Mechanism of co-seismic volumetric strain variation in the far field of great earthquakes

Abstract:

Several recent studies show that coseismic change of the volumetric strain measured in the far field of large earthquakes is often far greater than that predicted from static strain theory. The underlying reason for this disagreement between measurement and theory is not yet understood, but is important for the interpretation of strain data. Here we apply tidal analysis to the data of volumetric strain and water level documented in the Fuxin well, northeastern China, before and after the 2011 M_w 9.1 Tohoku earthquake, the 2012 M_w 8.6 Sumatra earthquake and the 2015 M_w 7.9 Nepal earthquake. Before each earthquake, water level fluctuations in the well had a negative phase shift due to the delay of groundwater flow to the well driven by the tidal volumetric strain of aquifers. Immediately following each earthquake, however, the phase of the water-level fluctuations began to lead that of the volumetric strain, suggesting that the coseismic change of strain may reflect the coseismic change of pore pressure in the aquifer. Furthermore, the hypothesis is consistent with the available data for the amplitude of the co-seismic volumetric strain and water level.