Letter | Published:

Aftershocks driven by a high-pressure CO2 source at depth

Abstract

In northern Italy in 1997, two earthquakes of magnitudes 5.7 and 6 (separated by nine hours) marked the beginning of a sequence that lasted more than 30 days, with thousands of aftershocks including four additional events with magnitudes between 5 and 6. This normal-faulting sequence is not well explained with models of elastic stress transfer1,2, particularly the persistence of hanging-wall seismicity3 that included two events with magnitudes greater than 5. Here we show that this sequence may have been driven by a fluid pressure pulse generated from the coseismic release of a known deep source4 of trapped high-pressure carbon dioxide (CO2). We find a strong correlation between the high-pressure front and the aftershock hypocentres over a two-week period, using precise hypocentre locations5 and a simple model of nonlinear diffusion. The triggering amplitude (10–20 MPa) of the pressure pulse overwhelms the typical (0.1–0.2 MPa) range from stress changes in the usual stress triggering models1,6. We propose that aftershocks of large earthquakes in such geologic environments may be driven by the coseismic release of trapped, high-pressure fluids propagating through damaged zones created by the mainshock. This may provide a link between earthquakes, aftershocks, crust/mantle degassing and earthquake-triggered large-scale fluid flow.

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Acknowledgements

We thank C. H. Scholz for comments, and D. Giardini, Y. Y. Podladchikov, J. A. D. Connolly, T. Kohl, K. Evans, G. Hillers and N. Deichmann for discussions.Authors' contributions C.C., structural geology and concept development; L.C. and M.C., aftershock data; M.B., structural geology; B.J.P.K., numerical model development.

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Correspondence to Stephen A. Miller.

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Figure 1: Geologic setting of the study area.
Figure 2: Map view of the seismicity and the rate of propagation.
Figure 3: Comparison of model results with initial conditions (top) to the hypocentres of aftershocks (bottom) shown in yellow in Fig. 2a.
Figure 4: Comparison of aftershock data to stress changes in the ΔCFS formulation and pore pressure changes.

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