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Lower-crustal earthquakes caused by magma movement beneath Askja volcano on the north Iceland rift

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Abstract

The lower crust of magmatically active rifts is usually too hot and ductile to allow seismicity. The Icelandic mid-Atlantic rift is characterized by high heat flow, abundant magmatism generating up to 25–30 km thick crust, and seismicity within the upper 8 km of the crust. In a 20-seismometer survey in July-August 2006 within the northern rift zone around the Askja volcano we recorded ~1700 upper-crustal earthquakes cutting off at 7–8 km depth, marking the brittle-ductile boundary. Unexpectedly, we discovered 100 small-magnitude (ML <1.5) earthquakes, occurring in swarms mostly at 14–26 km depth within the otherwise aseismic lower crust, and beneath the completely aseismic middle crust. A repeat survey during July-August 2007 yielded more than twice as many lower-crustal events. Geodetic and gravimetric data indicate melt drainage from crustal magma chambers beneath Askja. We interpret the microearthquakes to be caused by melt moving through the crust from the magma source feeding Askja. They represent bursts of magma motion opening dykes over distances of a few meters, facilitated by the extensional setting of the active rift zone.

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Acknowledgments

Seismometers loaned from the Natural Environment Research Council SEIS-UK facility were used for fieldwork. The SEIS-UK staff are thanked for assistance during all the phases of the project. E. Sturkell and H. Brynjólfsson provided logistical help and advice. N. Borner, M. Coffin, J. Eccles, K. Harland, D. Hawthorn, J. Hólmjárn, A. Lillington, A. Mikkola, J.-C. Molina Santana, A. Nowacki and S. White participated in the fieldwork. The rangers in the mountain huts of the Askja-Herðubreið nature reserve, and the Akureyri branch of the Icelandic Touring Association are thanked for friendly help in many practical matters. J. Drew helped in data processing and provided the CMM algorithm for earthquake detection. Generic Mapping Tools software (Wessel and Smith 1998) was used for drawing the figures. Comments by S. De Angelis and S. Moran improved the manuscript. Dept. Earth Sciences contribution no. ES.9418.

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Correspondence to Heidi Soosalu or Robert S. White.

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Editorial responsibility: M. Ripepe

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Soosalu, H., Key, J., White, R.S. et al. Lower-crustal earthquakes caused by magma movement beneath Askja volcano on the north Iceland rift. Bull Volcanol 72, 55–62 (2010). https://doi.org/10.1007/s00445-009-0297-3

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  • DOI: https://doi.org/10.1007/s00445-009-0297-3

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