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The Aravalli–Delhi Fold Belt (ADFB) is one of three prominent ridges on the continental part of the Indian plate extending northward from central India and subducting beneath the Himalayan arc. Though it is seismogenic along its entire NE–SW structural trend, activity varies spatially with a noteworthy region of higher seismicity in the vicinity of Delhi. We analyse historical and modern instrumented seismicity. Using recomputed focal mechanisms for events such as the 1956 Bulandshahr earthquake, we found that the maximum principal stress (σ1) in this region is oriented in the N–S to NE–SW direction, similar to that in the neighbouring Garhwal–Kumaun Himalayas. Our analysis also indicates that the earthquakes along the ADFB occur on steep faults which are parallel or oblique to its NE–SW structural trend and that the entire crust appears to be seismically active as the earthquakes occur up to a depth of ~40 km. Within the better instrumented Delhi region, we observed a pattern in the average monthly frequency of earthquakes recorded that we speculate was modulated by monsoonal recharge of the local water table. The cause of seismicity in the AFDB is still a question for debate, pending better seismological and geodetic instrumentation, but we build on evolutionary models of the ADFB to suggest that seismicity is the consequence of the continued relative motion between the Bundelkhand and Marwar cratons in the northern Indian shield. The nature and causes of earthquakes in this region notwithstanding, the proximity of large cities such as Delhi, Jaipur and Udaipur to poorly-studied or unknown sources of seismic hazard is a concern and warrants further scientific and policy intervention. [ABSTRACT FROM AUTHOR]

Seismic activity parameters (a $a$ and b $b$ values in a Gutenbdataer–Richter relationship, maximum magnitude Mmax ${M}_{\max }$, and predominant focal depths H $H$) have been estimated for a grid of square cells of size 0.1° in latitudes and longitudes covering a large region bound by 24°−33°N and 72°−82°E to define input seismicity for microzoning of the Delhi metropolitan area. For this purpose, seven area types of seismic sources are first identified from a detailed seismotectonic evaluation of the region. Two of these sources form part of the Northwestern Himalaya and the remaining five encompass the intraplate area including and surrounding the National Capital Region. The seismic activity parameters are then estimated for each source zone by compiling a comprehensive catalog of 4483 past earthquakes with magnitudes of 2.0 or more covering the 1720–2020 period. All the grid cells in each source are initially assigned the same values of parameters b $b$, Mmax ${M}_{\max }$, and H $H$ as those for the source zone; whereas parameter a $a$ has been redefined for each grid cell using the cumulative occurrence rate of earthquakes above the minimum magnitude of completeness for the source zone. To account for possible uncertainties in past earthquake locations, values of the activity parameters thus assigned to all the grid cells in the region are spatially smoothed using an elliptical Gaussian kernel with a major axis along the predominant strike direction for the source zone of each grid cell. The grid cells across the source zone boundaries are also included in the smoothing process to normalize the effects of any subjectivity introduced in defining the source zone boundaries. Our estimations of seismic activity parameters thus obtained can be considered in the development of a robust and physically realistic seismicity model for practical engineering applications, which was made possible by our use of a sufficiently long duration of the earthquake catalog along with complete reporting of data up to magnitudes very close to the expected maximum magnitude in each source zone. We have also estimated the seismic activity parameters for the Hindu Kush subduction zone, which is then idealized by a point source. [ABSTRACT FROM AUTHOR]

Construction of metro transport networks has been on the ascent in Indian cities like New Delhi, since early 2000s. Metro tunnels in New Delhi traverse through various geotechnical profiles of altering stiffness at different cover depths. High water tables (< 5 m) are encountered in New Delhi, especially in north and north-eastern parts. Another concern is the possibility of tectonic activities, as New Delhi is placed in a zone of high seismic intensity. These issues could ultimately result in tunnels being constructed in liquefiable deposits, especially at low cover depths. Therefore, a study was performed for New Delhi, using site-specific artificial ground motion at different cover depths to identify the significant concerns. Tunnel uplift and thrust increments on the tunnel liner have been identified as issues to be addressed. The study culminated with suitable design suggestions like grouting, which could avoid these detrimental effects. [ABSTRACT FROM AUTHOR]