A total of 333 aftershocks have been registered through Thursday morning, since a massive 6-degree earthquake hit western Bulgaria in the wee hours Tuesday.

This number includes all so-called micro-quakes that cannot be directly felt by humans, said Emil Botev, researcher from the Geophysical Institute at the Bulgarian Academy of Sciences.

In the area of the city of Pernik, the most hard hit region, people have felt some 31 shakes total.

This includes tremors of magnitude larger than 2-2.5 degrees on the Richter scale, which can be directly felt by sensitive people, explained Botev.

A total of 4 aftershocks after the initial shake-up Tuesday have been around or exceeded magnitude 4, including an aftershock early Thursday.

"In comparison to other seismic areas, in this case the strong quakes were few and we hope that they will decrease and aftershocks will become ever more feeble," said the geophysicist.

Botev added that the seismology unit of the Geophysical Institute will continue to register microtremors perhaps over the period of a month ahead, but said he hopes no significant shakes are in store as aftershocks of the initial earthquake.