FAULT SEGMENTATION AND STRESS TRIGGERING IN THE 2006/03/31 SILAKHOUR EARTHQUAKE

The 6.1 Mw Silakhour earthquake occurred on March 31, 2006, in Lorestan province in the Zagros region of western Iran. It was preceded by two relatively large foreshocks of 4.8 and 5.2 mb on March 30 and followed by two relatively large aftershocks of 4.7 and 4.9 mb on March 31, and a large number of smaller aftershocks. The earthquake sequence occurredalong the right-lateral strike-slip Main Recent Fault (MRF). Our moment tensor inversion shows a mostly right-lateral strike-slip faulting with a strike favorably parallel to MRF for the mainshock, the two large foreshocks and the larger aftershock. Using accelorometric stations, we estimate the depth of mainshock and the two large foreshocks to be around 7 km. To understand the spatial relationship between the mainshock and the high level of foreshock and aftershock activity, we relocated the events using a multiple event relocation method. The relocated epicenters of the earthquake sequence lie up mainly on the northeast side of the MRF, and spread parallel to the MRF over a distance of about 55 km.This is much longer than the 10-15 km fault dimension to be expected of a magnitude 6.1 earthquake. The relocated epicenters reveal a segmentation where the MRF steps up to the north, nearby Borujerd. The seismicity of the 2006 sequence is divided into two parts, an eastern patch of 10-15 km on the Borujerd-Dorud segment of the MRF that is directly associated with the rupture of the March 31 mainshock, and a western patch, 20-25 km long, on the Nahavand-Borujerd segment of the MRF. The two patches are offset by about 5 km in the same sense as the MRF, and separated by a nearly aseismic gap. The seismicity on the Nahavand-Borujerd segment, which includes the two largest aftershocks, began almost simultaneously with the mainshock rupture. Our Coulomb stress modeling suggests that seismicity on the this segment have been triggered by static stress changes related to the mainshock.