More than a drumbeat – Towards a new suite of periodic patterns in volcano-seismology

Volcano observatories often rely on their ability to accurately decipher volcano-seismic signals to assess the state of a specific volcanic system. For this purpose, well-established patterns such as VT and LP event sequences during pre-eruptive unrest phases – or their periodic manifestation as ‘drumbeats’ – constitute a trusted and reasonably well-understood parameter. We present a suite of similarly distinct, but less commonly observed periodic patterns recorded at various volcanic systems, notably consisting of stable and dynamic drumbeat-like seismicity, as well as pulsed and spiked tremor episodes. Within that, we focus on a long-term pulsed tremor signal recorded during a rare dome-extrusion phase at Whakaari/White Island (New Zealand). Considering the resemblance between this and other instances of pulsed tremor observed at comparably phreatic systems in Indonesia and Costa Rica, we interpret the occurrence of such periodic seismicity as the mechanical response of partially sealed hydrothermal systems upon the influx of magmatic and non-magmatic fluids. These often short-lived patterns are very hard to trace using conventional Volcano Observatory monitoring tools, such as EQ detectors and tremor-based metrics (e.g., RSAM, SSAM, DSAR), and their significance for volcanic hazard assessment is largely unknown. As Machine Learning techniques are becoming increasingly accessible, we explore in how far they constitute an opportunity to track these elusive seismic patterns. Using this case study as a starting point, we push towards further investigation of similar periodic signals and their underlying physical source processes. We seek to discuss how common such subtle patterns are, and how they can be detected and interpreted within their respective volcano-environmental contexts.