Conditional Probability Assessment of Earthquake Occurrences in Himalayas

Abstract

The first step in evaluating the danger posed to a region by seismic activity is to conduct a Seismic Hazard Assessment (SHA). It is dependent on the region's geology and tectonic setting, large data sets, knowledge of the earth's underlying structure and physical processes, and the application of statistical and mathematical models. The SHA has been carried out for the Himalayan region utilising Time-dependent statistical models in this study. Since the instrumental era, there has been a considerable increase in interest in and relevant studies connected to the Himalayan region's seismic danger, and each destructive earthquake motivates a re-evaluation. The pattern of stress building and subsequent release in the Himalayan arc has never been consistent. The Himalaya has a discordant form of seismicity, resulting in both seismically active parts and seismic gaps, which store stress over time without releasing it, and thus a single mathematical or statistical model cannot capture the seismicity of the entire mountain belt. As a result, the seismic hazard is estimated using four time-dependent models: Weibull, Brownian Passage Time (BPT), Gamma, and Log-Normal distributions. The study area is divided into five zones by considering approximately 380 years of earthquake catalogue which are treated for Homogenization, Declustering and Completeness which are required before using the earthquake data. The non-Poissonian (or Period-dependent) probability of magnitude exceedance in a certain time in the future was investigated using the selected models for various elapsed timeframes based on historical earthquake recurrence time intervals in the Himalaya. To determine the adequacy of each stochastic model for each zone in order to describe the multiple physical processes involved in earthquake occurrence, the Kolmogorov Smirnov (K–S) test was utilised. The results show that the Weibull, BPT, Gamma, Log-normal and Log-normal were most suitable for Zones 1, 2, 3, 4 and 5, respectively, for Mw ≥ 6.0. The cumulative probability of recurrence intervals reaches up to 90% in 35 years for Zones 1 and 2, 60 years for Zone 3, 27 years for Zone 4 and 70 years for Zone 5. The most suitable models for Mw ≥ 7.0 were found to be the Log-normal for Zones 1 and 2 and 5, and Weibull for Zone 3. Only single earthquake of Mw ≥ 7.0 in zone 4 has occurred so corresponding graph cannot be traced for this zone. The cumulative probability for Mw≥7.0 reaches 90% in 65 years in Zone 1, 95 years in Zone 2, 90 years in Zone 3, and 110 years in Zone 5. The Himalayas' heterogeneous and complex tectonics, diverse plate movements, variable stress release patterns (spatially and temporally), and locking/unlocking of faults/thrusts all contribute to the various probability models that characterise earthquake occurrence events in these five zones. Finally, once the earthquake recurrence probabilities have been calculated using Time-dependent models, the Probabilistic Seismic Hazard Assessment (PSHA) for the Himalayas can be carried out to see how such modelling affects strong ground motion.