Does effective stress have reduced sensitivity to pore pressure at seismogenic depths?

Patrick M. Fulton, Szu-Ting Kuo, Hiroko Kitajima, & Xiaoda Liu

Submitted August 15, 2018, SCEC Contribution #8811, 2018 SCEC Annual Meeting Poster #180

The importance of fluids in fault mechanics is well-recognized. Pore pressure is known to affect elastic deformation and brittle failure through its control on effective stress, where Effective Stress = Stress – alpha * Pore Pressure.

However, the sensitivity of effective stress to pore pressure, controlled by the term alpha, may be less than the commonly assumed value of 1. Here we present an analysis of data from new rock deformation experiments conducted within the John W. Handin Laboratory for Experimental Rock Deformation at Texas A&M and a reanalysis of extant data to evaluate the pore pressure sensitivity of rocks under conditions representative of the brittle seismogenic zone.

Three key observations result from our analysis:

1) When considering the influence of effective stress on elastic deformation, we find that alpha is less than one for a variety of sandstones and for granite. In addition, this alpha for elastic deformation exhibits a stress dependence; it decreases with increasing confining pressure, consistent with theoretical considerations.

2) We also find alpha values less than one for the influence of effective stress on brittle failure. Similar to the effect on elastic deformation, we also find suggestions of a stress dependence on alpha for brittle failure.

3) Lastly, alpha increases as a result of rock damage. In other words, the same rock becomes more sensitive to pore pressure after it is damaged.

Together these results can help provide insight into properties and processes affecting the stress state on faults within Southern California and how they may respond to sudden transients.

Key Words
Effective stress, pore pressure

Fulton, P. M., Kuo, S., Kitajima, H., & Liu, X. (2018, 08). Does effective stress have reduced sensitivity to pore pressure at seismogenic depths?. Poster Presentation at 2018 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)