Abstract
Present study describes the statistical properties of aftershock sequences related with two major Nepal earthquakes (April 25, 2015, MW 7.8, and May 12, 2015, MW 7.2) and their correlations with the tectonics of Nepal Himalaya. The established empirical scaling laws such as the Gutenberg–Richter (GR) relation, the modified Omori law, and the fractal dimension for both the aftershock sequences of Nepal earthquakes have been investigated to assess the spatio-temporal characteristics of these sequences. For this purpose, the homogenized earthquake catalog in moment magnitude, MW is compiled from International Seismological Center (ISC) and Global Centroid Moment Tensor (GCMT) databases during the period from April 25 to October 31, 2015. The magnitude of completeness, MC, a and b-values of Gutenberg–Richter relationship for the first aftershock sequence are found to be 3.0, 4.74, 0.75 (±0.03) respectively whereas the MC, a and b-values of the same relationship for the second aftershock sequence are calculated to be 3.3, 5.46, 0.90 (±0.04) respectively. The observed low b-values for both the sequences, as compared to the global mean of 1.0 indicate the presence of high differential stress accumulations within the fractured rock mass of Nepal Himalaya. The calculated p-values of 1.01 ± 0.05 and 0.95 ± 0.04 respectively for both the aftershock sequences also imply that the aftershock sequence of first main-shock exhibits relatively faster temporal decay pattern than the aftershock sequence of second main-shock. The fractal dimensions, DC values of 1.84 ± 0.05 and 1.91 ± 0.05 respectively for both the aftershock sequences of Nepal earthquakes also reveal the clustering pattern of earthquakes and signifies that the aftershocks are scattered all around the two dimensional space of fractured fault systems of the Nepal region. The low b-value and low DC observed in the temporal variations of b-value and DC before the investigated earthquake (MW 7.2) suggest the presence of high-stress concentrations in the thrusting regimes of the Nepal region before the failure of faults. Moreover, the decrease of b-value with the corresponding decrease of DC observed in their temporal variations can primarily act as an indicator for possible prediction of major earthquakes in the study region.
Highlights
The physics of earthquakes can be understood with the help of aftershock sequences related with the main earthquakes [Kisslinger 1996]
The b-values, i.e., 0.75 (±0.03) and 0.90 (±0.04) for both the aftershock sequences that are found lower than the global mean value of 1.0, which implies the complexities in terms of heterogeneities present in the ruptured zones
The respective values of DC for both the aftershock sequences of first (1.84 ± 0.04) and second (1.91 ± 0.07) main-shocks of Nepal earthquake sequences indicate that the aftershocks are randomly scattered all around the two dimensional planar surface being filled up by fractures in the Nepal Himalaya
Summary
The physics of earthquakes can be understood with the help of aftershock sequences related with the main earthquakes [Kisslinger 1996]. We have seen a strong inconsistency between the observed aftershock activity and the forecasted curve during the forecasted period (Figure 2a) This implies that the earthquake sequence in the period of study cannot be fitted by a simple modified Omori law, thereby violating the self-similar process of aftershock occurrences [Utsu 2002, Shcherbakov and Turcotte 2004]. The statistical properties of both the aftershock sequences of the April 25 earthquake till before the occurrence of the May 12 earthquake have been studied carefully with respect to the GR frequency-magnitude statistics, the modified Omori law, and the fractal dimension, DC, to assess the seismic characteristics of the Nepal region. Grassberger and Procaccia [1983] formulated the correlation integral method for estimating DC and is given by
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