Effects of Pore Pressure Change on Coulomb Stress Change and Initiation of Induced Seismicity in the Raton Basin, Colorado–New Mexico

We investigate the effect of pore pressure on Coulomb static stress calculations of induced earthquakes in the Raton basin on the Colorado–New Mexico border. Change in Coulomb static stress (ΔCSS) on faults can bring the area closer or further from failure. Wastewater injection in the basin began in 1994, and multiple sequences of induced seismicity have occurred, especially since 2001. Overall Coulomb static stress includes the pore pressure increase from wastewater injection and Coulomb static stress transfer from induced earthquakes. We use three models to account for these two components: (1) constant apparent friction model in which pore pressure effect is folded into friction coefficient; (2) isotropic model in which pore pressure is computed as a function of volumetric strain; and (3) diffusion model in which pore pressure is obtained from a groundwater diffusion model. These three models are used to calculate the ΔCSS caused in part by Mw 4.0 or larger earthquakes that occurred prior to the largest Mw 5.3 event in 2011. The differences in modeled results clearly show that pore pressure is an important causal mechanism for initiating seismicity. In particular, the diffusion model best explains the locations of the seismicity. Model results indicate that deliberate inclusion of pore pressure in ΔCSS calculations is necessary; oversimplification of the pore pressure component can significantly change the distribution and value of ΔCSS in model results.