Your application calls serialized versions of math functions when you use the strict floating point model. To fix: Do one of the following:

Use the fast floating point model to enable more aggressive optimizations or the precise floating point model to disable optimizations that are not value-safe on fast transcendental functions.
Windows* OS Linux* OS
/fp:fast -fp-model fast
/fp:precise /Qfast-transcendentals -fp-model precise -fast-transcendentals
CAUTION: This may reduce floating point accuracy.
Use the precise floating point model and enforce vectorization of the source loop using a directive: !$OMP SIMD

Windows* OS	Linux* OS
/fp:fast	-fp-model fast
/fp:precise /Qfast-transcendentals	-fp-model precise -fast-transcendentals

Example

gfortran program.for -O2 -fopenmp -fp-model precise -fast-transcendentals

!DIR$ OMP SIMD COLLAPSE(2)
do i = 1, N
...

gfortran program.for -O2 -fopenmp -fp-model precise -fast-transcendentals

!DIR$ OMP SIMD COLLAPSE(2)
do i = 1, N
    a(i) = b(i) * c(i)
    do j = 1, N
        d(j) = e(j) * f(j)
    enddo
enddo