What are the reason fore exist stress?

We know that the Earth's under crust forces change the form of the crust's material, but we do not know where they happen and what is the reason of their existence.

 The explication of the most earthquake in the last years falls under the group of plate tectonics. While two plates move in opposite direction at their boundaries they put on forces on each other. The strength of reaction motive chemical and physical changes at their boundaries. The movements of plates are up and down, side to side, and in opposite direction.  Earthquakes happen in area that old plates are being carried and new  plates are being made into the Earth's deep parts.

 Interplate earthquake are named because of the interaction of plates. These earthquakes are happened because of the native systems of forces, like changes in temperature or lack of power below the earth's crust. Majority of them are because of the movement of pre-existing faults.