what triggers an earthquake?

Answer 1

Um ok, I've never hear the moon theory before. Perhaps it is a minor contributing factor, but I think that accepted theory says the it is due to shifting tectonic plates.

Most naturally occurring earthquakes are related to the tectonic nature of the Earth. Such earthquakes are called tectonic earthquakes. The Earth's lithosphere is a patchwork of plates in slow but constant motion caused by the release to space of the heat in the Earth's mantle and core. The heat causes the rock in the Earth to become flow on geological timescales, so that the plates move slowly but surely. Plate boundaries lock as the plates move past each other, creating frictional stress. When the frictional stress exceeds a critical value, called local strength, a sudden failure occurs. The boundary of tectonic plates along which failure occurs is called the fault plane. When the failure at the fault plane results in a violent displacement of the Earth's crust, the elastic strain energy is released and seismic waves are radiated, thus causing an earthquake.

Answer 2

Most earthquakes are caused by the sudden slip along geologic faults. The faults slip because of movement of the Earth’s tectonic plates. This concept is called the elastic rebound theory. The rocky tectonic plates move very slowly, floating on top of a weaker rocky layer. As the plates collide with each other or slide past each other, pressure builds up within the rocky crust. Earthquakes occur when pressure within the crust increases slowly over hundreds of years and finally exceeds the strength of the rocks. Earthquakes also occur when human activities, such as the filling of reservoirs, increase stress in the Earth’s crust.

Answer 3

The moon,
Googled moon earthquakes
to find
http://news.nationalgeographic.com/news/2005/05/0523_050523_moonquake.html

http://www.moonlightsys.com/themoon/phases.html
says
When the Moon is full, earthquakes, storms and floods increase in frequency

but a major dam could also (change in weight of earth's crust)
and
we expect quakes as iceland melts and all the glacier
drains into the ocean.