Relocation of the 2006 Mw 6.1 Silakhour, Iran, Earthquake Sequence: Details of Fault Segmentation on the Main Recent Fault

Abstract

The M w 6.1 Silakhour earthquake occurred on 31 March 2006 in the Zagros region of Iran. It was preceded by two relatively large foreshocks of m b 4.8 and 5.2 on 30 March and followed by an extensive aftershock sequence. The earthquake sequence occurred along the right‐lateral strike‐slip Main Recent Fault (MRF). To understand the spatial relationship between the mainshock and the high level of foreshock and aftershock activity, we have merged all seismic data recorded by Iranian seismic networks with data from the global catalogs and relocated the events using a multiple‐event relocation method. We also conducted source studies of the mainshock and larger foreshocks and aftershocks, which confirm dominantly right‐lateral strike‐slip faulting for the mainshock, the two large foreshocks, and the largest aftershock. The relocated epicenters of the 2006 earthquake sequence lie close to the MRF and spread parallel to the MRF over a distance of ∼55 km, much longer than the 15–20‐km rupture length to be expected of a magnitude 6.1 strike‐slip earthquake. The relocated epicenters reveals that the seismic sequence is divided into two parts, an eastern patch on the Borujerd–Dorud segment of the MRF that is directly associated with the rupture of the mainshock and a second patch, 20–25 km long, on the Nahavand–Borujerd segment of the MRF. The two patches are offset by ∼5 km in the same sense as the MRF. The pattern of the western patch of seismicity strongly suggests a seismically active lineation that does not correspond closely to the mapped faults of the area. We take this as evidence of ongoing evolution of fault geometry on this section of the MRF. Coulomb stress calculations suggest that the seismicity on the Nahavand–Borujerd segment may be a separate earthquake sequence, triggered by static stress changes related to the mainshock rupture.