Seismic microzoning of the Delhi metropolitan area, India—II: Hazard computation and zoning maps

Abstract

We present seismic microzonation maps of New Delhi, India, using the probabilistic seismic hazard analysis (PSHA) method with input seismic activity parameters estimated in Part-I of this two-part paper. Our calculations required three different attenuation equations for amplitude scaling of strong ground motion from three principal contributing sources: the National Capital Region (NCR), Northwestern Himalaya (NWH), and the Hindu Kush subduction (HKS). We show that uniform hazard spectral (UHS) amplitudes are dominated only by the local seismicity in the NCR at high frequencies beyond about 2 Hz. For intermediate and long periods, UHS amplitudes are dominated by contributions from the NWH and HKS earthquakes. Our results show that specifying strong motion amplitudes for use in engineering design by means of peak ground acceleration can lead to serious errors for buildings higher than four to five stories and hence should not be used. Our results also show that the shape of design spectra must depend on the nature of contributing earthquake sources, their relative activity, potential to create large magnitudes, and geographical placement relative to the site of interest, and thus a standard design-spectrum shape cannot satisfy all these requirements. We use local geological site condition parameters directly in all calculations and present hazard maps for three different soil site conditions ("rock," stiff soil sites, and deep soil sites). Thus, the user can extract the UHS amplitudes of pseudo relative velocity spectra at any site in the National Capital Territory by experimentally determining the local soil site conditions and then applying the corresponding maps shown in this paper. K E Y W O R D Scontribution from a large distant earthquake to the design strong motion amplitudes; computation of seismic hazard for earthquake sources, which follow different attenuation laws; microzonation maps that include a description of site geology and site soil properties

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

“…The trends shown in Figures 8−11 are similar to what we found for seismic hazard mapping in Delhi 6 and in Guwahati, 11 where the UHS was influenced by the earthquakes from regional seismicity and large distant earthquakes. This is also similar to what was found for seismic hazard mapping in Serbia, where the calculated UHS had contributions from local seismicity and from distant large earthquakes in the Vrancea source zone 10 .…”

“…Such a large distance is unusual for any analysis of seismic microzonation, and the recorded strong motion is also rarely available at such large distances as well. However, our scaling equation for HKS events works at these large distances, which was also illustrated in Gupta et al, 6 by comparing the recorded and predicted PSV amplitudes at three stations-Chamoli, Dhanaulti, and Roorkee-at distances of 1010, 940, and 966 km, respectively (Figure 2A).…”

“…As in Gupta et al, 6,11 the probability that some spectral amplitude S ω ( ) will be exceeded at least once in Y years is 41,42 p S ω…”

“…As in Gupta et al, 6,11 the probability that some spectral amplitude $$S(\omega )$$ will be exceeded at least once in Y years is 41,42 $$$p[S(\omega )]=1-{{\rm{e}}}^{-{N}_{{\rm{E}}}[S(\omega )]}$$$where $${N}_{{\rm{E}}}[S(\omega )]$$ is the expected number of times that $$S(\omega )$$ will be exceeded at the site in Y years. The recurrence time of a given amplitude $$S(\omega )$$, $$T[S(\omega )]$$, is then $$$T[S(\omega )]={N}_{{\rm{E}}}{[S(\omega )]}^{-1}$$$for the same time unit of Y years.…”

“…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.…”