Structural Analysis of the Dasht-e Bayaz (Iran) Earthquake Fractures

Abstract

A structural analysis is presented of the fractures formed in the fault zone associated with the Dasht-e Bayaz earthquake of August 31, 1968. The segment of the fault zone studied in detail here is 25 km long, 2 to 3 km wide, and located in the Quaternary sediments of the Nimbluk Valley. The maximum relative displacements ohserved in the fault zone (up to 450 cm left-lateral and 250 cm vertical) are concentrated in an east-west principal displacement zone 2 to 100 m wide. This zone is in turn, on a larger scale, composed of en echelon shear zones. The fault zone contains also many small fracturesdispersed throughout the area and reflecting, to a certain extent, the dominant trends of the principal displacement zone. Both on the scale of the whole fault zone and on the larger scale of the principal displacement zone, the structures are characteristic of a simple shear type of deformation. Their sense of movement and initial directions can be interpreted in terms of the Coulomb failure criterion applied to a material with an angle of shearing resistance of 35 to 40. The analysis also shows that the Nimbluk Valley contains many fault lineaments that extend into the mountains around the valley- In many places the cast-west principal displacement zone follows very precisely one of these lineaments. Evidence of fault reactivation is also found on another important lineament that crosses the principal displacement zone in a WNW-ESE direction. The earthquake fracture pattern in the Nimbluk Valley is compatible with a predominant movement along the east-west lineament, followed by stress readjustments along the WNW-ESE lineament.