Mapping the near-surface trace of the seismically active Kachchh Mainland Fault and its lateral extension in the blind zone, Western India

The W-WNW-striking Kachchh Mainland Fault (KMF) is the largest intra-basinal fault in the Kachchh Rift Basin (KRB) located at the western continental margin of the Indian plate, and is responsible for several historic earthquakes, including the 2001 Bhuj earthquake and the 1956 Anjar earthquake. The objective of the study is to constrain the precise location and near-surface characteristics of the KMF using field and ground penetrating radar (GPR) studies. Efforts were also made to ascertain and map the eastward continuity of the KMF. The fault has a prominent geomorphic expression up to the east of Devisar, where the KMF scarp finally dies out. The KMF marks the lithotectonic contact between Mesozoic rocks in the south and Tertiary rocks in the north. The segmented nature of the KMF has been previously attributed to transverse faults, complex deformation zones and heterogeneous assemblages of unconformable Quaternary deposits. Eight survey sites between Nirona and Sikra were chosen for near-surface investigation of KMF using GPR. Comparatively, high-amplitude radar reflections were received from well-compacted sand- and shale-rich Mesozoic rocks, whereas low-amplitude weak reflections were observed from softer and clay-rich Tertiary rocks. In the upper section of the GPR profiles, Quaternary deposits display low- to moderate-amplitude semi-continuous to wavy reflections. The traceable length of the KMF has been extended eastward by similar to 20 km through the current study, which is considered a blind zone as feeble surficial evidence is present, but the fault trace is confirmed near the surface using GPR. The eastern part of the KMF is inferred to be a near-vertical north-dipping normal fault based on GPR data analysis. The tendency of the KMF to transform into a reverse fault in the vicinity of the intersection with transverse faults implies zones of relatively higher stress accumulation that might activate and trigger devastating earthquakes in the future.