Supplementary Information for: SUPPLEMENTARY INFORMATION · File “Supplementary_movie_2.mp4” included within submission. Movie displaying the time-dependent deformation model

Supplementary Information for:

Triggered earthquakes suppressed by an evolving

stress shadow from a propagating dyke

Robert G. Green1

Tim Greenfield1

Robert S. White1

*Corresponding Author: Green, Robert G. [email protected]

1Bullard Laboratories, Department of Earth Sciences, University of Cambridge, Madingley

Road, Cambridge, UK, CB3 0EZ.

SUPPLEMENTARY INFORMATIONDOI: 10.1038/NGEO2491

NATURE GEOSCIENCE | www.nature.com/naturegeoscience 1

© 2015 Macmillan Publishers Limited. All rights reserved

http://dx.doi.org/10.1038/ngeo2491

Contents

Dyke seismicity and opening

Supplementary Movie 1 | Animation of seismicity delineating the dyke and triggered earthquakes.

Supplementary Movie 2 | Animation of dyke opening model and geometry

Supplementary Table 1 | Table of dyke segments and opening parameters

Seismicity-Stress relationships

Supplementary Figure 1 | GPS displacement misfit for a deflating sill beneath Bárðarbunga caldera

Supplementary Figures 2–4 | Daily coulomb stress maps for the triggered regions

Supplementary Figure 5 | Parameter search for Aσ and ̇ in Seismicity-Rate equation

Supplementary Figure 6 | Stress – seismicity evolution for varying effective coefficient of friction, μ

Supplementary Figure 7 | Kistufell shut off on day 230

Supplementary Figure 8 | Kverkfjöll cluster on day 230

Supplementary Table 2 | Parameters used for predicted cumulative seismicity

Earthquake locations and focal mechanisms

Supplementary Table 3 | Table of velocity model used in earthquake locations

Supplementary Figure 9 | Network coverage of the seismically active area

Supplementary Figure 10 | Earthquake depths

Supplementary Figure 11 | Example fault-plane solutions from the three regions

Supplementary Figure 12 | Bárðarbunga – two sets of focal mechanisms but consistent stress evolution

Electronic reference material

Supplementary Files | Electronic reference material

Automatic Earthquake Catalogue files:

autolocation_bardeqs.csv autolocation_kisteqs.csv autolocation_kverkeqs.csv Manually Refined Earthquakes – locations and fault-plane solutions:

manual_locations_Kistufell_fps.csv manual_locations_Kverkfjoll_fps.csv manual_locations_NEbard_fps.csv Dyke Opening and Bárðarbunga deflation Model:

dyke_and_deflation_model.csv Velocity Model:

cambridge_velocity_model.csv Animation Movies:

Supplementary_movie_1.mp4 Supplementary_movie_2.mp4


Supplementary Movie 1 | Animation of seismicity delineating the propagating dyke and triggered earthquakes.

File “Supplementary_movie_1.mp4” included within submission

The rapidly migrating line of earthquakes tracks the leading edge of the propagating dyke. The dyke grows at a variable rate with each burst forward marking a new segment. Further seismicity is triggered on the north-east flank of Bárðarbunga central volcano and at the surrounding volcanic edifices of Askja, Kistufell and Kverkfjöll geothermal field. This seismicity then terminates abruptly in each region at different times as the stress field imposed by the propagating dyke changes.

MOVIE


Supplementary Movie 2 | Animation of dyke opening model and geometry

File “Supplementary_movie_2.mp4” included within submission

Movie displaying the time-dependent deformation model and its fit to the GPS data. Frames are as in the snapshot above. Ice sheets are in blue-white, observed GPS displacements are black vectors with 95% confidence intervals, and modelled displacements in red. Scales in bottom left corner. Bottom right inset shows a perspective view of the opening dyke segments viewed from the south-east. Active dyke segment is represented by a purple line, and purple dot is the deflation source Bárðarbunga beneath caldera.

MOVIE


Supplementary Table 1 | Table of final dyke segments and opening parameters

SegStart UTM Easting / km

SegStart UTM Northing / km

SegEnd UTM Easting / m

SegEnd UTM Northing / m reverse Dip Top / km Bottom / km

385484.9 7168423 390397.5 7165298 0.86 90 2 8 390397.5 7165298 401156.1 7172313 0.92 90 2 8 401156.1 7172313 404326.2 7176456 1.8 90 2 8 404326.2 7176456 408086.5 7179943 3.33 90 2 8 407191.9 7179113 407679.2 7183000 0.17 90 2 8 407346.4 7180344 406611.3 7181513 0.05 90 4 8 407679.2 7183000 409255.1 7188947 2.78 90 2 7.5 409255.1 7188947 411753.7 7193708 5.09 90 1 7 411753.7 7193708 413168.1 7198011 1.12 90 1 7

Time dependent model of both deflation and dyke opening is available in dyke_and_deflation_model.csv

Supplementary Figure 1 | GPS displacement misfit for a deflating sill beneath Bárðarbunga caldera

RMS misfit results for GPS displacements on day 244, using the best dyke opening model, and varying the depth, location, size and amount of contraction in a horizontal sill beneath Bárðarbunga caldera. The above misfit space is for a 12.25 km2 sill positioned as shown by the stars in Supplementary Figures 2–4. The red cross marks the minimum misfit solution for a deflation source at 16.8 km depth. This location and geometry of the contracting source was then used in our time-dependent modelling of the Bárðarbunga deflation and dyke opening.


Supplementary Figure 2 | Daily coulomb stress maps for the Bárðarbunga triggered target faults

Panel figure of the coulomb stress field imparted on each day computed on the Bárðarbunga target fault (020/90/-25) using an effective coefficient of friction of 0.4. Solid black lines mark the ice limit, dashed lines delineate central volcanoes, and ticked lines mark calderas. Green circles highlight earthquakes occurring in the current 24 hour period, and grey circles are earthquakes since dyke propagation onset. Green lines show the current dyke geometry. Yellow star marks deflation source location. Seismicity shuts off as soon as the coulomb stress begins decreasing on day 230 at Bárðarbunga. A strong negative coulomb stress (stress shadow) then remains through the rest of the dyke propagation.


Supplementary Figure 3 | Daily coulomb stress maps for the Kistufell triggered target faults

Panel figure of the coulomb stress field imparted on each day computed on the Kistufell target fault (033/87/-25) using an effective coefficient of friction of 0.4. Solid black lines mark the ice limit, dashed lines delineate central volcanoes, and ticked lines mark calderas. Green circles highlight earthquakes occurring in the current 24 hour period, and grey circles are earthquakes since dyke propagation onset. Green lines show the current dyke geometry. Yellow star marks deflation source location. Seismicity shuts off as soon as the coulomb stress begins to decrease on day 231 at Kistufell. The area then remains in a coulomb stress shadow through the rest of the dyke propagation.


Supplementary Figure 4 | Daily coulomb stress maps for the Kverkfjöll triggered target faults

Panel figure of the coulomb stress field imparted on each day computed on the Kverkfjöll target fault (185/75/144) using an effective coefficient of friction of 0.4. Solid black lines mark the ice limit, dashed lines delineate central volcanoes, and ticked lines mark calderas. Green circles highlight earthquakes occurring in the current 24 hour period, and grey circles are earthquakes since dyke propagation onset. Green lines show the current dyke geometry. Star is deflation source location. Seismicity shuts off as soon as the stress begins to decrease on day 231. The region then remains in a deep Coulomb stress shadow.


Supplementary Figure 5 | Parameter search for Aσ and ̇ in Seismicity-Rate equation

As detailed in the methods section, the stress-transfer law of Dieterich (1994) can be used to generate predicted seismicity rates to compare with the observed cumulative seismicity. The calculation is dependent on the unknown parameters ̇, the background stressing rate, and Aσ, the product of the effective normal stress and a fault constitutive parameter. This figure presents the results of a grid search over those two variables, defined by a root-mean-square (rms) misfit between the predicted and observed cumulative seismicity. There is a clear trade-off between these two parameters, illustrated by the blue line which tracks the minimum misfit well. Consequently we do not constrain the value of these variables but use them only as fitting parameters which are optimised to demonstrate the evolution of seismicity as a function of the stressing history.


Supplementary Figure 6 | Stress – Seismicity evolution for varying effective coefficient of friction, μ

Variation in the coulomb stress evolution and therefore seismicity rate evolution for different values of the effective coefficient of friction (μ) is presented here for the three regions. Black lines are observed cumulative seismicity and grey bars show hourly rates. Dashed lines show the coulomb stress evolution and coloured solid lines show the cumulative seismicity predicted using the seismicity-rate equation (Hainzl et Al. 2010) for values of μ of 0.2(blue), 0.4(red), 0.6(green). The parameters used are best fit values from the parameter search and are specified in Supplementary Table 2.


Supplementary Figure 7 | Kistufell shut off on day 230

At Kistufell, the shut off in seismicity is observed to occur at 10:00 am on 18th August (day 230). GPS displacement solutions are however made for 24 hour time periods, and the timestamp for each displacement solution is noon of the given day. However, 10:00 am on 18th August was the instant at which the dyke broke through after a stalled period and rapidly propagated 5 km north-east (blue circles on panel c above). This opening has a significant influence on the GPS solution for that day, and that opening imposes a decrease in calculated stress at the triggered region in Kistufell. The seismicity halt coincides with this dyke surge as earthquakes at Kistufell occur up to but not after 10 am on day 230. As we assume a linear stress variation between the 12:00 time-stamps of each day, the stress rate is consequently incorrectly determined to be falling from 12:00 noon on 17th August (day 229) onwards. This results in a predicted seismicity shut-down which is 22 hours earlier than that which is actually observed (Figure 3f). Figure panels a,b,d as in Figure 3.


Supplementary Figure 8 | Kverkfjöll seismicity surge on day 230

At Kverkfjöll the seismicity halts just before noon on 18th August (day 230). At 10:00 am on 18th August the dyke broke through after a stalled period and rapidly propagated 5 km north-east (blue circles on panel c above). This opening has an influence on the GPS solution for that day, and that opening imposes the negative stress region which the western Kverkfjöll cluster lies on the edge of. These earthquakes however all occurred before 10:00 am (panel c) and so panel b and main figure 3h present an ambiguity because of the24 hour GPS resolution and the time frame of the earthquakes plotted in green. The argument however remains sound, as the Kverkfjöll cluster halts abruptly with the rapid segment propagation from 10:00 am on day 230. The time evolution is not affected (panel d) as the stress function for this was fortuitously calculated at the more eastern cluster.


Supplementary Table 2 | Parameters used for predicted cumulative seismicity

Parameters used to calculate coulomb stress and then predict cumulative seismicity using the seismicity-rate equation of Dieterich (Dieterich1994, Hainzl2010). From the parameter search over Aσ and ̇ in Supplementary Figure 4 there is an unconstrained trade-off between these values. For consistency we use the combination of parameters with an Aσ of 3Kpa for each predicted seismicity curve (Figure 3 c,f,i).

Region Fault-plane solution μ Aσ (Kpa) ̇ (Kpa/day) Bárðarbunga 188/78/-138 0.2 3 0.015 Bárðarbunga 188/78/-138 0.4 3 0.0155 Bárðarbunga 188/78/-138 0.6 3 0.0155 Kistufell 033/87/-25 0.2 3 0.0225 Kistufell 033/87/-25 0.4 3 0.017 Kistufell 033/87/-25 0.6 3 0.0135 Kverkfjöll 185/75/144 0.2 3 0.011 Kverkfjöll 185/75/144 0.2 3 0.015 Kverkfjöll 185/75/144 0.2 3 0.019

Supplementary Table 3 | Table of velocity model used in earthquake locations

Depth / m Vp / ms-1 Vs / ms-1 -3000 1500 843 1000 4000 2247 3000 5200 2921 5000 6100 3427 6000 6404 3598 7000 6583 3698 8000 6688 3757 9000 6792 3816

10000 6896 3874 13000 7038 3954 14000 7057 3965 15000 7076 3975 16000 7095 3986 17000 7114 3997 18000 7133 4007 22000 7210 4051 26000 7286 4093 30000 7362 4136 42000 7400 4157

Velocity model derived from the ICEMELT refraction experiment (Darbyshire1998) in the Vatnajökull region for manual and automatic earthquake locations.


Supplementary Figure 9 | Network coverage of the seismically active area

Our local seismic network provided excellent coverage of the triggered and dyke-propagation seismicity. All ray azimuths are covered by both nearby and distant (see inset) seismic stations, with especially good sampling to the north. Blue triangles indicate the network of University of Cambridge seismometers, and green triangles where Cambridge seismometers are at stations operated by the Icelandic Meteorological Office.


Supplementary Figure 10 | Earthquake depths

The triggered earthquakes (blue – automatic locations, red manual locations) occur between 4 and 10 km depth, with an average depth of 6 km. Coulomb stresses are calculated at 6 km depth at Kistufell and 7 km depth at Bárðarbunga and Kverkfjöll. Dyke earthquakes are black dots.


Supplementary Figure 11 | Example fault-plane solutions from the three regions

Individual fault-plane solutions from each of the three triggered regions, showing good coverage of the focal sphere (lower hemisphere equal area stereographic projection). Open circles show dilatational P-wave first motion and filled circles are compressional first motion arrivals. Red triangle is P axis, and blue inverted triangle is the T axis.


Supplementary Figure 12 | The Bárðarbunga cluster – Two focal mechanism sets but consistent stress

evolution

Of the focal mechanisms at Bárðarbunga there are two sets, with left lateral type fault-plane solutions in the north-west of the cluster, and right-lateral type fault-plane solutions in the south-east of the cluster (Figure 2a). This figure presents the range of possible fault-plane solutions (black great circles) for some of these events along with the maximum probability solution in green. Compressional P-wave polarities are in red and dilatational arrivals in blue. For both sets there is a range of possible rakes and so we used a range of these to calculate the coulomb stress. We find that for the right-lateral set (top row) the stress evolution always shows an increase and then decrease on the second day (for varying μ). For the left-lateral set (bottom row) with all but the most southern rakes, the stress evolution is also a single day increase followed by immediate decrease. The stress evolution displayed in figure 3c is the coulomb stresses resolved on the representative right-lateral fault.


Documents

Supplementary Information for: SUPPLEMENTARY INFORMATION · File “Supplementary_movie_2.mp4” included within submission. Movie displaying the time-dependent deformation model