I’ll answer you from your last demand, to understand the mathematics behind the InSAR, please have a look at these literature from ESA,
Creating Single Interferogram
Also please have a look and you could also use this Processing Steps of Creating Single Interferogram In case you are applying the processes through out gpt
Coming back to the interferogram is posted,
High magnitude earthquake, would certainly develops fringes in the whole effected area, but still there are many problems prevents these fact to be seen, the land cover of the study area, in case of highly vegetated area, and few buildings were collapsed as example of this case, so the higher resolution image more than S-1 could gives these precise details, however in this case assumingly not. Moreover to the aforementioned reason, the Atmospheric effects, are not entirely removed from the interferogram, and this also other reason to the mess-interferogram, please have a look at this processing results of single interferogram of the high magnitude results of the earthquake,
Earthquake Modeling using InSAR
Concerning the Reference point
Some conditions of selecting the reference point,
1- Should be stable (Type of geological materials is matter, the type of object is selected as rp)
2- Point should be high coherent point,
3- It’s better to select the point out of the range of the displacement area of some kilometers,
The elevation has no effect to the selection of RP, because through out he processing the elevation phase should be removed.