Seismolosical studies of magma injection processes: volcano monitoring and imaging of magma chambers

Abstract

The processes associated with magma injection at shallow depths within the crust have been the topic of many geophysical studies, some investigating the seismicity that accompanies volcanic activity and others attempting to map the subsurface extent and geometry of the resulting magma bodies. The aim of this study is to obtain a better understanding of these processes by investigating the nature of seismic signals that accompany volcanic eruptions and by seismically imaging a magma body beneath a mid-ocean ridge, both located on, or adjacent to Iceland. The seismic phenomena associated with the 1996 Vatnajӧkull subglacial eruption in central Iceland, have been studied using data recorded by both temporary (HOTSPOT) and permanent (SIL) seismic networks. These networks comprise 60 broadband and short-period three-component seismographs and cover most parts of the country. Two very active volcanic systems, Bárdarbunga and Grimsvӧtn, are situated underneath the Vatnajokull ice cap. The volcanoseismic signals recorded there were categorised according to their waveform shape and frequency content, into three groups: (a) low-frequency events (1-2 Hz); (b) mixed-frequency events (1-4 Hz); and (c) volcanic tremor. The eruption was preceded by intense seismic activity which began with a = 5.6 earthquake located at the Bárdarbunga volcanic system. The epicentres of the earthquake swarm that followed the M(_w), = 5.6 event initially delineated the Bárdarbunga caldera rim and then migrated towards Grimsvӧtn, to a place where a fissure was later observed. Pre-eruptive tremor started at least two days before the eruption as a harmonic signal around five narrow frequency bands (0.5-0.7, 1.6, 2.2, 2.8 and 3.2 Hz). Co-eruptive tremor started as a broadband, continuous signal which evolved into low-amplitude background tremor interrupted by high-amplitude, cigar-shaped bursts. Further analysis revealed that continuous tremor and the cigar-shaped bursts had all the characteristics of low- dimensional chaotic signals. Geophysical and geochemical evidence suggest that a lateral migration of magma from Bárdarbunga facilitated the rupture of the roof of a magma chamber, situated at the fissure area, which subsequently erupted as tephra on the glacier. The second phase of the RAMESSES (Reykjanes Ridge Axial Melt Experiment: Structural Synthesis from Electromagnetic and Seismics) experiment involved the acquisition of multichannel seismic reflection data from 39 along- and across-axis lines shot over the magmatically active 57º 45'N axial volcanic ridge. The data from one along-axis line were processed using a variety of techniques that mainly aimed at reducing the large amount of coherent noise present, a result of scattered energy at the rough seabed. The final processed section revealed a number of reflection events that could be interpreted as intra-crustal reflections, originating from the interface between pillow lavas and sheeted dykes, and from the top part of a thin melt lens.