Stochastic Modeling of Earthquake Interevent Counts (Natural Times) in Northwest Himalaya and Adjoining Regions

Abstract

In the driven nonlinear complex dynamical earthquake system in which the event occurrences are distributed along a frequency-magnitude spectrum, “natural time” statistics can be utilized to evaluate the contemporary state of earthquake hazards in a region. The natural times, in contrary to the clock/calendar times, are nothing but the interspersed number of small magnitude counts between successive large earthquake events in a fixed area. Natural times are positive and often random in nature. In this paper, our aim is to investigate the best-fit probability distribution in order to develop natural time statistics in the seismogenic northwest Himalayan orogen including some part of north-central India, east-northeast Pakistan and its contiguous regions. We consider eight continuous probability distributions to fit the observed natural time data. We use maximum likelihood strategy for model parameter estimation and several goodness-of-fit measures for model prioritization. Results based on the natural times corresponding to M≥3 events between M≥6 events reveal that the exponential, exponentiated exponential, Weibull and exponentiated Weibull distributions provide the best fit to the observed natural times in the study area. In addition, assuming that the seismicity statistics of larger northwest Himalaya region is indifferent from the “local” regions (e.g., cities) embedded in the larger area, we calculate “nowcast” values for a number of cities, namely Jammu, Ludhiana, Chandigarh, Shimla, Dehradun and New Delhi, to assess the current state of earthquake hazards in these cities. It is found that their earthquake potential scores (%) are 99, 89, 86, 87, 83 and 58, respectively. From these results, we argue that the concept of natural times and thereby nowcasting technique provide a rapid, alternative and effective way to analyze earthquake hazards in a seismic region.