| dc.description.abstract | In September 30, 1996, a subglacial eruption, the Gjalp eruption, occurred along a 7-km-long fissure between two volcanic centres, Grimsvotn and Barearbunga in Iceland. A strong earthquake in Barearbunga (Mw 5.6) preceded the eruption, one and a half days before. The eruption continued for 13 days and it was followed by an intense earthquake swarm with a large number of small earthquakes (ML < 4) and volcanic tremor. To study these events in detail, we analyzed the volcanoseismic signals that were recorded by the temporary seismic network, HOTSPOT, using modern seismological methods. In order to obtain highly accurate
locations, a reliable minimum 1D velocity model with accompanying station correction terms was computed. All these events were then located by calculating non-linear, probabilistic locations using the newly derived model. We correlated the waveforms of these 301 events to define linked events based on the catalogue and the similarity of their recorded seismograms. Subsequently, we calculated the relative travel time difference between event pairs and solved the hypocentral separation between these events using the double-difference method. With this approach, 192 events were successfully relocated and we were able to improve the
location accuracy by showing a tight clustering and producing relative horizontal location uncertainties of less than 1 km and less than 2 km for the vertical location uncertainties. In this study, we also calculated a spectral parameter, known as the centroid frequency for all earthquake signals. Furthermore, the moment tensor for the main event and 10 events that followed were calculated in order to understand their source processes. All of events we
analyzed in this study were best characterized with non-double-couple solutions and insignificant isotropic components. The events along the Barearbunga caldera rim show thrust mechanisms while the events located in the fissure are dominated by strike-slip mechanisms. From the seismicity patterns, the shift of epicentres from the Barearbunga caldera towards the eruption site was an indication that there was a lateral migration through a dyke extending from the shallow magma chamber underneath the Barearbunga volcano. Centroid frequencies indicate that most events along the Barearbunga ring fault and along the fissure contained high-frequencies implying rupture of rock, rather than fluid-related source processes. This result thus we consider as an explanation for the insignificant isotropic component of the moment tensors derived in this study. | en_US |