Are you confused yet? I was at first when I started reading about HTDP, but it is actually a very interesting Geodetic program that was developed and put out by the National Geodetic Society.
HTDP is a tool that enables users to predict horizontal displacements and/or horizontal velocities related to crustal motion in the United States.
Wordy and scientific explanation of the software:
The software employs models that address both the continuous and the episodic components of crustal motion. For characterizing continuous motion, the models assume that points on the Earth’s surface move with constant horizontal velocities. This assumption is generally acceptable except for the accelerated motion experienced during the years immediately following a major earthquake and for the motion associated with volcanic/magmatic activity. For characterizing the episodic motion associated with earthquakes, the models use the equations of dislocation theory.
While it sounds confusing though, the output is actually fairly easy to understand, the blue shades are slow movement and colors going to the red are faster movements.

You can see in the output, where the San Andreas Faultline lays and why they have so many earthquakes.