HR: 14:25h
AN: V23C-04    [Abstracts]
TI: Temporal variations in seismic velocities obtained from seismic tomography in Southwest Iceland
AU: * Tryggvason, A
EM: ari.tryggvason@geo.uu.se
AF: Dep. of Earth Sciences, Uppsala University, Villavägen 16, Uppsala, 75236 Sweden
AU: Bödvarsson, R
EM: reynir.bodvarsson@geo.uu.se
AF: Dep. of Earth Sciences, Uppsala University, Villavägen 16, Uppsala, 75236 Sweden
AU: Björnsson, G
EM: grb@isor.is
AF: ISOR, Grensásvegi 9, Reykjavik, 108 Iceland
AU: Roberts, R G
EM: roland.roberts@geo.uu.se
AF: Dep. of Earth Sciences, Uppsala University, Villavägen 16, Uppsala, 75236 Sweden
AB: Local earthquake (LE) seismic tomography is generally performed assuming the seismic velocity structure is stationary with time. Seismic velocities are in turn controlled by mechanical properties that may change with time, because of temporal variations of e.g. temperature, pressure, and fluid saturation. The reasons not to consider temporal variations in the seismic velocities when performing LE tomography should therefore mainly be attributed to a shortage of seismic stations and events. During the year 2000 more than 110,000 earthquakes occurred in the area of the Reykjanes Volcanic Zone (RVZ) and the South Iceland Seismic Zone (SISZ). This seismicity rate is much larger than normal, and is related to the two magnitude 6 earthquakes that occurred there in June 2000. Events were selected and divided into suitable time windows and inverted in sequence. The starting P- and S-wave seismic velocity models were based on data from 1973-98, and are considered to be very robust. Within the region considered well-resolved, variations occur mainly in four regions. Two of those coincide with two volcanic systems. One of them is the Hengill volcanic area, which consists of an active central volcano within a 70 - 80 km long fissure system. North of the volcano is the Nesjavellir hydrothermal field, in which a 400MWth geothermal power plant is operated. The changes in Vp/Vs ratio we observe in this region are gradual. For example, a decrease lasting approximately one year is observed, starting in March 1999. These observations are correlated with temperature observations from the hydrothermal wells. The other two regions where we observe variations with time are located within the SISZ, near the two large June 2000 earthquake hypocenters. Instead of a gradual change, a sudden increase is observed in the near source region after the large earthquakes. Work that involves relative locations of earthquakes is ongoing, something which we expect will refine the models further.
DE: 7223 Earthquake interaction, forecasting, and prediction (1217, 1242)
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 7270 Tomography (6982, 8180)
DE: 8135 Hydrothermal systems (0450, 1034, 3017, 3616, 4832, 8424)
DE: 8180 Tomography (6982, 7270)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005