High resolution upper crustal velocity structure beneath Qeshm Island (SE Zagros) from surface wave, first arrival, and interevent interferometry tomography methods

Abstract The Qeshm earthquake with magnitude Mw 6.0 occurred on November 27, 2005, on a southern island in Iran and was followed by many aftershocks. We analyzed the aftershock waveforms and the first arrival traveltimes to calculate the 3D velocity model. The tomographic procedure was conducted in three different depth ranges from the subsurface to a depth of 17 km beneath Qeshm Island. The subsurface was probed by the Rayleigh wave tomographic method. The waveforms were filtered in the period range of 0.6 to 6 s, the dispersion curves were calculated for the selected signals, and a 2D tomography procedure was then performed. The resulting group dispersion measurement maps were inverted to obtain the shear wave velocity model to a depth of 5 km. However, the first arrival traveltimes were applied to obtain the VP and VP/VS models using the SIMULPS program in the depth range of 5 to 14 km, while depths ranging from 14 to 17 km were considered by the event interferometric approach. All appropriate interevent empirical Green\u0027s functions (EGFs) were extracted in the 4 different slices/depths, and the Rayleigh wave tomography processing was then repeated for these layers at the period of 1.2 s. Our results demonstrate that analysis of the interevent EGFs can yield useful information from depths for which a velocity model cannot be obtained via classic techniques. In addition to the Qeshm Fault, which appears as a stretched low-velocity anomaly, the Hormuz salt and evaporites may be other sources with low velocity. Although finding a direct relation between faulting and folding and/or any other mechanism of folding is difficult on Qeshm Island, the role of the Hormuz salt in generating them is undeniable. The rise of the Hormuz salt and evaporites, which appear as a low-velocity (weak) zone, can directly affect crustal layering (lower sedimentary cover and basement), fault dip and anticline formation and deformation at depths to 14 km. According to the effect of these tectonic features, folding and secondary faults, the soft sedimentary thickness varies between 2 and 3 km.