Segmented geoelectrical characterization of the Kachchh Mainland Fault (Western India) and significance for seismic Hazard

Characterizing seismic sources is crucial for assessing seismic hazards, particularly for active faults like the Kachchh Mainland Fault (KMF), a 150 km long fault in the Kachchh region. The KMF is laterally displaced by transverse faults with different orientations (NW-SE to NE-SW). To better understand the KMF, a joint interpretation of the five North-South trending Magnetotelluric (MT) profiles (two recently acquired profiles and three earlier published ones) is conducted across various segments of the fault. These profiles covered diverse stretches across the fault, ranging from 15 to 81 km in length. Analysis of the geoelectric sections derived from 2-D MT data inversion revealed that the KMF dips to the south in the vicinity of the transverse faults while it takes on a steep north-dipping orientation farther away from the transverse faults. The central and eastern parts of the KMF are seismically active. Therefore, seismic hazard assessments is carried out by considering a magnitude Mw 7.6 scenario earthquake with a northward dip for all four segments of the KMF. To account for uncertainty, parametric testing was conducted, exploring a range of stress drop values, Kappa values, and quality factors (Q) as proposed by various studies in the Kachchh region. The maximum peak ground acceleration (PGA), of 0.85 g (under soft rock conditions with Vs of 500 m/s), is estimated due to the considered scenario earthquake along all four segments. The study revealed that the PGA decreased by 14–38% at sites south of the KMF (such as Bhuj and Bhachau) and increased by 30–47% at sites located north of the KMF (like Rapar and Khavda) compared to estimates based on a southward dipping KMF. This underlines the significance of considering and estimating variations in fault dip along its length and how such variations can impact seismic hazard assessments within tectonic plate interiors.