  We discuss the properties of stellar mass black hole (BH) mergers induced by
tidal encounters with a massive BH at galactic centres or potentially in dense
star clusters. The tidal disruption of stellar binaries by a massive BH is
known to produce hypervelocity stars. However, such a tidal encounter does not
always lead to the break-up of binaries. Since surviving binaries tend to
become hard and eccentric, this process can produce BH mergers in principle.
For initially circular binaries, we show that the gravitational wave (GW)
merger times become shorter by a factor of more than $10^{2}$ ($10^5$) in
$10\%$ ($1\%$) of the surviving cases. This reduction is primarily due to the
growth in binary's eccentricity at the tidal encounter. We also investigate the
effective spins of the survivors, assuming that BH spins are initially aligned
with the binary orbital angular momentum. We find that binary orientations can
flip in the opposite direction at the tidal encounter. For the survivors with
large merger time reduction factors, the effective spin distribution is rather
flat. We estimate the merger rate due to the tidal encounter channel to be
$\sim 0.6\ \textrm{Gpc}^{-3}\textrm{yr}^{-1}$. This mechanism is unlikely to be
the dominant formation channel of BH mergers. However, the current and
near-future GW observatories are expected to detect an enormous number of BH
mergers. If mergers are found in the vicinity of massive BHs (e.g. the
detection of GW lensing echoes or preceding extreme-mass-ratio bursts), this
mechanism would provide a possible explanation for their origin.
