Here's the whole scenerio of the sixth seal:
"12. I watched as he opened the sixth seal. There was a great earthquake. The sun turned black like sackcloth made of goat hair, the whole moon turned blood red,
13 and the stars in the sky fell to earth, as late figs drop from a fig tree when shaken by a strong wind.
14 The sky receded like a scroll, rolling up, and every mountain and island was removed from its place."REV 6:12-14
Sounds like a volcanic eruption of incredible scale. The earthquake would be caused by the enourmous amount of energy released from the eruption, The sun turning black and the moon turning red would be caused be the amount of ash thrown into the atmosphere,enough to block out the sun, and as it is blown around the earth by atmoshperic winds, those on the night side could see the moon actually turn red, like when there is alot of dust in the upper atmosphere. Sunrises and sunsets are red due to solid particles in the atmosphere,like dust or pollutants, so volcanic ash could certainly cause the same effect. And the stars falling could be pieces of molten rock thrown up into the air and falling back to Earth. The sky rolling back is puzzling. Every mountain and island being moved from its place I think is the product of the huge earthquake.
If the earthquake is not volcanic, and purely techtonic, than the mention of exteme famine could indicate a time without rain, which could result in all that dry dust being cast into the air, having the same effect as the volcanic ash.
Another possibility and the most likely to me after doing a little research, is that the moon and sun changing color and the earthquake are two totally unrelated yet simultaneous events. The earthquake could occur durring a total lunar eclipse, which would turn the moon red, and the sun black at the same time. We had one this year, and the moon did take on a reddish hue. Here's an excerpt from a webpage desribing different types of lunar eclipses:
"During a total lunar eclipse, the Earth blocks the Sun's light from reaching the Moon. Astronauts on the Moon would then see the Earth eclipsing the Sun. (They would see a bright red ring around the Earth as they watched all the sunrises and sunsets happening simultaneousely around the world!) While the Moon remains completely within Earth's umbral shadow, indirect sunlight still manages to reach and illuminate it. However, this sunlight must first pass deep through the Earth's atmosphere which filters out most of the blue colored light. The remaining light is a deep red or orange in color and is much dimmer than pure white sunlight. Earth's atmosphere also bends or refracts some of this light so that a small fraction of it can reach and illuminate the Moon."-http://www.mreclipse.com/Special/LEprimer.html