No, a massive earthquake did not also strike the East Coast today and cause thousands of refugees to flee inland. That said, while the lines do not represent refugees, this is still a massive migration of people.
What you're seeing is the net population migration, both inward and outward, from Middlesex County, Massachusetts. My immediate family still lives there, so they're not represented by a line, but should my parents ever act on their Colorado retirement plans, well, they'd join the slightly heavy red line tracing from Middlesex to somewhere in Douglas or El Paso. Black is inbound.
The map is interactive and covers nine major U.S. cities by default (but you can click any of the individual counties as well). The darker the line, the heavier the migration. Detroit, for sad and probably obvious reasons, is nearly all red. Seattle? Nearly all black. Good for Seattle!
Note: The data is from 2008, during which 10 million Americans moved from one county to another. [Forbes]