Abstract
Large earthquakes can generate small changes in static stress: increases that trigger aftershock swarms, or reductions that create a region of reduced seismicity—a stress shadow1,2. However, seismic waves from large earthquakes also cause transient dynamic stresses that may trigger seismicity3,4. This makes it difficult to separate the relative influence of static and dynamic stress changes on aftershocks. Dyke intrusions do not generate dynamic stresses, so provide an unambiguous test of the stress shadow hypothesis. Here we use GPS and seismic data to reconstruct the intrusion of an igneous dyke that is 46 km long and 5 m wide beneath Bárðarbunga Volcano, central Iceland, in August 2014. We find that during dyke emplacement, bursts of seismicity at a distance of 5 to 15 km were first triggered and then abruptly switched off as the dyke tip propagated away from the volcano. We calculate the evolving static stress changes during dyke propagation and show that the stressing rate controls both the triggering and then suppression of earthquake rates in three separate areas adjacent to the dyke. Our results imply that static stress changes help control earthquake clustering. Similar small static stress changes may be important for triggering seismicity near geothermal areas, regions being hydrofractured and deflating oil and gas fields.
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Acknowledgements
Seismometers were borrowed from the Natural Environment Research Council (NERC) SEIS-UK (loans 968 and 1022), and the work was financially supported by research grants from the NERC and FutureVolc, with graduate studentships from the NERC and Shell. We thank T. Ágústsdóttir, B. Brandsdóttir, H. Soosalu, S. Steinþórsson and all those who assisted with fieldwork in Iceland. We are very grateful to J.-P. Avouac for detailed discussions on earthquake stress triggering and stress shadows, and to P. Segall for constructive comments. C. Bean (University College Dublin), the British Geological Survey and Icelandic Meteorological Office (IMO) kindly provided additional data from their seismometers in northeast Iceland: data delivery from IMO seismic database 20141124/01. Department of Earth Sciences, Cambridge contribution number ESC3285.
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All authors participated in data collection from seismometers deployed in Iceland by the Cambridge group, in processing and analysing the seismic data and in interpretation of the results. Geodetic modelling was carried out by T.G. and R.G.G. analysed the seismicity and stressing rates. All authors contributed to preparation of the manuscript.
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Green, R., Greenfield, T. & White, R. Triggered earthquakes suppressed by an evolving stress shadow from a propagating dyke. Nature Geosci 8, 629–632 (2015). https://doi.org/10.1038/ngeo2491
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DOI: https://doi.org/10.1038/ngeo2491
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