Report

Continuous Permeability Measurements Record Healing Inside the Wenchuan Earthquake Fault Zone

Science  28 Jun 2013:
Vol. 340, Issue 6140, pp. 1555-1559
DOI: 10.1126/science.1237237

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Water at the Bottom of a Well

Earthquakes generate numerous fractures as they propagate through an underground fault zone. These fractures strongly influence the way in which fluids flow in the subsurface, and the permeability of fault zones is often used as a proxy for the extent of fracturing. Following the 2008 Mw 7.9 Wenchuan earthquake in central China, several wells were drilled in and around the fault zone to understand the mechanics of the earthquake. Because the bottoms of these deep boreholes were open, the water levels in the wells were sensitive to tidal forces acting on the surrounding rock. Through continuous measurements of water levels over 1.5 years, Xue et al. (p. 1555) found that the rate at which water was pumped in and out of the borehole was proportional to the permeability of the fault zone, providing a direct way to measure the evolution of the hydrologic properties of a fault zone following a major earthquake. Permeability decreased ∼25% during that time, suggesting that fractures generated in fault zones heal relatively rapidly.

Abstract

Permeability controls fluid flow in fault zones and is a proxy for rock damage after an earthquake. We used the tidal response of water level in a deep borehole to track permeability for 18 months in the damage zone of the causative fault of the 2008 moment magnitude 7.9 Wenchuan earthquake. The unusually high measured hydraulic diffusivity of 2.4 × 10−2 square meters per second implies a major role for water circulation in the fault zone. For most of the observation period, the permeability decreased rapidly as the fault healed. The trend was interrupted by abrupt permeability increases attributable to shaking from remote earthquakes. These direct measurements of the fault zone reveal a process of punctuated recovery as healing and damage interact in the aftermath of a major earthquake.

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