When working with SI units in earthquake scenarios numbers might get very large. Rescaling the equations might be advantageous, e.g. when working with single precision arithmetic. In this section, we show how to properly scale the elastic wave equation.

The elastic wave equation in velocity-stress form with source terms \(s_i\) and \(f_i\) is given by

\[\begin{split}\begin{aligned} \frac{\partial\sigma_{ij}}{\partial t} - \lambda \delta_{ij}\frac{\partial u_k}{\partial x_k} - \mu \left( \frac{\partial u_i}{\partial x_j} + \frac{\partial u_j}{\partial x_i} \right) &= s_i \\ \rho\frac{\partial u_i}{\partial t} - \frac{\partial \sigma_{ij}}{\partial x_j} &= f_i \end{aligned}\end{split}\]

We define scaled quantities

\[\bar{x}_i = \frac{x_i}{L}, \quad \bar{u}_i = \frac{u_i}{u_c}, \quad \bar{\sigma}_{ij} = \frac{\sigma_{ij}}{\sigma_c},\]

where \(L, u_c, \sigma_c\) are scaling constants. Inserting these into the elastic wave equation gives

\[\begin{split}\begin{aligned} \sigma_c\frac{\partial \bar{\sigma}_{ij}}{\partial t} - u_cL^{-1} \lambda \delta_{ij}\frac{\partial \bar{u}_k}{\partial\bar{x}_k} - u_c L^{-1} \mu \left( \frac{\partial \bar{u}_i}{\partial\bar{x}_j} + \frac{\partial \bar{u}_j}{\partial\bar{x}_i}\right) &= s_i \\ \rho u_c\frac{\partial \bar{u}_i}{\partial t} - L^{-1}\sigma_c \frac{\partial \bar{\sigma}_{ij}}{\partial \bar{x}_j} &= f_i \end{aligned}\end{split}\]

Multiplying the first equation with \(\sigma_c^{-1}\), multiplying the second equation with \(L\sigma_c^{-1}\), and defining

\[\begin{split}\bar{\rho} = L\sigma_c^{-1} u_c\rho, \quad \bar{\lambda} = \sigma_c^{-1}u_cL^{-1}\lambda, \quad \bar{\mu} = \sigma_c^{-1}u_cL^{-1}\mu, \\ \bar{f}_i = L\sigma_c^{-1} f_i, \quad \bar{s}_i = \sigma_c^{-1} s_i\end{split}\]

leads to

\[\begin{split}\begin{aligned} \frac{\partial \bar{\sigma}_{ij}}{\partial t} - \bar{\lambda} \delta_{ij}\frac{\partial \bar{u}_k}{\partial\bar{x}_k } - \bar{\mu} \left( \frac{\partial \bar{u}_i}{\partial\bar{x}_j} + \frac{\partial \bar{u}_j}{\partial\bar{x}_i}\right) &= \bar{s}_i \\ \bar{\rho}\frac{\partial \bar{u}_i}{\partial t } - \frac{\partial \bar{\sigma}_{ij}}{\partial \bar{x}_j } &= \bar{f}_i\end{aligned}\end{split}\]


We change units with the scaling constants

\[L = 10^3, \quad u_c = 1, \quad \sigma_c = 10^6\]

In the rescaled equations, the spatial dimension of the mesh is [km], velocities are in [m/s], and stresses are in [MPa]. Parameters and source terms are scaled with

\[\bar{\rho} = 10^{-3}\rho, \quad \bar{\lambda} = 10^{-9}\lambda, \quad \bar{\mu} = 10^{-9}\mu, \quad \bar{f}_i = 10^{-3} f_i, \quad \bar{s}_i = 10^{-6} s_i\]

The wave speeds are given in [km/s]:

\[\bar{c}_s = \sqrt{\frac{\bar{\mu}}{\bar{\rho}}} = 10^{-3}c_s, \quad \bar{c}_p = \sqrt{\frac{\bar{\lambda} + 2\bar{\mu}}{\bar{\rho}}} = 10^{-3}c_p,\]

Acoustic-elastic with water layer

The acoustic wave equation is a special case of the elastic wave equation, therefore the scaling of parameters is identical. However, when using a water layer one needs to adjust gravitational acceleration in the parameters file:

GravitationalAcceleration = 0.00981