• SUMER CHOPRA

      Articles written in Journal of Earth System Science

    • A local magnitude scale $M_{L}$ for the Saurashtra horst: An active intraplate region, Gujarat, India

      VISHWA JOSHI SUMER CHOPRA SANTOSH KUMAR

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      The calibration of local magnitude scale to local tectonics is essential for seismic hazard assessment and quantifying the seismicity in active regions. In the present study, we have developed a local magnitude scale $M_{L}$, for the Saurashtra region, which is a horst, located in the western continental margin of India. The local magnitude scale is developed using 1968 amplitude measurements from horizontal component recordings of 319 earthquakes, obtained from sites in the Saurashtra region, with hypocentral distances ranging from 3 to 298 km. All the 1968 amplitude measurements were inverted simultaneously to determine attenuation curve, magnitudes and station corrections for the studied region. The resultant distance correction term for the Saurashtra is $-log(A_{0}) = 1.31 log(r/100) + 0.0002 (r - 100) + 3$ for 100 km normalization, where $A_{0}$ is the distance correction and $r$ is the hypocentral distance. The distance correction term ($-log A_{0}$) suggests that the attenuation in the Saurashtra region is lower as compared to neighbouring Kachchh region. The station corrections obtained in the present study varies from $-0.31$ to $+0.24$. Overall, standard deviation of the magnitude residuals without station correction is 0.28, while with station correction, it is 0.23, which indicates that applying station correction reduces the variance by 31% and brings the average residual closer to zero.

    • Multi-criteria approach using GIS for macro-level seismic hazard assessment of Kachchh Rift Basin, Gujarat, western India – First step towards earthquake disaster mitigation

      VASU PANCHOLI NISARG BHATT PAWAN SINGH SUMER CHOPRA

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      The earthquakes have the most dominating societal and economic impact on the built environment. The earthquakes in an intraplate region are infrequent but often damaging. The uncontrolled urban growth in cities due to population explosion and migration makes it necessary to assess seismic hazards in an active region. It provides parameters for seismic safety and helps in disaster mitigation. The Kachchh Rift Basin (KRB) of western India is a seismically active intraplate region where many damaging earthquakes have occurred in the past (Mw 7.8 in 1819, Mw 7.6 in 2001). The KRB hosts many economic corridors and ports. Though the region has been put in a category with highest seismic hazard, the entire region is not prone to high hazards. The primary objective of the study is to integrate major attributes that influence seismic hazard on a GIS platform and prepare a multi-criteria-based hazard map by multi-criteria decision process named as analytical hierarchy process (AHP) developed by Saaty. In this study, the information about some of the attributes like peak ground acceleration (PGA), geology and geomorphology, and tsunami hazard is taken from published literature, whereas shear wave velocity to 30 m depth (Vs)$_{30}$ and amplification factor were obtained through empirical relationships. The integration of these different attributes was performed, and weights were assigned depending on their contribution to the seismic hazard. The multi-criteria approach reveals that the southwestern part comprising of Kachchh mainland has a low hazard as compared to central and northern parts and almost 1 million people and around 0.18 million houses are exposed to moderate to high hazard. Large swaths of land are prone to liquefaction hazard. The corridor comprising of Bhuj, Bhachau and Rapar needs seismic microzonation. This macro level hazard map will be beneficial for the urban planners and government authorities to decide the areas, where seismic microzonation or site-specific studies are required that would help in mitigating earthquake disasters in the future.

    • Integrated analysis of the gravity and the magnetic data to infer structural features and their role in prospective mineralisation in and around the Ambaji–Deri– Danta–Chitrasani region, NW India

      AVINASH KUMAR CHOUHAN SUMER CHOPRA HIMANSHU CHAUBE DINESH SINGH ABHISHEK KUMAR MISHRA

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      The Ambaji–Deri region is located in the northeastern part of the Gujarat state of India and is well-known for hosting lead–zinc–copper minerals deposits. Recently, gravity and magnetic data are collected in the region with the objective of geological and structural mapping of the area. This data is further processed using upward continuation, derivative analysis, and 2.5-dimensional gravity modelling to understand the subsurface geometry for mineral exploration. The upward continued regional gravity anomaly reveals high value in SW part of the region. The residual gravity and magnetic anomaly show the NE–SW trend, which is sympathetic with the general trend of Delhi supergroup. The high values of the residual Bouguer and the magnetic anomaly at the junction of the Jaisalmer–Barwani and the Chambal–Jamnagar lineaments are inferred as possible potential sites for sulphide mineralisation. The horizontal gradient of the tilt derivative (HGTD) of both the gravity and the magnetic anomalies reveals NE–SW trending lineaments. Based on the results of HGTD, several new structural features have been identified and a refined lineament map of the study area is proposed. The gravity modelling using residual Bouguer anomaly could delineate a high-density intrusive body in the upper crustal level. The result of the gravity model also confirms that the middle crust is uplifted by 1–3 km in the eastern part of the study area. In this study, three prospective zones for base metal mineralisation have been identified.

    • Investigation of hydrological characteristics of the Kachchh Mainland Fault (KMF) Zone, Gujarat, Western India using time domain electromagnetic study

      RAKESH NIKAM SUMER CHOPRA G PAVAN KUMAR INDU CHAUDHARY MEHUL NAGAR HIMANSHU CHAUBE DINESH SINGH DURGA PRASAD

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      The East–West oriented Kachchh Mainland Fault (KMF) is a major primary fault in an active Kachchh rift basin in the western part of India. In the present work, we made an attempt to understand the tectonic features of the KMF system and the hydrological characteristics of the fault zone using a time domain electromagnetic (TDEM) survey. The TDEM investigations are carried out at 52 sites, distributed along six profiles, three across the fault and three along the fault zone, to study the hydrological phenomenon. The resistivity sections of these six profiles are correlated with well log and lithology data provided information up to a depth of around 250 m and suggest a multilayer aquifer system. Theresistivity sections also show KMF as a sharp contact between the Mesozoic rocks and the Quaternary alluvium overlain on Tertiary rocks. Most of the groundwater potential aquifer zones are towards the South and very few groundwater potential aquifer zones are found in the North. In this region, streams are flowing towards Rann of Kachchh. We infer that the KMF zone is impervious that possibly bisects the aquifer zones and acts as a barrier for the groundwater flow in the North–South direction and conduit for the parallel flow in the East–West direction. The study highlights the significant tectonic controls of the groundwater flow in the Kachchh rift.

      $\bf{Highlight}$

      $\bullet$ The study provides the hydrological characteristics of Kachchh Mainland Fault zone.

      $\bullet$ Time domain electromagnetic investigations for Fault zone hydrology and tectonic controls of the groundwater flow in the Kachchh rift.

      $\bullet$ Delineation of groundwater potential zones around Fault zone.

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