From 1973 to 1996, earthquake and eruption frequencies were almost stable, increasing only slightly year after year, but from 1996 onwards, an acceleration is noticeable. Volcanic eruptions show an increase from about 59 eruptions per year at the end of the 1990s to roughly 75 eruptions per year in the period 2007 - 2010 (+30%).

Today, the increase in volcanic activity has reached such a level that, by late November 2013, 35 volcanoes were actively erupting , including volcanoes that had been dormant for decades.

It could be argued that the increase in both the frequency and intensity of earthquakes and volcanic eruptions is, at least partly, a result of the slowdown and 'opening up' processes:

1) The Earth's minute slowdown exerts mechanical stress on the crust (compression at low latitudes and extension at high latitude). This stress deforms the crust. This deformation is more pronounced and can even lead to partial ruptures around the weakest spots of the crust, i.e. the fault lines (boundaries between tectonic plates) which are the typical location of seismic and volcanic activity.

2) The mantle has a higher density than the crust and therefore has a higher momentum and won't slow down as fast as the crust. The difference in rotation between the crust and the mantle is equal to the crustal slippage. The fluidity of the mantle enables slippage induced by the different momentum carried by the crust, the upper mantle and the core.

This speed difference can cause friction at the interface between the crust and the mantle. This friction can locally deform the crust and cause earthquakes and eruptions.

3) The decrease in the surface - core E-field reduces the binding force and loosens the tectonic plates relative to each other. The plates are then free to move relative to each other. It is this very relative movement (divergence, convergence or sliding) which is one of the main causes for earthquakes and volcanic eruptions:

[Change] in Earth's speed of rotation would induce changes in the magma tide as it adjusted to the new equator or altered rotational speed. Such changes, however, might not be uniform throughout, owing to a 'drag' factor deep in the magma itself, although, overall, they would certainly impose terrible strains on the lithosphere generally.

4) A final factor involved in earthquakes and volcanic eruptions is electromagnetism:

Some scientists have become aware of a correlation between sunspots and earthquakes and want to use sunspot data to help predict earthquakes. The theory is that an intensification of the magnetic field can cause changes in the geosphere [i.e. crust]. NASA and the European Geosciences Union have already put their stamp of approval on the sunspot hypothesis, which suggests that certain changes in the Sun-Earth environment affect the magnetic field of the Earth, which can then trigger earthquakes in areas prone to them. It is not clear how such a trigger might work.