Is the crust in intraplate regions critically stressed?

The emergence in 2008 of seismicity induced by energy industry practices in the Central United States (CUS) has presented both a challenge and an opportunity to address pressing questions about intraplate deformation. Human activity is reactivating slip on long-dormant faults by perturbing the state of stress of these faults through either wastewater injection or hydrocarbon production stimulation. By illuminating the presence, orientation and dimension of faults that are near failure and favorably oriented to the present stress field, induced seismicity provides a window into the stress conditions of intraplate faults and into the processes that drive seismicity in stable continental interiors. Thus, the emergence of induced seismicity can be viewed as one of the largest intraplate earthquake and tectonic experiments at the continental scale of our history.

Today we understand that subsurface pressure changes resulting from fluid injections can trigger earthquakes over a range ofdistances and times. The resulting earthquake productivity also varies markedly between sedimentary basins. A key observation is that even small fluid pressure perturbations can initiate slip on preexisting faults. This corroborates the concept of a criticallystressed crust, in which faults sit close to frictional failure. This, together with the observation that fluid pressures appear to remain at hydrostatic levels, is proposed to explain the occurrence of fault slip in intraplate regions. The hypothesis implies that faults rupture repeatedly, thereby preserving permeability and dissipating overpressure in the crust. Much of the research on intraplate seismicity is, in fact, framed within this hypothesis.

But there’s the rub: this hypothesis appears to be inconsistent with other key observations emerging from regions affected byinduced seismicity. In this presentation we analyze and compare the long-term fault displacement in regions of the CUS where seismicity is interpreted to be anthropogenic versus of natural origin. In regions of natural seismicity, faults exhibit a long deformation history, in agreement with the hypothesis of a critically stressed crust. But in regions of induced seismicity, we employ high resolution seismic reflection data to show that faults failing today due to wastewater injection had little to no activityfor the past 300 million years. Thus, while these latter faults must have been close to failure, as predicted by the critically stressed crust hypothesis, it does not explain their quiescence over such long time.

As research progresses and data availability improves, this contradiction is becoming more acute. We are learning that the changesin pressure necessary to cause faults to slip are vanishingly small, indicating that faults are precariously close to failure. At the same time, high-quality data show that these faults have been largely inactive for millions of years. Reconciling these observations requires moving from the macro scale of the seismic reflection images to the micro scale of rock mechanics of the faulting process, to understand the conditions that favor slip in the basement of the Central US in particular, and of other regions in the world in general.