The 2006 March 25 Fin earthquakes (Iran)-insights into the vertical extents of faulting in the Zagros Simply Folded Belt

Abstract

SUMMARY We investigate the depth of faulting and its connection with surface folding in the Zagros Simply Folded Belt of Iran. Our focus is a sequence of earthquakes (Mw 5.7, 5.5, 5.2, 5.0, 4.9) that struck the Fin region, in the south-eastern Simply Folded Belt, on 2006 March 25. Modelling ground displacements measured with radar interferometry, we find that either N- or S-dipping model reverse faults can reproduce the observed fringe patterns. Despite the uncertainty in fault orientation, we can constrain the vertical extents of rupture to between a top depth of ∼5–6 km and a bottom depth of ∼9–10 km, consistent with the ∼8 km centroid depth of the largest earthquake. We suggest that the faulting ruptured the thick ‘Competent Group’ of Paleozoic and Mesozoic conglomerates and platform carbonates, which makes up the lower part of the sedimentary cover. The rupture probably terminated within the Precambrian Hormuz salt at its base, and the Cretaceous Gurpi marls at its top. These mechanically weak layers act as barriers to rupture, separating faulting within the Competent Group from deformation in the layers above and below. The pattern of coseismic surface uplift is centred on the common limb of the Fin syncline and Guniz anticline, but is oblique (by 20 ◦ ) to the trend of these open, symmetric, ‘whaleback’ folds, and also overlaps a section of the Fin syncline axis. These observations suggest that locally, surface folding is decoupled from the underlying reverse faulting. Although the Fin syncline and Guniz anticline are symmetric structures, some other nearby folds show a strong asymmetry, with steep or overturned southern limbs, consistent with growth above N-dipping reverse faults. This suggests that the Simply Folded Belt contains a combination of forced folds and detachment folds. We also investigate the distribution of locally recorded aftershocks in the weeks following the main earthquakes. Most of these occurred at depths of ∼10–30 km, with a particularly high concentration of events at ∼20–25 km. These aftershocks therefore lie within the crystalline basement rather than the sedimentary cover, and are vertically separated from the main rupture. This study confirms earlier suggestions that earthquakes of Mw 5–6 are capable of being generated within the thick ‘Competent Group’ of Paleozoic and Mesozoic sediments, as well as in the basement below the Hormuz Salt Formation.