Earthquake hazard potential of Indo-Gangetic Foredeep: its seismotectonism, hazard, and damage modeling for the cities of Patna, Lucknow, and Varanasi

Abstract

The Indo-Gangetic Foredeep region lies in close proximity to the Himalayan collision tectonics and the Peninsular Shield thereby subjecting it to repeated strong ground shaking from large and great earthquakes from these active tectonic regimes. An attempt is, therefore, made to understand the seismotectonic regime of the Indo-Gangetic Foredeep region while performing probabilistic seismic hazard modeling of its important cities of Patna and Lucknow and the religious city of Varanasi based on consideration of seismogenic source characteristics, smoothened gridded seismicity zoning, and generation of next generation ground motion attenuation models appropriate for this region along with other existing region-specific ground motion prediction equations in a logic tree framework. In the hazard modeling, peak ground acceleration (PGA) and 5% damped pseudo-spectral acceleration (PSA) at different time periods for 10 and 2% probability of exceedance in 50 years with a return period of 475 and 2475 years have been estimated at firm rock site condition (site class B/C) with an average shear-wave velocity of about 760 m/s, of which, however, the results of only 475 years of return period have been presented here for urban development and earthquake engineering point of view. Surface-consistent probabilistic seismic hazard is modeled using the International Building Code-compliant short and long period site factors corresponding to topographic gradient-derived shear-wave velocity-based site classes. The estimated surface-consistent PGA is seen to vary in the range of 0.222–0.238 g in Patna City, while it varies in the range 0.257–0.295 g in Lucknow and 0.146–0.172 g in the city of Varanasi. The cumulative damage probabilities in terms of ‘none,’ ‘slight,’ ‘moderate,’ ‘extensive,’ and ‘complete’ have been assessed using the capacity spectrum method in the Seismic Loss Estimation Approach (SELENA) using both the fragility and capacity functions for six model building types in these cities. The discrete damage probability exhibits that the building types ‘IGW-RCF2IL (PAGER/FEMA:C1L),’ ‘IGW-RCF21M (PAGER/FEMA:C1M),’ and ‘IGW-RCF11L (PAGER/FEMA:C3L)’ will suffer minimum damage, while ‘IGW-RCF21H (PAGER/FEMA:C1H),’ ‘IGW-RCF11M (PAGER/FEMA:C3M),’ and ‘PAGER/FEMA:C3H’ will suffer extensive damage in the event of a maximum earthquake of M_w 7.2 impacting the terrain as predicted from median nodal maximum magnitudes in a heuristic search in the probabilistic protocol. The estimated probabilistic seismic hazard and damage scenario are expected to play vital roles in the earthquake-inflicted disaster mitigation and management of these cities for better predisaster prevention, preparedness, and postdisaster rescue, relief, and rehabilitation. The results may also be incorporated in the building codal provisions of these smart cities as intended by the Federal Government of India.

Appendix

The computation procedure followed in the design response as given by IBC (2006, 2009) is as given below:

• (1) Compute the maximum considered earthquake spectral response acceleration at 0.2 s and 1.0 s periods as,

  $$ {S}_{\mathrm{MS}}={F}_{\mathrm{a}}{S}_{\mathrm{s}} $$

  (21)
  $$ {S}_{\mathrm{ML}}={F}_{\mathrm{v}}{S}_{\mathrm{l}} $$

  (22)

  F_a and F_v correspond to the amplification factors for acceleration response spectra at 0.2 and 1.0 s periods as listed in Table 8. S_s and S_l denote the spectral accelerations at the respective periods.

• (2) Compute the design basis earthquake spectral response acceleration at 0.2 and 1.0 s periods as,

  $$ {S}_{\mathrm{DS}}=2/3\ {S}_{\mathrm{MS}} $$

  (23)
  $$ {S}_{\mathrm{DL}}=2/3\ {S}_{\mathrm{ML}} $$

  (24)

• (3) Determine the characteristic time periods as,

  $$ {T}_0=0.2\ {S}_{\mathrm{DL}}/{S}_{\mathrm{DS}} $$

  (25)
  $$ {T}_{\mathrm{S}}={S}_{\mathrm{DL}}/{S}_{\mathrm{DS}} $$

  (26)

• (4) Construct the design response spectra as

  $$ {S}_{\mathrm{a}}=\left\{\begin{array}{c}0.6\left({S}_{\mathrm{DS}}/{T}_0\right)T+0.4{S}_{\mathrm{DS}},\kern0.5em \mathrm{for}\kern0.5em T\le {T}_0\\ {}{S}_{\mathrm{DS}},\begin{array}{cc}\mathrm{for}& T\ge {T}_0\begin{array}{cc}\ \mathrm{and}& T\le {T}_{\mathrm{s}}\end{array}\end{array}\\ {}\begin{array}{cc}& \end{array}{S}_{\mathrm{DL}}/T,\begin{array}{ccc}& \begin{array}{cc}\begin{array}{cc}\begin{array}{cc}& \mathrm{for}\end{array}& \end{array}& \end{array}& T\ge {T}_{\mathrm{s}}\end{array}\end{array}\right\} $$

  (27)

Where S_a is the design spectral response acceleration and T is the fundamental time period of the structure.