults. Active faults are like injuries that haven’t
completely healed yet. They’re weaker than other parts of the ground
and frequently cause earthquakes.
Intra-plate earthquakes do not cause tsunamis, and their magnitudes rarely
exceed 7. However, they occur at relatively shallow depths, within 20km
from the ground. Therefore, intra-plate earthquakes can also cause large
damage.
There are 6 known active faults in the Sendai area, the Nagamachi-Rifu
Fault Zone, the Dainenjiyama Fault, the Shishiochizaka Fault, the Tsubonuma
Fault, the Ayashi Fault, and the Sakunami-Yashikidaira Fault. |
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Magnitudes and Seismic
Intensity |
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The magnitude of an earthquake is the measurement of an earthquake’s
energy. When the magnitude of an earthquake increases by one step, its
energy increases by a factor of 32. If the magnitude increases by two
steps, the energy increase is a factor of 32 times 32 – approximately
1,000.
Seismic intensity refers to the strength of shaking experienced in
a particular location. The seismic intensity of an earthquake can vary
according to location and geological conditions. The Japan Meteorological
Agency Seismic Intensity Scale is used in Japan. The scale runs from
0 to 7 and has 10 different measurements because intensities of 5 and
6 are divided in to “weak” and “strong” intensities.
Seismic intensity used to be measured by human perception and damage
to buildings, but it is now quickly measured by mechanical seismic scales.
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