Interesting, Glen - I encountered the term "backslip" for the first time (in a seismological setting) just a couple of days ago, and, from the context (and, it was one word, not two), determined that it referred to doing computations on data from seismic recordings to run the slip backwards. This would be for the purpose of getting information about the rupture process (I think), or, kinematics. This was while I was reading up on characteristic earthquakes and seismic gap modeling.

Also, Googling "Savage" and "backslip" brings up a wealth of information, including that Savage is a person.

Michael F. Williams
Arroyo Grande, CA USA