Lower-crustal earthquakes caused by magma movement beneath Askja volcano on the north Iceland rift |
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Authors: | Heidi Soosalu Janet Key Robert S White Clare Knox Páll Einarsson Steinunn S Jakobsdóttir |
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Institution: | 1. Bullard Laboratories, University of Cambridge, Madingley Road, Cambridge, CB3 0EZ, UK 2. Institute of Earth Sciences, University of Iceland, Sturlugata 7, 101 Reyjavík, Iceland 3. Icelandic Meteorological Office, Bústaeavegur 9, 150 Reykjavík, Iceland
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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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