Recent jokulhlaups from western Vatnajokull, Iceland: hydrologic insights from seismic tremor measurements and aerial observations
In July and September 2002, jokulhlaups were recorded in Skafta, the river draining the south-western flank of Vatnajokull, Iceland. Skafta jokulhlaups (Skaftarhlaups) have a recurrence interval of 1-4 years, and create occasional downstream hazards. Previous research demonstrated that Skaftarhlaups originate from two possible sub-glacier lakes, delineated by surface cauldrons, within an extended region of the Grimsvotn geothermal field. Glaciological evidence points to ice-dam flotation as the trigger mechanism for Skaftarhlaups. Analysis of seismic intensity plots derived from continuous ground motion (tremor) measurements at selected digital seismic stations on and around Vatnajokull permits inferences about the timing of onset and relative transit times of the 2002 Skaftarhlaups. Using tremor intensity, supplemented by multiple aerial observations of each jokulhlaup, known hydraulic potential for the sub-glacier flood path, and published downstream rating-curves, we reconstruct the hydrodynamics of the July and September 2002 Skaftarhlaups. Compositely, these data provide a basis for understanding (i) sub-glacier drainage instabilities following jokulhlaup initiation; (ii) the initial velocity of sub-glacier and proglacial jokulhlaup propagation; and (iii) probable ice-proximal peak discharge and its timing relative to downstream measurements. Tremor plots from five seismic stations, ranging from 17 to 88 km distance from the Skafta cauldrons, reveal pronounced, continuous tremor (0.5 1 Hz bandwidth) for six days during the July Skaftarhlaup, with a sudden onset 47 hours before arrival of detectable flood conditions at a gauging station 25 km downstream from the ice margin. We interpret this regional seismicity as the product of jokulhlaup-induced hydraulic resonance, first beneath Vatnajokull, and then in Skafta. Empirical reconstructions of peak ice-proximal discharge give an estimate of about 1,500 m3 s-1. Initial peak discharge at the ice margin occurred about 24 hours before receipt of the same flood peak 25 km downstream. In combination with other environmental data, real-time tremor plots could prove to be a useful tool for facilitating early warning of jokulhlaup hazard potential in Skafta and adjacent rivers.
EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003