Earthquakes indicated lava viscosity during the 2018 eruption of Kīlauea Volcano, Hawai‘i

Abstract:

Identification of precursory signals indicative of bulk magma viscosity would allow forecasts of eruption style and the scale of associated hazards. We show that the orientation of fault-plane solutions for earthquakes preceding and accompanying Kı̄lauea’s 2018 eruption indicate a 90° local stress field rotation, a phenomenon previously observed only at high-bulk viscosity eruptions, and never before at Kı̄lauea. Although the seismic network downrift of Pu‘u ‘Ō‘ō is relatively sparse, we were able to determine 62 well-constrained fault-plane solutions (FPS) for swarm events at and downrift of Pu‘u ‘Ō‘ō throughout the eruption (May 1 - September 4). At Puʻu ʻŌʻō, rotated FPS occur from the onset of swarm propagation on May 1 to the end of Phase 1 on May 17. In the upper LERZ, rotated FPS occur on May 1 and 4 (early Phase 1). In the lower LERZ, rotated FPS occur from May 11-15 (approximately late Phase 1). No rotated FPS occur during Phase 2 or the majority of Phase 3. After the Fissure 8 vent shut down, a final mobilization of residual cool and crystal-rich magma occurred, leading to a final burst of rift-normal stress on August 7-13. Bulk viscosities (calculated based on experimentally-determined liquid viscosity and known crystallinity) for 2018 products and earlier lavas from Pu‘u ‘Ō‘ō vent tightly constrain the bulk viscosity threshold required for local stress field reorientation to between Phase 1 and Phase 2 viscosities, or ~10^2.5 Pa s. We argue that rotated fault-plane solutions in earthquake swarms at Kīlauea and other volcanoes worldwide provide an early indication that unrest involves relatively viscous magma, and thus real-time monitoring of fault-plane solutions orientations provides critical information on impending eruption style.