Research ArticleGEOPHYSICS

Shear-wave anisotropy reveals pore fluid pressure–induced seismicity in the U.S. midcontinent

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Science Advances  13 Dec 2017:
Vol. 3, no. 12, e1700443
DOI: 10.1126/sciadv.1700443

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Seismicity in the U.S. midcontinent has increased by orders of magnitude over the past decade. Spatiotemporal correlations of seismicity to wastewater injection operations have suggested that injection-related pore fluid pressure increases are inducing the earthquakes. We present direct evidence linking earthquake occurrence to pore pressure increase in the U.S. midcontinent through time-lapse shear-wave (S-wave) anisotropy analysis. Since the onset of the observation period in 2010, the orientation of the fast S-wave polarization has flipped from inline with the maximum horizontal stress to inline with the minimum horizontal stress, a change known to be associated with critical pore pressure buildup. The time delay between fast and slow S-wave arrivals exhibits increased variance through time, which is common in critical pore fluid settings. Near-basement borehole fluid pressure measurements indicate pore pressure increase in the region over the earthquake monitoring period.

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