Seismic velocity changes during the 2024-26 fountaining sequence at Kīlauea, Hawaiʻi

Kīlauea, Hawaii, one of the world's most active volcanoes, has experienced numerous (>40) eruptive episodes since December 2024 with remarkable lava fountain heights (up to 450m) in Halemaʻumaʻu crater. Following a dike intrusion within the Halemaʻumaʻu crater in December 2024, the eruption entered a stable pressurization and release pattern from January 2025 onwards with lava flows during the sequence confined to Halemaʻumaʻu crater.We study the 2024-26 sequence focusing on relative seismic velocity changes (dv/v). We use ~20 seismic stations located within 10 km of the Halemaʻumaʻu crater and process the data using the traditional cross-station and less conventional single-station approach and estimate the dv/v using the wavelet approach. The dv/v patterns highlight at least three distinct phases of activity during the 2024-26 eruption sequence, as well as some interesting velocity decreases prior to the onset of the sequence in December 2024 although these are spatially confined. We inspect the differences between our new results with previous seismic velocity patterns (2015-2024) and explore the nature of the changes using complementary observations (seismic and geodetic data), as well as numerical modeling. Our study suggests a change in strain patterns at the shallow Halemaʻumaʻu reservoir which implies a dynamic evolution of the magmatic system feeding the eruption. Additionally, we show how deformation (deflation) of the deeper South Caldera reservoir contributes to the observed dv/v patterns. Our study sheds light on the dynamics between different magma reservoirs and links to surface processes.