Anatomy of a Caldera Collapse: Kīlauea 2018 Summit Seismicity Sequence in High Resolution

The 2018 Kīlauea eruption and caldera collapse generated intense cycles of seismicity tied to repeated large seismic (Mw ~ 5) collapse events associated with magma withdrawal from beneath the summit. To gain insight into the underlying dynamics and aid eruption response, we applied waveform‐based earthquake detection and double‐difference location as the eruption unfolded. Here, we augment these rapid results by grouping events based on patterns of correlation‐derived phase polarities across the network. From April 29 to August 6, bracketing the eruption, we used ~2,800 events cataloged by the Hawaiian Volcano Observatory to detect and precisely locate 44,000+ earthquakes. Resulting hypocentroids resolve complex, yet coherent structures, concentrated at shallow depths east of Halema'uma'u crater, beneath the eventual eastern perimeter of surface collapse. Based on a preponderance of dilatational P wave first motions and similarities with previously inferred dike structures, we hypothesize that failure was dominated by coupled shear and crack closure. Plain Language Summary We used high‐resolution methods to examine seismicity associated with the 2018 Kīlauea eruption and summit collapse, as magma drained underground from the summit region and eventually erupted toward the east. In total, we detected and precisely located more than 44,000 earthquakes from late April to early August 2018. The location patterns of these earthquakes reveal numerous seismically active structures, both on the boundary and within the collapsing zone. Based on the characteristics of these earthquakes, we hypothesize that most events were generated by a combination of fault slip and crack closure as the surface rocks collapsed into the underground region previously occupied by magma. Key Points We apply high‐precision earthquake detection, location, and phase polarity analysis to illuminate the 2018 Kīlauea summit collapse sequence Seismicity was most active near the eventual eastern perimeter of surface collapse but also occurred in the interior of the collapse area A preponderance of dilatational P wave first motions suggests that seismicity was largely generated by coupled shear and crack closure