Deflation and Deformation of the Askja Caldera Complex, Iceland, Since 1983: Strain and Stress Development on Caldera Boundaries Prior to Tsunami Generating Rockslide in 2014 at Lake Öskjuvatn

Abstract:

The relation between ground deformation and caldera development can be studied at the Askja caldera complex at the divergent plate boundary in Iceland. The Lake Öskjuvatn caldera, 4-5 km wide and about 250 m deep, began forming shortly prior to a major explosive eruption in 1875 and continued to grow rapidly for over 2 decades. The boundaries of the caldera collapse remain unstable. A large rockslide on 21 July 2014 of the order of 30-100 million cubicmeters generated a lake tsunami with run up heights of over 30 m. Geodetic measurements at the volcano since 1983 suggest that instability of the basal plane of failure of the rockslide may have steadily increased as the volcano deflated throughout this period. Deformation (subsidence and horizontal contraction) has been mapped by levelling, distance measurements, GPS and satellite radar interferometry (InSAR) using the ERS, Envisat, Radarsat, TerraSAR-X and Cosmo-SkyMed satellites. An intermediate digital elevation model has been delivered by the TanDEM-X mission. Initial subsidence rate was up to 7 cm/yr, decaying to 2-3 cm in recent years. A large part of the signal can be reproduced by a model with pressure decrease in a spherical source at 3 km depth, interpreted as a pressure drop in a magma chamber, or subsidence may occur over a structurally weak region in relation to plate spreading. The 2014 rockslide has a basal plane of failure passing through intact rock. A section of the basal plane is exhumed by the slide, revealing impressive striations and slip marks. A simple model, assuming uniform elastic halfspace with a rectangular failure plane extending to the surface, indicates that Coulomb stress on the failure plane or a deeper weakness zone under the rockslide, increased by over 2 MPa since 1983 due to the deflation, exceeding the change often associated with stress triggering of earthquakes. Furthermore, the slide occurred in an area of persistent geothermal and seismic activity. Although gravity, geothermal alteration, topography and climate factors are likely to dictate mostly the 21 July 2014 rockslide, our observations and modelling indicate the rockslide is an integral event in the development of the Öskjuvatn caldera, with deflation of the volcano in recent decades contributing to instability of the caldera boundary and failure on the basal plane of the rockslide.