Sudden excitation and prolonged relaxation of Strokkur geyser, Iceland

Geysers are iconic examples of self-organized hydrothermal systems, renowned for their regularity and apparent predictability. Yet even these archetypes of cyclic behavior can undergo abrupt and persistent transitions, the drivers of which remain poorly understood. On 18 October 2024, we recorded such a transition at high temporal and spatial resolution at Strokkur geyser, Iceland, using dense seismic instrumentation, a catalog of 794,000 fountains and infrared Unoccupied aircraft system surveys. Repeating, shallow and local seismic events accompanied the change for five days, indicating localized subsurface changes. Dormant hot springs immediately activated and at Strokkur 65% more water fountains, less single eruptions and up to 16 water fountains within an eruption occurred. After five days, Strokkur stabilized in a new regime characterized by 18% more fountains than in previous years. From January 2025 onward, Strokkur started relaxing towards its former state in terms of eruption number, while in the wider area, new hot springs formed. A heat loss model suggests that heat input from depth increased by 24%, possibly delivered as a pulse of hot gas or steam. These observationsprovide rare, high-resolution evidence of how modest subsurface perturbations can reorganize hydrothermal systems over timescales of days to months.