A high‑resolution water fountain catalogue reveals an abrupt hydrothermal change at Strokkur geyser, Iceland

Strokkur geyser in the Haukadalir valley in south Iceland is an erupting hot spring that allows studying hydrothermal processes. Since March 2020, we have continuously monitored Strokkur using three seismometers located ~40 m from the conduit. This long-term dataset has enabled the creation of a high‑resolution catalogue containing more than 760,000 individual water‑fountain events, which has previously been used to investigate eruption types, driving mechanisms, and the influence of air temperature and wind on geyser dynamics.In this contribution, we present a striking change in Strokkur’s behaviour that occurred on 18 October 2024 at 18:00. Following this moment, the geyser began producing a larger number of water fountains per eruption, more water fountains per hour and exhibited a markedly shorter recharge cycle. Simultaneously, several neighbouring hot springs activated or increased their activity. Because the onset of this transition was captured seismically, the dataset offers a rare opportunity to examine the triggering mechanism and its implications for subsurface fluid pathways.By analysing the spatio‑temporal evolution of seismic signals associated with this behavioural shift, we explore the underlying processes driving the system’s reorganisation. The study highlights the value of dense seismic monitoring and detailed event catalogues for understanding hydrothermal dynamics, and it provides insights into geothermal systems and their time‑dependent changes.