Comparison of hydrological responses to the Wenchuan and Lushan earthquakes

- Water-level changes by two earthquakes are used to test proposed mechanisms.
- These changes are inconsistent with the static strain hypothesis.
- Enhanced permeability may be the dominant mechanism for the changes.
- Liquefaction was far more sensitive to the first earthquake than to the second.

On 20th April, 2013, a large earthquake (Lushan Mw 6.6) occurred in Sichuan, China, in the same fault zone as the devastating 2008Mw 7.9 Wenchuan earthquake. The two earthquakes have similar focal mechanisms and both caused large hydrological changes in the region. The similarity of focal mechanisms, fault zone geology and the abundance of hydrological responses provide a rare opportunity for testing proposed mechanisms for hydrological responses to earthquakes. Using data from wells installed in hard rocks, we find that both the magnitude and the sign of water level changes are inconsistent with those predicted by the coseismic strain hypothesis in the near-field and in the intermediate-field. Instead, permeability change may be the dominant mechanism for the coseismic changes in water level. We also find that the minimum seismic energy required to trigger liquefaction for the Wenchuan earthquake is only 1/20 of that for the Lushan earthquake, suggesting either that the occurrence of liquefaction is more sensitive to low seismic frequencies or that the Wenchuan earthquake changed the sensitivity of unconsolidated materials, and properties had not completely recovered during the 5-year interval between the two earthquakes.