Why does the earthquake depth change?

Is earthquake depth at a specific location variable/subject to change?

• I realize certain edges of a tectonic plates (fault lines) are classified as shallow, intermediate, and deep. What I want to know is much more specific. For example, in Japan on January 17th (2012) there was an earthquake in Ibaraki Prefecture that was recorded as being at a depth of 50 km. Is it possible that another earthquake could occur at that epicenter (more or less) at a different depth, or is that particular spot restricted to earthquakes at or around 50 km beneath the surface? Could one occur at a shallower depth (such as 20 km, or 10 km)? If it is possible to have a different depth, but not probable, then I'd like to hear that too.

• Answer:

  The deepest epicenters are 725 to 750 km down. Yes, depths of epicenters ARE variable because the stresses/forces that cause earthquakes vary over time . I don't think there are more specific designations than shallow, intermediate and deep. The specificity is when the depth of the epicenter of an earthquake is stated. Yes, it is possible to have different depths for epicenters even withing the same general area. Earthquakes are caused by rocks and minerals fracturing because the stresses on the minerals exceed stress capacity and strain limits of the minerals in the rocks. Once a group of minerls starts to gracture, the fracture just propagate to relieve the bult up stresses.