For Mr Red
Earthquakes are caused by stress release in rocks strained beyond their failure envelope. The causes of stress are mainly attributed to the dynamics of plate tectonics over periods of time. What actually triggers the failure is a matter of both study and debate, Tidal events, storm events, flooding, water injection etc. etc can all give a shock event to bring about failure. These events however only contribute to a premature failure that would eventually occur anyway.
This is because regional stress is in a stasi equilibrium and when failure occurs the entire spectrum of stress may be modified over a long/short timescale and area and cause additional failures for this reason.
This is the likely explanation of aftershocks.
Quantification of degree is possible(post event) but prediction is not yet feasible. There is a school that feels annimals can predict earthquakes and modify their behaviour. This does not have general acceptance, although annimals senses have mostly a greater range than humans so may be able to sense factors we are unaware of.

It is a fascinating field to study and prediction would be a tool all would wish for in order to reduce fatalities.



http://all-geo.org/highlyallochthonous/2011/05/the-many-faces-of-earthquake-triggering/

http://petrowiki.org/Stress_strain_relationships_in_rocks