Post-earthquake ground movements correlated to pore-pressure transients

Abstract

Large earthquakes alter the stress in the surrounding crust, leading to triggered earthquakes and aftershocks. A number of time-dependent processes, including afterslip, pore-fluid flow and viscous relaxation of the lower crust and upper mantle, further modify the stress and pore pressure near the fault, and hence the tendency for triggered earthquakes. It has proved difficult, however, to distinguish between these processes on the basis of direct field observations, despite considerable effort. Here we present a unique combination of measurements consisting of satellite radar interferograms and water-level changes in geothermal wells following two magnitude-6.5 earthquakes in the south Iceland seismic zone. The deformation recorded in the interferograms cannot be explained by either afterslip or visco-elastic relaxation, but is consistent with rebound of a porous elastic material in the first 1-2 months following the earthquakes. This interpretation is confirmed by direct measurements which show rapid (1-2-month) recovery of the earthquake-induced water-level changes. In contrast, the duration of the aftershock sequence is projected to be approximately 3.5 years, suggesting that pore-fluid flow does not control aftershock duration. But because the surface strains are dominated by pore-pressure changes in the shallow crust, we cannot rule out a longer pore-pressure transient at the depth of the aftershocks. The aftershock duration is consistent with models of seismicity rate variations based on rate- and state-dependent friction laws.

The earthquake deformation cycle

“…Poroelastic deformation was most clearly observed after two moderate earthquakes in Iceland in June 2000 (Jonsson et al, 2003), where independent observations from hydrothermal water wells could confirm that water was flowing. During the earthquakes, the water level rose in wells that were in regions around the fault that contracted, and dropped in wells that were in regions that dilated.…”

Groundwater level changes induced by the 2011 Tohoku earthquake in China mainland

“…Step-like changes of groundwater level in the near field of an earthquake can often be quantitatively explained by the poroelastic pressure response to the earthquake's static strain field (Wakita 1975;Quilty & Roeloffs 1997;Jonsson et al 2003). However, the static strain mechanism can hardly explain groundwater level changes in the intermediate or far field.…”

Gravity changes observed between 2004 and 2009 near the Tokai slow-slip area and prospects for detecting fluid flow during future slow-slip events

“…This result indicates that the effects of near-surface groundwater and pore-fluid flow driven by elastic deformation (e.g. Jónsson et al, 2003) would be too small to explain the observed trends because changes in fluid density are at most of the same order of magnitude as the elastic volumetric changes. The gravity changes estimated by the model of Ogawa and Heki (2007), which is based on elastic deformation, also turned out to be too small.…”