  We study properties of gravitational waves based on the three-dimensional
simulations, which demonstrate the neutrino-driven explosions aided by the
standing accretion shock instability (SASI). Pushed by evidence supporting slow
rotation prior to core-collapse, we focus on the asphericities in neutrino
emissions and matter motions outside the protoneutron star. By performing a
ray-tracing calculation in 3D, we estimate accurately the gravitational
waveforms from anisotropic neutrino emissions. In contrast to the previous work
assuming axisymmetry, we find that the gravitational waveforms vary much more
stochastically because the explosion anisotropies depend sensitively on the
growth of the SASI which develops chaotically in all directions. Our results
show that the gravitational-wave spectrum has its peak near $\sim 100$ Hz,
reflecting the SASI-induced matter overturns of $\sim O(10)$ ms. We point out
that the detection of such signals, possibly visible to the LIGO-class
detectors for a Galactic supernova, could be an important probe into the
long-veiled explosion mechanism.
