HR: 15:10h
AN: V23C-07    [Abstracts]
TI: Controlling factors on earthquake swarms associated with magmatic intrusions; constraints from Iceland
AU: * Pedersen, R
EM: Rikke@hi.is
AF: Nordic Volcanological Center, Institute of Earth Sciences, University of Iceland, Sturlugata 7, Reykjavik, 107 Iceland
AU: Sigmundsson, F
EM: fs@hi.is
AF: Nordic Volcanological Center, Institute of Earth Sciences, University of Iceland, Sturlugata 7, Reykjavik, 107 Iceland
AU: Einarsson, P
EM: palli@raunvis.hi.is
AF: Institute of Earth Sciences, University of Iceland, Sturlugata 7, Reykjavik, 107 Iceland
AU: Brandsdottir, B
EM: bryndis@raunvis.hi.is
AF: Institute of Earth Sciences, University of Iceland, Sturlugata 7, Reykjavik, 107 Iceland
AU: Arnadottir, T
EM: thora1@hi.is
AF: Nordic Volcanological Center, Institute of Earth Sciences, University of Iceland, Sturlugata 7, Reykjavik, 107 Iceland
AB: Intrusion of magma into the Earth's crust is frequently associated with seismic activity, often occurring as distinct earthquake swarms. Understanding the nature of these swarms is important for evaluating crisis situations in volcanic areas. However, there often seem to be little correlation between the amount of seismic energy release, the spatial extent of the volume of rock affected by the stress perturbations, and the volume of magma on the move, which complicates the immediate risk evaluation. A number of factors may influence the evolution of a magmatically induced seismic swarm and the resulting seismic energy release. A number of factors need to be evaluated in each individual case. These are, in random order: the crustal thickness, presence/absence of a crustal magma chamber, geothermal gradient, magmatic flow rate/stressing rate, intrusion volume, depth of intrusion, tectonic setting of the intruded area, regional stresses and tectonic history. Based on three case studies, where seismic swarm activities have been confirmed through deformation measurements to be related to magmatic movements, we attempt to evaluate the relative importance of the assumed controlling factors. All case examples are located within Iceland, but in different tectonic settings. 1. The Hengill triple junction, situated where two extensional plate boundaries join a transform zone. The area experienced a period of unusually persistent earthquake activity from 1994 to 1999, contemporaneously with ground uplift at a rate of 1-2 cm/yr. The uplift was modeled as a response to magma injection at about 7 km depth. 2. The Eyjafjallajokull volcano, situated in a volcanic flank zone where extensional fractures are only poorly developed. Two minor seismic swarms, in 1994 and 1999; were associated with a cumulative surface uplift of more than 35 cm. The two uplift events were modeled as sill intrusions at depths of 4.5 to 6.5 km. 3. The Krafla rift segment, forming part of an extensional plate boundary, which experienced an active rifting period with numerous intrusive and extrusive events during 1975 to 1984. The cumulative horizontal expansion was up to 9 meters. Seismic activity was clearly related to two distinct processes: inflow of magma into a crustal magma chamber and rifting of the plate boundary in discrete dike-forming events. The relationship between the volume change and the associated cumulative seismic energy release is strikingly different for these three cases, making them suitable targets for demonstrating the significance of the above mentioned controlling factors.
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 7280 Volcano seismology (8419)
DE: 8100 TECTONOPHYSICS
DE: 8178 Tectonics and magmatism
DE: 8400 VOLCANOLOGY
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005