Fluids play a prominent role in the genesis of earthquakes, particularly in intraplate settings. In this study, we present evidence for a highly heterogeneous nature of electrical conductivity in the crust and uppermost mantle beneath the Kachchh rift basin of northwestern India, which is host to large, deadly intraplate earthquakes. We interpret our results of high conductive zones inferred from magnetotelluric and 3‐D local earthquake tomography investigations in terms of a fluid reservoir in the upper mantle. The South Wagad Fault (SWF) imaged as a near‐vertical north dipping low resistivity zone traversing the entire crust and an elongated south dipping conductor demarcating the North Wagad Fault (NWF) serve as conduits for fluid flow from the reservoir to the middle to lower crustal depths. Importantly, the epicentral zone of the 2001 main shock is characterized as a fluid saturated zone at the rooting of NWF onto the SWF.
The 2001 Bhuj earthquake (Mw 7.7) occurred in northwestern region of Indian peninsula has reactivated a couple of transverse faults to its surroundings. Intermediate to moderate magnitude earthquakes are occurring along these faults which includes recent Dholavira earthquake (Mw 5.1, 2012) suggesting distinct tectonic scenario in the region. We present the results of magnetotelluric (MT) impedance tensors analyses of 18 sites located along a profile cutting various faults in the uplifted Wagad block of the Kachchh basin. The MT time series of 4-5 days recording duration have been processed and the earth response functions are estimated in broad frequency range (0.01-1000 s). The observed impedance tensors are analyzed by using three decomposition techniques as well as by the phase tensor method constraining with the induction arrows. The analyses suggest distinct tectonic feature within the block bounded by the South Wagad Fault (SWF) and the North Wagad Fault (NWF) particularly in the period band of 1-10 s. In the south of NWF, the telluric vectors and the major axes of the phase ellipses are aligned in the NNW-SSE to NW-SE direction where as a dominant E-W strike is obtained for northern side of the NWF. The transverse geo-electric strike coincides with the prominent clustering of seismicity after the Bhuj earthquake and trend of the Manfara transverse fault is located in close vicinity of the study area. We therefore suggest the presence NNW-SSE trending transverse structural feature in the Wagad uplift of the basin appears to play significant role in the current seismicity of the active intraplate region.
The Kachchh Mainland Fault (KMF) in the central position of the Kachchh paleo rift basin of the northwestern India is the main fault system where the reverse movement started along preexisting normal fault planes during postrift tectonic inversion at end of the Tertiary period. We present the analyses of transient electromagnetic data at 68 sites, distributed along nine traverses across the KMF zone within the Kachchh Mainland. The resistivity sections across the KMF zone do not only image the shallow subsurface geometry of the fault but also delineated synthetic and antithetic splays of the primary faults. An experimentally estimated scaling factor from the late‐time TEM response, which is a proxy for fracture density of the geological formation, is well coincides with the fault zones that are delineated in the present study. The subsurface resistivity structure infers in general north‐dipping reverse faulting along the various segments of the KMF. Some of the en echelon splays form flower structure scenario, suggesting either presence of localized preserved transtensional features in this regional compressive regime or a combination of both positive and negative flower structure scenarios. The observed anomalous deep conducive zones in the fault zone could qualitatively suggest neotectonic activity in the region. The study supports the presence of oblique to transverse faults that bisects the primary KMF. We opined that these transverse structures along with the splays could play a key role in transferring part of stress that being accumulating in the KMF zone.
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