Probing earthquake dynamics through seismic radiated energy 1 rate : illustration with the M 7 . 8 2015 Nepal earthquake

11 Dynamic characterizations of earthquakes focus on whole-event representations, that is 12 whether the total radiation of seismic waves is more or less energetic. Denolle et al. [2015] 13 and Yin et al. [2018] suggest to use the source spectrogram in order to analyze the radia14 tion during the rupture itself. Here, we take a retrospective view on these studies to better 15 establish the methodology of source spectrogram, and highlight its strengths and limita16 tions. We provide clear interpretation of the temporal evolution of the source spectrogram 17 through time-variant high-frequency falloff rate and radiated energy rate using canonical 18 kinematic and pseudo-dynamic examples. The radiated energy rate provides the amount 19 of energy radiated through time and its integral is the total radiated energy. It is most sen20 sitive to fault heterogeneities in the local slip-rate function and its peak, and in rupture 21 velocity. The high-frequency falloff rate peaks at times of zero moment acceleration, but 22 remains constant otherwise and theoretically equal to one. The M7.8 2015 Nepal earth23 quake exemplified the propagation of a slip pulse and is thus perfectly suited to demon24 strate this approach. We use 3D empirical Green’s functions to remove wave propagation 25 effects and construct the P-wave source function. We then construct spectrograms and ex26 plore the variations in the radiated energy rate functions. We find that the Nepal earth27 quake radiated seismic waves at the beginning and at the end of the rupture, but not dur28 ing the phase of high moment release. Finally, we interpret our results in light of rupture 29 dynamics, i.e. the earthquake initiation, propagation, and arrest. 30