EGU2020-16388
https://doi.org/10.5194/egusphere-egu2020-16388
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

The Great Geysir and tectonic interactions in South Iceland

Bergthóra S. Thorbjarnardóttir, Ingi Th. Bjarnason, and Revathy M. Parameswaran
Bergthóra S. Thorbjarnardóttir et al.
  • University of Iceland, Institute Earth Sciences, Reykjavik, Iceland (ingib@hi.is)

The Great Geysir is within a tectonically active region bordering the eastern flank of the Western Volcanic Zone (WVZ), south of Langjökull glacier. The geothermal area has been active at least throughout the Holocene (Torfason, 1985). It is a high-temperature system, which is not common in Iceland for geothermal areas located outside the neovolcanic zones. Its longevity suggests continuously active tectonics in the region. Indeed, half a century of seismic monitoring shows relatively high activity of minor earthquakes (magnitude<4.0). The general pattern of seismicity is rather constant through time, but comes in bursts of activity. We attempt to elucidate the driving forces in this unusual and poorly tectonically understood area, by analyzing the most modern seismic data collected in the years 1995-2016 within a study area ~25x25 km2 enclosing the Geysir area. It is, for instance, observed how the large (Mw~6.5) earthquakes in June 2000, located ~45 km south and southwest of Geysir in the South Iceland Seismic Zone (SISZ), induced seismicity kilometers away within hours after their occurrence. The heightened level of activity, an order of magnitude in terms of number of earthquakes, lasted half a year after the 2000 events in large parts of the study area and finally tapered down in 2001. Within the first two weeks of the 2000 events, the main activated faults are within 5 km of the Great Geysir. The activation is mostly at shallow depth (< 4 km). However, none pass directly through the Geysir geothermal area. That may explain the only minor change observed in the dormant state of the Great Geysir, which has now lasted approximately a century. There are historic accounts on how several large South Iceland earthquakes in the SISZ activated the Great Geysir, lasting for years or decades. The last such activation was in 1896. In its full might, it erupts up to a height of 70-80 m (Torfason, 1985).  Its currently active neighbor, Strokkur geysir, usually erupts to heights of 15-20 m.  Cross-sections of the seismicity near Geysir suggest several near vertical  right-lateral ~NNE trending faults.  Focal mechanisms indicate strike-slip movements, but also oblique-normal and thrust events in between. This may suggest fault jogs and high horizontal stresses. Approximately 6 km north of Geysir, in the Sandfell and Sandvatn area, there is a persistent ~ENE trending ~5 km long seismic pattern with main activity between 4-8 km depth. This seismicity has occurred, on and off, through the history of seismic observations. Here the faulting is also complicated (strike-slip and thrust), but focal mechanisms suggest the main component to be normal to oblique normal. Cross-sections, although unclear, suggest possible dip to the ~SSE. We intend to calculate stress inversions in the study area prior to the conference.

How to cite: Thorbjarnardóttir, B. S., Bjarnason, I. Th., and Parameswaran, R. M.: The Great Geysir and tectonic interactions in South Iceland, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16388, https://doi.org/10.5194/egusphere-egu2020-16388, 2020

Comments on the presentation

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Presentation version 1 – uploaded on 07 May 2020
  • CC1: Comment on EGU2020-16388, Eva Eibl, 07 May 2020

    Hi Ingi,

    pretty cool to see more work in the area.

    Is that cross section scheme of a geyser with a bubble trap just a generic one or one for Great Geyser in specific? Do you know whether there is a model/ mapping published about Strokkur or Great Geyser? I've located seismic signals from Strokkur and think I can see its bubble trap location.

    Thanks,

    Eva

    • AC1: Reply to CC1, Ingi Þorleifur Bjarnason, 07 May 2020

      Hello Eva,

      I downloaded the animation from wiki geysers. I am no expert in the geysers eruptions, but it is something that I will read more up on. I wounder if the animation is misleading in showing that recharge is from the surface. I had assumed that recharge is from below. I was just downloading your presentation. I will try to have some ready questions for you. See you on the chat,

      Ingi

      • CC2: Reply to AC1, Eva Eibl, 07 May 2020

        Hi Ingi,

        I see what you mean. It is indeed a bit odd that the water pipe goes downwards to the bubble trap. Part of the surficial water flows back into the conduit, but there is hot water supply from depth as well since some water is lost on the surface e.g. due to wind.

        Thanks,

        Eva

    • AC2: Reply to CC1, Ingi Þorleifur Bjarnason, 07 May 2020

      Eva, have you seen the booklet by Helgi Torfason on the geysers at Geysir? If not I should have scanned pdf copy that I could send you.

      Ingi

      • CC3: Reply to AC2, Eva Eibl, 07 May 2020

        Hi Ingi,

        I have a copy of Torfason 1999 "Geology of the geysir area in southern Iceland" (7 pages) and

        Torfason 1995 "Eyrar i eldhafi" (9 pages)

        Is this what you mean?

        Thanks,

        Eva

  • CC4: Comment on EGU2020-16388, Eva Eibl, 07 May 2020

    "Ingi Unv. Iceland author (17:33) Thank you Eva. Can earthquake predtion learn anything from your work?"

    Hi Ingi,

    We are more focussed on similarities to volcanic eruptions since the system is quite similar with a heat source at depth, chamber filled by a multi-phase fluid and conduit linking it to the surface. I am not sure how similar earthquakes behave.

    Best,

    Eva