Question

Why did the time period of rotation of Earth decrease by microseconds after the Japan 2011 earthquake?

• A. The oceanic floor and land disturbances caused the effect of rate of rotation of Earth.
• B. Redistribution of mass caused MOI to increase.
• C. Redistribution of mass caused MOI to decrease.
• D. The kinetic energy of Earth increased due to earthquakes.

Answer 1

The Correct Option is B

The time period of rotation of Earth did not decrease after the Japan 2011 earthquake. In fact, the earthquake caused a slight increase in the Earth's rotation period, resulting in a shortening of the day.

The calculations also show the Japan quake should have shifted the position of Earth's figure axis (the axis about which Earth's mass is balanced) by about 17 centimeters 
During large-scale earthquakes, the distribution of mass on Earth can change. The Japan 2011 earthquake, also known as the Great East Japan Earthquake, was a massive undersea earthquake that occurred off the northeastern coast of Japan on March 11, 2011. This earthquake had a significant impact on the Earth's rotation due to the redistribution of mass caused by the shifting of tectonic plates.
The earthquake caused the Pacific tectonic plate to move and resulted in a redistribution of mass, primarily by shifting a significant amount of Earth's crust from the ocean floor to land. This shift of mass closer to the Earth's axis of rotation caused a slight increase in the planet's moment of inertia. To be more precise, the earthquake's effect on Earth's rotation was estimated to have shortened the day by approximately 1.8 microseconds (1.8 millionths of a second) by redistributing the Earth's mass closer to the axis. This change is relatively small and would not be perceptible to human experience.

Answer 2

The time period of rotation of Earth did not decrease after the Japan 2011 earthquake. Instead, the redistribution of mass caused the moment of inertia (MOI) to increase. Here's an expanded and unique explanation:

 Explanation:

Moment of Inertia and Earth's Rotation
The Earth's rotation period is influenced by its moment of inertia (MOI), which is a measure of how mass is distributed relative to the axis of rotation. The formula for the rotational period \(T\) of the Earth is given by:

\[ T = 2\pi \sqrt{\frac{I}{L}} \]

where \(I\) is the moment of inertia and \(L\) is the angular momentum. Since \(L\) remains nearly constant for the Earth, any change in \(I\) affects \(T\).

Impact of the 2011 Japan Earthquake
The 2011 earthquake in Japan, also known as the Tōhoku earthquake, had a significant impact on the distribution of Earth's mass. This seismic event caused the Earth's crust to deform, redistributing the mass closer to the Earth's axis of rotation.

 Redistribution of Mass
- Crustal Deformation**: The earthquake resulted in large-scale deformation of the Earth's crust, including subsidence and uplift of land masses. This redistribution of mass caused a slight change in the Earth's shape.
- Subduction Zone: The earthquake occurred in a subduction zone where the Pacific Plate is being forced under the North American Plate. This movement altered the distribution of mass in the Earth's crust and mantle.

 Increase in Moment of Inertia
As mass moved closer to the Earth's axis, the moment of inertia increased. The moment of inertia \(I\) for a rotating body is calculated as:

\[ I = \sum m_i r_i^2 \]

where \(m_i\) is the mass of a particle and \(r_i\) is the distance of the particle from the axis of rotation. By bringing mass closer to the axis, \(r_i\) decreases, but the overall \(I\) can still increase depending on the specific redistribution of mass.

 Conclusion:
The increase in the Earth's moment of inertia caused by the redistribution of mass during the 2011 Japan earthquake led to a change in the rotational period. Although the change is extremely small, it illustrates the sensitivity of the Earth's rotational dynamics to internal and surface mass redistributions.

Therefore, the correct interpretation is that the redistribution of mass caused the moment of inertia to increase, which in turn affected the Earth's rotation period.