Earth's free oscillations excited by the 2011 Tohoku Mw 9.0 earthquake detected with a groundwater level array in mainland China

Abstract

Earth's free oscillations excited by a mega-thrust earthquake were observed by a continentscale array of groundwater monitoring sites for the first time. After the occurrence of the 2011 Tohoku Mw 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 0Sl (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 sitespecific aquifer response may provide further insight into local hydrogeological conditions.