VT fault-plane solution orientations as a precursory indicator of lava bulk viscosity during the May 2018 Leilani Estates eruption at Kīlauea Volcano, Hawai'i

Foreknowledge of the bulk viscosity of magma is necessary to anticipate eruption hazards, as magma viscosity is a primary control on eruption style and on how far lava flows may travel from the vent. Here, we demonstrate that fault-plane solutions (FPS) for a swarm of volcano-tectonic (VT) earthquakes that migrated from Kīlauea Volcano's Pu'u 'Ō'ō vent into the lower east rift zone on May 1, 2018, immediately before the onset of eruption in Leilani Estates on May 3 and an M6.9 decollement earthquake on May 4, are consistent with a high bulk viscosity, as observed in Phase I lavas (which have liquid viscosities 0.5 log units higher than Phase II lavas). We repicked first-motion polarities for 138 events recorded by the Hawaiian Volcano Observatory on May 1 (UTC) and listed in the NEIC COMCAT to calculate double-couple FPS based on HVO hypocenter locations. Although the seismic network downrift of Pu'u 'Ō'ō is relatively sparse, we were able to obtain 17 well-constrained FPS for swarm events at and downrift of Pu'u 'Ō'ō. These FPS (all located at or up to 13 km downrift of Pu'u 'Ō'ō) are dominated by strike-slip and thrust components, and P-axis orientations form two trends, one rift-parallel (similar to FPS for UERZ swarms) and another that is approximately perpendicular to the rift and propagation direction. Detailed polarity analysis of lower-magnitude swarm events confirms these trends, and additional LERZ FPS from May 11-June 1 indicate that the rotation persisted through the first weeks of eruption. As ~90°-rotated VT FPS, indicative of strong dike-normal stresses, have previously only been observed in association with eruptions of high-bulk-viscosity magmas at volcanoes worldwide (Roman and Cashman, 2006), we suggest that FPS for VT earthquake swarms at Kīlauea and other similar basaltic volcanoes may provide a useful indication that unrest involves remobilized pockets of crystalline and evolved magma.