Title:  Periodic seismicity: from the laboratory to Nepal

Abstract:
Small transient stress perturbations are prone to trigger earthquakes and a better understanding of the dynamics of earthquake triggering by transient stress perturbations is essential in order to improve our understanding of earthquake physics and our consideration of seismic hazard. In the Earth’s crust, these transient stress changes can be caused by various sources (passing of seismic waves, forcing by tides, hydrological load, and other seasonal climatic loads).
At the laboratory scale, we describe an experimental fault system, which mimics the natural seismic cycle, at the laboratory scale. Periodic perturbations of stresses (of variable amplitude and frequency) are superimposed to a far field loading during the frictional shearing of a granular gouge. The experimental system is instrumented so that acoustic emissions (AEs), deformations and stresses, can be continuously recorded during the experiments. Experimental results show that both background seismicity and modulation increase with loading rate, increasing perturbation amplitude and period. The magnitude-frequency distribution of AEs is also computed and we show that the Gutenberg-Richter b-value oscillates with stress oscillations.
At the field scale, Nepalese seismicity has been reported as having seasonal variations. We here re-investigate this seasonal nature by analyzing Nepalese seismicity over a period of 20 years, as detected by the Nepalese national network, restricting ourselves to earthquakes located along the eastern and central sections of the Nepalese Main Himalayan Thrust.  In the light of our experimental observation of b-value dependency to transient stressing, we investigate potential Gutenberg-Richter b-value annual variations. We use GRACE and GRACE-FO large-scale mass distribution data to resolve the stress tensor variations generated at depth by transient surface loads and find that that mean annual b-value peaks when Coulomb stress reaches its recurring lowest value during the summer monsoon, suggesting a periodic clamping of the Main Himalayan Thrust.
Our observations may help complement our understanding of the influence of low inertia stress phenomena on the distribution of seismicity, such as observations of dynamic triggering and seismicity modulation by tides or hydrological loading.