Kernels of trace operators via fine continuity

We study traces of elements of fractional Sobolev spaces $H_p^α(\mathbb{R}^n)$ on closed subsets $Γ$ of $\mathbb{R}^n$, given as the supports of suitable measures $μ$. We prove that if these measures satisfy localized upper density conditions, then quasi continuous representatives vanish quasi everywhere on $Γ$ if and only if they vanish $μ$-almost everywhere on $Γ$. We use this result to characterize the kernel of the trace operator mapping from $H_p^α(\mathbb{R}^n)$ into the space of $μ$-equivalence classes of functions on $Γ$ as the closure of $C_c^\infty(\mathbb{R}^n\setminus Γ)$ in $H_p^α(\mathbb{R}^n)$. The measures do not have to satisfy a doubling condition. In particular, the set $Γ$ may be a finite union of closed sets having different Hausdorff dimensions. We provide corresponding results for fractional Sobolev spaces $H_p^α(Ω)$ on domains $Ω\subset \mathbb{R}^n$ satisfying the measure density condition.