The great earthquake of Revelation, one of three that is predicted to occur during the Tribulation, is a powerful, worldwide disaster that impacts the entire globe simultaneously.  It's power is believed to potentially exceed 12.0 on the Richter scale, or perhaps even higher, and have the power to level any and all standing structures.  Even the most sound and secure will not remain untouched.  The exact extent of the damage and the death toll is unknown.  However, it is believed to possibly kill as much as 1/4th of the entire world's population and render everyone homeless.  It may also cause massive tsunamis that wipe entire coastlines clean of all civilization while at the same time sinking ships by the thousands.

The exact answer as to what causes this quake is unknown, only that its effects will be felt worldwide and they will be massive.  Some of the resulting consequences of this quake include the sun being darkened (turned black, or heavily shaded) and the moon appearing deep red (blood moon).  There is also the potential during this event for a considerable number of large asteroids to fall from the sky.  There will also occur a rather unusual event in the sky where the heavens will appear to "depart as a scroll" or "roll up".  It's uncertain what this is referring to other than, as one commentator has stated, perhaps the gates of Heaven appearing in the sky or opening briefly for all to see.

It's uncertain why this would need to occur, other than to possibly give further incentive to those still on the Earth to choose God over Satan.  IE, it would be as if God were saying, "Yes, I'm real, and so is Heaven, and if you want to live here, you'd better make the right choice.  Sadly, given the other verses which speak of how they will instead shake their fists at God and further defy Him, it's unlikely that this expanded effort to reach the lost will net many converts.  However, it's also possible that it may result in a great soul harvest.  Another side effect of this quake, and the sheer intensity of it, the Apostle John tells us that it will cause every mountain and island to be moved.  One commentator believes this means they will literally be shifted off their foundations.