Seismic microzoning of the Delhi metropolitan area, India— I: Seismicity modeling

Abstract

Seismic activity parameters ( a $a$ and b $b$ values in a Gutenbdataer–Richter relationship, maximum magnitude M max ${M}_{\max }$, 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 $b$, M max ${M}_{\max }$, 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.

Characterization of strong‐motion recording sites in the Himalayas and adjoining areas

“…The major structural features in the IGP include numerous basement faults, ridges, and basins, such as the Great Boundary Fault, Moradabad Fault, Patna Fault, Lucknow Fault, Delhi‐Haridwar Ridge, Faizabad Ridge, and Munger‐Saharsa Ridge 53 . The IGP is characterized by much lower seismic potential than the Himalayan region, but the great Himalayan earthquakes may contribute significantly to the seismic hazard in this region also 54–56 …”

“…53 The IGP is characterized by much lower seismic potential than the Himalayan region, but the great Himalayan earthquakes may contribute significantly to the seismic hazard in this region also. [54][55][56] Details of all the 200 recording sites considered in this study are given in Table A1 in Appendix A. The various columns from left to right in this table give the serial number, the full name and a three-character code, location in terms of latitudes and longitudes, elevation from mean sea level, number of three-component strong-motion records obtained, and the surface geology in brief for each of the strong motion recording sites.…”

Seismic zoning maps of the National Capital Region (NCR) of India

“…Gupta and Trifunac 14 The NCR is also vulnerable to seismic hazards due to earthquakes in the western Himalayas and the HKS sources, in addition to local earthquakes in close proximity. The three different types of seismic sources considered to contribute to the NCR seismic hazard are described by separate path-dependent attenuation of PSV amplitudes similar to that in Gupta et al 5,6 for the seismic microzonation of NCT of Delhi. This has helped in developing a very realistic characterization of the spatial variation in the seismic hazard due to seismicity alone.…”

“…The seismic sources and spatial distribution of the seismic activity parameters used in this study are the same that Gupta et al 5 detailed and used for the microzonation of the Delhi metropolitan area. 6 The modeling of the seismic activity for the local seismic sources covering the intraplate NCR area and its vicinity, as well as the north-western Himalaya (NWH) plate boundary sources is described in Section 4, while those of the HKS source at distances of about 1000 km is described in Section 5 of Gupta et al 5 The method of describing seismicity is similar to that reported in Lee et al, 7,8 Lee and Trifunac, 9,10 and Gupta et al 11 The path-dependent attenuation of spectral amplitudes has been accounted realistically using scaling equations based on attenuation models developed using strong motion data recorded from local events in the NCR of India, 12 events in the western part of the Himalayan Plate Boundary region recorded within the Himalaya and the adjacent Gangetic plains, 13 and from deep events in the HKS zone recorded at very long distances of around 1000 km in western Himalaya. 14 Lacking both the exact and detailed information on the local geological and soil site parameters to incorporate directly into the hazard computation, we have generated zoning maps of NCR for nine possible combinations.…”