Changes in Permeability Produced By Distant Earthquakes

Abstract:

Oscillations in stress, such as those created by earthquakes, can increase permeability and fluid mobility in geologic media. In natural systems, strain amplitudes as small as 10^-6 can increase discharge in streams and springs, change the water level of wells, and enhance production from petroleum reservoirs. Enhanced permeability typically recovers to pre-stimulated values over a period of months to years.

This presentation will review some of the observations that indicate that dynamic stresses produced by seismic waves change permeability. We use the response of a set of wells distributed throughout China to multiple large earthquakes to probe the relationship between earthquake-generated stresses and water-level changes in wells. We find that dynamic stresses dominate the responses at distances more than 1 fault length from the earthquake and that permeability changes may explain the water level changes. Regions with high deformation rates are most sensitive to seismic waves. We also consider the response of a large alluvial fan in Taiwan to the 1999 M7.5 Chi-Chi earthquake where there were sustained changes in groundwater temperature after the earthquake. Using groundwater flow models, we infer that permeability increased by an order of magnitude over horizontal scales of tens of km, and vertical scales of several km. Permeability returned to the pre-earthquake value over many months. As much as half the total transport in the fan occurs during the short time periods with enhanced permeability.