Fast report of Bastak Earthquake, Zagros-Iran Mw 5.5 (EMSC), fD ~10 km, 02-Jan-14 local time: 06:43
By: Behnam Oveisi
The Zagros fold belt results from active collision of the Arabian plate with central Iran, and is characterized by the development of a spectacular >200 km-wide fold-train in its sedimentary cover. Lacomb et al. (2006) suggest 025° (±15°) late folding compression regime across the Zagros belt and the southern Iranian Plateau. Their result supports an overall mechanism of buckling of the detached Zagros cover. There is evidence that deformation within the SFB cover has migrated from NE to SW through time. Hessami et al. (2001) but seismicity distribution pattern mostly cover the internal portion of the Zagros Mountain between Mountain Front Fault (MFF) and the High Zagros Fault. GPS and geochronological data suggest that shortening in central Fars is concentrated within ?100 km of the Persian Gulf coastline (Walpersdorf et al. 2006; Oveisi et al. 2009). Oveisi et al. (2009) suggest that the sedimentary cover of the frontal Zagros is decoupled from the basement, most probably at the level of the Hormuz Salt. This weak basal detachment level, together with several intermediate décollement levels, appears to be responsible for the overwhelmingly aseismic deformation of the Zagros sedimentary cover, and also to control the development of a large panel of fold structures, from detachment to fault-propagation folds with varying wavelength and rooting depth. Suppe and Medwedeff (1990) have mentioned that buried fault models indicate that forming and growing fault propagation folds (FPF) depends on fault geometry. The Bastak earthquake event occurred 70±10 km from coastline and below the Lavar master fold structure. This moderate magnitude seismic event shows that growing of the Lavar fold structure is not completely aseismic process. We don’t expect, however, a surface rapture due to the Bastak Earthquake (Centroid depth ~10 km, EMSC) along the frontal limb of the anticline or along the fold axis which is inclined toward frontal limb, but it can be suggested that some changes create decreasing in internal angle of the fold axis. In other word, we expect that distribution of axial plane displacements did not reach to the surface but the fold structure push to be more closed (decreasing in internal fold angle). A systematic morphotectonic studies and/or a local geodetic network may show evidence of fold shortening mechanism. It suggests that there is an interaction between normal fold growth (tectonic shortening) and salt diapirism. Based upon the fast remote sensing on the backward limb of the anticline, it seems that an overhang of a diaper is close to the surface (see amphitheatre structure at the backward fold limb).