A Geological Study of Earthquakes in Kutch, Gujarat, India

In this paper an attempt has been made to work out a geological model explaining the cause of earthquake rupture nucleation. Kutch rift is characterised by foothill uplifts and hanging wall half-grabens bound by quasi-vertical faults, which are resurgent primordial faults of Mid-Proterozoic Delhi fold belt. The structural style, tectonic cycles, fault kinematics and seismological data are reviewed to identify the causative fault and the critical zone for repeated earthquakes in Kutch. The Kutch Mainland fault (KMF) along the rift axis is the principal weak zone within the rift. Initiated as normal fault during Mesozoic rifting the near vertical KMF became a right lateral strike slip fault during post-Cretaceous inversion. The fault side steps to the left and continues to the east as the South Wagad Fault (SWF). This SWF is identified as the most active segment for repeated earthquake nucleation. The over step zone between them is a convergent transfer zone and is presently under compressive stress.

Two first order basement ridges orthogonal to the rift axis are the unique features of this rift basin: the Median High occurs across the middle of the basin along its hinge zone and Radhanpur-Barmer arch occurs along the eastern rift margin separating it from the N-S Cambay rift. The Radhanpur-Barmer arch along the western rift shoulder of Cambay rift, acts as a ramp for the present northeast-directed compressive stress. Due to this the strain build up in the eastern part of Kutch rift is relatively more than in the western part, west of the Median High. The mounting stress in the eastern part is responsible for repeated thrusting along SWF. Presumably, the SWF would tend to flatten towards a detachment surface close to the base of the seismogenic layer where thrusting is expected. Thus, SWF becomes the active segment (eastern trace of KMF) of the principal fault causing repeated thrust at depth in mid-crustal region. The fading KMF is likely to converge with SWF at dcpth. Aftershock data corroborate this interpretation. The transfer zone where the epicenters of both 1956 Anjar and the recent 2001 Bhuj earthquakes are located close to each other is the critical area for the occurrence of earthquakes. The focal depths (15-20 km) seem to be occurring in the zone of flattening of SWF, which appears to be the critical site for rupture nucleation. Accordingly this segment is interpreted as the vulnerable sire for repeated ruptures causing earthquakes.