  The relativistic jet in M87 offers a unique opportunity for understanding the
detailed jet structure and emission processes due to its proximity. In
particular, the peculiar jet region HST-1 at ~1 arcsecond (or 80 pc, projected)
from the nucleus has attracted a great deal of interest in the last decade
because of its superluminal motion and broadband radio-to-X-ray outbursts,
which may be further connected to the gamma-ray productions up to TeV energies.
Over the last five years, we have been doing an intensive monitoring of HST-1
with EVN at 5GHz in order to examine the detailed structural evolution and its
possible connection to high-energy activities. While this program already
yielded interesting results in terms of the detailed mas-scale structure,
proper motion measurements and structural variations, the recent HST-1
brightness is continuously decreasing at this frequency. To counter this, we
have shifted our monitoring frequency to 1.7GHz from October 2013. This
strategy successfully recovered the fainter emission that was missed in the
last 5GHz session. Moreover, we again discovered the sudden emergence of a new
component at the upstream edge of HST-1, demonstrating that the use of EVN
1.7GHz is indeed powerful to probe the current weak nature of HST-1. Here we
report early results from the 1.7GHz monitoring as well as further progress on
the long-term kinematic study.
