Why are sedimentary rocks primarily deposited flat?

Pressure form the water depth makes the sediment compact.

Explanation:

Oceanic crust is more dense than continental crust, and that is due to the compact sediment.

true. earthquakes are felt whenever there is a slip around a fault in the rocks below the earth. this reorientation of faults create sesmic waves and is known as earthquake, the severity of which is measured in richter scale. these are more prominent in hilly regions as the rocks usually form faults to balance the pressures on it.