Seasonal modulation of seismicity in the Himalaya of Nepal

[1] For the period 1995–2000, the Nepal seismic network recorded 37 ± 8% fewer earthquakes in the summer than in the winter; for local magnitudes ML > 2 to ML > 4 the percentage increases from 31% to 63% respectively. We show the probability of observing this by chance is less than 1%. We find that most surface loading phenomena are either too small, or have the wrong polarity to enhance winter seismicity. We consider enhanced Coulomb failure caused by a pore-pressure increase at seismogenic depths as a possible mechanism. For this to enhance winter seismicity, however, we find that fluid diffusion following surface hydraulic loading would need to be associated with a sixmonth phase lag, which we consider to be possible, though unlikely. We favor instead the suppression of summer seismicity caused by stress-loading accompanying monsoon rains in the Ganges and northern India, a mechanism that is discussed in a companion article. Citation: Bollinger, L., F. Perrier, J.-P. Avouac, S. Sapkota, U. Gautam, and D. R. Tiwari (2007), Seasonal modulation of seismicity in the Himalaya of Nepal, Geophys. Res. Lett., 34, L08304, doi:10.1029/ 2006GL029192.