  The 2018 rift zone eruption of K\={\i}lauea volcano was accompanied by a
remarkable and episodic collapse of its summit. Between May-August the eruption
and collapse sequence included over 70,000 earthquakes (M$\geq$0) and 54 major
earthquakes (M$\geq$5). We analyzed the seismicity in the K\={\i}lauea summit
region and estimated seismic full moment tensors with their uncertainties for
the 54 M$\geq$5 events. These events occurred at almost daily intervals and
were accompanied by intense seismicity which was concentrated between 0-3 km
depths beneath the Halema`uma`u pit crater. The hypocenters reveal partial
elliptical patterns (map view) that migrated downward by $\sim$200 m. The
moment tensors reveal remarkably consistent mechanisms, with negative isotropic
source types and localized uncertainties, and vertical P-axis orientations.
From the moment tensors we derived Poisson's ratios which are variable
($\nu=0.1-0.3)$ for the first half of the collapse events and converged to
$\nu\sim0.28$ from June 26 onward.
