Jenny S. Nakai

Seismicity of the Rocky Mountains and Rio Grande Rift from the EarthScope Transportable Array and CREST temporary seismic networks, 2008‐2010

We developed a catalog of small magnitude (ML −0.1 to 4.7) seismicity across Colorado and New Mexico from the EarthScope USArray Transportable Array and CREST (Colorado Rocky Mountains Experiment and Seismic Transects) seismic networks from 2008 to 2010 to characterize active deformation in the Rio Grande Rift. We recorded over 900 earthquakes in the Rio Grande Rift region, not including induced earthquakes and mine blasts, and find that the rift is actively deforming both broadly and in distinct regions. Seismic events that are likely induced, mostly in the Raton Basin, make up 66% of the catalog (1837 earthquakes). Neogene faults in the northern rift in north central Colorado are seismically active in the North Park Basin and northwestern Colorado. The central rift from the San Luis Basin (southern Colorado) to south of the Socorro Magma Body is the most seismically active rift region, and seismicity delineates the deformation in the Colorado Plateau transition zone, which is spatially correlated with volcanic vents, dikes, and faults within the western Jemez Lineament. The eastern Jemez Lineament is nearly aseismic and surrounded by a halo of seismicity culminating in boundaries defined by recent moderate (Mw 3.9 and Mw 3.3) earthquakes. The southern rift is characterized by diffuse seismicity in Texas and Mexico. This study provides an updated seismic catalog built with uniformity in seismometer coverage and low epicentral uncertainties (~2 km) that allows for regional evaluation of seismicity. During this time period, clusters of seismicity and moderate magnitude earthquakes characterize deformation in a low-strain rate extensional environment.

Evaluating the effectiveness of induced seismicity mitigation: Numerical modeling of wastewater injection near Greeley, Colorado

Mitigation of injection-induced seismicity in Greeley, Colorado, is based largely on proximity of wastewater disposal wells to seismicity and consists of cementation of the bottom of wells to eliminate connection between the disposal interval and crystalline basement. Brief injection rate reductions followed felt events, but injection rates returned to high levels, >250,000 barrels/month, within six months. While brief rate reduction reduces seismicity in the short term, overall seismicity is not reduced. We examine contributions to pore pressure change by injection from twenty-two wells within 30 kilometers of the center of seismicity. The combined injection rate of seven disposal wells within 15 kilometers of the seismicity (Greeley Wells) is correlated with the seismicity rate. We find that injection from NGL-C4A, the well previously suspected as the likely cause of the induced seismicity, is responsible for ~28% of pore pressure increase. The other six Greeley Wells contribute ~28% of pore pressure increase, and the fifteen Far-field Wells between 15 and 30 kilometers from the seismicity contribute ~44% of pore pressure increase. Modeling results show that NGL-C4A plays the largest role in increased pore pressure, but shows the six other Greeley Wells have approximately the same influence as NGL-C4A. Furthermore, the fifteen Far-field Wells have significant influence on pore pressure near the seismicity. Since the main mitigation action of cementing the bottom of wells has not decreased seismicity, mitigation based on reduced injection rates and spacing wells farther apart would likely have a higher potential for success.