Source Dynamics of Ruapehu's 2022 Volcanic Unrest: Insights from drumbeat seismicity, tremor, and crater lake signals

Active volcanoes exhibit a broad range of behaviours that can act as potential heralds of an impending eruption. One of Aotearoa New Zealand's most active andesitic volcanoes, Mt Ruapehu, had its Volcanic Alert Level raised to Level 2 on March 21, 2022 following heightened volcanic tremor, the initiation of a new heating phase at the crater lake Te Wai ā-moe, and increases in gas plume emissions. Observed during the unrest was a sequence of highly periodic, low-frequency earthquakes known as `drumbeats'. These signals have been observed around the world to often precede and/or accompany the ascent of magma and volcanic eruptions. This research seeks to understand why these signals were observed at Mt Ruapehu with no accompanying eruption and what source processes drove them so that, should this behaviour re-emerge, future monitoring practices may be better informed. Approximately 43,000 discrete drumbeat events and 89 days of continuous volcanic tremor were identified over the 121-day unrest period, which were analysed in the time, amplitude, and frequency domains. Drumbeats were cross-correlated and grouped into families of highly similar events to assess source similarity. A distinct temporal continuum is observed between drumbeat and tremor signals, with drumbeat events becoming variably discretised in time to form continuous tremor. Groups of drumbeat families appear to be clustered in time during different stages of the unrest episode and possess different characteristics. We argue that increases in volcanic tremor, lake temperatures, and gas throughput are the result of magma ascent into the shallow system immediately prior to or contemporaneous with the onset of tremor. We construct a conceptual model for the generation of drumbeat, tremor, and lake temperature signals that consists of shallow magma storage, a gas cavity, a permeable plug, and the crater lake. The presence of repetitive drumbeat earthquakes results from transient sealing and failure within the fracture pathways of the permeable plug. This is driven and regulated primarily by pressure accumulation from persistently degassing magma and the strength of the sealing mechanism.