  Sunquakes are helioseismic power enhancements initiated by solar flares, but
not all flares generate sunquakes. It is curious why some flares cause
sunquakes while others do not. Here we propose a hypothesis to explain the
disproportionate occurrence of sunquakes: during a flare's impulsive phase when
the flare's impulse acts upon the photosphere, delivered by shock waves,
energetic particles from higher atmosphere, or by downward Lorentz Force, a
sunquake tends to occur if the background oscillation at the flare footpoint
happens to oscillate downward in the same direction with the impulse from
above. To verify this hypothesis, we select 60 strong flares in Solar Cycle 24,
and examine the background oscillatory velocity at the sunquake sources during
the flares' impulsive phases. Since the Doppler velocity observations at
sunquake sources are usually corrupted during the flares, we reconstruct the
oscillatory velocity in the flare sites using helioseismic holography method
with an observation-based Green's function. A total of 24 flares are found to
be sunquake active, giving a total of 41 sunquakes. It is also found that in
3-5 mHz frequency band, 25 out of 31 sunquakes show net downward oscillatory
velocities during the flares' impulsive phases, and in 5-7 mHz frequency band,
33 out of 38 sunquakes show net downward velocities. These results support the
hypothesis that a sunquake more likely occurs when a flare impacts a
photospheric area with a downward background oscillation.
