Seismicity and the State of Stress in the Dezful Embayment, Zagros Fold
and Thrust Belt
Abstract
This study focuses on determining the orientation and constraining the
magnitude of the present-day stress in the Dezful Embayment in Iran’s
Zagros Fold and Thrust Belt. Two datasets are used: the first includes
petrophysical data from 25 wells (3 to 4 km), and the second contains
108 earthquake focal plane mechanisms mostly occurring in blind active
basement faults (5 to 20 km). Formal stress inversion analysis of the
focal plane mechanism demonstrates that the major basement faults are
reverse faults with ( =2.0-2.2). The seismologically determined
SHmax direction is 37{degree sign}{plus
minus}10{degree sign}, nearly perpendicular to the strike of most
faults in the region. However, borehole geomechanics analysis using rock
strength and drilling evidence leads to the counterintuitive result that
the shallow state of stress is a normal/strike-slip regime. These
results are consistent with the low seismicity level in the sedimentary
cover in the Dezful Embayment, and may be evidence of stress decoupling
due to the existence of salt layers. This finding also aligns with the
Mohr-Coulomb faulting theory in that the N-S strike-slip basement
Kazerun fault has an unfavourable orientation for slip in a reverse
fault regime with an average SW-NE SHmax orientation.
The stress state situation in the field was used to identify the
optimally oriented fault planes and the fault friction factor. The
results are useful for determining the origin of seismic activity in the
basin and better assessing fault-associated seismic hazards in the area.