# Fluid injection interruption causes temporary changes in local stress field and induced seismicity at Krafla caldera, Iceland

## TL;DR

Stopping fluid injection at a geothermal site in Iceland temporarily changed the local stress field and triggered seismic activity.

## Contribution

The study links injection interruptions to rapid changes in seismic anisotropy and induced seismicity.

## Key findings

- A 90° flip in fast S-wave polarization was observed within hours of injection stoppage.
- Strike-slip seismic events increased sharply during injection interruptions.
- Restarting injection led to seismic quiescence and increased time delays in S-wave components.

## Abstract

Induced seismicity related to fluid injection in the upper crust is a major concern in the context of geothermal energy production. For exploited high-temperature geothermal systems in tectonically and volcanically active areas, such as Krafla caldera in NE Iceland, understanding the processes that trigger seismicity and changes in the local stress field can be difficult to unravel. We observe a link between anthropogenic activity and changes in the local stress field, since the appearance of a strike-slip cluster coincides with changes in the seismic anisotropy around an injection well during an injection interruption period. By analyzing the shear-wave splitting phenomenon, 90\documentclass[12pt]{minimal}
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\begin{document}$$^\circ$$\end{document} flips of the fast S-wave polarization and a decrease in the time delays between the fast and the slow S-wave component is observed, starting within hours after an injection stop, coinciding with a sharp increase of strike-slip events in the vicinity of the well. When the injection restarts, the seismic quiescence and increase of time delays may suggest a resumption of the previous state. The changes in seismicity patterns and variations of the seismic anisotropy might be linked to changes in the pore pressure and a possible activation of a shear-fault due to anthropogenic activity.