Micro-seismicity in the Hverahlíð high-temperature geothermal field, Hengill, SW-Iceland

The Hverahlíð high-temperature geothermal field is located in the southern part of the Hengill volcanic complex in southwest Iceland. Prior to the onset of geothermal production in 2016, seismic activity in the area was limited. Since then, persistent micro-seismicity has been detected, characterised by a diffuse spatial pattern and only minor swarm activity. Despite covering just ~2 km², Hverahlíð hosts some of Iceland’s most productive geothermal wells, with measured temperature exceeding 300°C at around 1.5 km depth.

In this study, we analyse seismicity in Hverahlíð from 2016 to 2025, recorded by a varying number of seismometers (14 to 40) deployed across the wider Hengill area. The core network consists of permanent stations operated by Iceland GeoSurvey (ÍSOR) for ON Power, supplemented by the regional SIL-network of the Icelandic Meteorological Office. Additionally, 30 temporary stations were installed during the COSEISMIQ project (2018–2021), significantly improving the local detection capability and spatial resolution.

Seismicity in Hverahlíð is dominantly micro-seismicity, with ~90% of the activity of M_L < 1.0, and a magnitude range of M_L -0.3 to 3.5. High-resolution relative relocations show that seismicity is confined to 2-3.5 km depth below sea level, i.e., located slightly below the bottom of the production wells and organised in one main cluster and another significantly smaller cluster, both trending NNE-SSW within the production area.

Although the Hverahlíð area is highly fractured with cross-cutting faults trending from NNE-SSW to ENE-WSW, the observed seismicity does not directly illuminate known surface faults. Instead, the earthquake distribution reflects the geothermal production zone, closely matching the geometry of the geothermal system as inferred from existing resistivity models. The earthquake depth distribution may reflect, at least partially, cooling and thermal contraction of the hot host rock induced by deep fluid convection linked to the heat source of the geothermal system. Comparison with other high-temperature geothermal systems in Iceland suggests that the seismicity may delineate the base of a highly permeable convective geothermal reservoir.

Despite considerable production driven pressure draw-down in Hverahlíð, only around 18% of earthquake source mechanisms show pure normal faulting, whereas 55% show pure strike-slip faulting. As the production area will grow in lateral extent in coming years through planned step-out-wells, a corresponding increase in the lateral extent of seismicity is possible.