HR: 10:50h
AN: S32B-03    [Abstracts]
TI: Fault Patterns in the South Iceland Seismic Zone Revealed by Double-Difference Mapping of Microeartquakes
AU: * Hjaltadottir, S
EM: slauga@vedur.is
AF: Icelandic Meteorological Office, Bustadavegur 9, Reykjavik, IS-150 Iceland
AU: Vogfjord, K S
EM: vogfjord@vedur.is
AF: Icelandic Meteorological Office, Bustadavegur 9, Reykjavik, IS-150 Iceland
AU: Slunga, R
EM: slunga@foi.se
AF: Uppsala University, Geophysics Department, Villavagen 16, Uppsala, 75236 Sweden
AB: The South Iceland Seismic Zone (SISZ) is an approximately 70 km long 15 km wide left lateral shear zone connecting the two rift zones in southern Iceland, the Western Volcanic Zone and the Eastern Volcanic Zone. Historically earthquakes of magnitude 6-7 have occured in this shear zone a few times per century. Even though the trend of the SISZ is east, major faulting in the zone takes place on right lateral, N-S trending faults, leading to "book-shelf" type faulting. Many of the major historic faults have been mapped on the surface, but until now their sub-surface construction has never been revealed. Following two M=6.5 earthquakes in the SISZ in June 2000, seismic activity in SW-Iceland greatly increased. The roughly nineteen thousand microearthquakes recorded in the area during the following six months have been relocated, using a double-difference relocation method, with cross-correlation of wave forms to determine relative travel times with increased accuracy. Due to the high clock-accuracy of the Icelandic SIL-network, this can increase location accuracy to the order of tens of meters, and therefore enable fault mapping by grouping together events that form apparent lineaments. Mechanisms are calculated for each event using polarities and spectral amplitudes. Joint interpretation of the event distribution with focal mechanisms can confirm the assumption of a common fault and enables the determination of slip direction on it. Through this type of mapping, a high resolution image of the two M=6.5 events' fault surfaces is revealed. The aftershock activity on the N-S striking, near vertical June 17 fault is mainly confined to its margins and center, and the fault is composed of three patches, each striking a few degrees east of the overall fault strike. The June 21 fault, on the other hand, is more linear, but with varying dip. South of the epicenter the fault is vertical but north of it, dip changes to 77 degrees. The different characteristics observed on the two faults may help explain the different source-time functions observed from the two events. Several other smaller faults in SW-Iceland have also been mapped. These include separate patches of historical faults in the SISZ, and the rough fault-outlines of two earthquakes on Reykjanes Peninsula dynamically triggered by S-waves from the J17 event. Size determination of these two faults has proven problematic by other means.
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 7250 Transform faults
DE: 8123 Dynamics: seismotectonics
SC: Seismology [S]
MN: Fall Meeting 2005