  Aims: We aim to constrain the structural variations within the HST-1 region
downstream of the radio jet of M87, in general as well as in connection to the
episodes of activity at very high energy (VHE). Methods: We analyzed and
compared 26 VLBI observations of the M87 jet, obtained between 2006 and 2011
with the Very Long Baseline Array (VLBA) at 1.7 GHz and the European VLBI
Network (EVN) at 5 GHz. Results: HST-1 is detected 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 ~4c. 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 -65deg to -90deg, 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 VHE events.
