  We show how the star formation activity of galaxies is progressively
inhibited from the outer region to the center of the massive cluster A2142.
From an extended spectroscopic redshift survey of 2239 galaxies covering a
circular area of radius $\sim 11$~Mpc from the cluster center, we extract a
sample of 333 galaxies with known stellar mass, star formation rate, and
spectral index $D_n4000$. We use the Blooming Tree algorithm to identify the
substructures of the cluster and separate the galaxy sample into substructure
galaxies, halo galaxies and outskirt galaxies. The substructure and halo
galaxies are cluster members, whereas the outskirt galaxies are only weakly
gravitationally bound to the cluster. For the cluster members, the star
formation rate per stellar mass decreases with decreasing distance $R$ from the
cluster center. Similarly, the spectral index $D_n4000$ increases with $R$,
indicating an increasingly average age of the stellar population in galaxies
closer to the cluster center. In addition, star formation in substructure
galaxies is generally more active than in halo galaxies and less active than in
outskirt galaxies, proving that substructures tend to slow down the transition
between field galaxies and cluster galaxies. We finally show that most actively
star forming galaxies are within the cluster infall region, whereas most
galaxies in the central region are quiescent.
