Breaking the Seal: Tracking Eruptive and Flood Tremor in Iceland with Seismic Arrays

Abstract:

Bárðarbunga volcano in Iceland is located beneath Vatnajökull glacier. It erupted from August 29th, 2014 to February 27th, 2015 outside the ice cap after a ca. 50 km long dike had formed, mainly in the subsurface beneath the ice. Both the dike formation and the following effusive eruption were accompanied by continuous, harmonic tremor and tremor bursts. During the eruption these tremor bursts were interpreted as magma-ice interaction, flowing water and magma. This analysis was mainly based on the seismic frequency content and amplitude distributions on the regional seismic network. It was sometimes supported by the detection of cauldrons in the ice on subsequent days.

 Here we analyse the tremor using three seven-element arrays south- and northwest of the glacier cap (Jökulheimar, Innri Eyrar and Urðarháls). Using array analysis we can better constrain event locations, estimate tremor migration speed and use wavefield slowness tohelp estimate the source depth which can help to constrain the source type.

We show that the backazimuths of the main harmonic tremor with dominant energy around 1 Hz correlate well with the extent of the growing lavafield for at least 27 days. Another one day long harmonic tremor episode contained energy up to 5 Hz but dominant at 1 to 2 Hz. The source was located beneath the ice and moved upwards and horizontally (about 42 deg). We compare it to tremor during smaller floods underneath the glacier and discuss a moving water or magma front as source.

Elastic full wavefield simulations in models including ice thickness and ice basal topography allow us to strip out path effects thus yielding information about the tremor sources. Indications are that (i) parts of the dike were still evolving well after it had been breached by advancing magma and (ii) that eruptive lava is a significant source of seismic tremor.