Seasonal modulation of seismicity: the competing/collaborative effect of the snow and ice load on the lithosphere

Seasonal patterns associated with stress modulation, as evidenced by earthquake occurrence, have been detected in regions characterized by present day mountain building and glacial retreat in the Northern Hemisphere. In the Himalaya and the Alps, seismicity is peaking in spring and summer; opposite behaviour is observed in the Apennines. This diametrical behaviour, confirmed by recent strong earthquakes, well correlates with the dominant tectonic regime: peak in spring and summer in shortening areas, peak in fall and winter in extensional areas. The analysis of the seasonal effect is extended to several shortening (e.g. Zagros and Caucasus) and extensional regions, and counter-examples from regions where no seasonal modulation is expected (e.g. Tropical Atlantic Ridge). This study generalizes to different seismotectonic settings the early observations made about short-term (seasonal) and long-term (secular) modulation of seismicity and confirms, with some statistical significance, that snow and ice thaw may cause crustal deformations that modulate the occurrence of major earthquakes.