You see for four and a half billion years, Earths entire lifetime, our rotation has been gradually slowing. The speed of average day has grown longer by (between 15 millionths and 25 millionths of a second every year) and you probably think that would not be large enough to have an effect, and you’d be wrong. What we have is a constant struggle between the planet's gravity and its centrifugal force as its angular motion slows. Due to the loss in the Earth Kinetic Energy from all forms of forces acting on it from the tides, or galactic space dust, include the solar wind, also the space weather, the geomagnetic storms, all working to slow it down.
This slow down permits gravity to pull the planet's shape into a more perfect circle or sphere. Like a daisy chain of one thing causing another, tremendous stresses within the Earth and in its crust struggle to fit on the slowly changing mantle. Were the Earth's rotation not slowing, there would be none of this stress, and the Earth would maintain its girlish curves throughout time. Its rotation is gradually slowing however, (even taking the conservation of momentum into consideration), causing its angular momentum to become progressively weaker. Gravity remains just as strong as ever and continues to exert the same amount of inward pressure.
This continuous struggle between gravity's centripetal force and angular momentum's centrifugal force results in tremendous dynamic stress within the Earth as it attempts to adjust to its trimmer shape. The Earth finds itself gradually shifting its shape from that of an oblate spheroid with a bulging equator & flatter pole regions to that of a more perfect sphere. Throughout the ages, since day one, the Earth has been going through this constant gradual change.
The surface of the Earth's mantle, especially in the tropical and subtropical regions of the Earth is becoming smaller while the pole regions have uplifted and become rounder. The fact that the Earth is getting rounder, even today, has been confirmed by the satellite "Lageos" which is being monitored by scientists at the University of Texas Center for Space Research and at the Jet Propulsion Laboratory.
What it comes down to is an imbalance exists between the angular momentum and gravity that cause the subduction motions of the continental crustal plates due to their crusts shifts and crunches as they continually try to fit what has become a diminishing area of mantle. There are hundreds of GPS sensors along the Pacific Coast which indicate North America is moving westward at about 3 inches per year and that the Pacific plate is subducting under the continent's Pacific coast. Then the subducting ocean plate is replaced by the central Pacific ridge of mantle oozing up to form a new ocean bottom.
And guess what, it is this very activity, this relentless, extremely slow, tremendously powerful, shrinking equatorial region that then cause earthquakes to occur, volcanoes to erupt and the Earth's vast mountain ranges to rise.
Dr. Roger Bilham from the University of Colorado has said that the intense correlation between the changing rotation and the earthquake and volcanic activity suggest there is going to be an increase in numbers of intense earthquakes next year. During these periods the Earth's mantle sticks a little more to the crust, which changes how outer core flows, creating a mismatch between the speed of the solid crust and the mantle.
Experts warn we 'had it easy this year' with just six severe earthquakes and claim that next year we could have at least 20 serious earthquakes, and the most intense ones are expected to occur in tropical regions, home to around one billion people and plenty of vacationing resorts.