  We have undertaken a survey of H alpha emission in a substantially complete
sample of CGCG galaxies of types Sa and later within 1.5 Abell radii of the
centres of 8 low-redshift Abell clusters. Some 320 galaxies were surveyed, of
which 116 were detected in emission (39% of spirals, 75% of peculiars).
Detected emission was classified as `compact' or `diffuse'. From an analysis of
the full survey sample, we reconfirm our previous identification of compact and
diffuse emission with circumnuclear starburst and disk emission respectively.
The circumnuclear emission is associated either with the presence of a bar, or
with a disturbed galaxy morphology indicative of on-going tidal interactions.
The frequency of such tidally-induced (circumnuclear) starburst emission in
spirals increases from regions of lower to higher local galaxy surface density,
and from clusters with lower to higher central galaxy space density. We
conclude that tidal interactions are likely to be the main mechanism for the
transformation of spirals to S0s in clusters. Finally, for regions of
comparable local density, the frequency of tidally-induced starburst emission
is greater in clusters with higher central galaxy density. This implies that,
for a given local density, morphological transformation of disk galaxies
proceeds more rapidly in clusters of higher central galaxy density. This effect
is considered to be due to subcluster merging and could account for the
previously considered anomalous absence of a significant type - local surface
density relation for irregular clusters at intermediate redshift.
