Probabilistic Seismic Hazard Analysis of Sitamarhi near Bihar-Nepal Region

: 28 This article presents the results of a probabilistic seismic hazard analysis (PSHA) for Sitamarhi, Bihar 29 considering the region-specific maximum magnitude and ground motion prediction equation (GMPEs). North 30 Bihar region is one of the seismically unstable areas in India facing several destructive earthquakes for the 31 Himalayan Mountains that was created by the collision of Indian and Eurasian plate. The Gutenberg-Richter (G- 32 R) seismic hazard parameter ‘a’ and ‘b’ have been evaluated by considering the available local earthquake data. 33 Earthquake data were collected from the United States geological survey (USGS), Indian Meteorological 34 Department (IMD), New Delhi, Seismotectonic Atlas of India (GIS 2000) within 500 km radius of the study 35 area, and 62 seismotectonic sources were identified and considered in this study. Seismic source zones for the 36 region have been defined based on large-scale geological features, which are used for assigning the maximum 37 possible earthquake potential. Estimated PGA values are 0.89 g and 0.61 g for the 2% and 10% probabilities of 38 exceedance in 50 years. The results showed that West Patna fault and Sitamarhi Fault are the two main faults, 39 which contribute maximum in the peak ground acceleration (PGA) values for Sitamarhi region. 40


MS
influenced by all the seismic sources with different magnitudes and distances. The widely used approach to 106 estimate seismic-design loads for engineering projects is probabilistic seismic-hazard analysis (PSHA). The 107 PSHA consists of four steps (Reiter 1990): i) identification and characterization of earthquake sources 108 contributing to the seismic hazard of a study area, ii) seismicity or temporal distribution of earthquake 109 recurrence must be characterized using suitable recurrence relationship, iii) use of suitable attenuation 110 relationships to predict the distribution of ground motion intensities, and iv) the uncertainty in earthquake 111 location, earthquake size and ground motion parameter prediction are combined to obtained the probability that 112 the ground motion parameter (e.g., PGA) will be exceeded during a particular time period. The primary output 113 from PSHA is the hazard curve showing the variation of selected ground-motion parameters, such as peak 114 ground acceleration (PGA) or spectral acceleration (SA), against the annual frequency of exceedance (or its 115 reciprocal, return period). The design value is the ground-motion level that corresponds to a pre-selected design 116 return period (Bommer and Abrahamson 2006). PSHA can reflect the actual hazard level due to bigger earthquakes along with smaller events, which are also crucial in the hazard estimation, due to their higher 118 occurrence rates (Das et al. 2006 After converting all earthquake records in Mw, earthquake catalogue has been rearranged based on the moment 196 magnitude as shown in Table 3.  Table   218 2) for each earthquake sources using the following relationships between Mmax and L (Wells and Coppersmith  where lnY, M, R and σ are the logarithm of ground motion, magnitude, hypocentral distance and standard 292 deviation; a , a , a , a , a , a are the corresponding regressions co-efficient. The coefficient a is equal to a 293 when M < 6.0 and R < 300, else equal to a . The comparison between the above three GMPEs has been shown 294 for a moment magnitude of 6.5 in Fig.7.

311
where, N (m ) is the earthquake frequency of seismic source n above a minimum magnitude mo; f (m) is the 312 probability density function for the minimum magnitude mo and maximum magnitude mu; f (r|m) is the 313 conditional probability density function for the distance to earthquake rupture; and P(Z > z|m, r) is the 314 probability that given a magnitude 'm' earthquake at a distance 'r' from the site, the ground motion exceeds 315 level z.

369
 Uniform hazard spectrum (Fig. 11) for the 2 % and 10 % probability of exceedance in 50 years has 370 been presented for the region which is situated at zone V as per the Indian seismic zonation map.

371
 The present result is more region-specific and advanced than the previous studies and can be used     whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors.

Figure 2
Seismic source map of the study area Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors.

Figure 3
Chart showing the no. of earthquakes vs magnitude (4.0 and above) of earthquakes, from 1900 to 2020. Comparison of PGA (g) with distance (km) at moment magnitude (MW) of 6.5 and Vs30 = 1500m/s (wherever applicable) Total hazard curves calculated using three GMPEs for Sitamarhi Uniform Hazard Spectrum (UHS) with 2% and 10% probability of exceedance in 50 years