What are the Causes Earthquakes And its relationship with tsunami

The main cause of earthquakes is plate tectonics. the plate tectonics theory states that the Earth's lithosphere (the outermost part of the earth) is broken up into 8 main parts called plates and many minor ones. All those plates are carried upon a layer below them called the asthenosphere (the ductile part of the earth). According to the continental drift theory; the lithospheric plates move on top of the asthenosphere carrying continents and oceans on top of them due to deriving forces underneath in the asthenosphere called convection currents (like the ones you see when you're boiling some water). This plate motion is the primary cause of earthquakes, that's why the areas on Earth of highest earthquake activity are the places that lie on tectonic plate boundaries (the part between two or more adjacent tectonic plates). Depending on the relative type of motion between the plates a "Fault" ( breaking followed by displacement of ground layers) may occur. That break is the beginning of the earthquake and sometimes also leads to volcanic activities.

Tsunamis occur when two plates that are carrying oceanic crusts on top of them keep moving against each other for such a long time and pressure keeps building up between them with no pressure release in the form of an earthquake. This continues until the ground can't take more pressure so it rebounds ( just like when you apply pressure to a plastic ruler by bending it, it will break at a certain point and rebound) causing a great earthquake and enormous sea waves as a result.

you can also check these out:

http://en.wikipedia.org/wiki/Structure_of_the_Earth
http://en.wikipedia.org/wiki/Earthquake

I hope this helped :) .

Earthquakes are caused from underground shifting or volcano activity. And a tsunami happens when those things happen under water.