Abstract

Magnitude ~ 5 earthquakes are often observed at caldera volcanoes undergoing unrest. These earthquakes stand out in regional and teleseismic moment tensor catalogues for their shallow depths and non-double-couple source mechanisms. Intriguingly, the orientation of the moment tensor (vertical-dilatational or vertical-compressional) appears to correlate with the stage of volcanic unrest (inflation building up to eruption, or deflation as magma leaves the edifice), but we don’t yet have a general understanding of what source processes lead to their distinctive nature.

In this talk I will explore how local measurements of such earthquakes at Bárðarbunga caldera, central Iceland, can be used to investigate this further. Bárðarbunga has hosted the longest sequence of such anomalous earthquakes observed at any volcano globally, and in 2014-15 hosted a spectacular eruption that featured more than 70 of these Mw 5+ events over a period of just 6 months. Analysis of detailed observations of seismicity and ground deformation delivered important new constraints on models of the caldera system and ring faults, but also new questions. Since 2015 the edifice has been re-inflating, accompanied by around 1-2 large earthquakes per year. We have sought to exploit this by supplementing the long-running regional network with temporary dense deployments of seismometers above the ice-filled caldera, to provide further improved constraints on earthquake locations and focal mechanisms. I will describe the 10’s of thousands of earthquakes we have located from these few months of data, and the new insights they provide into caldera ring fault seismicity at Bárðarbunga.