Electric Aftershocks

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Science  20 Jul 2007:
Vol. 317, Issue 5836, pp. 296
DOI: 10.1126/science.317.5836.296a

Earthquake ruptures are expected to generate electromagnetic activity within the surrounding rocks, but direct evidence for this effect has been lacking. Laboratory experiments on real rocks do generate currents due to fluid movement and piezoelectric effects, but they are weak and in the geological setting it is hard to disentangle them from anthropogenic signals or more ambient electronic noise. Park et al. report possible detection of a characteristic electrical signal using an electrode array placed on the San Andreas Fault at Parkfield, California. Electrical disturbances lasting 3 hours were picked up within 250 m of the fault immediately after a magnitude 6.0 earthquake that occurred in September 2004; signals of opposite polarity were subsequently detected after two magnitude 5.0 aftershocks. Although similar electromagnetic changes do occur on a daily basis in this area, the team argue that the localization, timing, and unusual polarity of their signals support association with the earthquake rupture process. They propose fluid movements as the most likely cause of the electrical signals, although they are unable to explain the rapid onset. No precursor signals were observed, so this technique may not ultimately help with earthquake prediction. — JB

J. Geophys. Res. 112, 10.1029/2005JB004196 (2007).

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