A2 Knútur Árnason The DRG seismic experiment in Krafla

DRG

Knútur Árnason and Karl Gunnarsson

ISOR

The DRG seismic experiment in Krafla

GGW2016, 24.11. 2016

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The DRG seismic experiment in KraflaThe aim is to study the propagation of seismic waves andlook for indications of magma in the Krafla volcano :Tomography, reflections, attenuation, S-wave shadows.

1.Scrutinize data from the permanent seismic network2.Deploy 20 additional seismic stations and record for 4 months3.Undershooting by distant earthquakes and explosives4.Detailed analyses of all data (seismic, EM and bore- hole data) to characterize the deep roots of the the Krafla geothermal system.

The proposal:

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Seismic experiment in Krafla (cont.)

Fieldwork managed by ISOR and UU

Most of the data processing and interpretation was to bedone by a PhD student at UU and ISOR expert(s)

Phenomenological interpretation in connection withthe COTHERM project.

DRGActual setup and data acquisition• Decided to deploy the 20 stations on a dense profile

between the test wells during the IMAGE VSP experiment, late May to early June 2014

• From June 25th to 31st the 20 stations were installed

at 200-300 m intervals on two profiles, 14 stations on a 2.5 km SSW-NNE profile and 7 stations on a 1.3 km NW-SE profile

• August 5th to 7th and Sept. 1st to 2nd all the stations were serviced

• The Network was pulled out Oct. 6th and 7th

• 90% data recovery

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Permanentseismic networkin Krafla operatedby ÍSOR, 11 stations(red dots)

DRGThe DRG seismic network

Blue stars are seismicstations during VSP

Red stars during DRGrecording

Black dots arePermanent stations

Purple lines showboundaries ofS-wave shadows

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Krafla area(green circle)

Distant earth Quakes(yellow stars)

Explosives (4 to 9September 2014)100-300 kg TNT(red stars)

Undershooting and distant quakes

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Processing steps1. Load all the data recorded by the DRG network into the SeisComp seismic software at ÍSOR.2. Load all information about the seismic stations into SeisComp.3. Look for and pick P and S phases for distant earthquakes and the undershooting in DRG network, the permanent seismic network and nearby SIL stations.4. Set up record sections to look qualitatively at S-wave shadow(s) which can give information of the presence and distribution of melt.5. Systematically inspect local earthquakes and explosions (from the VSP experiment) looking for intermediate phases that could be reflections form melt at depth.

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Picked shots and distant earthquakes

100 to150 distantearthquakes havebeen picked and4 shots (the figureshows examples)

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05/01/2023

Distant earthquake in Bárðarbunga

DRGDistant shotsMánáreyjarVopnafjörður

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Picked local earthquakes during DRG installed

Yellow picked and locatedby the permanentNetwork

Light bluepicked on theDRG network

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05/01/2023

Local earthquake in Krafla

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05/01/2023

3 km/s

Some other seismic studies in KraflaP-wave velocity in Krafla from seismicrefraction (Brandsdóttir et.al., 1997)

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05/01/2023

Recent seismic tomography (Schuler et al., J. Geophys. Res., 2015)Just earthquakes

Earthquakes andRefraction data

CalderaIDDP-1

S-waveshadows

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05/01/2023

DRGConclusions

• In the DRG seismic experiment in Krafla, a high quality data were recorded for three mots on dense profiles over an S-wave shadow in Krafla. • The data have been loaded and calibrated in SeisComp database at ISOR.• Extensive undershooting data, both from explosives and distant earthquakes, with good azimuthal coverage exist.• ISOR operates, for LV, an 11 station permanent continuously recording seismic network in Krafla.• ISOR has very extensive seismic data, and other complimentary data, to study the inner structure of the Krafla volcano and its geothermal system.

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EpilogueAs stated earlier, most of the analysis and interpretationof the DRG seismic data from Krafla was to be done bya PhD at UU.Unfortunately, no PhD student is available at UU at themoment for the work.The GEORG grant only assumed limited work for ISORexperts.ISOR has limited own funds for research projects.The seismic group at Cornell University, USA, hasanalyzed the data to do “Virtual Refraction Profiling”(VRP) looking for reflections from local earthquakes.This work is presented in a separate presentationhere at this workshop (Doyeon Kim, et al., 2016)