Well you got a core file, which if you have a debugger, you can view the backtrace or see information about program variables and such at the time that the segmentation fault occurred. You can use GDB for this. You should have compiled your program with the -g flag, if you haven't, then re-compile with the -g flag, re-run and if/when you encounter the segmentation fault and get a core file you can then load that up using GDB.
gdb <executable-file> core
I haven't done this for FORTRAN but I'd imagine that you'd have a backtrace or some other capability to give you a report on what happened. If all else fails, create debug in various points of the code to document program progress so you can better zero in on where you hit the segfault. Or just run from within the debugger and when it hits the fault, you should be able to determine where in your program you were before hitting the fault.