On April 25, 2015, Nepal was struck by the M(W)7.8 Gorkha earthquake followed by an
intense aftershock sequence. It was one of the most destructive earthquakes in the
Himalayan arc, causing more than 8900 fatalities. In this study, we analyzed the dataset
(429 events, magnitude of completeness (M-c) >= 4.2 local magnitude) of the first
45 days after the Gorkha earthquake to estimate the seismicity parameters b-value,
D-value, and p-value. We used the maximum likelihood method to estimate the b-value
and Omori-Utsu parameters, whereas the correlation integral method was applied to
estimate the fractal dimension (D-value). The analysis was carried out using running
and sliding window techniques. The lowest b-value (0.57 +/- 0.04) and the highest
D-value (1.65 +/- 0.02) were computed at the time of the Gorkha earthquake, after
which the b-value significantly increased to a maximum of 1.57. It again dropped to
0.93 at the time of the major aftershock on May 12, 2015. The D-value showed an initial
quick drop and then decreased in a wavy pattern until the end of the study period,
indicating the clustering and scattering of earthquakes in a fault region. The b-value
contour map identified the eastern part of the study area as a high stress region
(b = similar to 0.8), implying that the stress shifted to that region. The D-value
contour map reveals that the seismogenic structure shifted from linear to planar in
the region. The rate of aftershock decay (p = 0.86 +/- 0.04) for a short period reflects
that the level of stress decreased rapidly. This study helps to understand the level
of stress and seismicity pattern of a region, which could be useful for aftershock
studies.
