Study on the Tidal Reponse of Underground Water Level below 800m

Abstract: Tidal response of underground water level is useful to study damage zones of causative faults of large earthquakes. A detailed analysis on the spectrum characteristics of the water level data acquired from the borehole of over 800m deep inside the Wenchuan earthquake fault zone suggests that water level correlates well with the earth tide. The tidal constituents of O1 (25.819h), K1 (23.934h), M2 (12.421h), S2 (12.000h) can be easily recognized from the spectrum map of the water level data. The tidal constituent M2 may play the most important role on the water level variation among these four tidal constituents because of its strongest spectrum amplitude. Further analysis shows that the tidal constituents of Mf (13.661d) and Mm (27.555d) may also play a role in the water level variation. According to a newest study of our group which suggests that the tidal force M2 has a close relationship with a special type of georesistivity pulses which appeared before large earthquakes, we suppose that there may exist a correlation between the tide force related water level variations and formation of the pulses.Key words: Tidal force, Water Level, Spectrum Characteristics, Earthquake