Characterizing the large M4.7+ seismic events during the 2018 Kīlauea eruption is important to understand the complex subsurface deformation at the Kīlauea summit. The first 12 events (May 17 - May 26) are associated with long-duration seismic signals and the remaining 50 events (May 29 - August 02) are accompanied by large-scale caldera collapses. Resolving the source location and mechanism is challenging because of the shallow source depth, significant non double-couple components, and complex velocity structure. We demonstrate that combining multiple geophysical data from broadband seismometers, accelerometers and infrasound is essential to resolve different aspects of the seismic source. Seismic moment tensor solutions using near-field summit stations show the early events are highly volumetric. Infrasound data and particle motion analysis identify the inflation source as the Halema’uma’u reservoir. For the later collapse events, two independent moment tensor inversions using local and global stations consistently show that asymmetric slips occur on inward-dipping normal faults along the northwest corner of the caldera. While the source mechanism from May 29 onwards is not fully resolvable seismically using far-field stations, infrasound records and simulations suggest there may be inflation during the collapse. The summit events are characterized by both inflation and asymmetric slip, which are consistent with geodetic data. Based on the location of the slip and microseismicity, the caldera may have failed in a ‘see-saw’ manner: small continuous slips in the form of microseismicity on the southeast corner of the caldera, compensated by large slips on the northwest during the large collapse events.