We present the results of a long and dense monitoring of the radio jet in M87 performed with Very Long Baseline Interferometry (VLBI). We gathered data since 2006 at 1.6 GHz with the VLBA and at 5 GHz with the EVN; We focus on the jet feature HST-1, located about 80 parsec downstream the jet. We detect HST-1 at all epochs; we model-fitted its complex structure with two or more components, the two outermost of which display a significant proper motion with a superluminal velocity around ~4 c. The motion of a third feature that is detected upstream is more difficult to characterize. The overall position angle of HST-1 has changed during the time of our observations from -65° to -90°, while the structure has moved by over 80 mas downstream. Our results on the component evolution suggest that structural changes at the upstream edge of HST-1 can be related to the very high energy events in 2005, 2008, and 2010.