Seismic Velocity Monitoring Reveals Complex Magma Transport Dynamics at Kīlauea Volcano Prior to the 2018 Eruption

Abstract Magma and pressure transport between Kīlauea's summit reservoirs and along its East Rift Zone (ERZ) are dynamic even in the absence of surface eruptions. However, these processes do not always produce surface manifestations and may sometimes elude detection by current geological and geodetic monitoring. Here we monitor subsurface seismic velocity changes across Kīlauea's system from 2013 to 2018 and integrate these observations with concurrent measurements of ground deformation and lava lake elevation. We corroborate years‐long seismic velocity decreases around the summit caldera, which are particularly pronounced at southern stations, consistent with sustained pressurization of the South Caldera reservoir (SCR) from a deep magma supply. Following the 2015 summit intrusion, accelerated rates of velocity decrease, summit inflation, and lava lake rise suggest an increased magma supply to the SCR. Notably, we identify an anomalous 7‐month period (late 2016–mid 2017) of disrupted magma/pressure transfer between the SCR and Halema'uma'u magma reservoir (HMR), as evidenced by dropping lava lake levels despite continued summit inflation and SCR pressurization. This period coincided with pressurization observed beneath Pu'u'ō'ō, indicating pressure/magma diversion from the summit toward the ERZ and the episode terminated with a M5.3 flank earthquake in June 2017 that restored the connectivity between the SCR and HMR and triggered shallow crustal pressurization beneath the summit caldera for the subsequent 2–3 months. Our findings reveal significant perturbations in Kīlauea's magmatic plumbing system approximately one year before the catastrophic 2018 eruption, highlighting seismic velocity monitoring's value for detecting subtle changes of the volcano.