Morphometric and Seismic Hazard Analysis of Achankovil Shear Zone in Part of Kerala and Tamil Nadu States, India

Numerous studies have considered Achankovil shear zone as NW-SE trending Precambrian crustal scale structure. Two major faults namely Thenmala and Thenmala south faults are also identified as associated with this shear zone by earlier studies. The present study identified segmented lineaments in these zones. The major drainages in this zone are flowing in a general NW-SE trend. The rock units surrounding these faults are also trending in NW-SE directionThe present study applied both conventional and recent geomorphic parameters to identify anomalies in the terrain. Morphometric results suggest that the area between Thenmala fault and Thenmala South fault especially the central part exhibits anomalies in parameters for example elongation ratio, bifurcation ratio and stream frequency. Recent study identified continuities of NW-SE trending faults as brittle deformation in the southeastern continuity, away from hill ranges and identified as geologically young deformation. The M=6.0 Coimbatore earthquake of 1900 is the largest event reported in the region which occurred in Palghat Cauvery shear zone. The nearest seismic source zone identified is the one located in KKPT shear zone located 70-80 km from the study area which produced a M=5.0 event. There are also several instances of historic and recent earthquakes reported in the study area. Considering the general trend of the seismic source zone reported in the peninsular India the NW-SE trending faults can generate a Magnitude M >5.0. We calculated peek ground acceleration for a magnitude of M=5.5 as the maximum credible earthquake that can generate by these two faults. The peak ground acceleration that Thirunelveli, the nearest city, would experience from Thenmala fault is of the range of 0.287-0.262.