  We determine the spin of a supermassive black hole in the context of
discseismology by comparing newly detected quasi-periodic oscillations (QPOs)
of radio emission in the Galactic centre, Sagittarius A* (Sgr A*), as well as
infrared and X-ray emissions with those of the Galactic black holes. We find
that the spin parameters of black holes in Sgr A* and in Galactic X-ray sources
have a unique value of $\approx 0.44$ which is smaller than the generally
accepted value for supermassive black holes, suggesting evidence for the
angular momentum extraction of black holes during the growth of supermassive
black holes. Our results demonstrate that the spin parameter approaches the
equilibrium value where spin-up via accretion is balanced by spin-down via the
Blandford-Znajek mechanism regardless of its initial spin. We anticipate that
measuring the spin of black holes by using QPOs will open a new window for
exploring the evolution of black holes in the Universe.
