Fault Interaction and -Propagation in the South Iceland Seismic Zone in the Current Major Earthquake Sequence, On-Going since 1998.

Plate spreading of approximately 1.9 cm/yr and the eastward shift of the rift zones across Iceland builds up stress in the two transform zones connecting the displaced rift zones. Every century through historical time the 80-km-long E-W trending southern shear zone, or South Iceland Seismic Zone (SISZ), has experienced episodes of major earthquakes; each sequence often lasting a few years. The current one started in 1998 and is still on-going. The previous sequence lasted from 1896 to 1912. Even though the underlying motion is E-W oriented left-lateral slip, the earthquakes occur on N-S oriented right-lateral strike-slip faults, distributed along the shear zone. The present episode started with two M>5 earthquakes five months apart in 1998 at the western margin of the SISZ. In 2000, two ~M6.5 earthquakes occurred 3 days and 17 km apart in the center of the SISZ. Four additional events of M>5 followed the first event, of which two were dynamically triggered by the event's shear waves. On May 29 2008 the most recent earthquake of the sequence occurred in the western SISZ. It was generated by slip on two parallel N-S oriented vertical strike-slip faults, 4 km apart, with a combined magnitude of M6.3. The earthquake started on the eastern, 10-km-long Ingolfsfjall fault and waves from this event triggered slip on the western, 19-km-long Kross fault, presumably at its center. Slip on an E-W oriented fault, west of the Kross fault and active in 1998, may have contributed to the event, but so far aftershocks on this fault have not been found in the data until 53 minutes after the main event, suggesting it was not. The event was felt over most of SW Iceland and strongly felt in the near-by towns Hveragerdi and Selfoss. No human fatalities occurred, but over 30 houses were terminally damaged and live-stock killed. The closest seismic stations were saturated, but 3 cm/s peak ground velocity was recorded at 46 km distance. The event was well recorded on the continuous GPS network, with maximum horizontal displacement of 19 cm measured 2.5 km SE off the Ingolfsfjall fault. Details of the fault structures are mapped through relative location of aftershocks, showing two parallel 8-km-wide, vertical faults. Seismic data from the first few seconds are analyzed to search for the onset of slip on the Kross fault. An overview of the evolution of faulting and fault patterns of the previous earthquakes in the sequence is also presented.