Time-dependent seismicity analysis in the Northwest Himalaya and its adjoining regions

The Northwest Himalaya and its adjoining regions are one of the most seismically vulnerable regions in the Indian subcontinent which have experienced two great earthquakes [1902 Caucasus of magnitude M S 8.6 and 1905 Kangra, India of M S 8.6 (M W 7.8)] and several large damaging earthquakes in the previous century. In this study, time-dependent seismicity analysis is carried out in five main seismogenic zones in the Northwest Himalaya and its adjoining regions by considering earthquake inter-arrival times using a homogeneous and complete earthquake catalogue for the period 1900–2010 prepared by Yadav et al. (Pure Appl Geophys 169:1619–1639, 2012a). For this purpose, we consider three statistical models, namely Poisson (time independent), Lognormal and Weibull (time dependent). Fitness of inter-arrival time data is investigated using Kolmogorov–Smirnov (K–S) test for Lognormal and Weibull models, while Chi-square test is applied for the Poisson model. It is observed that the Lognormal model fits remarkably well to the observed inter-arrival time data, while the Weibull model exhibits moderate fitting. The parameters A and B of the time-dependent seismicity equation $$\ln {\text{IAT}} = A + BM \pm C$$ ln IAT = A + B M ± C (where ln IAT is the log of inter-arrival times of earthquakes exceeding magnitude M and C is the standard deviation), developed by Musson et al. (Bull Seismol Soc Am 92:1783–1794, 2002) are evaluated in each of the five main seismogenic zones considered in the region. The mean of the inter-arrival times for the Lognormal distribution is found to be linearly related to the lower-bound magnitude (M min). Values of the slope (B) of the mean vary from 2.34 to 2.57, while the parameter A ranges from −9.06 to −7.01 in the examined seismogenic zones with standard deviation ranging from 0.21 to 0.38. It is observed that the Hindukush–Pamir Himalaya and Himalayan Frontal Thrust exhibit higher seismic hazard (i.e., high seismic activity and low recurrence periods), while the Sulaiman–Kirthar ranges show the lowest. The variation in estimated seismicity parameters from one zone to another reveals high crustal heterogeneity and seismotectonic complexity in the study region.