HR: 09:15h
AN: S51D-05    [Abstracts]
TI: Fault Mapping in the Hengill Region, SW Iceland by Joint Interpretation of Microearthquake Distribution and Collective Focal Mechanisms.
AU: * Vogfjord, K S
EM: vogfjord@vedur.is
AF: Icelandic Meteorological Office Physics Department, Bustadavegur 9, Reykjavik, 150 Iceland
AU: Slunga, R
EM: slunga@foi.se
AF: Uppsala University, Department of Geophysics, Villavagen 16, Uppsala, S-752 36 Sweden
AB: Joint interpretation of microearthquake distribution and individual focal mechanisms is used to map faults and slip directions during a period of uplift and large-scale faulting in the Hengill region, Southwest Iceland. Earthquake locations are obtained through cross-correlation of P and S waveforms and subsequent inversion of relative arrival times; focal mechanisms are obtained by inversion of spectral amplitudes of P and S waves. The tectonic activity, during the years 1993 to 1998, at the intersection of the Western Volcanic Zone and the South Iceland Seismic Zone (SISZ) induced over 80 thousand microearthquakes. The cause has been attributed to emplacement of magma into the crust, under the Hengill region. The often episodic seismic activity was mostly confined to the region north of the center of uplift, which is at the northern margin of the SISZ, but occasional swarms also occurred at the southern margin of the SISZ. Faulting, however, did not ruptured through the Seismic Zone until June 1998, when the seismic activity culminated in in a three-day swarm producing over two thousand earthquakes. The activity started north of the uplift center and finally ruptured south, through the SISZ on the second day. This breaking through the SISZ barrier produced a M~5 event and two M~4 events. Later the same year the volcanio-seismic episode finally ended in another three-day swarm confined to the southern margin of the SISZ, which produced a second M~5 earthquake. In the region north of the uplift center, the faults revealed by the microearthquake analysis show an interweaving pattern of strike directions, mostly varying between NE and E, but with some short N trending sections. The fault sections are mostly near-vertical and commonly a few hundred meters to a kilometer long. Slip directions are generally right lateral on NE striking faults and left lateral on E striking faults. Normal faulting is also observed. In the SISZ, West and South of the center of uplift, the predominant fault direction is N and fault patches can be as long as 3 km. Slip direction on these is predominantly right-lateral strike-slip, characteristic for faulting in the SISZ. Event distribution around the epicenter of the two main shocks indicate that they occurred on near-vertical N striking faults in agreement with the focal mechanisms. An approximately 1 km long arching fault scarp, which was mapped on the surface, 1 km northeast of the M~5 June epicenter, therefore is probably not due to the main event.
DE: 7230 Seismicity and seismotectonics
DE: 7215 Earthquake parameters
SC: Seismology [S]
MN: 2004 AGU Fall Meeting