  We explore the well-posedness of the fractional version of Zener's wave
equation for viscoelastic solids, which is based on a constitutive law relating
the stress tensor $\boldsymbol{\sigma}$ to the strain tensor
$\boldsymbol\varepsilon(\bf u)$, with $\bf u$ being the displacement vector,
defined by: $(1+\tau D_t^\alpha) {\boldsymbol{\sigma}}=(1+\rho D_t^\alpha)[2\mu
{\boldsymbol\varepsilon}({\bf u})+\lambda\text{tr}(\boldsymbol\varepsilon(\bf
u)) \bf ]$. Here $\mu,\lambda\in\mathrm{L}^\infty(\Omega)$, $\mu$ is the shear
modulus bounded below by a positive constant, and $\lambda\geq 0$ is first
Lam\'e coefficient, $D_t^\alpha$, with $\alpha \in (0,1)$, is the Caputo
time-derivative, $\tau>0$ is the characteristic relaxation time and
$\rho\geq\tau$ is the characteristic retardation time. We show that, when
coupled with the equation of motion $\varrho \ddot{\bf u} =
\text{Div}{\boldsymbol\sigma} + \bf F$, considered in a bounded open Lipschitz
domain $\Omega$ in $\mathbb{R}^3$ and over a time interval $(0,T]$, where
$\varrho\in \mathrm{L}^\infty(\Omega)$ is the density of the material, bounded
below by a positive constant, and $\bf F$ is a specified load vector, the
resulting model is well-posed in the sense that the associated
initial-boundary-value problem, with initial conditions ${\bf u}(0,\mathbf{x})
= {\bf g}(\mathbf{x})$, $\dot{\bf u}(0,\mathbf{x}) = \bf h(\mathbf{x})$,
${\boldsymbol\sigma}(0,\mathbf{x}) = {\bf s}(\mathbf{x})$, for $\mathbf{x} \in
\Omega$, and a homogeneous Dirichlet boundary condition, possesses a unique
weak solution for any choice of ${\bf g }\in [\mathrm{H}^1_0(\Omega)]^3$, ${\bf
h}\in [\mathrm{L}^2(\Omega)]^3$, and ${\bf S} = {\bf S}^{\rm T} \in
[\mathrm{L}^2(\Omega)]^{3 \times 3}$, and any load vector ${\bf F}
\in\mathrm{L}^2(0,T;[\mathrm{L}^2(\Omega)]^3)$, and that this unique weak
solution depends continuously on the initial data and the load vector.
