During the 2015 Gorkha earthquake (Mw7.8)
sequence, numerous aftershock events recorded by the local seismic network of
Nepal provide useful dataset for studying the detailed spectral characteristics
of small to moderate earthquakes in central Nepal. We analyzed P-wave spectra
from 280 events (4.0 ≦ ML ≦ 5.7), recorded at 12 short-period stations within
one year after the mainshock. We performed the spectral inversion method using
the reference site approach to isolate source, path and site terms from the recorded
spectra. From the result of the inversion, we found out that the quality factor
of P-wave (Qp) depends on frequency and fits well to the frequency-dependent Qp
model (Qp =90f 0.7) in the frequency range up to 16 Hz. Displacement source
spectra obtained by correcting frequency dependent path and site effects fairly
follow the omega-squared source model over the entire magnitude range of
analysis. By considering circular Brune-type source model, source parameters
such as seismic moment, corner frequency, source radius and stress drop were
estimated. The estimated stress drop values range from 2.1 to 95.4 MPa showing
positive dependency on size of events. We also evaluated the spatial
distribution of stress drop values with relation to co-seismic slip during the
mainshock. The result does not indicate any significant variation between
stress drop of events in the large co-seismic slip area than those in the small
slip area.
Keywords: Spectral inversion method,
Q-factor, source parameters, Gorkha earthquake aftershocks