HR: 0830h
AN: S21D-0323    [PDF]
TI: Precise Hypocenter Relocation of Microearthquakes in the Torfaj\"{o}kull Volcanic System, Iceland
AU: * Lippitsch, R
EM: regina@esc.cam.ac.uk
AF: University of Cambridge, Bullard Labs, Madingley Road, Cambridge, CB3 0EZ United Kingdom
AU: White, R S
EM: rwhite@esc.cam.ac.uk
AF: University of Cambridge, Bullard Labs, Madingley Road, Cambridge, CB3 0EZ United Kingdom
AU: Soosalu, H
EM: heidi@hi.is
AF: Nordic Volcanological Institute, Grensasvegur 50, Reykjavik, 108 Iceland
AB: The Torfaj\"{o}kull volcanic system is one of about 30 active volcanoes comprising the neovolcanic zones of Iceland. It is located at the rift-transform junction between the Eastern Volcanic Zone and the South Iceland Seismic Zone. The central volcanic part of the system is the largest silicic centre in Iceland with a caldera of about 12 km diameter. It's high-temperature geothermal system is one of the most powerful in Iceland. Torfaj\"{o}kull is the source of persistent seismicity, where both high- and low-frequency earthquakes occur. To study the microseismicity of the volcanic area in detail a temporary array of 20 broad-band seismic stations was deployed between May and November 2002. These temporary stations were embedded in the permanent South Iceland Lowland (SIL) network, and data from nine adjacent SIL-stations were included in the study. A 'minimum one-dimensional velocity model' with station corrections was computed for earthquake relocation by inverting manually picked P- and S- wave arrival times from events occurring in the Torfaj\"{o}kull volcanic centre, beneath Myrdalsj\"{o}kull glacier south of the temporary array, and in the South Iceland Seismic Zone in the west. High-frequency earthquakes from the Torfaj\"{o}kull volcanic centre were then relocated using the program NonLinLoc, which calculates a non-linear, probabilistic solution to the earthquake location problem. From several hundred earthquakes in the Torfaj\"{o}kull area, 122 were well locatable (gap $<$ 180 degrees, more than 10 observations). Subsequently, we correlated the waveforms of this sub-dataset (around 2000 obseravtions) to define linked events, calculated the relative travel time difference between event pairs, and solved for the hypocentral separation between these events with HypoDD. The resulting high-resolution pattern shows a tighter clustering in epicenter and focal depth when compared to original locations. All earthquakes are located beneath the caldera with hypocenters between 1 and 6 km depth and most earthquakes lie within the geothermal system. The deeper earthquakes occur directly underneath the western caldera rim and seismic activity shallows towards the center of the caldera. No seismic activity has been observed in the fissure swarms to the northeast and southwest of the volcanic centre.
DE: 7220 Oceanic crust
DE: 7230 Seismicity and seismotectonics
DE: 7280 Volcano seismology (8419)
SC: Seismology [S]
MN: 2003 Fall Meeting