Unexpected far-field hydrological response to a great earthquake.

Permeability, an important crustal property that controls transport processes in Earth's shallow crust, is known to change during large earthquakes, with significant implications on the groundwater resources and contaminant transport. Analysis of the response of groundwater in wells to Earth tides has been increasingly used to detect such changes, but often multiple interpretations of the response changes are possible, making it difficult to ascertain its hydrological implications. Here we use data from a groundwater well in North China to illustrate the use of barometric response as a method to resolve this uncertainty. The tidal response of water level in this well shows abrupt coseismic increases of both phase shift and amplitude ratio after the far-field great 2011 M w 9.1 Tohoku earthquake, which may be interpreted either as a large increase in the horizontal permeability of a confined aquifer or as an occurrence in the vertical permeability of an unconfined aquifer. Using data for the water-level response to barometric pressure we show that the aquifer was confined both before and after the Tohoku earthquake; thus the coseismic changes of the tidal response in this well are more likely to be due to an increase in the horizontal permeability. Interpretation of the tidal response shows a hereto unidentified change of poroelastic property in the far field of an earthquake, which stayed constant for ∼4 years after the earthquake until silt-cleaning in the well interrupted data collection. Thus the dynamic shaking during the Tohoku earthquake may have caused permanent deformation in the shallow crust at epicentral distances >1500 km. • The 2011 M w 9.1 Tohoku earthquake caused lasting hydrologic response in the far field. • Barometric response is used as an extra check of the post-seismic aquifer confinement. • Post-seismic hydraulic & poroelastic property changed for >4 yr in a far-field aquifer. • The earthquake may have caused permanent crustal deformation at distances >1500 km. [ABSTRACT FROM AUTHOR]