Changes in Seismicity and Stress in Response to Fluid Injection, Paradox Valley, Colorado
Abstract
The Bureau of Reclamation injects brine 4.3 to 4.8 km below the surface into the Leadville limestone along the southwest margin of the Paradox Valley, Colorado, initially at a rate of 1300 L/min, which was reduced to 800 l/min in mid 2000. Seismicity in this region was low until injection began in 1991. Injection has produced abundant seismicity within the Wray-Mesa fault system flanking Paradox Valley. By 2003, about 3.44 million cubic meters of brine had been injected, excess fluid pressure had increased to 32.23 MPa, inducing more than 4000 surface recorded seismic events. Through 2003, induced seismicity produced 89% strike-slip focal mechanisms with average P- and T-axes oriented coincident with the average P- and T-axes orientation from tectonic earthquakes in the surrounding area (Ake et al., 2005). Between 2003 and 2009, an additional 2.15 million cubic meters of brine was injected. Excess pressure increased by 1.55 MPa and the average P-axis azimuth of induced strike-slip earthquakes within the Wray-Mesa fault system rotated 10-15 degrees counterclockwise toward the normal to the primary normal faults of the Wray-Mesa fault system near the injection well. The proportion of strike slip events also decreased, comprising only 78% of the 2006-2009 focal mechanisms. In 2006-2009 there was a marked increase in near-vertical dip-slip and reverse slip earthquakes, with one reverse event in 2006, four in 2008, and six in 2009. These changes in seismicity reflect local rotation of stress orientation coincident with increases in fluid pressure near the injection well. Since the beginning of 2002 to the present magnitude recurrence has also markedly changed over time (Block and Wood, 2010). Prior to 2002, b-values were about 0.8 (Ake et al., 2005), consistent with observations of earthquake occurrence in the Colorado Plateau. The b-value dropped to 0.6 in the 2002-2003 time period, suggesting a transition from a phase of release of pre-existing tectonic shear stress to a regime where total injected volume and fluid pressure began to significantly change stresses near the injection well (Ake et al., 2005). The period from 2001 to 2008 produced only 0.5 M 2.5+ events/year, but 2009 produced eight M 2.5+ events and a very small b-value (Block and Wood, 2010). Finite element modeling of the stress changes support the interpretation that there has been minimal leakage of fluid from the fault zone into the host rock. Rather, the counterclockwise rotation of induced event average P- and T-axis azimuths and decrease of b-value as total injected volume increases all indicate that long-term injection has compressed the fluid in the fault system, inflating the Wray Mesa fault system and significantly altering the stresses that induce earthquakes within the Wray-Mesa fault system within 3-4 km of the injection well.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.S13B1999D
- Keywords:
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- 7200 SEISMOLOGY;
- 7230 SEISMOLOGY / Seismicity and tectonics