Seasonal modulation of seismicity in the Himalaya of Nepal

Abstract

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 six‐month 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.