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Acta Geodynamica et Geomaterialia

 
Title: COUPLING MECHANISM OF VOLUME STRAIN AND WATER LEVEL IN THE FUXIN WELL LOCATED IN A GEOTHERMAL AREA BEFORE AND AFTER THE 2011 MW 9.1 TOHOKU EARTHQUAKE
 
Authors: Zhang Yan, Fu Li-Yun, Zhao Lian-feng and Chen Xuezhong
 
DOI: 10.13168/AGG.2016.0028
 
Journal: Acta Geodynamica et Geomaterialia, Vol. 14, No. 1 (185), Prague 2017
 
Full Text: PDF file (1.3 MB)
 
Keywords: coupling mechanism; tele-seismic waves; volume strain; water level; 2011 Mw 9.1 Tohoku earthquake.
 
Abstract: According to previously reported observations, low-frequency (~0.01–500 Hz) seismic waves can enhance permeability and alter the distribution of pore pressure, which in turn, enhance geothermal energy/oil production. To quantify the effects of low-frequency, dynamic stress stimulation on fluid flow and in-situ pore pressure variations in porous media, observations were conducted in the Fuxin well (located in a geothermal area in north-eastern China), before and after the far-field Mw 9.1 Tohoku huge earthquake. In doing so, precise measurements of earthquake-induced stress, strain, aquifer characteristics and fluid flow parameters could be obtained. We apply tidal analysis to the volumetric strain and water level data of the well. We demonstrate that the tele-seismic waves with a central frequency of 0.02 Hz hardly had an impact on the undrained poroelastic parameters of the aquifer, indicating that the tele-seismic waves cannot induce the disruption of the aquifer medium directly, although it can easily enhance permeability and induce the transfer of pore pressure in the aquifer. Together with the phases, the signs of the co-seismic amplitude change as well as the co-seismic and post-seismic patterns of volumetric strain and water level, we obtain the coupling mechanism of the volume strain change and the water level change: Before the earthquake, tidal volumetric strain induced the change of water level (the phase of the water level lagged behind that of the volume strain). In comparison, during the earthquake, the permeability enhanced by tele-seismic waves, water level (pore pressure) transferred from high pressure region to the Fuxin well, and induced the dilatation of volume strain (phase shifts increased sharply, indicating the change of water level caught up with that of volume strain). Finally, during the short time span after the earthquake, the water level and volumetric strain interacted with each other and changed consistently (phase shifts approach 0°).