Determining injection-induced earthquake magnitude and ground motion exceedance probability using Paradox Valley, Colorado, injection and seismicity data including parametric sensitivities
Proceedings of the Thirty-Eighth Workshop on Geothermal Reservoir Engineering, SGP-TR-198, Stanford University, (February 2013)Abstract
Using the injection volume and earthquake magnitude exceedance relations outlined in Shapiro et al. (2010) and an extensive Paradox Valley (Northern Colorado) injection and seismic dataset first documented in Ake et al. (2005) we computed an Mmax of 1.2 at the 95\% exceedance probability. This result is remarkably close to the observed Mmax of 0.9 (at the 92\% exceedance probability) for the Paradox Valley 1991 14-day 11,000 m3 initial injection sequence. In an effort to understand the difference between predicted Mmax (5.7 at the 95\% exceedance probability) and observed Mmax (4.3 at the 55\% exceedance probability) for the Paradox Valley 1996-2000 >2,000,000 m3 injection sequence, we investigate the impact of parametric variability on hazard estimates. Key parameters in these injection- related seismicity hazard estimates are the maximum pore pressure necessary to create displacement along randomly-oriented cracks, Cmax, poroelastic uniaxial storage coefficient, and cumulative injection volume at the time of the end of injection. Predicted induced Mmax is much less sensitive to crack density, background earthquake activity rate, and earthquake recurrence b values derived from later injection. Real-time seismicity monitoring during injection is essential to confirm or update these key parameters to obtain robust estimates of Mmax.