Voronoi noise. You'll start with a displacement shader. Add a scalar to colour node into the blend by input. Connect the voronoi noise called 'cell' into the input of the scalar to colour node. The voronoi noise will need two inputs. Get position in geometry for the input and a constant scalar for the scale input. The seed value, just type a number up to 6 digits long.
The constant scalar controls the size/scale of the voronoi noise. The displacement shader controls the amount of displacement. The image you posted would need a negative value for the displacement. Try 500 for the constant scalar and -750 for the displacement value. These can be altered to fit your scene, but it's a start.
You'll need to add more shaders for the 'texturing' of the landscape. I would use distribution shaders and/or surface layers to control where the further displacements have effect.
