To show how to read in velocity boundary conditions from files,
example files are provided.

First, untar the given 'velbc_regional.tar.gz'.
(e.g., type 'tar xzvf velbc_regional.tar.gz')

Each of the velbc files has the following naming scheme: {root}{age}.

{root} is 'velbc_full' in this example, but can be defined by 'file_vbcs' in the input file.
{age} is each million years from 'start_age' defined in the input file to 0 M.y. In this example, the start_age is 4 (M.a.)