I read in forbes :
Earth's rotation, as with many things, is cyclical, slowing down by a few milliseconds per day then speeding up again. We will never notice this very slight variation in the rotational speed of Earth. However, we will certainly notice the result, an increase in the number of severe earthquakes. Scientists believe a slowdown of the Earth's rotation is the link to an observed cyclical increase in earthquakes.
While scientists aren't exactly sure about the mechanisms that produce this variation, there are a few hypotheses.
One hypothesis involves Earth's outer core, a liquid metal layer of the planet that circulates underneath the solid lower mantle. The thought is that the outer core can at times "stick" to the mantle, causing a disruption in its flow. This would alter Earth's magnetic field and produce a temporary hiccup in Earth's rotation.
Currently, the data only notes a striking correlation, but no causation. Hence, scientists are still unsure whether this change in Earth's rotation is the cause of an uptick in earthquakes.
“Each day provides its own gifts.” —Marcus Aurelius