Tide-factor anomalies from observations of well level in the Sichuan Province prior to the great Wenchuan earthquake of 2008

We systematically calculated the tide-factors of M₂ and O₁ waves of the earth tide using water level data from 16 wells in the Sichuan Province acquired 2 years prior to the great Wenchuan earthquake of 2008. Our purpose was to extract anomalies in the tide-factors and analyze their variations in time and space. The results indicate that these anomalies are related to the Wenchuan quake of 2008. The numbers of wells that exhibited anomalies of the O₁ wave tide-factor increased gradually as time approached the occurrence of the quake. These anomalies were distributed in space in a discontinuous manner. In terms of such signals, it is possible to exclude the interferences from other factors such as rainfall and exploitation to some extent when we attempt to use anomalous well level variations to predict future earthquakes.