Investigation of crustal stress, fractal dimension, beta value and strain rate distribution before the mainshock of the Nepal earthquake, 2015, Mw7.8: a case study of Nepal Himalaya region

On April 25, 2015, an earthquake of magnitude Mw 7.8 occurred in Nepal, revealing that the main shock occurred in a region with high crustal stress. Our analysis of the Nepal Himalaya region shows that it can be divided into two blocks based on comparative low and high b values. We found that the main shock ruptured in the eastern Nepal Himalayan region, where the crustal stress was higher than that in the western Nepal Himalayan region. The temporal variation of the b value in the region shows that from 2005 to 2015, lower b values, ranging from 0.88 ± 0.08 to 1.11 (± 0.11), were predominant. This indicates a buildup of higher crustal stress compared to the period before 2004 when b values ranged from 1.05 (± 0.10) to 1.56 (± 0.15). In addition, we observed that the eastern block region is associated with a low fractal correlation dimension (Dc) value of 1.33, which implies that the region is comparatively more stressed than the western block with a Dc value of 1.57. We analyzed the seismicity rate change in the two blocks to identify precursory signals. The beta value over time showed variations in seismicity rate change for the western and eastern blocks. A decrease in the beta value from 2003 to 2015 corresponds with a reduction in the b value, indicating a positive correlation in the eastern Nepal region during this time period. We further analyzed the geodetic measurements for the geodynamic processes occurring in the tectonic collision zone between the Eurasian and Indian plates in this part of Nepal Himalaya. The result indicates that the high shear strain rate is reaching up to about 65 nanostrains per year. The occurrence of the main event between the two high strain rate regions and in the high crustal stress area is noteworthy. Seismicity analysis and geodetic-based crustal motion monitoring are observed to be very helpful tools for estimating precursory signals of this large earthquake.