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

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 (M L <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.

Dynamics of the Askja caldera July 2014 landslide, Iceland, from seismic signal analysis: precursor, motion and aftermath

“…This cluster at Öskjuvatn has been seen in seismic data since 1975 (Einarsson, 1991;Jakobsdóttir, 2008;Greenfield and White, 2015) and was hypothesised to be caused by hydrothermal circulation above a shallow magma body (Soosalu et al, 2010) or by thermal cracking and heat extraction in the crust 30 (Einarsson, 1991 Relocation of 86 earthquakes in this southeastern hydrothermal area at lake Öskjuvatn showed that the events are concentrated at depths between 2-4 km b.s.l. (Greenfield et al, in press).…”

“…In the region of the Askja volcanic system, earthquakes occur at two levels in the crust, as shallow crustal seismicity between the surface and mostly 5 km depth, and as deep seismicity in the ductile lower crust at depths of 10-35 km, with magnitudes of usually M L <3 (Jakobsdóttir et al, 2002;Soosalu et al, 2010;Greenfield et al, 2015). 20…”

“…The deep crustal earthquakes are located in distinct regions, beneath Kollóttadyngja shield volcano to the north, beneath the hyaloclastite mountain Upptyppingar to the east, location of a dyke intrusion in -2008White et al, 2011), and beneath Askja volcano, attributed to melt migration (Soosalu et al, 2010;Key et al, 2011;Greenfield and White, 2015).…”

Geophysical constraints on the dynamics of spreading centres from rifting episodes on land

“…Although eruptions occurred in Askja between 1921 and 1929 and in 1961, the most recent episode of major rifting occurred in 1874-1876 81 . Current deformation at Askja has been well documented by GPS, InSAR and levelling data [82][83][84][85][86] , and seismicity has been mapped using a dense local network 80,87,88 . The zones of horizontal and vertical strain accumulation seem to act on different length scales: extension, at the expected full plate-spreading rate, is distributed over a zone ~80 km wide, whereas rift-axis subsidence is concentrated within the central ~20-km-wide zone of faulting and fissures (Fig.…”

“…Recent deployments of dense seismic arrays around Askja have detected several clusters of micro-earthquake activity in the lower crust, which is normally ductile 87,88 . These have been interpreted as areas where high rates of melt movement generate strain rates that are sufficient to cause brittle failure.…”