ALBERT

Description: Earth's free oscillations excited by a mega-thrust earthquake were observed by a continent-scale array of groundwater monitoring sites for the first time. After the occurrence of the 2011 Tohoku M w 9.0 earthquake, water level records at 43 out of 216 wells in the China mainland revealed long-period free oscillation signals. In the time domain, these free oscillations exhibit globe circling Rayleigh surface waves. In some single wells, even the globe-circling Rayleigh wave R7 was visible, which travels three times around the Earth after the first arrival and appears about 10 hr after the earthquake occurrence in the present case. The spectral analysis shows that the principal oscillatory fluctuations seen in the water level records correspond to the spheroidal modes 0 S l ( l  = 2–31 for frequencies between 0.3 and 5.0 mHz) of the Earth's free oscillation. Especially at quiet sites, the spheroidal modes at very low frequencies (〈1.5 mHz) can be identified with high signal-to-noise ratios. Using signal enhancement methods (product spectrum over 43 wells), even the gravest modes of these oscillations can be detected. The results suggest that groundwater level arrays can be considered as a low-cost complementary tool to study the Earth's free oscillations excited by great earthquakes. Additionally, the site-specific aquifer response may provide further insight into local hydrogeological conditions.

Description: Groundwater level changes induced by the 2011 M w = 9.0 Tohoku earthquake were observed by the Chinese mainland hydrological network dedicated to earthquake prediction research at epicentral distances between 1300 and 5400 km. Out of 216 wells operating during the time of the event, 73 showed sustained changes that are evenly distributed between those experiencing water level rises and falls. Water level oscillations during the passage of the seismic waves were recorded at another 85 wells. At the remaining 58 wells, no response to the earthquake was observed. No spatial pattern in the different well response types is evident, neither in terms of distance nor in terms of azimuth with respect to the epicentre. About 80 per cent of the well-aquifer systems are sensitive to Earth tides. Statistical analysis revealed that those wells that showed a response to the Tohoku earthquake are characterized by admittance factors (with respect to the major tidal constituents M 2 and O 1 ) above 0.5 m of water level change per microstrain—corresponding to 5 kPa water pressure change per microstrain or 5 GPa, whereas the majority of the non-responding wells are characterized by lower tidal factors. However, no systematical difference in tidal admittance factors or phases can be seen between wells that displayed sustained changes and those showing coseismic oscillations/fluctuations. Postseismic phase shifts in the M 2 tide were observed at 31 sites, positive in 22 cases and negative in nine cases. Such phase shifts may indicate changes in the aquifer permeability. Thus, earthquake-induced temporal variations in the permeability might have occurred at about 43 per cent of those wells that displayed sustained water level changes, but less than 15 per cent of all observed wells. Statistical methods were used to analyse the relationships between the different types of groundwater level changes, tidal amplitude and phase, well-epicentre distance and azimuth, well depth and aquifer lithology. The statistical analysis did not indicate any obvious significant relationships between water level changes and any other parameter—except the tidal admittance—indicating that the processes behind groundwater level changes induced by a distant great earthquake are complex. Further detailed studies are required to understand these underlying mechanisms.